Rotating Spaceships

[Rossco]

Quick question for our space guys;

In lots of fiction/close to reality films including Stowaway, they depict rotating spaceships with each 'end' of the rotating ring being huge.
Is there a reason for this? (Other than it looks cool?) is it something to do with speed? i.e the smaller the ring the faster it has to turn to achieve the same G levels?

[KelvinZero]

Do you mean why is the radius large? There is evidence that a small radius would make people feel nauseous because the force would be different between head and toes, and there would be weird effects to you balance sense when you turn your head.

Here is a nice youtube clip that talks about the issues and makes a guess at how small a radius we could get away with:



(but generally speaking, the bigger, the more comfortably earthlike the sensation of gravity would be.. we don't know for certain what radius humans would readily adapt to)

[Rossco]

That's great, thanks for the reply, I'll have a watch of the video later.

My question was based on a video from SpaceXvision of their artificial gravity Starship concept, they show two sides extending out of each side of the Starship, which in principle is a fine idea, however they extend theirs out massively which then got me thinking if it would be that necessary or if just expanding it closer to the body would give the same result.

[daedalus1]

That's great, thanks for the reply, I'll have a watch of the video later.

My question was based on a video from SpaceXvision of their artificial gravity Starship concept, they show two sides extending out of each side of the Starship, which in principle is a fine idea, however they extend theirs out massively which then got me thinking if it would be that necessary or if just expanding it closer to the body would give the same result.

There is no need to rotate the spacecraft around its longitudinal axis thereby requiring large diameter. It can be tumbled end over end . As the spacecraft is probably already quite long there is no need for extra hardware.

[jbenton]

That's great, thanks for the reply, I'll have a watch of the video later.

My question was based on a video from SpaceXvision of their artificial gravity Starship concept, they show two sides extending out of each side of the Starship, which in principle is a fine idea, however they extend theirs out massively which then got me thinking if it would be that necessary or if just expanding it closer to the body would give the same result.

There is no need to rotate the spacecraft around its longitudinal axis thereby requiring large diameter. It can be tumbled end over end . As the spacecraft is probably already quite long there is no need for extra hardware.

I heard that there was an idea to have 2 Starships launch together, and have them dock (during the coast phase) from the aft end (which is designed to dock with a tanker anyways). That would give them twice the length for end over end tumbling.

[Rossco]

I think its the thought of having the whole thing tumble end over end is somewhat less elegant - forgetting of course, there is no 'up down forward or back' in space & the need for aerodynamics is also not required. 

[Paul451]

they depict rotating spaceships with each 'end' of the rotating ring being huge.
Is there a reason for this? (Other than it looks cool?) is it something to do with speed? i.e the smaller the ring the faster it has to turn to achieve the same G levels?

Yes. Centripetal acceleration is inversely proportional to radius, and proportional to the square of RPM. Ie, double the speed of rotation, you get four times the g-load. Halve the radius at the same RPM, you halve the g-load.

In addition, there was a lot of early research which said people couldn't adapt to higher rates of rotation. (There was also concern about having a large difference in g-levels between your head and feet.) So if you wanted higher g-loads, you would have to have a very large radius. For example, if you were limited to 1 RPM and wanted 1g, you needed a spacecraft more than a mile wide.

If you want to play with the numbers, there's an online calculator at: https://www.artificial-gravity.com/sw/SpinCalc/

Play with radius in units you're comfortable with (and remember it's radius not diameter), acceleration in "g" (Earth gravities) and angular velocity in rotations/minute (RPMs). Use different sizes, spins, etc, to see the relationships between size/spin/g-load. (Ignore the tangential velocity. Useful for catapults, but not relevant here.) The coloured indicators will also give you the "safety/comfort" according to the older research.

However, more recent research suggests people can adapt to extremely high RPMs, and that differences between head/feet aren't an issue. Throw in that we don't have any data of the lowest acceptable levels of gravity to avoid micro-gravity issues. It will likely turn out that the only thing limiting how small you can go is the physical space you need for the astronauts. But the maths of size/spin/g-load still applies.

[Rossco]

That's great info, thanks for the reply. Interesting stuff.

[Vultur]

However, more recent research suggests people can adapt to extremely high RPMs

That's really cool/promising! Do you have any links or names of papers?

[Paul451]

However, more recent research suggests people can adapt to extremely high RPMs

That's really cool/promising! Do you have any links or names of papers?

If you go to Spincalc, Ted discusses old vs new research in his sources section. You want Lackner and DiZio as your starting point. However, most of their work is on trying to "break" the vestibular system in order to figure out how it works, pure research. But some of their early stuff was directly related to spin-gravity. For example: https://www.researchgate.net/publication/8607002_Adaptation_to_rotating_artificial_gravity_environments (semi-paywalled.)

Abstract
A series of pioneering experiments on adaptation to rotating artificial gravity environments was conducted in the 1960s. The results of these experiments led to the general belief that humans with normal vestibular function would not be able to adapt to rotating environments with angular velocities above 3 or 4 rpm. By contrast, our recent work has shown that sensory-motor adaptation to 10 rpm can be achieved relatively easily and quickly if subjects make the same movement repeatedly. This repetition allows the nervous system to gauge how the Coriolis forces generated by movements in a rotating reference frame are deflecting movement paths and endpoints and to institute corrective adaptations. Independent mechanisms appear to underlie restoration of straight movement paths and of accurate movement endpoints. Control of head movements involves adaptation of vestibulo-collic and vestibulo-spinal mechanisms as well as adaptation to motor control of the head as an inertial mass. The vestibular adaptation has a long time constant and the motor adaptation a short one. Surprisingly, Coriolis forces generated by natural turning and reaching movements in our normal environment are typically larger than those elicited in rotating artificial gravity environments. They are not recognized as such because self-generated Coriolis forces during voluntary trunk rotation are perceptually transparent. After adaptation to a rotating environment is complete, the Coriolis forces generated by movements within it also become transparent and are not felt although they are still present.

I suspect the reason the early research got such different results (both from newer results and from each other, where the nausea limit could be as low as 1RPM or as high as 6RPM) is that they missed the importance of movement to retrain your brain. We also have a default assumption, which I've heard in many spin-gravity/rotating-spacestation threads, that we could tolerate higher RPMs if we limit head movement. We probably also limit head movement if we experience nausea, which compounds the variation between research.

The trick during training seems to be to reduce RPM below the point of nausea, then move around to adapt. Then increase the RPM to the new limit of nausea, move around again. Rinse-repeat until you reach the desired rate of spin. So the early research that produced higher limits, I suspect that when trying to find "the" limit, their testing protocol was to test at low RPM, have the test subject move their head/body around to test whether they were experiencing nausea, if not, they increased the RPMs to a higher level, repeated testing, increase/test, increase/test. Inadvertently, they were giving the test subjects increased tolerance. Whereas researchers who went straight to the target RPM, or tested at high RPM then dropped down until the subjects stopped feeling nauseous, the subjects didn't get a chance to develop increased tolerance.

[KelvinZero]

However, more recent research suggests people can adapt to extremely high RPMs

That's really cool/promising! Do you have any links or names of papers?

That youtube clip from David Kipping at Colombia University also seemed to be quite optimistic and includes a bunch of academic references in the text below it. I would guess anything claimed in the clip is justified in one of those references.

[Vultur]

If you go to Spincalc, Ted discusses old vs new research in his sources section. You want Lackner and DiZio as your starting point. However, most of their work is on trying to "break" the vestibular system in order to figure out how it works, pure research. But some of their early stuff was directly related to spin-gravity. For example: https://www.researchgate.net/publication/8607002_Adaptation_to_rotating_artificial_gravity_environments (semi-paywalled.)

Cool, thanks! That does look more promising than other numbers I've seen.

[Twark_Main]

Ignore the tangential velocity. Useful for catapults, but not relevant here.

The tangential velocity tells you how much delta-v the spacecraft needs to reserve for spin-up or spin-down. This is helpful for calculating the total mass penalty of the AG system, including fuel.

[Shevek23]

Another use of the tangential velocity--it is another way to calculate the radius needed for a given G effect.

The formula is pretty simple for this--A=(V^2)/R where A is acceleration, and V is this purportedly to be ignored "tangential velocity" and R is the radius. This means the square of V is radius times the G we want, so the square root of those two latter is V. We can also swap around to say R=(V^2)/A. All this is just another way to express the same thing as A=R*(Omega^2) where Omega is angular speed.

So say we decide that it darn well would bug us to have our head at half the G our feet are at, but we won't mind if it is just a 25 percent difference. So we want radius to be more than 4 anyway, and we want G to be 10 m/sec^2, a hair over Earth surface G. We know R and we know G, they multiply to 40 so we can see that V would be a bit over 6 m/sec. That means it is 1.5 radians per second, and there are 2Pi radians in the whole circle, so it cycles round every 4 seconds or so which means RPM is 15.

You ask me, that seems like a heck of a fast RPM to adapt to but maybe we can. Such a speed would give us a full G spinning a Starship on its long axis (which is not dynamically stable, we'd need to be tweaking it with reaction wheels or pulsing the reaction engines or it will tend to tumble, shimmying badly on the way to this).

Another example--believing as I do that people cannot adapt to continual low G and need to at least often be in full G in the long run, a Lunar surface colony could not sustain human life unless people get on some kind of centrifuge under a full G much of the time. Minimum size centrifuge is what we are talking about here mostly, but suppose we conservatively guess that say 100 m/sec is the maximum safe speed relative to the surface we can sustain safely--this is comparable to high speed trains in Japan or France OTL. That gives us V=100, thus V squared is 10,000, we want a full G (a hair less but 1/6 G at right angles adds very little per the Pythagorean addition we do here) about 10, so dividing that square by our desired full G we get 1 kilometer in radius--here's where we get 1 RPM, because now the speed is 1/10 radian per second, and it takes 2pi or a bit over 60 seconds to go around, hence roughly 1 RPM. This is what they thought was the maximum RPM in the old days.

Of course we could probably go a lot faster than 100 m/sec relative to the ground, using tricks like magnetic levitation and so on, which would lower the RPM and raise the circle radius. We can also figure on lower G than a full G if it turns out lower G's such as Lunar surface do us some good anyway--I do suspect it is better to spend months in space at lunar G than free fall!

Anyway the tangential velocity is a very useful thing to pay attention to in many circumstances and should not be dismissed, I find it a lot easier to work with the velocity/radius/acceleration relationship than angular speed/Radius/acceleration myself.

It also comes in handy in orbital mechanics.

[spacester]

A 100 m spin radius at 3 rpm will provide almost exactly 1.0 "gee" with a clear expectation that Coriolis effects will not be too difficult to adapt to by healthy humans.

Here's the formula:

G = [R * [(pi*rpm) / 30]^2] / 9.81
OR
R = (9.81 * G) / [(pi*rpm) / 30]^2

Where:
G = Decimal fraction of Earth gravity
R = Radius from center of rotation in meters
pi = 3.14159
rpm = revolutions per minute

Example:
R = 99.4 meters
rpm = 3
plug these in:
G = [99.4 * [(3.14519 * 3) / 30]^2] / 9.81
G = [99.4 * [.0986958]] / 9.81
G = 1.0000


My derivation:
The centripetal acceleration will be A = V^2 / R
where V is the tangential velocity
and R is the radius
And V = W * R
where W is the angular velocity (W = "Omega")
So A = (W*R)^2 / R
A = R * W^2
W can be confusing on the units, it's in radians per second and there are 2*pi radians in a revolution, so
rpm = (W radians/sec) (60 sec/min ) /(2*pi radians / rev)
rpm = (W * 30) / pi
W = ( pi * rpm) / 30
Substituting for W
A = R * [(pi * rpm) / 30 ] ^2
G = A / g
where g = 9.81 m / s^2 (Earth gravity)
So
G = [R * [(pi*rpm) / 30]^2] / 9.81

[Twark_Main]

Ignore the tangential velocity. Useful for catapults, but not relevant here.

The tangential velocity tells you how much delta-v the spacecraft needs to reserve for spin-up or spin-down. This is helpful for calculating the total mass penalty of the AG system, including fuel.

This also tells you how much fuel you need in your rescue jet-pack in case you fall off the station during an EVA. 

[spacester]

Quick question for our space guys;

In lots of fiction/close to reality films including Stowaway, they depict rotating spaceships with each 'end' of the rotating ring being huge.
Is there a reason for this? (Other than it looks cool?) is it something to do with speed? i.e the smaller the ring the faster it has to turn to achieve the same G levels?

Rotation is of course about creating spin-gravity for people and the design of an actual vessel to take our space faring game to the next level is all about the cleverness of the mass distribution.

Generally, you want to put your mass - the human habitat part - as far away from the center of rotation as you can. The first geometry that pops out is a dumbbell, two cylinders separated from each other by a relatively low mass connector.  So two spent rocket stages connected by a tether.

The simple dumbbell does not even make it to back of napkin design stage. Especially if you use a tether, you cannot control the dynamics. Pretty much a nightmare in that category.

So you go to a rigid truss, that improves the spin control. But where are you mounting the thrusters to force that control? And what about a central hub, good idea or bad idea?

So the dumbbell is replaced by more units at the spin radius, so say four cylinders connected by trusses.

Choosing the spin radius and rpm is a big part of the design concept. For me, I settled on R=100 m and 3.0 RPM a long time ago.

So at a diameter of 200 m, pre-starship spent rocket stages will be pretty far apart and lonely. How do you circulate crew between your four habs? You probably don't.

So how about a connected ring of spent stages? Now we are talking, it doesn't have to be A.C.Clarke's wheel to be awesome.

Enter Starship, and its massive capability to deliver huge payloads on a rapid schedule.

There is a lack of discussion on these forums about what can be done with hundreds of Starship payloads. Still too soon?

For me, it is not too soon to look at how to construct a huge human refuge for up to 1000 people. In fact I have done so.

Minimum mass: 115 payloads at 115 T each = 13225 T
Maximum mass: 150 payloads at 150 T per payload = 22500 T
Design mass: 20000 T (for propulsion calcs)
Central design goal: Provide excellent conditions for robust human health for hundreds of humans, with plants all over the place, various partial gravity apartments, terrific radiation shielding everywhere, abundant energy and water, galley-cooked food, all the hot showers and hot-tub soaks you want.
Design lifetime: 100 years
Ultimate operating orbit: Mars - eight to twelve years in LEO first, then Cis-Lunar, then go help the Martians

[Paul451]

Choosing the spin radius and rpm is a big part of the design concept. For me, I settled on R=100 m and 3.0 RPM a long time ago.

If you go to 4 RPM, you cut the radius to 56m. Nearly half. Go to 6 RPM, radius drops to 25m, just a quarter. Why "settle" on 3?

[Edit: Also, your design might suffer from the intermediate axis problem, depending on how heavy that extended core thing is.]

[spacester]

Choosing the spin radius and rpm is a big part of the design concept. For me, I settled on R=100 m and 3.0 RPM a long time ago.

If you go to 4 RPM, you cut the radius to 56m. Nearly half. Go to 6 RPM, radius drops to 25m, just a quarter. Why "settle" on 3?

[Edit: Also, your design might suffer from the intermediate axis problem, depending on how heavy that extended core thing is.]

5 rpm requires more adaptation. I want 98% of people to be able to deal with the Coriolis no big deal. I once again reviewed the info we have available, which is scanty, and I just think that at 3 rpm no apologies will be needed. The idea is to provide the ultimate safe haven for space travelers. Also, I admit I like the slick math at 3 rpm - radius in m/100 equals gee force. Smaller radius designs are perfectly valid, but this design is firm on that choice.

You are correct about the intermediate axis problem. Well done.

I liked this design a lot until I finally got to the point where I could calculate the moments of intertia, and the ratio of the non-spin moments over the spin axis moment came out higher than I wanted. I don't think it would actually flip without active control, but I want a more inherently stable spin state. Also, the central hub is mostly empty, and I do not want to give that up (movie studio revenue).

So I actually did some work on this project today for the first time in a long while.

Dynamically, the base design is expected to be excellent for just spinning in space and station keeping. But I want propulsion built in and the rocket equation is a bitch so I needed to add a bunch of propellant tanks. I had the prior tanks too far from the plane of the habitat ring. They now form circular arrays all the way out to 80 m radius. I think it looks better to the eye of a dynamicist.

18 m diameter Starship 2.0 to the rescue! I added 80 tanks from 8 launches providing 12,000 T and now I need to run the numbers to check my guess. If there is interest I will update.

[edzieba]

The simple dumbbell does not even make it to back of napkin design stage. Especially if you use a tether, you cannot control the dynamics. Pretty much a nightmare in that category.

I see this asserted pretty often, but without analysis to back it up. For example, see the attached papers on modelling the tether dynamics of the OEDIPUS-A experiment (where unexpected nutation was observed), and the subsequent OEDIPUS-C experiment that validated the model and confirmed stable spinning tether dynamics, using only design parameters to allow passive stability without active control.

[spacester]

The simple dumbbell does not even make it to back of napkin design stage. Especially if you use a tether, you cannot control the dynamics. Pretty much a nightmare in that category.

I see this asserted pretty often, but without analysis to back it up. For example, see the attached papers on modelling the tether dynamics of the OEDIPUS-A experiment (where unexpected nutation was observed), and the subsequent OEDIPUS-C experiment that validated the model and confirmed stable spinning tether dynamics, using only design parameters to allow passive stability without active control.

Well of course I am sure we would all be delighted if someone flew an experiment and proved me wrong. The tether experiments I know about were all rather miserable failures. These were all some years ago and I am not aware of anything in the last 8 years or so. There appears to be quite a gap between the theory and the dynamics of flown hardware. These papers are welcome and I will be studying them to see what I can learn, but tethers are not for me. No doubt there is much I am ignorant of on the subject. I like rigidity.

The design I worked up is the result of making certain design choices. If the most fundamental choices are different, that would constitute different classes of spaceships. Certainly you can see that Aquarius is about as far afield from a tethered dumbbell design as you can get.

Alternate design approaches are something I would like to encourage, but I do not intend to explore all possibilities myself. If someone wants to help me start classifying all the different types of rotating spaceship concepts, I would be happy to be a part of that effort.

[TrevorMonty]

We need to define what the artificial gravity is trying to achieve before designing spacestation.
Here few ideas.
1) long term living ie years with children. Need 1g at low RPM.
2)keep astronaut health on extended trips of months may years. Probably 1/6g at higher RPMs.
3) make daily activities like, eating, drinking going to bathroom more pleasant where gravity helps keep everything in its place. This could be for short stays where we aren't using it reduce bone and muscle loss. Need 1/10 or less at moderate RPM.

Sent from my SM-T810 using Tapatalk

[spacester]

Also, I admit I like the slick math at 3 rpm - radius in m/100 equals gee force. Smaller radius designs are perfectly valid, but this design is firm on that choice.

And that's what I suspected. You latched onto a size, you made the RPM match, then you retrospectively justified that RPM.

It's what everyone does. Want giant station, "we must limit it to 1 RPM, see O'Neill". Feels too unrealistic? "2 RPM can be tolerated without adaptation, that's the obvious limit." Don't want a tether, "3 RPM can be tolerated by most people. Or was it 4? But definitely no more than that."

I don't object to you wanting to pick an arbitrary size, just don't pretend it's "necessary", that it's based on science.

And, IMO, if you stop pretending that the spin rate (and hence the size) is "necessary", you can also look at how to build up towards your final preferred size. Can you start smaller, but using the same modular components in lower quantities, and deliver a useful design earlier in the construction process? Can it be upgraded over time without significant disruption and dismantling? Even if it has to stop spinning to deploy new sections, can other maintenance and assembly work be done (safely) while spinning?

Wow you are a mind reader! Not.

It is not arbitrary whatsoever! It is based on science, what else? I want 1.0 gee on the outer ring (duh, what else, see the main objective), and 3 RPM is right in the center of the sweet spot. Find me any references that begin to say that it is a poor choice. I have done the research, I did it 15 years ago, have been posting about spin gravity ever since, on occasion.  Have you done the research? It doesn't sound like it. And so you seem to assume I have not either. Wrong. Rude.

I gave my reasons, address them. The central design concept is an ultra comfortable on-orbit refuge, I am not gonna spin it too fast for my market. If you want to have an informed and detailed discussion on why 3 RPM is not in the middle of the sweet spot, bring it on. It just is, and the math is slick, why fight it? Give the designer a starting point, do not spend a year identifying all the possibilities.

It is called a design decision. If you want to start a design with a different choice, be my guest. You gotta start somewhere and this is mine. I am gonna stick with it. What does your 50 m radius ship look like? I betcha I will like it. Show me, then we can talk tradeoffs. Show me exactly how you build a station of expanding size. I have a detailed plan for putting this together, and am looking forward to your alternative.

[spacester]

We need to define what the artificial gravity is trying to achieve before designing spacestation.
Here few ideas.
1) long term living ie years with children. Need 1g at low RPM.
2)keep astronaut health on extended trips of months may years. Probably 1/6g at higher RPMs.
3) make daily activities like, eating, drinking going to bathroom more pleasant where gravity helps keep everything in its place. This could be for short stays where we aren't using it reduce bone and muscle loss. Need 1/10 or less at moderate RPM.

Sent from my SM-T810 using Tapatalk

Agreed. But of course there are many answers. The question in the real world is of you can close a business case. Now, since prior to Starship there was essentially zero prospect of being able to orbit enough mass, let alone make the rest of the business case work, the projection of something possible must, for the time being, be based on Starship cargo delivery.

So I have a 100 m radius and the Starship user's guide. I roughed out an expanding three segment habitat payload that looks like a Newtonian telescope, and string 16 of them around the circumference connected by nodes.

This ship is way too audacious for NASA, being "too soon" for them. That's fine.

The idea here is simple: provide a fantastic safe haven for space travelers. Flip the whole minimum mass and deprivation model on its head. Luxury accommodations. People will be able to travel up and down the tubes and hang out at different levels, all at 3 RPM. Personal hygiene normalized and then you go up to the hub and play in zero gee and be sitting down again for lunch. Then a nice long hot shower.

Children will be welcome, but pregnancies might have to be banned at first. We need to make baby mice and rabbits first.

Why? Starship, that's why. Because we can finally work at scale, do big things, conquer space not just survive.

[spacester]

You know the spin-rate came from your design, not the other way around.

No the spin rate came from reading, comprehending and agreeing with the abstract you linked to, which in fact was a conclusion I was already in possession of, having read everything by Ted Hall many times  more than a decade prior and because the total body of information available is limited. That abstract is a simple review of the sparse research.

Remember the experiments they did over the last decade? No, because there are none. ISS goes roundy roundy and spin gravity has been out of order and I am wasting my time here, aren't I?

You read that abstract, and you get 3 RPM. It is as simple as that. Not 4, because too much motion sickness. Not 2 because at 3 RPM my visitors - who were in microgravity before arrival - will be just fine. 3 RPM is fast enough. 5 is right out. 2.8 or 3.2 or 3.1415926535? Well, maybe, but 3.0 is the design parameter, OK?

The first thing I did was design the expanding ring habs delivered by Starship, and stretch them around a 200 m  circle, and by golly 16 of them fit real nice with their nodes. Why fight it?

How is your design coming along?

[Twark_Main]

The first geometry that pops out is a dumbbell, two cylinders separated from each other by a relatively low mass connector.  So two spent rocket stages connected by a tether.

The simple dumbbell does not even make it to back of napkin design stage. Especially if you use a tether, you cannot control the dynamics. Pretty much a nightmare in that category.

So you go to a rigid truss, that improves the spin control. But where are you mounting the thrusters to force that control? And what about a central hub, good idea or bad idea?

So the dumbbell is replaced by...

So nightmare dynamics seems to be out.

I don't see how "thrusters" and "central hub yes/no" are problems for a dumbbell configuration. Can you elaborate?

Dumbbell configuration seems to be the AG implementation with "nothing left to take away."

[spacester]

The first geometry that pops out is a dumbbell, two cylinders separated from each other by a relatively low mass connector.  So two spent rocket stages connected by a tether.

The simple dumbbell does not even make it to back of napkin design stage. Especially if you use a tether, you cannot control the dynamics. Pretty much a nightmare in that category.

So you go to a rigid truss, that improves the spin control. But where are you mounting the thrusters to force that control? And what about a central hub, good idea or bad idea?

So the dumbbell is replaced by...

So "nightmare" dynamics seems to be out.

I don't see how "thrusters" and "central hub" are problems for a dumbbell configuration. Can you elaborate?

Basically all I am saying - besides trying to do a crash course on the concept of mass distribution for the OP - is that as a designer I see problems I cannot solve. They also are problems I have not seen anyone else solve. Maybe there are solutions, but I am going to move on and work on stuff I understand. If someone here knows how to guarantee that a dumbbell design's dynamics challenges can be met, I am all ears.

Meanwhile I am working on my thing. I am not going to work on all possibilities at once, OK?

So I have two cylinders and a tether. I need thrusters somewhere, yes? No? How no? So where exactly do I put these thrusters? What orientation are my cylinders at? How does that get the job done? How do I make the whole thing rotate and not lose control over the parts?

If the tether goes slack, how exactly do we react? How we make sure we do not overcompensate when bringing the tether back into tension and thus snap the tether? Can we live with so much active control that life will always be on the edge? Inherent stability is to be desired, yes?

Hub or no hub? I dunno, is there an obvious answer I am missing? Maybe so, this is the first time I have thought about this configuration in years. I simply am saying I have little basis to know whether the hub makes the dynamics better or worse.

If everything I am worried about is no big deal, great. (But what about nutation and precession and what not.) I am not an expert, but math is math and I have enough education and experience to see the challenges but not the solutions.

Citation? Failed flight tests with tethers, and the end of experiments with tethers, does that not count?

Replace the tether with a truss and it gets more manageable. The mass of the two will be very close to the same. I am sure some of you guys are working on that, can we get an update?

[Twark_Main]

The first geometry that pops out is a dumbbell, two cylinders separated from each other by a relatively low mass connector.  So two spent rocket stages connected by a tether.

The simple dumbbell does not even make it to back of napkin design stage. Especially if you use a tether, you cannot control the dynamics. Pretty much a nightmare in that category.

So you go to a rigid truss, that improves the spin control. But where are you mounting the thrusters to force that control? And what about a central hub, good idea or bad idea?

So the dumbbell is replaced by...

So "nightmare" dynamics seems to be out.

I don't see how "thrusters" and "central hub" are problems for a dumbbell configuration. Can you elaborate?

Basically all I am saying - besides trying to do a crash course on the concept of mass distribution for the OP - is that as a designer I see problems I cannot solve. They also are problems I have not seen anyone else solve. Maybe there are solutions, but I am going to move on and work on stuff I understand. If someone here knows how to guarantee that a dumbbell design's dynamics challenges can be met, I am all ears.

So I have two cylinders and a tether. I need thrusters somewhere, yes? No? How no? So where exactly do I put these thrusters? What orientation are my cylinders at? How does that get the job done? How do I make the whole thing rotate and not lose control over the parts?

If the tether goes slack, how exactly do we react? How we make sure we do not overcompensate when bringing the tether back into tension and thus snap the tether? Can we live with so much active control that life will always be on the edge? Inherent stability is to be desired, yes?

I think you might be confusing barbell configuration for bola-bolas configuration. Barbell is rigid (unless you know of any barbells on strings?), bola-bolas is tethered.

Even with tether designs there are obvious answers to the (subset of valid) problems you raised. I'd be happy to elaborate if you want, but I'm personally more interested in rigid designs.

Hub or no hub? I dunno, is there an obvious answer I am missing? Maybe so, this is the first time I have thought about this configuration in years. I simply am saying I have little basis to know whether the hub makes the dynamics better or worse.

Sure, but nothing about this is unique to the barbell. This same question has to be answered for any AG design.

If everything I am worried about is no big deal, great. (But what about nutation and precession and what not.)

Ditto. Every AG design has to contend with nutation and precession. Fortunately it's not terribly hard.

Citation? Failed flight tests with tethers, and the end of experiments with tethers, does that not count?

No it doesn't. No artificial gravity tether experiments have occurred, so N = 0.

Replace the tether with a truss and it gets more manageable. The mass of the two will be very close to the same. I am sure some of you guys are working on that, can we get an update?

Trusses (or even just inflation-rigidized tubes, which is basically the lightweight "balloon tank" tensegrity implementation of a truss) are indeed likely, yes. But jumping to a wheel space station because of that seems like a solution in search of a problem.

How much surface area does Aquarius have, vs a sphere with the same habitable volume? What effect does that have on the total radiation shielding mass needed? 

[spacester]

Hey you asked a question, thank you. You seem interested in the efficacy of the shielding, and see that the surface area to volume ratio can be improved by certain other design decisions. No doubt.

Just like with AG itself, when I search for wisdom on the subject, I find very little hard guidance for a designer. IOW I have never found a reference that says: well we don't know for sure, but it looks like we need this much plastic and this much water and here's how you would add lead foil to the mix and if you did that, you would have this level of safety and so let's use that as a baseline.

So I did my research and made a design choice. I admit I did not ask the permission of my fellow posters here to make this design decision, even though I knew well that you do not accept my authority to make such decisions. Guys, if no one else has done this before maybe you should let me get the ball rolling. I make zero statements about how anything has to be done "my way", I simply have decided on "a" way. I do not know if and when that becomes acceptable to y'all. I would rather sooner than later.

Every wall between the people and space consists of half a meter of polyethylene and water. The PE forms three nested tanks each of about 150mm thick walls of water, with a stainless steel skin inside and out, and metal foil between the PE tanks. This presents a path with more than 10 interfaces between materials and a pretty good gauntlet of hydrogen rich material for the particles to penetrate if they want to damage the ugly bags of mostly water inside.

Do I stand ready to *PROVE* that is the absolute best and only possible choice? Of course not, silly people. But I did my research and I crunched some numbers and got a result and am going with it to get things started.

It is a design decision. Every single decision is subject to revision. Certain design decisions are so fundamental that to change it is to turn this project into a similar project. I would love love love to be able to compare Aquarius to other fleshed out designs that come from other fundamental choices.

Please tell me I am now beating a dead horse on my design approach.

The Tubes are fabricated from flat-packed walls that are 9 m long and the half meter of water and PE thick. These monsters are wrangled together and assembled. They will be delivered with partial water loads, nominally the outer tanks only will be full (sealed) and the other two empty, pending completed construction followed by filling the radiation shield tanks with perhaps extra-terrestrial water. Maybe just the outer tank being full is enough for LEO but as soon as we boost it to cis-lunar space we want to fill those other two tanks up.

It is an enormous amount of water, and the design is not based on minimizing the water. It's called Aquarius because it is a water bearer. The comparison you ask for is not really relevant to my design process. (a waste of time from my point of view). There are programmatic benefits from the choice I made.

If maximizing the effect of water shielding is chosen as a primary design driver, that is a different design project. Whip out your alternative and let's compare. You will find me quite receptive to alternate ideas. Just please understand that certain changes constitute a not-Aquarius project. Which is great, just not Aquarius.

The central, founding, over-arching design imperative of Aquarius is comfort and safety. This led me to produce a design with absolutely every interior space being nearly equally well shielded with the half meter of hydrogen rich material. So all the habs and tubes and nodes and chutes and octo-walls all have that jacket.

And there are not only no windows, but no penetrations of the stainless steel skin anywhere on the habs, nodes, chutes and tubes. Two continuous skins of stainless, not even one bulkhead fitting anywhere below the central hub. All of it is "safe haven storm shelter". The flat walls of the central hub ("octowalls") will have plenty of bulkhead fittings, because you gotta have them.

No windows? Not sorry. The view would kinda suck anyway. Think about it.

edit: i just read your tag line at the end of your posts. Yeah, that.

[Roy_H]

I drew up plans for a rotating space station or it could easily be a space ship, just add appropriate thrusters to the non-rotating hub. Download complete description SpaceStation.pdf attached.

[Roy_H]

Do you mean why is the radius large? There is evidence that a small radius would make people feel nauseous because the force would be different between head and toes, and there would be weird effects to you balance sense when you turn your head.

Here is a nice youtube clip that talks about the issues and makes a guess at how small a radius we could get away with:



(but generally speaking, the bigger, the more comfortably earthlike the sensation of gravity would be.. we don't know for certain what radius humans would readily adapt to)

I couldn't listen to the whole video. The annoying loud music made it difficult to hear the words. I wish there was a way to remove music from these videos. This is a common problem as the people who add the music seem to think that it must be at least 10db louder than the voice.

[Twark_Main]

Hey you asked a question, thank you. You seem interested in the efficacy of the shielding, and see that the surface area to volume ratio can be improved by certain other design decisions. No doubt.

Just like with AG itself, when I search for wisdom on the subject, I find very little hard guidance for a designer. IOW I have never found a reference that says: well we don't know for sure, but it looks like we need this much plastic and this much water and here's how you would add lead foil to the mix and if you did that, you would have this level of safety and so let's use that as a baseline.

Agreed, the shortage of good publications on the subject is appalling.

If you're comfortable with about twice the levels of Ramsar, Iran (260 mSv/yr), you can achieve 470 mSv/year with about 20 g/cm^2 of polyethylene or water (Table 6a, p25). For a 40 year old male on a 6+6 month Mars round-trip, that's an extra 2% risk of death from cancer. This is probably acceptable IMO, despite the inevitable media hand-wringing over the "10x the allowable dose for radiation workers" sound bite.

For a spherical station with an interior volume of say 23,423 m³ (r = 17.75 m³), that's 800 tonnes of shielding, or 1,000 tonnes for double spheres (r = 14.1 m). For a torus (R = 100 m, r = 3.4 m), it jumps to 2,800 tonnes of shielding.

This extra 1800-2000 tones of mass also requires additional structural mass to support it, more resupply propellant to push it (including precess its axis of rotation), etc. And upmass ain't free, even with Starship.

This (for me) is one of the major appeals of barbell-type AG.

So I did my research and made a design choice. I admit I did not ask the permission of my fellow posters here to make this design decision, even though I knew well that you do not accept my authority to make such decisions. Guys, if no one else has done this before maybe you should let me get the ball rolling. I make zero statements about how anything has to be done "my way", I simply have decided on "a" way. I do not know if and when that becomes acceptable to y'all. I would rather sooner than later.

That's fair, and I have no problem with it. But what you said was "the simple dumbbell does not even make it to back of napkin design stage," and then went on to cite a bunch of (apparently non-existent) issues.

Glad we have cleared up the miscommunication!

Every wall between the people and space consists of half a meter of polyethylene and water. The PE forms three nested tanks each of about 150mm thick walls of water, with a stainless steel skin inside and out, and metal foil between the PE tanks. This presents a path with more than 10 interfaces between materials and a pretty good gauntlet of hydrogen rich material for the particles to penetrate if they want to damage the ugly bags of mostly water inside.

A half-meter (~40 g/cm²) doesn't do much beyond a quarter meter (~20 g/cm²) according to Table 6a linked above.

Do I stand ready to *PROVE* that is the absolute best and only possible choice? Of course not, silly people. But I did my research and I crunched some numbers and got a result and am going with it to get things started. [snip]

You don't have to prove anything to anyone (of course), but you were the one saying other types of designs were unworkable. That was the bit I took exception to. All water under the bridge now.

The Tubes are fabricated from flat-packed walls that are 9 m long and the half meter of water and PE thick. These monsters are wrangled together and assembled. They will be delivered with partial water loads, nominally the outer tanks only will be full (sealed) and the other two empty, pending completed construction followed by filling the radiation shield tanks with perhaps extra-terrestrial water. Maybe just the outer tank being full is enough for LEO but as soon as we boost it to cis-lunar space we want to fill those other two tanks up.

Water tanks are nice (vs PE) because they can be filled up and emptied without much human intervention. Water tanks are bad because they can leak water all over your ship without much human intervention.

Water is slightly less mass efficient than PE, but only slightly.

It is an enormous amount of water, and the design is not based on minimizing the water. It's called Aquarius because it is a water bearer. The comparison you ask for is not really relevant to my design process. (a waste of time from my point of view). There are programmatic benefits from the choice I made.

If maximizing the effect of water shielding is chosen as a primary design driver, that is a different design project. Whip out your alternative and let's compare. You will find me quite receptive to alternate ideas. Just please understand that certain changes constitute a not-Aquarius project. Which is great, just not Aquarius.

Point of clarification: is Aquarius supposed to be a space station, or a water delivery tanker ("water bearer" + no intent to minimize water mass)?

The central, founding, over-arching design imperative of Aquarius is comfort and safety. This led me to produce a design with absolutely every interior space being nearly equally well shielded with the half meter of hydrogen rich material. So all the habs and tubes and nodes and chutes and octo-walls all have that jacket.

And there are not only no windows, but no penetrations of the stainless steel skin anywhere on the habs, nodes, chutes and tubes. Two continuous skins of stainless, not even one bulkhead fitting anywhere below the central hub. All of it is "safe haven storm shelter". The flat walls of the central hub ("octowalls") will have plenty of bulkhead fittings, because you gotta have them.

No windows? Not sorry. The view would kinda suck anyway. Think about it.

Comfort's an "over-arching design imperative" and nice transparent water for shielding. So why no windows?

Moving shadows right? That's one of the big reasons I favor aligning/precessing the spin axis toward the Sun. If desired, you could even have lightweight film mirrors to reflect sunlight in the windows from a higher elevation angle.

edit: i just read your tag line at the end of your posts. Yeah, that.

Not a flashy design quote, but a good one. Guess who? No Google that's cheating.

Hint: the root word "design" (but not in that grammatical form) appears his most famous work (one that he didn't actually write)... 

[Roy_H]

Hey you asked a question, thank you. You seem interested in the efficacy of the shielding, and see that the surface area to volume ratio can be improved by certain other design decisions. No doubt.

Just like with AG itself, when I search for wisdom on the subject, I find very little hard guidance for a designer. IOW I have never found a reference that says: well we don't know for sure, but it looks like we need this much plastic and this much water and here's how you would add lead foil to the mix and if you did that, you would have this level of safety and so let's use that as a baseline.
...
Every wall between the people and space consists of half a meter of polyethylene and water. The PE forms three nested tanks each of about 150mm thick walls of water, with a stainless steel skin inside and out, and metal foil between the PE tanks. This presents a path with more than 10 interfaces between materials and a pretty good gauntlet of hydrogen rich material for the particles to penetrate if they want to damage the ugly bags of mostly water inside.

This looks very promising. I thought it would require much more thickness, but also I was thinking of a simple single material PE. I was under the impression that it was better at blocking radiation than water. What made you think of the multi-layer approach? Also I read that the aluminum on the ISS actually made the radiation problem slightly worse because of secondary radiation from aluminum molecules being hit. Does this not apply to stainless steel? What is the purpose of the stainless steel?

[spacester]

Hey you asked a question, thank you. You seem interested in the efficacy of the shielding, and see that the surface area to volume ratio can be improved by certain other design decisions. No doubt.

Just like with AG itself, when I search for wisdom on the subject, I find very little hard guidance for a designer. IOW I have never found a reference that says: well we don't know for sure, but it looks like we need this much plastic and this much water and here's how you would add lead foil to the mix and if you did that, you would have this level of safety and so let's use that as a baseline.

Agreed, the shortage of good publications on the subject is appalling.

If you're comfortable with about twice the levels of Ramsar, Iran (260 mSv/yr), you can achieve 470 mSv/year with about 20 g/cm^2 of polyethylene or water (Table 6a, p25). For a 40 year old male on a 6+6 month Mars round-trip, that's an extra 2% risk of death from cancer. This is probably acceptable IMO, despite the inevitable media hand-wringing over the "10x the allowable dose for radiation workers" sound bite.

For a spherical station with an interior volume of say 23,423 m³ (r = 17.75 m³), that's 800 tonnes of shielding, or 1,000 tonnes for double spheres (r = 14.1 m). For a torus (R = 100 m, r = 3.4 m), it jumps to 2,800 tonnes of shielding.

This extra 1800-2000 tones of mass also requires additional structural mass to support it, more resupply propellant to push it (including precess its axis of rotation), etc. And upmass ain't free, even with Starship.

This (for me) is one of the major appeals of barbell-type AG.

So I did my research and made a design choice. I admit I did not ask the permission of my fellow posters here to make this design decision, even though I knew well that you do not accept my authority to make such decisions. Guys, if no one else has done this before maybe you should let me get the ball rolling. I make zero statements about how anything has to be done "my way", I simply have decided on "a" way. I do not know if and when that becomes acceptable to y'all. I would rather sooner than later.

That's fair, and I have no problem with it. But what you said was "the simple dumbbell does not even make it to back of napkin design stage," and then went on to cite a bunch of (apparently non-existent) issues.

Glad we have cleared up the miscommunication!

Every wall between the people and space consists of half a meter of polyethylene and water. The PE forms three nested tanks each of about 150mm thick walls of water, with a stainless steel skin inside and out, and metal foil between the PE tanks. This presents a path with more than 10 interfaces between materials and a pretty good gauntlet of hydrogen rich material for the particles to penetrate if they want to damage the ugly bags of mostly water inside.

A half-meter (~40 g/cm²) doesn't do much beyond a quarter meter (~20 g/cm²) according to Table 6a linked above.

Do I stand ready to *PROVE* that is the absolute best and only possible choice? Of course not, silly people. But I did my research and I crunched some numbers and got a result and am going with it to get things started. [snip]

You don't have to prove anything to anyone (of course), but you were the one saying that other types of designs were unworkable. That was the bit I took exception to.

All water under the bridge now.

The Tubes are fabricated from flat-packed walls that are 9 m long and the half meter of water and PE thick. These monsters are wrangled together and assembled. They will be delivered with partial water loads, nominally the outer tanks only will be full (sealed) and the other two empty, pending completed construction followed by filling the radiation shield tanks with perhaps extra-terrestrial water. Maybe just the outer tank being full is enough for LEO but as soon as we boost it to cis-lunar space we want to fill those other two tanks up.

Water tanks are nice (vs PE) because they can be filled up and emptied without much human intervention. Water tanks are bad because they can leak water all over your ship without much human intervention.

Water is slightly less mass efficient than PE, but only slightly.

It is an enormous amount of water, and the design is not based on minimizing the water. It's called Aquarius because it is a water bearer. The comparison you ask for is not really relevant to my design process. (a waste of time from my point of view). There are programmatic benefits from the choice I made.

If maximizing the effect of water shielding is chosen as a primary design driver, that is a different design project. Whip out your alternative and let's compare. You will find me quite receptive to alternate ideas. Just please understand that certain changes constitute a not-Aquarius project. Which is great, just not Aquarius.

Point of clarification: is Aquarius supposed to be a space station, or a water delivery tanker ("water bearer" + no intent to minimize water mass)?

The central, founding, over-arching design imperative of Aquarius is comfort and safety. This led me to produce a design with absolutely every interior space being nearly equally well shielded with the half meter of hydrogen rich material. So all the habs and tubes and nodes and chutes and octo-walls all have that jacket.

And there are not only no windows, but no penetrations of the stainless steel skin anywhere on the habs, nodes, chutes and tubes. Two continuous skins of stainless, not even one bulkhead fitting anywhere below the central hub. All of it is "safe haven storm shelter". The flat walls of the central hub ("octowalls") will have plenty of bulkhead fittings, because you gotta have them.

No windows? Not sorry. The view would kinda suck anyway. Think about it.

Comfort's an "over-arching design imperative" and nice transparent water for shielding. Why no windows?

Hey you asked a question, thank you. You seem interested in the efficacy of the shielding, and see that the surface area to volume ratio can be improved by certain other design decisions. No doubt.

Just like with AG itself, when I search for wisdom on the subject, I find very little hard guidance for a designer. IOW I have never found a reference that says: well we don't know for sure, but it looks like we need this much plastic and this much water and here's how you would add lead foil to the mix and if you did that, you would have this level of safety and so let's use that as a baseline.

Agreed, the shortage of good publications on the subject is appalling.

If you're comfortable with about twice the levels of Ramsar, Iran (260 mSv/yr), you can achieve 470 mSv/year with about 20 g/cm^2 of polyethylene or water (Table 6a, p25). For a 40 year old male on a 6+6 month Mars round-trip, that's an extra 2% risk of death from cancer. This is probably acceptable IMO, despite the inevitable media hand-wringing over the "10x the allowable dose for radiation workers" sound bite.

For a spherical station with an interior volume of say 23,423 m³ (r = 17.75 m³), that's 800 tonnes of shielding, or 1,000 tonnes for double spheres (r = 14.1 m). For a torus (R = 100 m, r = 3.4 m), it jumps to 2,800 tonnes of shielding.

This extra 1800-2000 tones of mass also requires additional structural mass to support it, more resupply propellant to push it (including precess its axis of rotation), etc. And upmass ain't free, even with Starship.

This (for me) is one of the major appeals of barbell-type AG.

So I did my research and made a design choice. I admit I did not ask the permission of my fellow posters here to make this design decision, even though I knew well that you do not accept my authority to make such decisions. Guys, if no one else has done this before maybe you should let me get the ball rolling. I make zero statements about how anything has to be done "my way", I simply have decided on "a" way. I do not know if and when that becomes acceptable to y'all. I would rather sooner than later.

That's fair, and I have no problem with it. But what you said was "the simple dumbbell does not even make it to back of napkin design stage," and then went on to cite a bunch of (apparently non-existent) issues.

Glad we have cleared up the miscommunication!

Every wall between the people and space consists of half a meter of polyethylene and water. The PE forms three nested tanks each of about 150mm thick walls of water, with a stainless steel skin inside and out, and metal foil between the PE tanks. This presents a path with more than 10 interfaces between materials and a pretty good gauntlet of hydrogen rich material for the particles to penetrate if they want to damage the ugly bags of mostly water inside.

A half-meter (~40 g/cm²) doesn't do much beyond a quarter meter (~20 g/cm²) according to Table 6a linked above.

Do I stand ready to *PROVE* that is the absolute best and only possible choice? Of course not, silly people. But I did my research and I crunched some numbers and got a result and am going with it to get things started. [snip]

You don't have to prove anything to anyone (of course), but you were the one saying that other types of designs were unworkable. That was the bit I took exception to.

All water under the bridge now.

The Tubes are fabricated from flat-packed walls that are 9 m long and the half meter of water and PE thick. These monsters are wrangled together and assembled. They will be delivered with partial water loads, nominally the outer tanks only will be full (sealed) and the other two empty, pending completed construction followed by filling the radiation shield tanks with perhaps extra-terrestrial water. Maybe just the outer tank being full is enough for LEO but as soon as we boost it to cis-lunar space we want to fill those other two tanks up.

Water tanks are nice (vs PE) because they can be filled up and emptied without much human intervention. Water tanks are bad because they can leak water all over your ship without much human intervention.

Water is slightly less mass efficient than PE, but only slightly.

It is an enormous amount of water, and the design is not based on minimizing the water. It's called Aquarius because it is a water bearer. The comparison you ask for is not really relevant to my design process. (a waste of time from my point of view). There are programmatic benefits from the choice I made.

If maximizing the effect of water shielding is chosen as a primary design driver, that is a different design project. Whip out your alternative and let's compare. You will find me quite receptive to alternate ideas. Just please understand that certain changes constitute a not-Aquarius project. Which is great, just not Aquarius.

Point of clarification: is Aquarius supposed to be a space station, or a water delivery tanker ("water bearer" + no intent to minimize water mass)?

The central, founding, over-arching design imperative of Aquarius is comfort and safety. This led me to produce a design with absolutely every interior space being nearly equally well shielded with the half meter of hydrogen rich material. So all the habs and tubes and nodes and chutes and octo-walls all have that jacket.

And there are not only no windows, but no penetrations of the stainless steel skin anywhere on the habs, nodes, chutes and tubes. Two continuous skins of stainless, not even one bulkhead fitting anywhere below the central hub. All of it is "safe haven storm shelter". The flat walls of the central hub ("octowalls") will have plenty of bulkhead fittings, because you gotta have them.

No windows? Not sorry. The view would kinda suck anyway. Think about it.

Comfort's an "over-arching design imperative" and nice transparent water for shielding. Why no windows?

Moving shadows right? That's one of the big reasons I favor aligning/precessing the spin axis toward the Sun. If desired, you could even have lightweight film mirrors to reflect sunlight in the windows from a higher elevation angle.

edit: i just read your tag line at the end of your posts. Yeah, that.

Not a flashy design quote, but a good one. Guess who? No Google that's cheating.

Hint: the root word "design" (but not in that grammatical form) appears his most famous work (one that he didn't actually write)... 

Cool. I apologize for the lack of respect for the simplest designs. I see that I should not have used the word dumbbell.

So what do we call a simplest spin gravity configuration? This thread is an opportunity to organize design concepts and name them. Two bodies axially mounted is an "X", two bodies mounted at a T to the tether is a "Y", two bodies at a T turned out of the spin plane is a "Z", add a central hub for three more types, switch the tether out for a rigid truss gives you another class and six types.  I kinda sorta remember seeing this having been done but I am sure we could add clarity which would help add clarity to discussions if nothing else.

"Point of clarification: is Aquarius supposed to be a space station, or a water delivery tanker ("water bearer" + no intent to minimize water mass)?"

Yeah I can see the confusion. Perhaps the wackiest part of this proposal is that not only am I saying this would make sense as a very first spin gravity facility but that the thing is "built to last 200 years".

It is not a space 'station' because it has self-propulsion to take it from the construction and operational "station" in LEO, then to the Cis-Lunar "station" until being re-stationed at Mars.

So it is supposed to be a lot of things to a lot of different people. Ideally, all things to all people but that's too crazy even for me, but that's the line of thinking. The common attribute to all of its missions is that it is a safe haven.

So when a shiny new Aquarius is commissioned, she will have at least ~175 mm of PE/water in one of the three walls. Everywhere has that level of shielding, the whole thing is a safe haven to that extent. (Maybe there are storm shelters, btw.) That water will be beautiful pure Earth water, and for a long time it will just stay sealed up in that tank.

In addition, she will have an abundance of potable water distributed all throughout the ship. This store of water will be continuously recycled and purified. We have the PV power and the plumbing to do that, it is a main operational activity.

After some years of operation making money in LEO we will have amassed enough propellant to go to cis-lunar space. Once up there, she will want to immediately take on an additional jacket of water, from the moon or from asteroids to return to safe haven (to that extent) above the Van Allen belts.

Aquarius is also the water purifier. If you have dirty water from an asteroid, Aquarius wants it. That's a thing this water bearer does, she cleans her water and keep it fresh. The years in LEO let us mature the tech and we have the latest and greatest kit at the top of the hill and we are ready for the water we promised to buy.

We get all the shield tanks full. Maybe not pure wonderful water yet, but we can work on it on the trip to Mars and for years afterwards.

Now the water recycling is not perfect. We have human waste in the mix. It could be freeze dried but even then it needs to be stored.

So at some point, you are at Mars with a growing waste storage problem and with that original pure Earth water in the first shield tanks. Sewage should shield just about as well as pure water, I reckon. So yes Mars has water, but you now make available all that genuine Earth stuff. Aquarius has fulfilled that part of her water-bearer mission. She is full of crap but oh well.

So not a tanker in terms of moving water from place to place. A relocatable reservoir and water treatment plant that will pioneer the tech of cleaning up extra-terrestrial water. A safe haven where you go to hydrate and be among green growing things. Where you take a hot shower any time you want. Think low gravity hot tub for lunar robot techs. Likely the only place on orbit where water is abundant; the water bearer.

So why stainless steel and why no windows? Because I want that sheet of armor between me and space as a first defense. I want it to be impervious as possible. I want two of those skins, with leak-proof tanks filling the volume between. I do not want a bulkhead fitting for a video link to the 13th chute node's storage closet to ever spring a leak. Ever. 200 years of no chance of a penetration ruining everyone's day. Zero penetrations below the Hub.

Why no windows? The best bulkhead fitting is no bulkhead fitting. Give her excellent welding quality assurance and she will give you a 200-year lifetime. She would ring like a bell of you could hear it. A stainless steel armored interplanetary cruiser.

Or maybe down the road in the design process, someone proves that aluminum is a much better choice. Even with the program disruption. Fine, she gets made of aluminum. Every design decision is subject to review and revision.

You build half meter thick walls everywhere, paying the enormous price for that in the name of safe haven, and then you want to cut holes in it for people to stare at a spinning starfield with - at best - Earth whipping by every 20 seconds? Nah, bad idea. The overview effect is off the table for a spinning station IMO. They do that before and after visiting Aquarius.

I have not discussed orbital orientation. I am afraid to because maybe I am being stupid. But maybe you have a better understanding than I, so here goes.

I simply want to point the spin axis at the sun and have the thing orbit the Earth without coming out of that orientation. I am not 100 percent sure this actually works. Some days I convince myself it is easy, some days I think it fails.

What I do know is that I need to improve the rotational moments of inertia from what you have seen so far. I want the moment around the spin axis to be much higher than the other two orthogonal axes.

Aquarius will have a new look soon.

Also, believe it or not I am not here to hijack this thread. The thread title makes me want to work with others to describe and name the different configuration possibilities, just for starters.

[spacester]

Hey you asked a question, thank you. You seem interested in the efficacy of the shielding, and see that the surface area to volume ratio can be improved by certain other design decisions. No doubt.

Just like with AG itself, when I search for wisdom on the subject, I find very little hard guidance for a designer. IOW I have never found a reference that says: well we don't know for sure, but it looks like we need this much plastic and this much water and here's how you would add lead foil to the mix and if you did that, you would have this level of safety and so let's use that as a baseline.
...
Every wall between the people and space consists of half a meter of polyethylene and water. The PE forms three nested tanks each of about 150mm thick walls of water, with a stainless steel skin inside and out, and metal foil between the PE tanks. This presents a path with more than 10 interfaces between materials and a pretty good gauntlet of hydrogen rich material for the particles to penetrate if they want to damage the ugly bags of mostly water inside.

This looks very promising. I thought it would require much more thickness, but also I was thinking of a simple single material PE. I was under the impression that it was better at blocking radiation than water. What made you think of the multi-layer approach? Also I read that the aluminum on the ISS actually made the radiation problem slightly worse because of secondary radiation from aluminum molecules being hit. Does this not apply to stainless steel? What is the purpose of the stainless steel?

The radiation shield concept and design is the result of the deepest dive I could muster on the subject, and this was 8 to 10 years ago. I am an amateur but I find no experts. In addition to sheer thickness of hydrogen rich material, it seemed like having transitions between materials would allow a strong hit to spread out better. A GCR (galactic cosmic ray) can come from any direction and pack a punch. This is the case where I think you want it to hit a big heavy atom but ONLY if you are there to catch all the mess radiating from that collision.

So the idea is to provide geometry that can be assembled by the construction crew, provides multiple obstacles to high energy particles and in particular can be refined as we learn more. This design is this amateur's best shot at "hey let's just plan on this for now based on what do do know, and make it better as we go along. It's a start."

[Twark_Main]

[snip]

Cool. I apologize for the lack of respect for the simplest designs. I see that I should not have used the word dumbbell.

"Dumbwaiter?! Who you callin' dumb, buddy???" 

[Coastal Ron]

Just like with AG itself, when I search for wisdom on the subject, I find very little hard guidance for a designer.

Since there are no artificial gravity space stations yet, there is zero professional designers for such structures. All there is is concepts, which may or may not be grounded in reality.

So I did my research and made a design choice. I admit I did not ask the permission of my fellow posters here to make this design decision, even though I knew well that you do not accept my authority to make such decisions.

What? I hope this was an attempt at humor, because NO ONE on NSF can claim to be an "authority" for rotating space stations that generate artificial gravity. And there are plenty of us that have spent a LOT of time not just working on plans, but discussing and debating many approaches.

You have visited the various NSF threads for rotating space stations and spaceships? Because until someone actually builds a 1st generation rotating space station that generates some degree of artificial gravity, we're all just proffering our opinions. Just as you are.

Guys, if no one else has done this before maybe you should let me get the ball rolling.

You're kinda late to be volunteering to "get the ball rolling". The ball has been rolling for MANY years on NSF on this topic. Welcome to the club! 

Every wall between the people and space consists of half a meter of polyethylene and water. The PE forms three nested tanks each of about 150mm thick walls of water, with a stainless steel skin inside and out, and metal foil between the PE tanks. This presents a path with more than 10 interfaces between materials and a pretty good gauntlet of hydrogen rich material for the particles to penetrate if they want to damage the ugly bags of mostly water inside.

You're not the first to suggest using polyethylene - ultra-high-molecular-weight polyethylene (UHMWPE, UHMW) is a popular proposed construction material. I have designs that based on using Dyneema, but my stations are not planned for use in Earth orbit, where atmospheric oxygen attacks varieties of polyethylene.

I just don't understand what your structure looks like? Wheel? Star? Tube? What?

Water bags. Sure. But adding weight is one thing, building a structure that can handle that weight is something else.

Do I stand ready to *PROVE* that is the absolute best and only possible choice? Of course not, silly people. But I did my research and I crunched some numbers and got a result and am going with it to get things started.

Enough chest beating. Remember that until someone actually builds a rotating space station, and it works, then EVERYONE is just spitballing. You, me, and everyone else. Which means we ALL have opinions about what works and what doesn't - get used to it. 

[Paul451]

Dumbbell configuration seems to be the AG implementation with "nothing left to take away."

Or variations like the simple baton.

For example, taking Spacester's proposal: Using the rectangular panels in his eight, square-cross-section, "apartment tubes" you can build a hexadecagonal-cross-section baton with the same overall length. Same surface area (for radiation/MMOD shielding) but you now get 180 square metres of space on each floor, instead of 72 sq_metres (in 9 sq_metre sections). Same 2.5x increase in volume. And IMO, vastly more usable volume. (Or reduce the mass/surface area by 40% if you only want the same volume. Although still in a much more usable, open form.)

The "chutes" are eliminated entirely. As are most of the tensile cables and support structure. As are the weird telescoping rings. That mass can either be skipped entirely, or substituted for more construction material to make the main baton larger/wider. [Also, the hub can now be in the plane of rotation, as can the "process" modules, improving the mass distribution.]

Cover the sun-facing side with thin-film solar panels, offset from the baton to serve as a sun-shade, cover the back-side with radiators. (Given that it's an AG structure, might be able to get away with spiral droplet-radiators for greater mass-efficiency.)

The baton also makes your infrastructure easier. Running ECLSS and utilities through Spacester's design would be a nightmare.


I've included an artist's render of the station, below. Along with an earlier, hexagonal octagonal pathfinder station.

[Paul451]

[...]

I just don't understand what your structure looks like? Wheel? Star? Tube? What?

He posted it on page one.

[spacester]

Just like with AG itself, when I search for wisdom on the subject, I find very little hard guidance for a designer.

Since there are no artificial gravity space stations yet, there is zero professional designers for such structures. All there is is concepts, which may or may not be grounded in reality.

So I did my research and made a design choice. I admit I did not ask the permission of my fellow posters here to make this design decision, even though I knew well that you do not accept my authority to make such decisions.

What? I hope this was an attempt at humor, because NO ONE on NSF can claim to be an "authority" for rotating space stations that generate artificial gravity. And there are plenty of us that have spent a LOT of time not just working on plans, but discussing and debating many approaches.

You have visited the various NSF threads for rotating space stations and spaceships? Because until someone actually builds a 1st generation rotating space station that generates some degree of artificial gravity, we're all just proffering our opinions. Just as you are.

Guys, if no one else has done this before maybe you should let me get the ball rolling.

You're kinda late to be volunteering to "get the ball rolling". The ball has been rolling for MANY years on NSF on this topic. Welcome to the club! 

Every wall between the people and space consists of half a meter of polyethylene and water. The PE forms three nested tanks each of about 150mm thick walls of water, with a stainless steel skin inside and out, and metal foil between the PE tanks. This presents a path with more than 10 interfaces between materials and a pretty good gauntlet of hydrogen rich material for the particles to penetrate if they want to damage the ugly bags of mostly water inside.

You're not the first to suggest using polyethylene - ultra-high-molecular-weight polyethylene (UHMWPE, UHMW) is a popular proposed construction material. I have designs that based on using Dyneema, but my stations are not planned for use in Earth orbit, where atmospheric oxygen attacks varieties of polyethylene.

I just don't understand what your structure looks like? Wheel? Star? Tube? What?

Water bags. Sure. But adding weight is one thing, building a structure that can handle that weight is something else.

Do I stand ready to *PROVE* that is the absolute best and only possible choice? Of course not, silly people. But I did my research and I crunched some numbers and got a result and am going with it to get things started.

Enough chest beating. Remember that until someone actually builds a rotating space station, and it works, then EVERYONE is just spitballing. You, me, and everyone else. Which means we ALL have opinions about what works and what doesn't - get used to it. 

Hello again. My thing is to provoke new thoughts. That's what I do. Others do it too, but it's my thing.

As such, I am accustomed to much opposition when I present the results of my thought experiments. It does not help that all my best thought provokers are large scale ideas.

We played this game earlier this year, remember? Aquarius was presented in full and the reception was so underwhelming that I deleted it all. I am back with a new approach and a new attitude. Just let me get one thing off my chest, for the sake of a greater good on this thread.

You refer to previous threads on this topic. I may or may not have read all of them, I dunno. As an independent thinker, I am not committed to that task. Besides - and here is the point - if those threads are so important, than you guys must have come up with some conclusions and summations, right? No you didn't did you?  It's all attack all the time when I check in on those threads here and everywhere else. Anyone who presents an "idea" - not just me, anyone -  and expects praise is sorely disappointed but sometimes they stick around and join the attack chorus for all the next ideas. Let's stop with it already, OK? Say something nice first. Respect the work of others, of course their vision is way different from yours. Duh. Be nice.

So whenever I would try to get a spin-gravity concept across, the very smart people here would say show us the math. So I did Aquarius which means I did the math, it's on that overview pdf. 150 launches at 150 T ought to about do it for full construction and outfitting of Aquarius. It's high school math, mostly basic spreadsheet stuff, more on that later. The point is I called their bluff. That's how I know those previous threads didn't do the math.

So as far as chest thumping goes, dust off the results of your previous threads here, maybe do a bit more math, and let's compare. The current form of Aquarius is very much a preliminary one. Certainly a better design is out there, a whole set of them even. That's not the point. I have staked out the upper end in terms of ambition because that's a thing I like to do. Let's define the different kinds of "better" - optimization criteria - we are going to be using when we compare designs (those which are fleshed out enough to compare well). 

Me, I like this thread's title best of all. That's why I let it be until now. Let's start breaking down different design concepts into categories or classifications or whatever.

What's the difference between a bolo and a barbell?

OK so this while spitballing thing, let's look at that. To me, spitballing is what the Drill Sargent is doing to the pitiful maggot whose face he is spitting on while screaming. So I am not a big fan of that.

Nor am I interested in mere opinion proffering. Where does that get us? How disciplined would that have to be to actually work? Getting used to it is not gonna happen, I have better things to do than spend two days defending my right to choose a radius and rpm.

The request to show the math was a valid one, so let's all show the math, sort it out, and produce something in this thread.

What I am doing, by calling the bluff and doing the math and taking a deeper dive that you've seen before - because I can and I want to - my good sir, is designing. That is my specialty as a Mechanical Engineer.

[spacester]

Dumbbell configuration seems to be the AG implementation with "nothing left to take away."

Or variations like the simple baton.

For example, taking Spacester's proposal: Using the rectangular panels in his eight, square-cross-section, "apartment tubes" you can build a hexadecagonal-cross-section baton with the same overall length. Same surface area (for radiation/MMOD shielding) but you now get 180 square metres of space on each floor, instead of 72 sq_metres (in 9 sq_metre sections). Same 2.5x increase in volume. And IMO, vastly more usable volume. (Or reduce the mass/surface area by 40% if you only want the same volume. Although still in a much more usable, open form.)

The "chutes" are eliminated entirely. As are most of the tensile cables and support structure. As are the weird telescoping rings. That mass can either be skipped entirely, or substituted for more construction material to make the main baton larger/wider. [Also, the hub can now be in the plane of rotation, as can the "process" modules, improving the mass distribution.]

Cover the sun-facing side with thin-film solar panels, offset from the baton to serve as a sun-shade, cover the back-side with radiators. (Given that it's an AG structure, might be able to get away with spiral droplet-radiators for greater mass-efficiency.)

The baton also makes your infrastructure easier. Running ECLSS and utilities through Spacester's design would be a nightmare.


I've included an artist's render of the station, below. Along with an earlier, hexagonal octagonal pathfinder station.

I like it. These are the kind of reconfiguration options we can play with. I do not like the 3 m by 3m cubicles either, actually. Your octagons look nice.

I am not clear where the spin axis is and what is the value of the spin radius, will comment more with that info.

So the design basis for Aquarius is a ring station. What I am showing is not a Hollywood design, it is a bridge-like structural system spinning in space. It is firstly a ring of modules connected around the circumference of a 200 m diameter circle. That is its essence. The Nodes are part of that system but the Tubes and Chutes are not. They are added for circulation of people, air, water and everything else. The main reason for the chutes is they will nominally have an updraft. I haven't shown the 44 pipes yet, see attached.

The hab modules forming the ring are total placeholders in this design. Their design is not dumb, but I have no doubt it will get its ass kicked right off the bat. Still for the moment they are locked in as placeholders. Notably, no where on Aquarius have I used expandable structures. Except that these hab modules are designed to squeeze into the Starship payload envelope, and launch as three nested circular cylinders with half-meter walls of PE and water. After delivery they are expanded axially - the long axis - and you get what I show. 16 of them wrap that circle nicely; the connecting nodes are not too big to launch on Starship.

I show some strongbacks on the habs because I am really paranoid when it comes to structure. The strongbacks and the tension pipe add to the visual clutter on those pdfs, but this is a bridgework structure designed to last 200 years, not a Hollywood concept.

So yeah the 44 pipes per tube is a big part of the deal. Circulation galore, in any given location, you can access hot and cold water, warm dry air or warm moist air or a cool breeze. The central hub makes that all available in plenums attached to the 44 pipes. Each pipe is about 10 inch diameter, so lots of capabaility for smaller conduits for other things.

The 44 pipes house the CO2 scrubbers. Aquarius will have enormous CO2 capture capability.

The solar arrays will be cooled by ammonia based radiators. That's fine, but ammonia is totally banned inside the pressurized volume.

The central hub is the crown jewel, the luxury liner's grand ballroom. A huge shirtsleeve mini gravity playground for sports, movie making and what not. It exists to house the mechanical rooms, and is designed to be huge because we can. Because Starship.

[Coastal Ron]

So the design basis for Aquarius is a ring station.

Ring designs are very popular, though until we actually build such stations we have no idea if they are practical.

One of the things that catches my attention with them is how much mass they require per volume of usable space. In contrast, O'Neill cylinders are much more mass efficient - though I personally don't think O'Neill cylinders are buildable.

But hey, as long as we are debating theoretical designs here, we might as well have some way to measure them, and volume of usable space per mass of structure is one way. Other measurements to consider is the ability to survive collisions with space debris, or how many unique components need to be designed, since the more complexity you have the more costly the design likely is, and the more costly to maintain it once operational.

Notably, no where on Aquarius have I used expandable structures. Except that these hab modules are designed to squeeze into the Starship payload envelope, and launch as three nested circular cylinders with half-meter walls of PE and water. After delivery they are expanded axially...

Yeah, I think you contradicted yourself there. 

And when I saw your design the first thing I thought was that you were using nested cylinders that expand out. Which is a novel idea, but has the same limitations as other expandable structures, in that now you have to have outfit the insides, and outfitting in space is harder and more costly than outfitting on Earth. An alternative would be to build all the modules on Earth and then ship them to orbit - it would likely use less launches than having to launch the parts and workers to outfit them in space. Something to consider...

...but this is a bridgework structure designed to last 200 years, not a Hollywood concept.

You keep trying to distinguish your concepts from the concepts of "Hollywood". Until your design goes through an engineering review it is just a concept, no more or no less probable than "Hollywood" concepts. Same with my designs too.

As to service life, what we are all proposing are 1st generation rotating space stations, and none of them are truly big enough for colonies, so it would be unlikely that any 1st generation rotating space station would need to stay operational very long before it was superseded by a 2nd generation design that is a major improvement. Just a thought...

[spacester]

I drew up plans for a rotating space station or it could easily be a space ship, just add appropriate thrusters to the non-rotating hub. Download complete description SpaceStation.pdf attached.

Ooops, I almost missed this post, sorry. I am reading it now, might take a bit to react.

[spacester]

So the design basis for Aquarius is a ring station.

Ring designs are very popular, though until we actually build such stations we have no idea if they are practical.

One of the things that catches my attention with them is how much mass they require per volume of usable space. In contrast, O'Neill cylinders are much more mass efficient - though I personally don't think O'Neill cylinders are buildable.

But hey, as long as we are debating theoretical designs here, we might as well have some way to measure them, and volume of usable space per mass of structure is one way. Other measurements to consider is the ability to survive collisions with space debris, or how many unique components need to be designed, since the more complexity you have the more costly the design likely is, and the more costly to maintain it once operational.

Notably, no where on Aquarius have I used expandable structures. Except that these hab modules are designed to squeeze into the Starship payload envelope, and launch as three nested circular cylinders with half-meter walls of PE and water. After delivery they are expanded axially...

Yeah, I think you contradicted yourself there. 

And when I saw your design the first thing I thought was that you were using nested cylinders that expand out. Which is a novel idea, but has the same limitations as other expandable structures, in that now you have to have outfit the insides, and outfitting in space is harder and more costly than outfitting on Earth. An alternative would be to build all the modules on Earth and then ship them to orbit - it would likely use less launches than having to launch the parts and workers to outfit them in space. Something to consider...

...but this is a bridgework structure designed to last 200 years, not a Hollywood concept.

You keep trying to distinguish your concepts from the concepts of "Hollywood". Until your design goes through an engineering review it is just a concept, no more or no less probable than "Hollywood" concepts. Same with my designs too.

As to service life, what we are all proposing are 1st generation rotating space stations, and none of them are truly big enough for colonies, so it would be unlikely that any 1st generation rotating space station would need to stay operational very long before it was superseded by a 2nd generation design that is a major improvement. Just a thought...

Usable pressurized volume per mass of structure is a perfectly good metric. Aquarius in current form will not rank well on that metric, I am fine with that at this stage.

I am purposely leaving things wide open for Aquarius to be shown up on certain criteria. There is method to the madness.

You missed the word except. No expandability EXCEPT the stretching out of the hab payloads. I am trying to hint at a change a person could propose to my baseline reference design, and you are trying to find contradictions. Hint: Bigelow or BEAM or derivatives.

On complexity of parts, Aquarius will score very well indeed. This is not a science lab like ISS and EVAs are going to be very rare. KISS applies.

Structural design is not rocket surgery. I have experience in heavy steel fab and I have run some numbers and what I have is almost certainly over-designed. Certainly a professional analysis would be great but in the meantime we can proceed from conceptual design with no numbers to preliminary design with numbers. Structural revisions should be only favorable.

The distinction with Hollywood is about looking at their hardware and seeing a need for unobtanium while I am an engineer working with good old stainless steel and used to "when in doubt, build it stout." It is not about probability of actually getting built. Too soon for that discussion.

[Coastal Ron]

No expandability EXCEPT the stretching out of the hab payloads.

I was just pointing out that expanding by stretching is still expanding. No need to split hairs... 

On complexity of parts, Aquarius will score very well indeed. This is not a science lab like ISS and EVAs are going to be very rare. KISS applies.

Of course EVA's are going to rare, since how do you do an EVA on the exterior of a rotating space station? The space equivalent of mountain climbing, but with far more equipment. Which is why I foresee the need for robotic systems for doing "outside" maintenance on rotating space stations - which requires some technological improvements, but not a lot.

Structural design is not rocket surgery.

Right. Structural design is not truck driving either. Luckily we don't need to find imperfect analogies when the original thing works fine. Structural design is like structural design. Next!

I have experience in heavy steel fab and I have run some numbers and what I have is almost certainly over-designed. Certainly a professional analysis would be great but in the meantime we can proceed from conceptual design with no numbers to preliminary design with numbers. Structural revisions should be only favorable.

Since we're all working on concepts, there is no need to sweat design optimization, because that does come later.

The distinction with Hollywood is about looking at their hardware and seeing a need for unobtanium while I am an engineer working with good old stainless steel and used to "when in doubt, build it stout." It is not about probability of actually getting built. Too soon for that discussion.

The reason I mention it is that I think you are focused on something that doesn't matter, which makes it seem like you are trying to shift focus from your own design. Everyone should know that science FICTION is FICTION, and that entertainment shows are NOT sources of information about potential space hardware, but just sources of entertainment.

Rotating space stations will require engineering that has never been done before, but luckily a lot of the physics are known. I think the hardest part will be in keeping the rotating structure in a stable rotation, because I have yet to find a way for free-floating rotating structures to stop wobbling on their own.

And rotating structures will wobble, for many reasons, so I've been spending time working on what I hope are simple active control systems that don't use a lot of consumables.

[Paul451]

Of course EVA's are going to rare, since how do you do an EVA on the exterior of a rotating space station?

Same as you do on any tall industrial structure, walkways. Or do you leave it off and just hope that you never need it?

[spacester]

No expandability EXCEPT the stretching out of the hab payloads.

I was just pointing out that expanding by stretching is still expanding. No need to split hairs... 

On complexity of parts, Aquarius will score very well indeed. This is not a science lab like ISS and EVAs are going to be very rare. KISS applies.

Of course EVA's are going to rare, since how do you do an EVA on the exterior of a rotating space station? The space equivalent of mountain climbing, but with far more equipment. Which is why I foresee the need for robotic systems for doing "outside" maintenance on rotating space stations - which requires some technological improvements, but not a lot.

Structural design is not rocket surgery.

Right. Structural design is not truck driving either. Luckily we don't need to find imperfect analogies when the original thing works fine. Structural design is like structural design. Next!

I have experience in heavy steel fab and I have run some numbers and what I have is almost certainly over-designed. Certainly a professional analysis would be great but in the meantime we can proceed from conceptual design with no numbers to preliminary design with numbers. Structural revisions should be only favorable.

Since we're all working on concepts, there is no need to sweat design optimization, because that does come later.

The distinction with Hollywood is about looking at their hardware and seeing a need for unobtanium while I am an engineer working with good old stainless steel and used to "when in doubt, build it stout." It is not about probability of actually getting built. Too soon for that discussion.

The reason I mention it is that I think you are focused on something that doesn't matter, which makes it seem like you are trying to shift focus from your own design. Everyone should know that science FICTION is FICTION, and that entertainment shows are NOT sources of information about potential space hardware, but just sources of entertainment.

Rotating space stations will require engineering that has never been done before, but luckily a lot of the physics are known. I think the hardest part will be in keeping the rotating structure in a stable rotation, because I have yet to find a way for free-floating rotating structures to stop wobbling on their own.

And rotating structures will wobble, for many reasons, so I've been spending time working on what I hope are simple active control systems that don't use a lot of consumables.

My point on structural design was in response to you seeming to say that until a professional design review is done we don't know anything.

"Until your design goes through an engineering review it is just a concept, no more or no less probable than "Hollywood" concepts"

I reject that. We know what we know, I know what I know, you know what you know, math is math and steel is steel. Do the math, apply a factor of safety, use that result until the design is more mature.

I did the math so I am working with an engineering model. You want to restrict me to conceptual models only. Homie don't play that.

Hollywood does not worry about structural margins and failure theory. I am showing an engineering model to people who are used to seeing Hollywood models. It looks clunky, and I was explaining why. Also, I am not happy with the pdf drawings I can easily whip out from my CAD model.

Not every word I write in response to you is completely addressed only to you. I write to the entire audience.

We are on the same page with the wobbling question. I have taken great care in the construction design to allow for a very precisely balanced structure, and am going to get the moments of inertia fixed, but yeah, from what I understand there will be wobble no matter what. How hard it is to correct is very much guesswork for me at this time.

I do not see it as a showstopper by any means, whatever wobble we get will simply be included in the package of things people are going to need to get used to, along with the Coriolis. It's not like anyone is going to forget they are on a spinning ship.

As you say, how much active control is needed and the propellant cost is a big unknown. I can tell you that when I went to calculate the spin-up propellant needed for a fully equipped Aquarius, I did not like or believe the result. It's a lot. It will likely be desired to spin it up once and never stop spinning after that.

I will doff my cap to anyone who tackles the math of wobble correction control theory. I could possibly do it myself but it would take way too much time.

[spacester]

Of course EVA's are going to rare, since how do you do an EVA on the exterior of a rotating space station?

Same as you do on any tall industrial structure, walkways. Or do you leave it off and just hope that you never need it?

Well if you had to do an EVA on an operational (spinning) Aquarius, you would be using winches to lower the person down to the work site. Messy and expensive and not part of the program.

Aquarius is going to need a small army of robots to construct and maintain.

[Coastal Ron]

Of course EVA's are going to rare, since how do you do an EVA on the exterior of a rotating space station?

Same as you do on any tall industrial structure, walkways. Or do you leave it off and just hope that you never need it?

Come on Paul, you've seen spacesuits, yes? They weigh about 280 pounds (127 kg), and they are not balanced for standing in gravity.

Add to that the immobility of the suit due to the nature of inflated suits, and it should be clear that current generation spacesuits can't be used for EVA on rotating space stations. Something else will be needed, and it will likely be robotic in nature.

[Coastal Ron]

My point on structural design was in response to you seeming to say that until a professional design review is done we don't know anything.

Until your design goes through an engineering review it is just a concept, no more or no less probable than "Hollywood" concepts"

I reject that. We know what we know, I know what I know, you know what you know, math is math and steel is steel. Do the math, apply a factor of safety, use that result until the design is more mature.

First of all please learn to use HTML quotes, that way you don't have such a long post. Notice how most other people are able to cut out sections of text that aren't needed?

As to what you quoted from me, as far as interactions here on NSF my comment stands. You might be an engineering genius in real life, but here on NSF you are just poster "spacester", and no one knows if you know what you are talking about. Sorry.

I did the math so I am working with an engineering model. You want to restrict me to conceptual models only. Homie don't play that.

Well Homie, you can do whatever you want on NSF, but that doesn't mean anyone has to believe you. That is my point. And I don't expect everyone to believe me - just ask Paul451, who is someone I have come to know over many years, and although I don't always agree with him, I've come to value his input and ideas. But he certainly doesn't agree with everything I write (see above).

Hollywood does not...

You have a thing about comparing yourself to Hollywood. Just an observation, since I don't recall anyone else on NSF so concerned about how they are perceived against the entertainment industry in Southern California.

My assumption is that the first successful 1st generation rotating space stations won't look like anything we see in movies. Which shouldn't bother anyone building rotating space stations, since they were never competing with the graphic artists trying to make entertainment.

We are on the same page with the wobbling question. I have taken great care in the construction design to allow for a very precisely balanced structure, and am going to get the moments of inertia fixed, but yeah, from what I understand there will be wobble no matter what. How hard it is to correct is very much guesswork for me at this time.

You can design to the nth degree for balance, but once you put in humans and supplies, and people are walking around, and liquids are moving around the station, and hundreds of various inputs, the station won't be perfectly balanced. Some (not all) of that can probably be counteracted with moveable masses on the station itself, and propellant could be used too, but I'm hoping to find solutions that can take into account worst case scenarios. Too early to know if I've found them, so not talking about them yet.

I do not see it as a showstopper by any means, whatever wobble we get will simply be included in the package of things people are going to need to get used to, along with the Coriolis. It's not like anyone is going to forget they are on a spinning ship.

The studies I've seen (probably from Paul451) suggest that Coriolis force should not be a factor for humans, in that they should adapt fairly quickly, even on short diameter stations. Not sure I've seen any studies on wobble, so I can only guess that it would be kind of like being on a ship in heavy swells.

As you say, how much active control is needed and the propellant cost is a big unknown. I can tell you that when I went to calculate the spin-up propellant needed for a fully equipped Aquarius, I did not like or believe the result. It's a lot. It will likely be desired to spin it up once and never stop spinning after that.

Not surprised at your result. I suggest a quote from Archimedes that I think is valuable for solving this particular problem?

Give me a lever long enough and a fulcrum on which to place it, and I shall move the world.

In other words, mount your engines out beyond the diameter of the station. For instance, instead of chemical engines you can use Solar electric propulsion (SEP) on cables mounted around the perimeter of your station (hung off the "bottom" of the station). You'll only need 10% of the propellant, and time-wise it may not take that much longer considering the supply chain for refilling your chemical rocket engines.

I will doff my cap to anyone who tackles the math of wobble correction control theory. I could possibly do it myself but it would take way too much time.

I think the control system will be easy. The hard part is generating the physical inputs to counteract the wobble.

[jdon759]

As you say, how much active control is needed and the propellant cost is a big unknown. I can tell you that when I went to calculate the spin-up propellant needed for a fully equipped Aquarius, I did not like or believe the result. It's a lot. It will likely be desired to spin it up once and never stop spinning after that.

Not surprised at your result. I suggest a quote from Archimedes that I think is valuable for solving this particular problem?

Give me a lever long enough and a fulcrum on which to place it, and I shall move the world.

In other words, mount your engines out beyond the diameter of the station. For instance, instead of chemical engines you can use Solar electric propulsion (SEP) on cables mounted around the perimeter of your station (hung off the "bottom" of the station). You'll only need 10% of the propellant, and time-wise it may not take that much longer considering the supply chain for refilling your chemical rocket engines.

I will doff my cap to anyone who tackles the math of wobble correction control theory. I could possibly do it myself but it would take way too much time.

I think the control system will be easy. The hard part is generating the physical inputs to counteract the wobble.

I like this idea for the position of the engines.  It seems like it would be suitable for propulsion engines as well as spin-up engines.
However, for spin-up, why not have another ring - a counterweight or a living space - rotating in the other direction?  Use the power of electric motors instead of a consumable.
Perhaps a counterweight spinning ring could be used to tackle the problem of wobble too, if it had enough degrees of freedom?

[Paul451]

[...] and it should be clear that current generation spacesuits [...]

"Doctor, it hurts when I do this..."

We are either building better suits, or we aren't building much anywhere in space beyond a few pre-built modules berthed together.

[spacester]

My point on structural design was in response to you seeming to say that until a professional design review is done we don't know anything.

Until your design goes through an engineering review it is just a concept, no more or no less probable than "Hollywood" concepts"

I reject that. We know what we know, I know what I know, you know what you know, math is math and steel is steel. Do the math, apply a factor of safety, use that result until the design is more mature.

First of all please learn to use HTML quotes, that way you don't have such a long post. Notice how most other people are able to cut out sections of text that aren't needed?

As to what you quoted from me, as far as interactions here on NSF my comment stands. You might be an engineering genius in real life, but here on NSF you are just poster "spacester", and no one knows if you know what you are talking about. Sorry.

I did the math so I am working with an engineering model. You want to restrict me to conceptual models only. Homie don't play that.

Well Homie, you can do whatever you want on NSF, but that doesn't mean anyone has to believe you. That is my point. And I don't expect everyone to believe me - just ask Paul451, who is someone I have come to know over many years, and although I don't always agree with him, I've come to value his input and ideas. But he certainly doesn't agree with everything I write (see above).

Hollywood does not...

You have a thing about comparing yourself to Hollywood. Just an observation, since I don't recall anyone else on NSF so concerned about how they are perceived against the entertainment industry in Southern California.

My assumption is that the first successful 1st generation rotating space stations won't look like anything we see in movies. Which shouldn't bother anyone building rotating space stations, since they were never competing with the graphic artists trying to make entertainment.

We are on the same page with the wobbling question. I have taken great care in the construction design to allow for a very precisely balanced structure, and am going to get the moments of inertia fixed, but yeah, from what I understand there will be wobble no matter what. How hard it is to correct is very much guesswork for me at this time.

You can design to the nth degree for balance, but once you put in humans and supplies, and people are walking around, and liquids are moving around the station, and hundreds of various inputs, the station won't be perfectly balanced. Some (not all) of that can probably be counteracted with moveable masses on the station itself, and propellant could be used too, but I'm hoping to find solutions that can take into account worst case scenarios. Too early to know if I've found them, so not talking about them yet.

I do not see it as a showstopper by any means, whatever wobble we get will simply be included in the package of things people are going to need to get used to, along with the Coriolis. It's not like anyone is going to forget they are on a spinning ship.

The studies I've seen (probably from Paul451) suggest that Coriolis force should not be a factor for humans, in that they should adapt fairly quickly, even on short diameter stations. Not sure I've seen any studies on wobble, so I can only guess that it would be kind of like being on a ship in heavy swells.

As you say, how much active control is needed and the propellant cost is a big unknown. I can tell you that when I went to calculate the spin-up propellant needed for a fully equipped Aquarius, I did not like or believe the result. It's a lot. It will likely be desired to spin it up once and never stop spinning after that.

Not surprised at your result. I suggest a quote from Archimedes that I think is valuable for solving this particular problem?

Give me a lever long enough and a fulcrum on which to place it, and I shall move the world.

In other words, mount your engines out beyond the diameter of the station. For instance, instead of chemical engines you can use Solar electric propulsion (SEP) on cables mounted around the perimeter of your station (hung off the "bottom" of the station). You'll only need 10% of the propellant, and time-wise it may not take that much longer considering the supply chain for refilling your chemical rocket engines.

I will doff my cap to anyone who tackles the math of wobble correction control theory. I could possibly do it myself but it would take way too much time.

I think the control system will be easy. The hard part is generating the physical inputs to counteract the wobble.

I know how to do the html thing with the quotes but I do not do it because I am not like you in focusing on refutation. I am not here to refute everybody else. I am not interested in tearing apart your posts one point at a time. I find it childish. I put up the quote in full because I do not cherry pick. I do not break it apart because I am not obligated to organize my thoughts based on your presentation. We can all see what you said, and I trust the audience can see the relationship between your words and mine.

If you show me you are capable of reading my entire sentence before knee-jerking and cherry picking and firing off your rebuttal and refutation, I will do you the honor of breaking out the quotes.

I have explained the reference to Hollywood several times and in no case have you reflected back any level of comprehension. Apparently you see that word and don't even finish the sentence. Grow up.

When you ignore the answers, you are being intellectually dishonest.

When you pretend that math is only opinion, I just have to laugh.

As to the moment arm thing, are you telling me that we can produce more angular momentum from the same amount of propellant by increasing the distance of the thruster from the spin axis? Are you sure about that?

[spacester]

[...] and it should be clear that current generation spacesuits [...]

"Doctor, it hurts when I do this..."

We are either building better suits, or we aren't building much anywhere in space beyond a few pre-built modules berthed together.

I have designed the assembly and welding of Aquarius to be done with robotic arms operated by humans with direct line of sight supervision, in shirtsleeves. Payload zero provides that capability.

[Coastal Ron]

If you show me you are capable of reading my entire sentence before knee-jerking and cherry picking and firing off your rebuttal and refutation, I will do you the honor of breaking out the quotes.

Great, start now. I read your whole post on a 27" monitor, but many NSF members read posts on their mobile devices, so you are disrespecting THEM, not me, by loading your posts up with non-value added text. Because they know where to read the original post, which is why you don't have to completely copy it. Time to stop being a newbie Homie... 

When you ignore the answers, you are being intellectually dishonest.

We all have OPINIONS, and we can also have facts. But "answers" are not necessarily facts. And your opinions about your designs are just that, opinions. Same as my opinions about my designs are just opinions. You may think they are facts, but since we can't judge your work we have to treat them like opinions. That is just how the internet works.

When you pretend that math is only opinion, I just have to laugh.

You are laughing at your own joke, since I never said "math is only opinion". Don't make things up.

As to the moment arm thing, are you telling me that we can produce more angular momentum from the same amount of propellant by increasing the distance of the thruster from the spin axis? Are you sure about that?

Don't believe me, believe Archimedes - and math. Heck, you can prove this to yourself at home by doing a simple experiment with two levers of different lengths.

I have designed the assembly and welding of Aquarius to be done with robotic arms operated by humans with direct line of sight supervision, in shirtsleeves. Payload zero provides that capability.

My post wasn't about how to build a rotating station before it starts rotating, but how to MAINTAIN one that is already rotating. Unless you are assuming you are going to de-spin the rotating space station every time you need to do exterior inspection and maintenance.

My OPINION is that we won't use humans for such operations, but robotic systems. And for my rotating space station designs my assumption is that if humans have to go "outside", then they will need a lot of assistance (and I'm designing such systems). But I'm really hoping to eliminate the need for humans to need to go "outside" for normal maintenance needs.

[Twark_Main]

On complexity of parts, Aquarius will score very well indeed. This is not a science lab like ISS and EVAs are going to be very rare. KISS applies.

Of course EVA's are going to rare, since how do you do an EVA on the exterior of a rotating space station? The space equivalent of mountain climbing, but with far more equipment. Which is why I foresee the need for robotic systems...

...or a window-washing platform.

[Coastal Ron]

On complexity of parts, Aquarius will score very well indeed. This is not a science lab like ISS and EVAs are going to be very rare. KISS applies.

Of course EVA's are going to rare, since how do you do an EVA on the exterior of a rotating space station? The space equivalent of mountain climbing, but with far more equipment. Which is why I foresee the need for robotic systems...

...or a window-washing platform.

Sure, you can dangle someone down the outside of the station. But there is still the challenge of them being in a spacesuit that is likely as heavy as they are, or heavier, and the inflatable nature of those suits makes work difficult.

Not sure why there is so much resistance to the idea that we'll use robotic systems. In fact they would use teleoperation since humans can be nearby.

[spacester]

If you show me you are capable of reading my entire sentence before knee-jerking and cherry picking and firing off your rebuttal and refutation, I will do you the honor of breaking out the quotes.

Great, start now. I read your whole post on a 27" monitor, but many NSF members read posts on their mobile devices, so you are disrespecting THEM, not me, by loading your posts up with non-value added text. Because they know where to read the original post, which is why you don't have to completely copy it. Time to stop being a newbie Homie... 

I was trying to fit in with that procedure. I see it done a LOT here, they quote the whole nested stack and post just two sentences. I think it wastes space too, but I see it done a lot. The exceptions are guys like you who break it apart in order to tear it down. That was my observation, anyway.

We all have OPINIONS, and we can also have facts. But "answers" are not necessarily facts. And your opinions about your designs are just that, opinions. Same as my opinions about my designs are just opinions. You may think they are facts, but since we can't judge your work we have to treat them like opinions. That is just how the internet works.

One fact is that I presented a comprehensive design with specifications resulting from doing basic math. There is a CAD model and text descriptions of the design concept and execution. Another fact is that after all those years of arguing with each other here you have produced no comparable work product, even on a solo basis. If that is not a fact, just show me where to find it.

And maybe explain why you are incapable of judging my work. Unwilling, I get, but incapable? I take it that means you are incapable of judging anyone else's work? Maybe that's how the internet doesn't work.

Your statement

"until your design goes through an engineering review it is just a concept, no more or no less probable than "Hollywood" concepts"

is bogus. You are trying to dismiss my work.

You also continue to misconstrue my reason for referring to Hollywood, imposing your imagination on the subject of my thought process and failing to accept my answer, or react to it in any way that indicates you are listening to me. You are telling me what I think and why and I object. Who wouldn't?

Maybe even if no one at NSF worries about comparisons to Hollywood imagery and mythology, they should. Maybe explaining that to you is hopeless. Moving on then.

No one has built anything but that does not mean we know nothing. You might not be able to admit to knowing anything, which is weird, but we know that a guy named Ted Hall does know a lot. Also, at least one guy has presented some work on this thread that rises above mere opinion. We know that the object I depict and describe has been analyzed to the point where mass estimates and launch numbers have been produced. We know of the existence of one comprehensive design put forward by a person here ready to talk about it. We know that other designs have been discussed, and opinions offered, but we are not aware of any additional comprehensive designs being presented on this thread.

(snip)

Ron, you have blundered with this moment arm thing. You are making it look like you never took a physics class. It's called conservation of angular momentum. There is no such thing as a free lunch.

Rocket impulse transfers energy and momentum to the station. A given thrust will apply more torque at a greater moment arm, but work equals force times distance and since the work being done on the station by the rocket has to travel a longer distance over the same elapsed time the net effect is no change. The amount of impulse coming out the back of that rocket, ignoring losses, is the amount of impulse added to the rotational inertia (angular momentum) of the station, no matter where you mount it.

You go ahead and stick with Archimedes, I will go with Newton.

[Coastal Ron]

One fact is that I presented a comprehensive design with specifications resulting from doing basic math. There is a CAD model and text descriptions of the design concept and execution.

Gee, text descriptions too? Well, that must be proof that you have thought everything through then, right? 

Look, you aren't the only person that can use CAD, and just because someone uses CAD doesn't mean that what they designed is doable. And your design may in fact turn out to be viable, but what I have stated is still valid, in that none of our concepts can truly be assumed to be anything other than concepts until an engineering assessment is done. I can't tell you how many times in my manufacturing career that CAD drawings have led to parts that didn't work.

Another fact is that after all those years of arguing with each other here you have produced no comparable work product, even on a solo basis. If that is not a fact, just show me where to find it.

1. You are a newbie here, so are you stating that you have gone back over all the years of my posts to see if I have (or have not) released detailed drawings for my rotating space station designs? I think not.

2. Why would anyone feel they have to prove anything to you? Me personally I've defined two families of rotating space stations designs, but my preference is not to publish them until they have been validated enough to merit being shown to the public. Maybe others feel the same way too? Absence of proof is not proof of absence.

And maybe explain why you are incapable of judging my work. Unwilling, I get, but incapable?

At this point in time on NSF I don't critique rotating space station or rotating spaceship designs. I have previously, but I don't anymore.

And again, I haven't critiqued your design. Maybe it is workable, maybe not. However just because there is a design that holds together physically, that doesn't mean that it is affordable or practical.

There are LOTS of proposed designs for LOTS of things. At the end of the day though, if they can't get built then it doesn't matter. Hence my shift in focus to things that have a chance in getting built.

[spacester]

(snip)
There are LOTS of proposed designs for LOTS of things. At the end of the day though, if they can't get built then it doesn't matter. Hence my shift in focus to things that have a chance in getting built.

Newbie? I registered here in 2005. IIRC it was before there was even an L2 section. I posted for several years, but as it became increasingly clear that big ideas were not welcome, that amateurs were not welcome, and that this wonderful website was doing great things for lots of people besides me, I faded away. I checked in from time to time, and all I would see is the same old attack and refute and cherry picking.

I stayed away for years, lurking and waiting. Even with the rise of SpaceX my ideas were too big and too speculative.

Almost all eyes were on the past and almost none on the future. ISS was going to develop a magic pill to counteract the some 17 deleterious effects of micro-gravity. I was kinda being blasphemous with the whole AG thing.

I kept my eye open for AG threads, I may have missed one. And maybe there was one where people got along and worked together but I missed it. I doubt it though, culture is culture. I was sick and tired of that stuff 12 years ago. I have enormous respect for Chris B and everyone else here, but most people on the forums are not interested in the future like I am.

I am back because Starship is on the pad so they cannot tell me that my big projects are impossible any more. I am here to get stuff done. This threatens you. It shouldn't. I want to start really comparing the design choices. With numbers. That's what they told me here, without numbers they could not be bothered. So I am back with numbers and yeah, same old crap.

No one needs to prove anything to me, duh. As if, lol. I just want to not be dismissed again some more. You are trying to dismiss me by refusing to admit that conceptualization is not the end state of discussion.

If nothing matters until something gets built, why are you even here? It doesn't matter, right?

I want my efforts to matter. I want Paul's efforts to matter, I want to attract new voices. I want to talk to the other people on this thread besides you two. I want to demonstrate collective competency on the subject at NSF and start getting the public excited about AG. Which seems to threaten you. Whatever, I only have so much bandwidth for you.

The sooner you accept this the sooner you can start sharing all that knowledge you've got.

But that thing you just did there with the quotes? I call that cherry picking. The things you snip out matter too. Your physics blunder for example.

I applaud your wanting to work on things that get built. We do too, I have to suppose. But I want to collaborate.

[Coastal Ron]

(snip)
There are LOTS of proposed designs for LOTS of things. At the end of the day though, if they can't get built then it doesn't matter. Hence my shift in focus to things that have a chance in getting built.

Newbie? I registered here in 2005.

Great. Welcome back. But I used the reference because you weren't "up to speed" on the latest NSF posting norms. No longer an issue, so on to topical stuff.

I stayed away for years, lurking and waiting. Even with the rise of SpaceX my ideas were too big and too speculative.

Um, there are a LOT of people posting BIG ideas. Not me, though I might have one that is "medium", but in AG and other topics we have had some BIG ideas.

But remember that NSF is a discussion board, so if you don't want comments and critique about your BIG ideas, then I suggest publishing them on a personal blog and turn off the comments feature. Because if you post on NSF, then people will respond in their own way. It is human nature. And it is also why NSF has mods, because sometimes the discussions can get pretty rowdy. Your ideas won't be immune to that, just as mine are not immune to that.

I kept my eye open for AG threads, I may have missed one.

There are a number of them, and I've even started one or two.

And maybe there was one where people got along and worked together but I missed it.

Why does everyone have to get along? And who decides what ideas don't merit debate? Again, if you don't want feedback on your ideas, don't post them here, post them on a personal website or something. NSF is for discussions about all the topics.

...but most people on the forums are not interested in the future like I am.

You know, it's great to have enthusiasm, but it's kind of ignorant to think you are the only one on NSF who is interested in the future. Or in space. Or about rotating space stations and spaceships. I would imagine more than half the NSF members would take offense to your statement and challenge you. I know I would - just look at my byline. My focus is to support those things that help to expand humanity out into space, and if that isn't being "interested in the future" then I don't know what is.

I am here to get stuff done. This threatens you. It shouldn't.

Don't worry, it doesn't. Because why would I be threatened by an anonymous person on an NSF thread? Your ego is showing. 

But that thing you just did there with the quotes? I call that cherry picking.

I respond to what I respond to. If you want to carry on the conversation it is up to you.

I applaud your wanting to work on things that get built. We do too, I have to suppose. But I want to collaborate.

Actions speak louder than words is a great saying, and it applies here too. If you want to collaborate, then help others with their ideas or concepts. Give back. IRL I occasionally mentor people creating new businesses, and the best way to learn sometimes is to help others.

As to the topic at hand, in the past I have advocated that rotating spaceships are not yet needed, but I do have family of concepts that include slow travel rotating spaceships. But the mass is really high compared to spaceships with no artificial gravity, and humanity won't have the ability to support such spacecraft with their huge propellant needs for a very long time. So instead I've been focusing on rotating space stations.

If your concept is a rotating space station, then this is the wrong thread, since it is for rotating spaceships. Just thought I'd mention that...

[Twark_Main]

On complexity of parts, Aquarius will score very well indeed. This is not a science lab like ISS and EVAs are going to be very rare. KISS applies.

Of course EVA's are going to rare, since how do you do an EVA on the exterior of a rotating space station? The space equivalent of mountain climbing, but with far more equipment. Which is why I foresee the need for robotic systems...

...or a window-washing platform.

Sure, you can dangle someone down the outside of the station. But there is still the challenge of them being in a spacesuit that is likely as heavy as they are, or heavier, and the inflatable nature of those suits makes work difficult.

Not sure why there is so much resistance to the idea that we'll use robotic systems. In fact they would use teleoperation since humans can be nearby.

"KISS applies"

And the astronauts wouldn't be "dangling down." They're standing on a platform. If we can make suits for operating on a planetary surface, we make make suits for operating on a platform.

[Twark_Main]

Ron, you have blundered with this moment arm thing. You are making it look like you never took a physics class. It's called conservation of angular momentum. There is no such thing as a free lunch.

Rocket impulse transfers energy and momentum to the station. A given thrust will apply more torque at a greater moment arm, but work equals force times distance and since the work being done on the station by the rocket has to travel a longer distance over the same elapsed time the net effect is no change. The amount of impulse coming out the back of that rocket, ignoring losses, is the amount of impulse added to the rotational inertia (angular momentum) of the station, no matter where you mount it.

You go ahead and stick with Archimedes, I will go with Newton.

Holy physics error, batman!

So I can mount a thruster in-line with the center-of-mass (producing zero torque) and it will add the same amount of angular momentum? Because you said "no matter where you mount it." 

The unit mismatch should be a dead giveaway here. In physics, impulse is measured in newton seconds, while angular momentum is measured in newton meter seconds. So it can't be true that "the impulse coming out the rocket is... added to the rotational inertia of the station," because it's impossible to add two quantities with different dimensions of units.

As for "work equals force times distance," that's true, but a rocket (unlike a propeller or a wheel) uses the same amount of fuel/energy — while still producing the same amount of thrust — no matter how fast it's going.

Coastal Ron is right. Mount your thrusters twice as far from the CoM and you need half as much fuel to spin up, spin down, or precess your rotation axis in inertial space.

Placing thrusters as far from the satellite’s center of mass as possible gives them a larger moment arm and allows them to exert a greater torque for a given force. This is evident from the important concept we saw earlier. The greater the distance over which a force is applied, the more torque is delivered from the same force.

https://www.faa.gov/about/office_org/headquarters_offices/avs/offices/aam/cami/library/online_libraries/aerospace_medicine/tutorial/media/III.4.3.1_Space_Vehicle_Control_Systems.pdf

[Paul451]

Not sure why there is so much resistance to the idea that we'll use robotic systems. In fact they would use teleoperation since humans can be nearby.

Same reason I'm not counting on fusion to save the day. People have been promising me this since I first took an interest in space development as a child.

I've been watching all the demonstrations on lab-robots, and seen advances in actual useful robots, but the reality is the closest thing we have to "robot assembly and maintenance in space" is the Canadarm.

Sure, we might get there, one day. Musk might sprinkle his magic dust over that woman in a lycra suit and turn her into a useful product. And fusion might happen. Or nanobots and self-assembling smart-dust. I'm just not taking seriously any proposal that relies on it existing.

[Paul451]

Ron, you have blundered with this moment arm thing. You are making it look like you never took a physics class. It's called conservation of angular momentum. [...] Rocket impulse transfers energy and momentum to the station. A given thrust will apply more torque at a greater moment arm, but work equals [...]

So I can mount a thruster in-line with the center-of-mass (producing zero torque) and it will add the same amount of angular momentum? Because you said "no matter where you mount it."  [...]
Coastal Ron is right. Mount your thrusters twice as far from the CoM and you need half as much fuel to spin up, spin down, or precess your rotation axis in inertial space.

Or picture two identical thrusters, both in line with the centre of a beam. Forces exactly balance and cancels out; no net rotational acceleration, no net linear acceleration.

Move them slightly apart along a beam, in opposite directions, and most of their force is still balanced and cancels out, but some goes into rotational acceleration. Move the thrusters further apart and less force is lost, more goes into rotational acceleration, still no net linear acceleration of the beam overall. Rotates around the centre, but the location of the centre doesn't move.

Now repeat with one thruster removed. When it's at the centre, you only get linear acceleration, no rotational. Move the thruster slightly off-centre and you still get mostly linear acceleration, but now with some rotational. As you move the thruster further away from the centre, you get more rotational acceleration and less linear (although never zero, at least on a finite beam.) There's clearly more going on than "the amount of impulse is the amount of impulse."

None of this is rocket sci.... well, yeah, I mean....

[endlesslimitation]

Don't mean to throw fuel on the fire, but I think spacester is correct, though we need to consider the specific design when talking about whether the thrusters should be near or far. I've been casually skimming this thread but may have missed a key design element, apologies if that is the case.

The rotating structure will have kinetic energy:
KE= 0.5 I w^2 (I is moment of inertia and w is angular velocity)

The acceleration a human experiences sitting on the end of the structure which extends a distance r from CoM is
acentrifugal=v^2/r = w^2 r
for comfort let's fix that to the accel. due to gravity on earth, g.
g=w^2 r
thus w^2=g/r
sub that into the KE:

KE=0.5 I g/r
In the simplest design, with the two thrusters dominating the total mass and mounted at the ends of a massless truss of length 2r, the moment of inertia is
I=2 m r^2    where m is the mass of one thruster
In that case KE= m g r

Extending the total length increases the amount of energy in the system. With such a design you would probably be best keeping the whole system as small as possible while keeping differential spin gravity and coriolis effects at a tolerable level.

If, on the other hand, the thrusters make up a negligible fraction of the total mass, then the moment of inertia remains unchanged regardless of where the thrusters are. It's true that moving the thrusters out further will yield a greater torque for a given thruster force, but it will have to apply that torque for longer (longer time and distance) to spin up to nominal speed. It sounds like an engineering decision based on whether you can more easily build low-thrust long duration modules, or high-thrust short duration modules.

[endlesslimitation]

... and on  the third hand, we may not really care about the total energy requirements if we use thrusters that don't require mass or energy expenditure, eg. reflecting mirrors that steal small amounts of momentum from sunlight. In that case you would probably want to extend their lever arms as long as possible without it becoming the dominant contribution to system total mass.

[spacenut]

The only way I see Rotating Spaceships is when mankind goes beyond Mars to the asteroids and the moons of Jupiter and Saturn.  Then, they will probably have large rotating spaceships with nuclear power for transportation and operation.  Time and distance will probably require artificial gravity even with nuclear power.  This would be the mother ship with smaller landers attached for whatever place they were going to land and study, mine, or colonize. 

SpaceX has figured right now, refueling Starships in earth orbit, and flying to Mars during the 6 month synod Mars is closest to earth is adequate.  They will have gravity when they land on Mars.  However lots of studies will have to be made for 0.38g on the human body, even with exercise.  That will dictate how much gravity humans need to maintain body health. 

I am one who thinks a rotating space station would be a better idea in LEO for long term studies.  Moon gravity for a year, Mars gravity for say 2 years, and of course 1g.  The station could be made to spin up or down for these studies. 

I also think NASA should be in charge of this station with multiple companies and countries contributing like ISS.  It should have already been done before we went further than the moon, so we would know.  Then we would know how build the large rotating space ships for whatever journey they would take. 

[Twark_Main]

Don't mean to throw fuel on the fire, but I think spacester is correct

[snip]

In the simplest design, with the two thrusters dominating the total mass

Well 'errre's yer problem! You're setting up the problem unrealistically.

This "thrusters dominate the mass" setup is quite absurd. That's why I didn't set it up that way. Has anyone ever heard of a space station where the attitude control thrusters (of all things!) are the dominant mass of the whole system?

If, on the other hand, the thrusters make up a negligible fraction of the total mass, then the moment of inertia remains unchanged regardless of where the thrusters are. It's true that moving the thrusters out further will yield a greater torque for a given thruster force

Good.

but it will have to apply that torque for longer (longer time and distance) to spin up to nominal speed.

Not so good.

Angular momentum is a product of torque and time. By lengthening the thruster lever arm, the torque increases, which means that (all other things being equal) the thruster firing time needed to reach nominal rotation speed decreases proportionally.

Longer distance doesn't matter, for the reason I gave above. A rocket doesn't use any more fuel because it's going fast.

[endlesslimitation]

Perhaps i misunderstood. I was under the impression that y'all were discussing minimizing energy, not thruster fire time. If you are discussing energy, then really the rotational kinetic energy equation that I posted above is all that matters. You can choose to build up that energy over a short firing time with a strong thruster or a long period of time with weak thrusters, but the total energy expense is the same.

I presented the two limiting cases, one where the thrusters have no mass, in which case it doesn't matter how far away you place them, and secondarily one in which the thrusters dominate the mass in which it absolutely matters how far away you place the thrusters and you're much better off by placing them close in. Any realistic scenario will lie somewhere between those two limiting cases., Meaning that you're definitely not better off from an energy standpoint by moving the thrusters further out. There is no way getting around this. If, you are instead trying to minimize thruster firing time, then of course your analysis is correct.

[spacester]

Your ship is already spinning, right? So if you want to make it spin faster why would you put thrusters at the COG? You have turned a real world problem into a nonsense problem. You have made it a different physics problem. Forgive me for not covering all the nonsense scenarios.

That portion of the thrust which produces linear momentum will cancel out over a full revolution. So any (non-calculus) analysis needs to be done over a time period of a multiple of the rotational period so that this cancelling applies. If you go for an extra half revolution, you will get that much translation. But that is irrelevant to how the rpms are effected, which was the point, right?

So, wherever you put them except in the places where you know they won't work. Do I need to explain why you need multiple thrusters in radial symmetry?

So assuming you are not being intentionally idiotic, you have a lever arm radius and you have a distance defined by the circumference of the circle traveled at that radius multiplied by the number of rotations. You don't get your increased moment arm without accepting the increased circumference.

Ah but that is not the distance you use to calculate the work. It is a distance that would have been traversed anyway by a non burning rocket. The work imparted is related to the *extra* distance traveled as a result of the force's (F) application, expressed in the general work equation W = F * d

We need to solve the calculus problem to do this right, I am not up for that atm.

The work equation for this is W = F * deltadistance where we calculate the distance traveled minus the distance it would have traveled anyway, which we find from the starting rpm and ending rpm. (I think you are going to end up with a deltad / d term which is where your increased circumference cancels out the increased torque. Not sure.)

The energy of the rocket motor's exhaust does work on the space station's momentum. That is freshman engineering. People talk about energy a lot, but they do not get the concept of work.

Work is force times distance. Try to get around that at your peril. Momentum is conserved. Physics 101.

This post was not as easy to write as I would want you to think, lol. I totally see why a person would think you get a better result from putting the thrust further out. I wrote a couple of paragraphs that ended up seemingly proving myself wrong. It was fun and a little scary, lol. But natural laws are my friends, the conservation laws are the best natural laws and I know that there is only so much energy in that rocket motor. You cannot just move your thruster and get more or less momentum from the universe.

The thrust is the same, the burn time is the same and the revolutions during that burn is the same. The torque - not the thrust force - has increased linearly with the moment arm. The distance traveled by the rocket has increased linearly as well. That sounds great, right? But you have to look at the extra distance traveled in the work equation.

The reason you need the work equation is that you have conservation of both energy and momentum. The rocket has energy but the result we are looking for is in terms of momentum - and angular momentum at that (trying to steer clear of the complications there). You could do the full on physics problem solving both those equalities with the calculus, or you can thank James Watt and take the shortcut.

This is why physics has the concept of work. The work equation is physics' way of connecting the concept of energy with the physical world we experience. It is not an optional approach, it is the way you solve problems just like this where you are converting energies with motion involved.

I feel like I still have not written the paragraph needed to put this to bed, so debate away, that's my best shot for now.

[spacester]

Placing thrusters as far from the satellite’s center of mass as possible gives them a larger moment arm and allows them to exert a greater torque for a given force. This is evident from the important concept we saw earlier. The greater the distance over which a force is applied, the more torque is delivered from the same force.

https://www.faa.gov/about/office_org/headquarters_offices/avs/offices/aam/cami/library/online_libraries/aerospace_medicine/tutorial/media/III.4.3.1_Space_Vehicle_Control_Systems.pdf

So I studied that a bit, nice reference, bookmarked.

The section quoted is the best choice here, well done. But it does not talk about propellant usage.

The relationship between the torque delivered and the change in rotation rate is not fully developed there. One must be careful with angular velocity and angular acceleration.

I do not dispute that the applied torque is increased by increased distance from the spin axis.

The stored chemical energy of a given amount of propellant is converted into kinetic energy out the back and a momentum vector to the front. That momentum vector is expressed as a force vector, and that force vector gives us the torque we are applying to the spaceship. Yeah, we got a units problem here. And we need to fully understand the differences between linear and angular motion.  The equations are similar but the differences are tricky.

So that's a full on college level bunch of work there. Or we can turn to the concept of work.

I am confident that you cannot use the equations of motion to provide for free momentum just by changing your ship's geometry. In the extreme case you would use almost zero propellant to add all the momentum you want, which means you got free energy from somewhere. (momemtum  ~ m * v and energy ~ m * v^2). TANSTAAFL

[endlesslimitation]

If you are discussing energy, then really the rotational kinetic energy equation that I posted above is all that matters.

Ooops! My bad! I overlooked the very pertinent point that rockets are not simply magical machines that convert stored energy perfectly into kinetic energy of a payload. In fact they distribute that energy into the payload and the exhaust. No hard feelings on my end, I messed this one up and didn't fully appreciate Twark_main's point. In the situation where the thruster masses are small compared to the system mass, I now agree with Twark_main that placing the thrusters far out is best. I still really like the energy-consideration approach, so here's the line of thought that convinced me:

Start by assuming the system starts at rest but has a desired angular momentum, L. You can build that L up by applying some torque, tau, for some time, t.

L= tau t = r F t
where r is the distance away from CoM at which the force, F, is applied.

Thinking in terms of rockets, that force will be given in terms of the specific impulse ISP (in seconds) times the constant g, times the fuel burn rate, mdot (kg/s):
F=g ISP mdot

Therefore the targeted angular momentum will be given by:
L=r g ISP mdot t

It was at this point where I previously said, "ok, no big deal, if we decrease r we can simply choose a thruster with higher ISP. The kinetic energy needs of the rotating structure won't change as long as the moment of inertia doesn't change."
This rotational kinetic energy is:
Erot=0.5 I g/d (where d is the distance from CoM at which we want 1g acceleration)

And while that's true, I neglected to consider the energy carried away by the rocket exhaust.

This exhaust energy will be:
Eexhaust=0.5 mused ve^2   where mused=mdot t is the total fuel mass used and ve is the exhaust velocity.

ISP and exhaust velocity are related as such: ISP=ve/g
Therefore the wasted energy carried away is:
Eexhaust=0.5 mused g^2 ISP^2

The ratio of wasted energy to useful rotational energy is then:
Eexhaust/Erot=mused g ISP^2*d/I

subbing in L:
Eexhaust/Erot=L ISP*d/(I r)

and writing out L in terms of the moment of inertia and the required angular velocity, w:
Eexhaust/Erot=I w ISP*d/(I r)
Eexhaust/Erot= w ISP d/r

Recall d is the distance at which we feel 1g, r is distance at which the torque is applied. When r is huge the wasted to useful energy ratio is small. Therefore, to the extent to which it doesn't significantly affect the overall moment of inertia, it is indeed best to put the thrusters far away.

[cdebuhr]

If you are discussing energy, then really the rotational kinetic energy equation that I posted above is all that matters.

Ooops! My bad! I overlooked the very pertinent point that rockets are not simply magical machines that convert stored energy perfectly into kinetic energy of a payload.
[...snip...]

I was going to post as much earlier but got busy and didn't get the chance.  Glad to see you got here anyway!  I like to think of it this way: To a reasonably good approximation (and a very good approximation at low velocity ... around lift off and such), rocket engines don't so much do work on the rocket itself as they do on the reaction mass.  A hovering rocket is having exactly no work actually done on it - its all about the reaction mass.  Everything else is conservation of momentum.  Or to put it another way: balance momentum first - if it seems like you're loosing energy, it's probably in the reaction mass.

[Coastal Ron]

Not sure why there is so much resistance to the idea that we'll use robotic systems. In fact they would use teleoperation since humans can be nearby.

Same reason I'm not counting on fusion to save the day. People have been promising me this since I first took an interest in space development as a child.

Well, the same could be said for interplanetary spaceships, yet we think we may be on the cusp of that pretty soon.

I've been watching all the demonstrations on lab-robots, and seen advances in actual useful robots, but the reality is the closest thing we have to "robot assembly and maintenance in space" is the Canadarm.

You mean the Mobile Servicing System (MSS) on the ISS? That was launched 20 years ago, and designed years before that, so not exactly cutting edge technology. But certainly has elements that could be expanded upon, like the ability to move itself and plug-in for local power so it doesn't have to carry a battery. Power is one of the limitations of all robotic systems, so having the ability to just plug-in locally means you can have high-powered robotic systems that are more capable.

A friend of mine is actually building a robotic arm with the dexterity to do some assembly work, and just this morning we were just talking about how his arm could be used for maintenance in space. I don't think it is as far off as you fear.

Sure, we might get there, one day. Musk might sprinkle his magic dust over that woman in a lycra suit and turn her into a useful product. And fusion might happen. Or nanobots and self-assembling smart-dust. I'm just not taking seriously any proposal that relies on it existing.

I understand your concerns, but again there is an interplanetary spaceship being built in a field in Texas, and we are cheering them on. So I'm not sure why you don't think robotics is such a barrier, especially when industrial robots have been commonplace for decades.

The challenge is in building a rotating space station or spaceship so that teleoperated robotic maintenance can be done - from day one. Because robotic systems should be far less expensive to use than "spam in a can" humans. One human doing an EVA requires a whole team of people supporting them, but one robotic system crawling around outside of a space station only needs one operator (or maybe none). And if the robotic systems can plug-in as they work, they can work 24 hours a day, whereas humans can only stay out in a suit for far less time.

So sure, LOTS of technological hurtles to overcome, but I see this as an economic necessity to have robotic maintenance in space. Not to mention a safety issue too.

[Twark_Main]

Perhaps i misunderstood. I was under the impression that y'all were discussing minimizing energy, not thruster fire time.

It's the same thing.

If you are discussing energy, then really the rotational kinetic energy equation that I posted above is all that matters. You can choose to build up that energy over a short firing time with a strong thruster or a long period of time with weak thrusters, but the total energy expense is the same.

You can do the problem with rotational kinetic energy or angular momentum. Either way it comes out the same: longer lever arm = less fuel.

I presented the two limiting cases, one where the thrusters have no mass, in which case it doesn't matter how far away you place them

Funny enough, what you actually "proved" (read: asserted) is that it gets worse with a longer lever arm ("it will have to apply that torque for longer"). This is, of course, tragically incorrect.

and secondarily one in which the thrusters dominate the mass in which it absolutely matters how far away you place the thrusters and you're much better off by placing them close in.

Wrong again. You actually proved that in that case it doesn't matter where you put the thrusters. You said so yourself.

The model where the RCS thrusters dominate the mass of your large artificial gravity space station is, of course, absurd.

Any realistic scenario will lie somewhere between those two limiting cases., Meaning that you're definitely not better off from an energy standpoint by moving the thrusters further out. There is no way getting around this. If, you are instead trying to minimize thruster firing time, then of course your analysis is correct.

Nope. Wrong, wrong, and more wrong. Somehow you got more things wrong in this post than in your last post.

[Twark_Main]

Your ship is already spinning, right? So if you want to make it spin faster why would you put thrusters at the COG? You have turned a real world problem into a nonsense problem. You have made it a different physics problem. Forgive me for not covering all the nonsense scenarios.

The "nonsense" is when you said I could put the thrusters anywhere (including somewhere that produces no torque) and it has the same effect on angular momentum. My counterexample proves how ridiculous your statement is.

That portion of the thrust which produces linear momentum will cancel out over a full revolution. So any (non-calculus) analysis needs to be done over a time period of a multiple of the rotational period so that this cancelling applies. If you go for an extra half revolution, you will get that much translation. But that is irrelevant to how the rpms are effected, which was the point, right?

I don't think "cancel out" is the right model.

If you thrust forward (which requires adding kinetic energy) and then thrust backwards (which also requires adding kinetic energy), did you expend any energy? Did you gain any energy?

If the answer to those questions is not the same, why is that?

So, wherever you put them except in the places where you know they won't work. Do I need to explain why you need multiple thrusters in radial symmetry?

No, but you need to explain how you square your statement that I can put thrusters anywhere and they have the same effect on angular momentum, with your new statement that there are some places that "don't work."

Hint: your original statement was wrong, and the second statement is correct.

So assuming you are not being intentionally idiotic

Be nice.

Ah but that is not the distance you use to calculate the work. It is a distance that would have been traversed anyway by a non burning rocket. The work imparted is related to the *extra* distance traveled as a result of the force's (F) application, expressed in the general work equation W = F * d

We need to solve the calculus problem to do this right, I am not up for that atm.

No calculus needed. The distance is proportional to the radius. Double the radius, double the distance, double the torque, halve the fuel needed (which is just what I told you earlier ).

The work equation for this is W = F * deltadistance where we calculate the distance traveled minus the distance it would have traveled anyway, which we find from the starting rpm and ending rpm. (I think you are going to end up with a deltad / d term which is where your increased circumference cancels out the increased torque. Not sure.)

Don't overthink it. Just double the radius and halve the thrust, and the starting and ending RPM and burn duration is the same.

The energy of the rocket motor's exhaust does work on the space station's momentum. That is freshman engineering. People talk about energy a lot, but they do not get the concept of work.

Work is force times distance. Try to get around that at your peril. Momentum is conserved. Physics 101.

You'll let me quote the phrase "Physics 101" back at you after you've realized you're wrong, right?

No trick. All momentum is conserved, and all work is always force times distance.

This post was not as easy to write as I would want you to think, lol. I totally see why a person would think you get a better result from putting the thrust further out. I wrote a couple of paragraphs that ended up seemingly proving myself wrong. It was fun and a little scary, lol. But natural laws are my friends, the conservation laws are the best natural laws and I know that there is only so much energy in that rocket motor. You cannot just move your thruster and get more or less momentum from the universe.

You're so close!

The thrust is the same, the burn time is the same and the revolutions during that burn is the same. The torque - not the thrust force - has increased linearly with the moment arm. The distance traveled by the rocket has increased linearly as well. That sounds great, right? But you have to look at the extra distance traveled in the work equation.

The reason you need the work equation is that you have conservation of both energy and momentum. The rocket has energy but the result we are looking for is in terms of momentum - and angular momentum at that (trying to steer clear of the complications there). You could do the full on physics problem solving both those equalities with the calculus, or you can thank James Watt and take the shortcut.

This is why physics has the concept of work. The work equation is physics' way of connecting the concept of energy with the physical world we experience. It is not an optional approach, it is the way you solve problems just like this where you are converting energies with motion involved.

I feel like I still have not written the paragraph needed to put this to bed, so debate away, that's my best shot for now.

The longer lever arm means that more of the rocket's thrust goes into rotational energy, and less into linear kinetic energy translating sideways (plus kinetic energy in the exhaust; don't forget that!). Energy and momentum are conserved everywhere. There's no "free lunch" here.

[endlesslimitation]

Geeze dude...

Ooops! My bad! ... I messed this one up and didn't fully appreciate Twark_main's point.  ... I now agree with Twark_main that placing the thrusters far out is best.

Nope. Wrong, wrong, and more wrong. Somehow you got more things wrong in this post than in your last post.

A gracious educator you are not  I already acknowledged my error and even attempted to explain my mistake in a way that another energy- (rather than impulse) minded forum member might find more compelling. Let's move on to something more productive.

On that note, I will suggest that some of the recent mission parameters that have been suggested would lend themselves to using reaction wheels (or even better, dual opposite spinning artificial gravity wheels). These could be repeatedly spun up and decelerated to different RPM without worrying about reaction mass. This would be quite useful for the proposed use of it as a training area or lab  for understanding the effects of Earth/Mars/Moon/micro gravity. It would, of course, require very low friction bearings, but magnetic bearings and a small solar panel array to replenish the minor losses would probably be sufficient in the inner solar system.

[Twark_Main]

Placing thrusters as far from the satellite’s center of mass as possible gives them a larger moment arm and allows them to exert a greater torque for a given force. This is evident from the important concept we saw earlier. The greater the distance over which a force is applied, the more torque is delivered from the same force.

https://www.faa.gov/about/office_org/headquarters_offices/avs/offices/aam/cami/library/online_libraries/aerospace_medicine/tutorial/media/III.4.3.1_Space_Vehicle_Control_Systems.pdf

So I studied that a bit, nice reference, bookmarked.

The section quoted is the best choice here, well done. But it does not talk about propellant usage.

It doesn't have to. We all understand that a given thruster will produce a given amount of thrust and use a given amount of fuel per unit time, no matter where it's placed on the ship.

The relationship between the torque delivered and the change in rotation rate is not fully developed there. One must be careful with angular velocity and angular acceleration.

The relationship is straightforward: delta angular velocity = torque * time / moment of inertia.

Remember, rotational kinematics is just linear kinematics wrapped around a circle. The corresponding equation in linear kinematics would be delta velocity = force * time / mass.

Rotational kinematics cheat-sheet: https://www.physicstutorials.org/home/rotational-motion/rotational-motion-cheat-sheet

I do not dispute that the applied torque is increased by increased distance from the spin axis.

... and since the torque is increased, the thruster fires for less time. Less time = less fuel.

The stored chemical energy of a given amount of propellant is converted into kinetic energy out the back and a momentum vector to the front.

The "back" and "front" both have momentum. The propellant out the "back" has kinetic energy, as does the spaceship in "front." It's not a case of "energy in back and momentum in front." Both are everywhere.

That momentum vector is expressed as a force vector, and that force vector gives us the torque we are applying to the spaceship. Yeah, we got a units problem here. And we need to fully understand the differences between linear and angular motion.  The equations are similar but the differences are tricky.

Check the cheat-sheet I linked.

So that's a full on college level bunch of work there. Or we can turn to the concept of work.

I am confident that you cannot use the equations of motion to provide for free momentum just by changing your ship's geometry. In the extreme case you would use almost zero propellant to add all the momentum you want, which means you got free energy from somewhere. (momemtum  ~ m * v and energy ~ m * v^2). TANSTAAFL

Again, there's no free lunch. Just more efficient transfer of linear momentum into rotational momentum (with a long lever arm) or inefficient transfer (with a short lever arm).

You can't make the lever infinitely long, because once the tangential velocity exceeds the exhaust velocity of the thrusters, they no longer produces net impulse if fed from a central tank. The act of pumping the propellant to the rim will drain as much angular momentum as the thruster firing adds. If you feed the thruster from a local tank instead, then now we're dealing with a "rocket on a string" model, which has its own inherent limitations. If we want to refuel our rocket-on-a-string, now we're back to the first problem.

[Twark_Main]

I already acknowledged my error and even attempted to explain my mistake

Sorry bout that. It's not that I'm a sadist, I just didn't see that when I replied.


No "edu-ma-cators" 'round here, just a curmudgeonly old man...

[endlesslimitation]

No "edu-ma-cators" 'round here, just a curmudgeonly old man...

hahaha fair enough. Guilty of that myself often enough!

[Paul451]

So I'm not sure why you don't think robotics is such a barrier, especially when industrial robots have been commonplace for decades.

Because industrial robots have been commonplace for decades.

[Paul451]

Aside: If Relativity Space's 3d printed metal fabrication process ends up being as revolutionary as it seems, then the only thing shipped up might be weld-wire and replacement weld-heads. And the whole design concept (not just spacester's, I mean everyone's) will need to be completely re-thought.

I'm not hostile to the idea of robotic innovation. I'm just not taking seriously any proposal that relies on it already existing.

[Coastal Ron]

Aside: If Relativity Space's 3d printed metal fabrication process ends up being as revolutionary as it seems, then the only thing shipped up might be weld-wire and replacement weld-heads. And the whole design concept (not just spacester's, I mean everyone's) will need to be completely re-thought.

Living in a metal can in LEO, where Earth provides radiation protection, may be OK, but I wouldn't want to be living in a metal housing out in deep space - too much secondary radiation issues. Besides, I'm not sure why 3d printed metal fabrication is better for pressurized habitable space vs composite vessels - which are also additive manufacturing. Composite material provides more radiation protection, and can be just as strong, so bonus!

I'm not hostile to the idea of robotic innovation. I'm just not taking seriously any proposal that relies on it already existing.

Let me see how this sounds in a different context from 10 years ago:

I'm not hostile to the idea of reusable rockets. I'm just not taking seriously any proposal that relies on it already existing.

I think enough development has been done to show that the robotics industry has evolved enough that creating a robotic inspector, or robotic light-repair unit, for space applications won't be that big of a technology leap. All it needs is the right application, which space presents with its radiation environment. And I don't preclude humans being able to do repairs, just that the cost of sending a human into the harsh environment of space is significantly higher than the cost of sending a robotic system.

The motivation to use robotic systems in space will obviously be money. Capitalism, baby! 

[RonM]

The key to using robotic systems to repair a spacecraft is to design the spacecraft to be repaired by robots. If you're using parts or connectors that require humans in spacesuits then you're doing it wrong.

[Asteroza]

The key to using robotic systems to repair a spacecraft is to design the spacecraft to be repaired by robots. If you're using parts or connectors that require humans in spacesuits then you're doing it wrong.

The interesting question here though is do you want to be doing this only at the FRU level (such that you are basically only going to do module swaps or really easy cables, and return the modules to a workshop where humans will do the module repair), or get down to the boards and cables themselves. Elon Musk, in retrospect over the "alien juggernaut" debacle at Tesla, mentioned that using robotics for cables faced two problems. First was cable floppiness (which Tesla is now addressing with reduced and stiffened cables/cable harnesses), and the actual connectors which are designed for human installation/operation (and thus the flexibility/maneuverability of the human hand).

[Coastal Ron]

The key to using robotic systems to repair a spacecraft is to design the spacecraft to be repaired by robots. If you're using parts or connectors that require humans in spacesuits then you're doing it wrong.

The interesting question here though is do you want to be doing this only at the FRU level (such that you are basically only going to do module swaps or really easy cables, and return the modules to a workshop where humans will do the module repair), or get down to the boards and cables themselves. Elon Musk, in retrospect over the "alien juggernaut" debacle at Tesla, mentioned that using robotics for cables faced two problems. First was cable floppiness (which Tesla is now addressing with reduced and stiffened cables/cable harnesses), and the actual connectors which are designed for human installation/operation (and thus the flexibility/maneuverability of the human hand).

NOW we are getting to the core of the challenge for robotic servicing in space, but the same challenges apply to humans in inflated suits - they can't be doing the equivalent of brain surgery while hanging from a rotating spaceship and being stuck in a semi-rigid balloon.

So yes, designing the rotating vehicle or structure for as much simplicity as possible in repairs, which means Field Replaceable Units (FRU) where possible.

And we have to keep in mind that just like aircraft and ships that can get by with FRU support for normal operations, after a period of time there could be a need for a rotating spaceship to go through a much more comprehensive. The aviation industry breaks down such work as:

- Line Maintenance Checks - post-flight, maintenance pre-flight, service check, and overnight checks
- A Checks - every 400-600 flight hours
- B Checks - every 6-8 months
- C Checks - “heavy maintenance visit”
- D Checks - “heavy maintenance visit” every 6-10 years

We've all seen ISS spacewalks where they are fixing something, and though I'm sure we have learned a lot about how to design things for use in space, things will need monitoring and fixing.

[Twark_Main]

Composite material provides more radiation protection, and can be just as strong, so bonus!

"Composites" are usually composed of strong fibers (carbon fiber, kevlar, etc) mixed with weak plastic / epoxy.

Seems like there's an opportunity here to delete the plastic ("The best part is no part") and make it an inflatable.

This also greatly reduces the assembly of large components in-vacuo. Simply inflate the large exterior "envelope," then plug together LEGO interior components in a convenient shirt-sleeve environment.

No need for an EVA just to connect some plumbing. This is one of the major "lessons learned" from the ISS program.

[Paul451]

I'm not sure why 3d printed metal fabrication is better for pressurized habitable space vs composite vessels - which are also additive manufacturing.

In practice, composites are much harder to automate, requiring significantly more tooling and prep than Relativity's 3d full metal printing. (Indeed, I would expect their kind of manufacturing to see increasing use in making tooling/dies, vastly reducing the cost of setting up new production lines.)

I'm not hostile to the idea of robotic innovation. I'm just not taking seriously any proposal that relies on it already existing.

Can you please quote properly. I'm sick of having to fix people's lazy quoting.

Let me see how this sounds in a different context from 10 years ago:

I'm not hostile to the idea of reusable rockets. I'm just not taking seriously any proposal that relies on it already existing.

Actually yes. Before reusability was proven, I wasn't taking seriously anything that relied on reusability, or relied on massive price drops below SpaceX's expendables.

Look how anticipating development of other people's technology too early worked out for Robert Bigelow. (Should have launched the Genesis prototypes around the same time as Crew 1.)

SpaceX's party trick was to lower the cost of launch, even for expendables. That's ultimately what made (and with Starship, continues to make) reusables viable.

By contrast: The Space Shuttle orbiter was reusable. And it was absolutely cheaper to reuse the orbiter than build a new orbiter for each launch, in spite of the complexity of servicing. But "cheaper" didn't make its reusability "cheap". Indeed, it was so expensive it scared a lot of people off reusability entirely. (Even today, they don't seem to grok it. (See ULA.)) And the cost of "expending" a Shuttle orbiter was so high...and this is the core point... that it made even minor new versions too expensive to afford. You therefore couldn't iterate the Shuttle. Not just preventing MkII, but even Mk1.1. Similarly, you couldn't develop variants, even something seemingly as simple as Shuttle-C (and its successor Not-The-Shuttle-C.) Instead, you were stuck flying expensive prototypes for nearly 30 years.

An orbiter which was cheap to build and throw away could be developed iteratively into a reusable system that saved yet more money. Which is what Starship is doing. SN20 and they're not even in orbit. (And yes, I know the numbers aren't contiguous.)


Of course, none of this is relevant to the topic.

Except, perhaps, that: A) your first rotating space station isn't going to last 200 years. And B) if you count on technology that doesn't exist, you've just doubled the number of currently non-existent technologies you have to develop for your first rotating station.

[Paul451]

No need for an EVA just to connect some plumbing.

Provided there's nothing in the plumbing that you can't tolerate in your air supply. For example, it precludes many of the best cooling loop liquids, like ammonia. Even makes a steam/water cycle riskier. Realistically, for the operating temperature range, leaves you with water and non-phase-change heat exchanges. Bleh.

And at some point, you have to pass through to external radiators. So someone's goin' outside.

[Twark_Main]

No need for an EVA just to connect some plumbing.

Provided there's nothing in the plumbing that you can't tolerate in your air supply. For example, it precludes many of the best cooling loop liquids, like ammonia.

I almost mentioned that in my last post, too. "Don't use ammonia" was one of the other major lessons learned from ISS.

The Russian side uses silicone oil on the outside and water+antifreeze on the inside.

[Paul451]

The Russian side uses silicone oil on the outside and water+antifreeze on the inside.

In both cases, you don't get the enormous benefit of phase change. Which means your radiators/heat-exchangers are going to be lower efficiency and thus large and heavy. (On ISS, the US/international side deals with the majority of the station's excess heat.)

After all, they didn't pick ammonia because they like working with it. Some of the standard n-fluorocarbons might be a useful substitute, but pressure needs to be higher, compressors larger, etc, everything's heavier, noisier, higher maintenance, and with long lines it can be unfun to work with. And you still got to get the heat inside, outside to the radiators. Doesn't solve the "no external maintenance" requirement.

[Coastal Ron]

Composite material provides more radiation protection, and can be just as strong, so bonus!

"Composites" are usually composed of strong fibers (carbon fiber, kevlar, etc) mixed with weak plastic / epoxy.

Seems like there's an opportunity here to delete the plastic ("The best part is no part") and make it an inflatable.

1. Elon Musk did not invent the phrase "The best part is no part". It has been a philosophy used by engineers of all types since likely forever. Elon Musk uses the phrase to highlight points about what they are doing. But otherwise engineers are normally motivated to simplify things.

2. The epoxy in composites is not there for kicks and giggles, it is there to hold the flexible fibers in place and provide rigidity. So no, they are not "weak". Composite fibers are strong in tension, but not strong in compression. Hence the need for epoxy.

3. Making something inflatable does not magically make it lighter than a rigid structure, especially when you have to add layers of air barriers between the non-rigid fiber weave. And inflatables require mass needed for supporting the inflatables when they are deflated, which may not be needed when inflated - extra parts... 

This also greatly reduces the assembly of large components in-vacuo. Simply inflate the large exterior "envelope," then plug together LEGO interior components in a convenient shirt-sleeve environment.

Inflatables have some advantages, but with a rigid structure all of the interior can be assembled and tested before launch, and all of the outfitting material can be launched within the structure. You can't do that with inflatables, because they are launched compressed, so it requires a second launch for the rest of the interior outfitting "stuff" for an inflatable.

No need for an EVA just to connect some plumbing. This is one of the major "lessons learned" from the ISS program.

What? If anything we learned that pre-built rigid modules are easy to assemble as soon as they reach the station. Attach them, plug in the connections, and they are live. I'm not sure where you think we learned inflatables are better, because there are no permanently habitable inflatable structures on the ISS that have been used to compare to rigid structures - no such experiment was ever done.

[Coastal Ron]

I'm not sure why 3d printed metal fabrication is better for pressurized habitable space vs composite vessels - which are also additive manufacturing.

In practice, composites are much harder to automate, requiring significantly more tooling and prep than Relativity's 3d full metal printing.

Are you assuming that these modules are being built in space?

Because large composite structures are built on Earth in automated machinery. Just look at Boeing 787, where all fuselage sections are based on a single-piece barrel design, with prepregged carbon fiber deposited via automated fiber placement (AFP) around a collapsible mandrel.

Indeed, I would expect their kind of manufacturing to see increasing use in making tooling/dies, vastly reducing the cost of setting up new production lines.

I doubt it. I've scheduled tooling centers and hauled tooling sets around, so I'm familiar with the types of metal used in tooling and dies. The metals used for tooling and dies is different than what is stamped or drawn, which is what additive manufacturing is replacing.

Actually yes. Before reusability was proven, I wasn't taking seriously anything that relied on reusability, or relied on massive price drops below SpaceX's expendables.

An honest answer, and you certainly were not alone. And I was just pointing out that some technologies can be a generation or two away from use in space. I think robotics certainly will be.

Look how anticipating development of other people's technology too early worked out for Robert Bigelow.

I don't think Bigelow lacked the technology to succeed, he lacked the customer demand to succeed. Big difference.

By contrast: The Space Shuttle orbiter was reusable.

If we used the SpaceX definition of "reusable", no, the Shuttle was not reusable, it was "re-furbishable".

Except, perhaps, that: A) your first rotating space station isn't going to last 200 years.

Agreed. In fact my rotating space station designs anticipate being able to be disassembled easily for "recycling" (whatever that will be in the future, in space).

And B) if you count on technology that doesn't exist, you've just doubled the number of currently non-existent technologies you have to develop for your first rotating station.

Well, we are all in the same boat then as far as rotating spaceships and space stations, since those things don't exist. And it requires far more than one technology to actually build them, so I don't see that maturing existing robotic systems for use in space DOUBLES the technology needed for rotating spaceships and space stations. If anything just a small incremental amount.

But due to the harsh environment of space, robotic systems will be used more, not less, in space as we go into the future. They are less expensive, cheaper to operate compared to human certified equipment, and they will only get more and more capable.

OK, I'm done talking about robots on spaceships 

[Twark_Main]

The Russian side uses silicone oil on the outside and water+antifreeze on the inside.

In both cases, you don't get the enormous benefit of phase change. Which means your radiators/heat-exchangers are going to be lower efficiency and thus large and heavy. (On ISS, the US/international side deals with the majority of the station's excess heat.)

After all, they didn't pick ammonia because they like working with it. Some of the standard n-fluorocarbons might be a useful substitute, but pressure needs to be higher, compressors larger, etc, everything's heavier, noisier, higher maintenance, and with long lines it can be unfun to work with. And you still got to get the heat inside, outside to the radiators. Doesn't solve the "no external maintenance" requirement.

The ammonia loop on ISS is single-phase, not phase-change.

https://www.nasa.gov/pdf/473486main_iss_atcs_overview.pdf

The EATCS provides heat rejection capabilities for all U.S. pressurized modules and the main power distribution electronics on S0, S1 and P1. The system uses a single-phase anhydrous ammonia as its working fluid for its high thermal capacity and wide range of operating temperatures. Ammonia has an extremely low freezing point of -107 degrees ° F (-77 °C) at standard atmospheric pressure.

[Twark_Main]

Composite material provides more radiation protection, and can be just as strong, so bonus!

"Composites" are usually composed of strong fibers (carbon fiber, kevlar, etc) mixed with weak plastic / epoxy.

Seems like there's an opportunity here to delete the plastic ("The best part is no part") and make it an inflatable.

1. Elon Musk did not invent the phrase "The best part is no part". It has been a philosophy used by engineers of all types since likely forever. Elon Musk uses the phrase to highlight points about what they are doing. But otherwise engineers are normally motivated to simplify things.

I never said he did and I didn't think he did, but good clarification.

2. The epoxy in composites is not there for kicks and giggles, it is there to hold the flexible fibers in place and provide rigidity. So no, they are not "weak". Composite fibers are strong in tension, but not strong in compression. Hence the need for epoxy.

Clearly it's not needed, since inflatables do without it. The atmosphere (which is needed anyway) is the "structure" in compression, which is a lot more better than ~doubling the mass by impregnating with epoxy.

3. Making something inflatable does not magically make it lighter than a rigid structure, especially when you have to add layers of air barriers between the non-rigid fiber weave. And inflatables require mass needed for supporting the inflatables when they are deflated, which may not be needed when inflated - extra parts...)

Not "magic" of course, just the deletion of unnecessary parts.

Straps holding it together for launch and the thin membrane air bladders (which have come a long way from the TransHab days) are still a lot lighter than a huge mass of epoxy.

This also greatly reduces the assembly of large components in-vacuo. Simply inflate the large exterior "envelope," then plug together LEGO interior components in a convenient shirt-sleeve environment.

Inflatables have some advantages, but with a rigid structure all of the interior can be assembled and tested before launch, and all of the outfitting material can be launched within the structure. You can't do that with inflatables, because they are launched compressed, so it requires a second launch for the rest of the interior outfitting "stuff" for an inflatable.

Yes, this is one of the real trade-offs. Assembly of parts in space (a connection which equally can't "be assembled and tested before launch") vs. interior assembly in shirt-sleeves.

No need for an EVA just to connect some plumbing. This is one of the major "lessons learned" from the ISS program.

What? If anything we learned that pre-built rigid modules are easy to assemble as soon as they reach the station. Attach them, plug in the connections, and they are live. I'm not sure where you think we learned inflatables are better,

The lesson wasn't that. The lesson was that fluid and electrical connections should be run inside the pressure envelope, to avoid an EVA every time they need maintenance or reconfiguration.

because there are no permanently habitable inflatable structures on the ISS that have been used to compare to rigid structures - no such experiment was ever done.

BEAM is permanently habitable. There's always air inside. It's not permanently inhabited because they normally keep the door closed, but that's because....  it's an experiment to verify the suitability of inflatable structures.

[Coastal Ron]

2. The epoxy in composites is not there for kicks and giggles, it is there to hold the flexible fibers in place and provide rigidity. So no, they are not "weak". Composite fibers are strong in tension, but not strong in compression. Hence the need for epoxy.

Clearly it's not needed, since inflatables do without it. The atmosphere (which is needed anyway) is the "structure" in compression, which is a lot more better than ~doubling the mass by impregnating with epoxy.

I'm not sure you understand how tension works, since with composite construction the fibers are holding the structure together when in space and inflated, but the epoxy is holding the structure together when there is gravity. And what facts do you have to support your assertion that epoxy doubles the mass?

For inflatables, you need LOTS of layers to hold the structure together when in space, and other structure, not needed for in space, to keep the un-inflated structure together in gravity. Inflatables have their uses, but I think it is a narrow use case.

3. Making something inflatable does not magically make it lighter than a rigid structure, especially when you have to add layers of air barriers between the non-rigid fiber weave. And inflatables require mass needed for supporting the inflatables when they are deflated, which may not be needed when inflated - extra parts...)

Not "magic" of course, just the deletion of unnecessary parts.

You keep saying "unnecessary parts". I do not think it means what you think it means... 

For instance, for composites the epoxy is necessary to provide rigidity, and composite construction results in wall thickness far less than inflatables.

The Bigelow BEAM has multiple layers of soft fabric with spacing between layers, protecting an internal restraint and bladder system, and one of the layers is a closed-cell vinyl polymer foam. To me that is a LOT of layers, and a LOT of complexity. Plus inflatables would likely not be rigid enough to use as a structural member, like the ISS modules can used for. Meaning you have to add MORE complexity to provide the same function.

Straps holding it together for launch and the thin membrane air bladders (which have come a long way from the TransHab days) are still a lot lighter than a huge mass of epoxy.

It's not just the straps that are needed to hold everything together for launch, but also the structure required to keep the the whole inflatable payload from collapsing on launch, crushing whatever few items are packed inside.

As to what you call "a huge mass of epoxy", where are you getting your data from? And have you investigated to see how much an inflatable module will mass compared to a comparable composite one?

No need for an EVA just to connect some plumbing. This is one of the major "lessons learned" from the ISS program.

What? If anything we learned that pre-built rigid modules are easy to assemble as soon as they reach the station. Attach them, plug in the connections, and they are live. I'm not sure where you think we learned inflatables are better,

The lesson wasn't that. The lesson was that fluid and electrical connections should be run inside the pressure envelope, to avoid an EVA every time they need maintenance or reconfiguration.

You can't run ammonia lines inside a habitable space, unless you are OK with everyone dying if there is a leak. So NO, the ISS did not teach us that running coolant lines INSIDE was a good idea, not if you are using ammonia for coolant.

because there are no permanently habitable inflatable structures on the ISS that have been used to compare to rigid structures - no such experiment was ever done.

BEAM is permanently habitable. There's always air inside. It's not permanently inhabited because they normally keep the door closed, but that's because....  it's an experiment to verify the suitability of inflatable structures.

I chose my words carefully. The BEAM is not being inhabited, because it is just an experiment. They keep the door closed as part of the experiment - to measure leakage just in that module.

We have yet to see any long-term experiments of inflatable habits with humans inside. Why is that?

[Paul451]

Twark/Ron,

Before going too far in comparing rigid vs inflatable using BEAM walls and mass as a comparison, it's worth remembering its layers are also radiation and MMOD shielding. It's not just the containment layer. So comparing it with a rigid composite shell is misleading, since the latter will need all the other layers added between the pressure shell and the outside.

[edit: removed extra also more words]


Twark,

The density of epoxy is a touch over that of water. The density of any flexible gas-tight layer in an inflatable is going to be in the same range. Rigid vs inflatable won't make much difference to the final mass if the primary fibres are the same.

[Paul451]

Look how anticipating development of other people's technology too early worked out for Robert Bigelow.

I don't think Bigelow lacked the technology to succeed, he lacked the customer demand to succeed. Big difference.

No, he lacked affordable crew access to space. Without that, his modules & space-stations were worthless. My point being that he wasn't just dependent on the technology needed for the modules themselves, but also on an entirely separate technology that he didn't control.

Re: Composites being hard to automate.
Yeah, you're right, there's no complex custom tooling here at all.

[Twark_Main]

2. The epoxy in composites is not there for kicks and giggles, it is there to hold the flexible fibers in place and provide rigidity. So no, they are not "weak". Composite fibers are strong in tension, but not strong in compression. Hence the need for epoxy.

Clearly it's not needed, since inflatables do without it. The atmosphere (which is needed anyway) is the "structure" in compression, which is a lot more better than ~doubling the mass by impregnating with epoxy.

I'm not sure you understand how tension works

Unnecessary.

with composite construction the fibers are holding the structure together when in space and inflated, but the epoxy is holding the structure together when there is gravity.

The loads under gravity (including acceleration/vibration during launch) are a tiny fraction of the loads under the enormous internal atmospheric pressure. Why hit a fly with a sledgehammer?

And what facts do you have to support your assertion that epoxy doubles the mass?

I said "~doubles" actually. Let's not drop my error bars!

Carbon/epoxy prepregs for resin bleed processing are usually specified at resin contents ranging from 37% to 42% by weight resin, while the desired or nominal resin contents for these materials are 31% resin content for unidirectional prepregs and 35% resin content for woven cloth prepregs. A 57–60% final fiber volume is typically desired to balance both mechanical performance and part quality.

https://www.sciencedirect.com/science/article/pii/B9781856174152500071

So for carbon fiber it's 1.45-1.54x. Kevlar fiber has a lower density than carbon fiber (1.4 vs 1.9 g/cm³), so for kevlar the factor is 1.62-1.75x.

Other sources give bigger factors (Wikipedia's Specific strength article gives a factor of >3x!), so I was rounding down. Hence my tilde.

For inflatables, you need LOTS of layers to hold the structure together when in space

Rigid modules (metal or composite) you need to resist the exact same loads except it takes even more mass. "Holding the structure together [against internal pressure loading]" is the primary load experienced by pressurized space modules.

Most of those layers are thermal and micrometeoroid layers. The actual structure is just a kevlar web.

and other structure, not needed for in space, to keep the un-inflated structure together in gravity.

Minimal, compared to the mass using epoxy for that purpose.

But yes, increasing mass efficiency of inflatables is certainly a worthwhile pursuit! The technology hasn't been taken nearly as far as it could have been (mostly due to the same anti-capability / anti-Mars congressional factions that doomed NERVA and depots, for much the same reason).

Inflatables have their uses, but I think it is a narrow use case.

This is a symptom of a lack of vision, combined with an over-emphasis on historical implementations vs first-principles physics thinking.

3. Making something inflatable does not magically make it lighter than a rigid structure, especially when you have to add layers of air barriers between the non-rigid fiber weave. And inflatables require mass needed for supporting the inflatables when they are deflated, which may not be needed when inflated - extra parts...)

Not "magic" of course, just the deletion of unnecessary parts.

You keep saying "unnecessary parts". I do not think it means what you think it means... 

You keep using that quote like it's an argument. I do not think quoting a movie line means what you think it means... 

For instance, for composites the epoxy is necessary to provide rigidity, and composite construction results in wall thickness far less than inflatables.

"Necessary"... unless you can do the same function for less mass. 100% mass penalty (or even 50% mass penalty) is an awfully big hit.

Wall thicknesses, again, are mostly non-structural outer thermal/MMOD protective layers (which, once added, will also increase the wall thickness of your epoxy-fiber composite structure).

The Bigelow BEAM has multiple layers of soft fabric with spacing between layers, protecting an internal restraint and bladder system, and one of the layers is a closed-cell vinyl polymer foam. To me that is a LOT of layers, and a LOT of complexity.

"Number of layers" isn't the relevant metric, just like it's not a useful metric to count the number of plies in a composite structure.

I actually agree that the closed-cell foam should be replaced by more suitable (likely MLI-based) solutions.

Plus inflatables would likely not be rigid enough to use as a structural member, like the ISS modules can used for. Meaning you have to add MORE complexity to provide the same function.

There's that lack of vision again.

Is it possible to design an inflatable that is unsuitable as a structural member? Of course. Is it relatively trivial to make sure the design is suitable as a structural member? I think so.

These aren't party balloons or bouncy castles. Once inflated at 1 atmosphere they will be incredibly inflexible and rigid just due to proper layout of the restraining straps. If additional rigidity is needed (doubtful), high-pressure tubes can be added to increase compressive loads and raise the natural frequency at much lower mass budget than using composites for the same function.

Straps holding it together for launch and the thin membrane air bladders (which have come a long way from the TransHab days) are still a lot lighter than a huge mass of epoxy.

It's not just the straps that are needed to hold everything together for launch, but also the structure required to keep the the whole inflatable payload from collapsing on launch, crushing whatever few items are packed inside.

Yeah, the current designs try to "split the baby" by launching internal payload and the external envelope at the same time. Of course we've never tried to build an inflatable that would max out the payload with just the envelope... 

As to what you call "a huge mass of epoxy", where are you getting your data from? And have you investigated to see how much an inflatable module will mass compared to a comparable composite one?

See above.

No need for an EVA just to connect some plumbing. This is one of the major "lessons learned" from the ISS program.

What? If anything we learned that pre-built rigid modules are easy to assemble as soon as they reach the station. Attach them, plug in the connections, and they are live. I'm not sure where you think we learned inflatables are better,

The lesson wasn't that. The lesson was that fluid and electrical connections should be run inside the pressure envelope, to avoid an EVA every time they need maintenance or reconfiguration.

You can't run ammonia lines inside a habitable space, unless you are OK with everyone dying if there is a leak. So NO, the ISS did not teach us that running coolant lines INSIDE was a good idea, not if you are using ammonia for coolant.

Yes NASA knows that, as do I.

Already addressed; see my reply to Paul451 above.

because there are no permanently habitable inflatable structures on the ISS that have been used to compare to rigid structures - no such experiment was ever done.

BEAM is permanently habitable. There's always air inside. It's not permanently inhabited because they normally keep the door closed, but that's because....  it's an experiment to verify the suitability of inflatable structures.

I chose my words carefully. The BEAM is not being inhabited, because it is just an experiment. They keep the door closed as part of the experiment - to measure leakage just in that module.

Yep, I noticed you carefully choosing words to try to find some way to claim that we have't used man-rated inflatables on the ISS. Not carefully enough it seems: you originally said it wasn't "habitable" (false), but now you've switched to "inhabited." Also, so much for "no such experiment has been done [on the ISS]."

We have yet to see any long-term experiments of inflatable habits with humans inside. Why is that?

Because it's newer technology and NASA is inherently extremely cautious about human spaceflight. Even if the odds of failure are infinitesimally small, NASA doesn't want to find out about the failure because the crew died.

An extreme excess of caution (not in this case IMO, but just speaking in general) is a great trait to avoid getting your ass hauled in front of congressional hearings, but unfortunately it spells the death of technical progress.

[Twark_Main]

Twark/Ron,

Before going too far in comparing rigid vs inflatable using BEAM walls and mass as a comparison, it's worth remembering its layers are also radiation and MMOD shielding. It's not just the containment layer. So comparing it with a rigid composite shell is misleading, since the latter will need all the other layers added between the pressure shell and the outside.

[edit: removed extra also more words]

Yes, exactly.

Twark,

The density of epoxy is a touch over that of water. The density of any flexible gas-tight layer in an inflatable is going to be in the same range. Rigid vs inflatable won't make much difference to the final mass if the primary fibres are the same.

The mass of the epoxy overhead for a rigid structure scales as the volume (like any pressure vessel), but the mass of the gas bladder scales as the surface area. So the bigger you make the module, the bigger the savings from using inflatables.

[Twark_Main]

Look how anticipating development of other people's technology too early worked out for Robert Bigelow.

I don't think Bigelow lacked the technology to succeed, he lacked the customer demand to succeed. Big difference.

No, he lacked affordable crew access to space. Without that, his modules & space-stations were worthless. My point being that he wasn't just dependent on the technology needed for the modules themselves, but also on an entirely separate technology that he didn't control.

Good assessment IMO.

Re: Composites being hard to automate.
Yeah, you're right, there's no complex custom tooling here at all.

To be fair, just because it's not 100% manual doesn't mean that it's 100% automated either.

If nobody shows up for work in the building, how long does the complex custom tooling run before it needs human intervention? I'll bet those catwalks weren't just added for show.   I suspect it can do an entire job without intervention (assuming no problems occur), but what about de-molding? Cleaning the machine? Setting up for the next job? And then there's regular scheduled maintenance...

Naturally the low-hanging fruit has already been automated. The human touch points that remain are the difficult edge cases.

[Paul451]

Re: Composites being hard to automate.
Yeah, you're right, there's no complex custom tooling here at all.
[picture of complex custom machinery for a single part]

To be fair, just because it's not 100% manual doesn't mean that it's 100% automated either.

In case it wasn't clear, my comment was sarcasm. (Lowest form of wit, but it is mine.)

Everything in the image was set up for that single job, and for that specific part. Every other section of the fuselage, wings, etc needed its own, similar-but-unique manufacturing process. Composites are not "additive manufacturing", except in the meaningless sense that all manufacturing is additive, it is certainly not adaptive manufacturing.

The comparison was with Relativity Space's 3d metal printing. By comparison with that, composites are very difficult to automate production. As illustrated very much by Boeing's systems for the 787. Automation is great when you can lock down the process. Human are good when you can't. By mentioning Rel-Space's novel metal printing process, I was drawing attention to an exception to the rule. (And one I found interesting and potentially ground-breaking.) Ron, I think, missed the point of the comparison.


[So naturally I ended up going down the rabbit hole of how the 787 is made. Wow. Few of the nearly dozen sections of the main fuselage and wings are made in the same factory, same country or by the same companies. And none of the main fuselage by Boeing itself, only the vertical tail. Instead, Spirit and Vought (US), Fiji, Kawasaki, Mitsubishi (Japan) and Alenia (Italy) each make the individual sections. More countries for the major panels of the control surfaces. That's before you even touch on the actual "machinery" of the aircraft, just the bulk structure.]

[dondar]

Aside: If Relativity Space's 3d printed metal fabrication process ends up being as revolutionary as it seems, then the only thing shipped up might be weld-wire and replacement weld-heads. And the whole design concept (not just spacester's, I mean everyone's) will need to be completely re-thought.

I'm not hostile to the idea of robotic innovation. I'm just not taking seriously any proposal that relies on it already existing.

their method (just like of everybody else) requires gravity and uses convection cooling.

[Coastal Ron]

Everything in the image was set up for that single job, and for that specific part. Every other section of the fuselage, wings, etc needed its own, similar-but-unique manufacturing process.

I know you are enamored with Relativity Space's 3d printed metal fabrication process, and it could be revolutionary at some point in the future. But you are ignoring all the parts that go on that rocket that are NOT printed by the same exact machine.

As for the 787 tooling, dedicated tooling can be VERY cost effective and efficient if you have enough volume, which Boeing has.

Composites are not "additive manufacturing", except in the meaningless sense that all manufacturing is additive, it is certainly not adaptive manufacturing.

Ooh, using new buzzwords now - "adaptive manufacturing". Of course Relativity Space has to use that because of warpage, so if anything it is something necessary for Relativity Space but not for others. And BTW, most machining takes into account variations in the production process in order to achieve the correct result. Nothing new here, move along... 

The comparison was with Relativity Space's 3d metal printing. By comparison with that, composites are very difficult to automate production.

Relativity Space is standing on the shoulders of how many decades of innovation to get to the point they are at, and you think composite manufacturing is difficult? Paul, please take off your blinders, because by comparison composite manufacturing is not "very difficult to automate production", it is 3D printing large products that is difficult. There are factories all over the world that build composite parts in an automated way, including Boeing, and it is a known process. Perspective please!

Automation is great when you can lock down the process. Human are good when you can't.

I'm sure you thought that sounded logical when you said that, however in the real world humans need a repeatable process too if you want to make quality parts - in other words, you need a repeatable process regardless if you are using automated machinery or making something with human labor.

The job of manufacturing is to make a quality part, on time, at the least practical cost. Which means manufacturing is ALWAYS evaluating how they can do better on all three of those metrics. If automation helps, they will do that. But each change they make is a calculated effort to affect, in a positive way, one or more of those metrics.

And parts don't care how they are made, and a particular part can usually be made in MANY ways, it is just that Relativity Space is trying to use a novel manufacturing method to simultaneously reduce touch labor, reduce complexity, and hopefully cut overall cost while still achieving the same end result. Time will tell if that happens, but so far it is just PR.

For a rocket company that is actually launching, Rocket Lab has a machine that does subtractive manufacturing operations (i.e. drilling, machining) to their raw composite parts, and can process one Electron launch vehicle every 12 hours (down from 400 hours). Combine that with what SpaceX is doing with the Starship factory, and there is plenty of bets going on in the world of launchers to ultimately lower the cost to access space.

I support them all, but they all have their advantages and disadvantages.

As to how this relates to rotating spaceships, they have to get built somehow. I personally don't think we'll be 3D printing in space, because the amount of support needed to keep a 3D printing machine working and fed will exceed the usefulness of making parts in space. Instead I think parts will be made on Earth - in a variety of ways - and shipped up to space on a variety of launchers to be assembled into a spaceship that can eventually be spun up to create artificial gravity.

Maybe the 2nd or 3rd generation of rotating spaceships will use parts made in space, but we're not there yet.

[Paul451]

Composites are not "additive manufacturing", except in the meaningless sense that all manufacturing is additive, it is certainly not adaptive manufacturing.

As I've asked before: Please quote properly. It's painful to have to fix it when replying to you.

Composites are not "additive manufacturing", except in the meaningless sense that all manufacturing is additive, it is certainly not adaptive manufacturing.

Ooh, using new buzzwords now - "adaptive manufacturing". Of course Relativity Space has to use that because of warpage,

You're reading what you want into what I said.

By "adaptive", all I meant was that the same tool can produce more than one part. Any 3d printer, any CNC-machine, and any human-wielded tool, is adaptive. Every injection mould, every die-based stamp-press, is not. Composites, by their nature, are in the latter category. (There are some minor successes at "composite" 3d printing, but nothing yet that could replace conventional lay-up. It's one of the holy-grails of 3d printing.)

A lot of the rest of what you replied to wasn't similarly not based on what I actually said. But it's already off-topic and not worth another page of back-and-forth.

[Paul451]

[Also off-topic. But curiosity got the better of me.]

Aside: If Relativity Space's 3d printed metal fabrication

their method (just like of everybody else) requires gravity and uses convection cooling.

I could debate how much cooling they need, but I'm much more curious about why you think the process needs gravity? It seems ideally suited to being used in low or zero gravity.

[Twark_Main]

"adaptive manufacturing". Of course Relativity Space has to use that because of warpage, so if anything it is something necessary for Relativity Space but not for others.

I don't know why you think material warpage would be an issue unique to Relativity Space.

[Coastal Ron]

"adaptive manufacturing". Of course Relativity Space has to use that because of warpage, so if anything it is something necessary for Relativity Space but not for others.

I don't know why you think material warpage would be an issue unique to Relativity Space.

Because they said it was. Did you watch their video about how their process works?

[Coastal Ron]

[Also off-topic. But curiosity got the better of me.]

Aside: If Relativity Space's 3d printed metal fabrication

their method (just like of everybody else) requires gravity and uses convection cooling.

I could debate how much cooling they need, but I'm much more curious about why you think the process needs gravity? It seems ideally suited to being used in low or zero gravity.

The Relativity Space Stargate 3D printer uses Earth gravity today, and it is operated in a normal Earth atmosphere, and from what we have seen the temperature they operate in is amendable to humans. In the manufacturing world if you change the conditions of how and where your machinery is supposed to work, you should not be surprised to find that it doesn't work properly anymore.

Maybe the Stargate printer will work better in space, or maybe it won't. I would assume they could get it to work, but what it would take to do that is a complete unknown.

[Coastal Ron]

Composites are not "additive manufacturing", except in the meaningless sense that all manufacturing is additive, it is certainly not adaptive manufacturing.

As I've asked before: Please quote properly. It's painful to have to fix it when replying to you.

I have no idea what you are talking about. I quoted you directly. Explain yourself or stop complaining.

Composites are not "additive manufacturing", except in the meaningless sense that all manufacturing is additive, it is certainly not adaptive manufacturing.

Ooh, using new buzzwords now - "adaptive manufacturing". Of course Relativity Space has to use that because of warpage,

You're reading what you want into what I said.

By "adaptive", all I meant was that the same tool can produce more than one part. Any 3d printer, any CNC-machine, and any human-wielded tool, is adaptive.

Having a manufacturing background I had no idea what you meant by "adaptive manufacturing", so I did some research and found that it is a term used, with the definition being:

Most of the time, basic automated systems use robots. These are repeatable, but not really accurate. To overcome this lack of accuracy, an adaptive process needs to be implemented. Adaptive means that the system is able to measure and adapt its process in real-time.

So if you want to make up your own definition for words and phrases, fine, but just make sure you explain they are your own definitions, and what they mean. Otherwise readers will look up the REAL definition and come to different conclusions.

Every injection mould, every die-based stamp-press, is not. Composites, by their nature, are in the latter category. (There are some minor successes at "composite" 3d printing, but nothing yet that could replace conventional lay-up. It's one of the holy-grails of 3d printing.)

Yeah, you really don't like composites, and seem to be going out of your way to make them look like they are horrible to use, and hardly worth the effort. Suffice it to say that the world of manufacturing would disagree with you... 

And all of this is off topic to rotating spaceships, unless you can actually tie in what you saying to how relates. Just sayin'...

[Twark_Main]

"adaptive manufacturing". Of course Relativity Space has to use that because of warpage, so if anything it is something necessary for Relativity Space but not for others.

I don't know why you think material warpage would be an issue unique to Relativity Space.

Because they said it was. Did you watch their video about how their process works?

Assume I haven't and please point me to the specific video / statement.

[Twark_Main]

Composites are not "additive manufacturing", except in the meaningless sense that all manufacturing is additive, it is certainly not adaptive manufacturing.

As I've asked before: Please quote properly. It's painful to have to fix it when replying to you.

I have no idea what you are talking about.

He means incorrectly using [​quote] instead of [​quote author=Paul451 link=topic=53918.msg2291632#msg2291632 date=1632022998].

[Coastal Ron]

"adaptive manufacturing". Of course Relativity Space has to use that because of warpage, so if anything it is something necessary for Relativity Space but not for others.

I don't know why you think material warpage would be an issue unique to Relativity Space.

Because they said it was. Did you watch their video about how their process works?

Assume I haven't and please point me to the specific video / statement.

This video, and they talk about mitigating warpage around 5:30:

[Coastal Ron]

Composites are not "additive manufacturing", except in the meaningless sense that all manufacturing is additive, it is certainly not adaptive manufacturing.

As I've asked before: Please quote properly. It's painful to have to fix it when replying to you.

I have no idea what you are talking about.

He means incorrectly using [​quote] instead of [​quote author=Paul451 link=topic=53918.msg2291632#msg2291632 date=1632022998].

It is not "incorrect", since I have years of conversations with Paul where he never mentioned this supposed issue, and since I was only responding to one single post he made, the norm on NSF is to only reference the header HTML code at the beginning.

If Paul wants to change that standard on NSF, fine. But he is the first person I can remember to complain about it, so I don't feel a need to change.

[Twark_Main]

"adaptive manufacturing". Of course Relativity Space has to use that because of warpage, so if anything it is something necessary for Relativity Space but not for others.

I don't know why you think material warpage would be an issue unique to Relativity Space.

Because they said it was. Did you watch their video about how their process works?

Assume I haven't and please point me to the specific video / statement.

This video, and they talk about mitigating warpage around 5:30:

[snip]

Weird. I just got done watching that video before reading your post.

Nothing in that video suggests that this warpage problem is unique to Relativity Space, however. On the contrary they say it's a general material property related to thermal expansion.

What they describe isn't a problem unique to Relativity Space, but a solution unique to Relativity Space.

It is not "incorrect", since I have years of conversations with Paul where he never mentioned this supposed issue, and since I was only responding to one single post he made, the norm on NSF is to only reference the header HTML code at the beginning.

If Paul wants to change that standard on NSF, fine. But he is the first person I can remember to complain about it, so I don't feel a need to change.

Add me as the second person to complain; it's really annoying to fix those broken quotes! Not sure where your "norm" is coming from. Almost everyone else uses the quotes feature correctly.



Can we get back on-topic to rotating spaceships?

Summarizing thus far, it seems that we fall into three camps: modules, inflatables, and 3D printed. The construction method, naturally, has a huge influence on how any rotating spaceship design ultimately looks and works.

We also can categorize into deep space vs. LEO. For deep space the radiation shielding needs are more significant, which effects the choice of layout. Maximizing volume to surface area ratio becomes more important in deep space.

Lastly we have long-term vs. short-term habitation. Long-term space colonization demands even lower radiation levels, putting even more design pressure on volume to surface area ratio.

[Coastal Ron]

This video, and they talk about mitigating warpage around 5:30:

Weird. I just got done watching that video before reading your post.

Nothing in that video suggests that this warpage problem is unique to Relativity Space, however.

I never said it was unique to Relativity Space. It is something that needs to be monitored on large 3D printed parts, especially since the walls are so thin and the length so long. And warpage is a concern in many manufacturing processes, so it isn't unique to 3D printing.

Add me as the second person to complain; it's really annoying to fix those broken quotes!

They aren't "broken". The link at the top is still there. This "debate" is about convention - not unlike the "debate" about how many spaces after a period. But it is a debate about a convention, not about a law being broken.

And yes, rotating spaceships... 

[Twark_Main]

This video, and they talk about mitigating warpage around 5:30:

Weird. I just got done watching that video before reading your post.

Nothing in that video suggests that this warpage problem is unique to Relativity Space, however.

I never said it was unique to Relativity Space. It is something that needs to be monitored on large 3D printed parts, especially since the walls are so thin and the length so long. And warpage is a concern in many manufacturing processes, so it isn't unique to 3D printing.

Ok, I guess I got confused by "of course Relativity Space has to use that because of warpage, so if anything it is something necessary for Relativity Space but not for others."

Add me as the second person to complain; it's really annoying to fix those broken quotes!

They aren't "broken". The link at the top is still there. This "debate" is about convention - not unlike the "debate" about how many spaces after a period. But it is a debate about a convention, not about a law being broken.

It harms readability, navigability, and usability. Also you're welcome; I fixed your broken quote above for the zillionth time in this thread.

Also one space after a period is correct; all else is blasphemy.

And yes, rotating spaceships... 

Yes!

Unless anyone has a fantastic electromagnetic GCR radiation thingy, we're stuck with geometric constraints.

Do we have a fantastic (near-term) electromagnetic GCR radiation thingy, by any chance? Bueller? Bueller?

[Coastal Ron]

They aren't "broken". The link at the top is still there. This "debate" is about convention...

It harms readability, navigability, and usability.

I went back and looked at the conversations you, me and Paul were having two years ago in the rotating space station thread. I was doing then what I am doing now, and you two where doing then what you are doing now. And no one mentioned that there was a problem until now. And until Paul mentioned it NO ONE had mentioned it to me. Lots of writing conventions the are in use on NSF threads, we we represent two of them...

Unless anyone has a fantastic electromagnetic GCR radiation thingy, we're stuck with geometric constraints.

Do we have a fantastic (near-term) electromagnetic GCR radiation thingy, by any chance? Bueller? Bueller?

The big challenge with rotating spaceships is the ratio of mass to thrust, which not only affects radiation shielding, but also the type of artificial gravity that can be provided.

For instance, how immobile does a vehicle have to be to still be termed a "ship"? We don't think of a self-propelled barge as a ship, but it has some of the characteristics. Would we consider a spaceship a spaceship if it doesn't have the ability to go from LEO to, say, L2 without refueling AND getting there in less than 3 months?

I ask because we could all probably dream up a structure that has enough mass and machinery to reduce GCR radiation outside of Earth's protection, but it would likely have too much mass to be termed a "spaceship" due to the lack of propulsions systems that could move it around enough to be recognized as a "spaceship".

Even the SpaceX Starship will need to be refueled once it launches from Earth before it can leave LEO for other destinations, but it doesn't have the additional mass needed to protect from GCR or to provide artificial gravity. Additional mass to do those would likely require refueling in multiple places in space for the Starship to go anywhere - which would not be unusual, since I have to plug in my Tesla multiple times on long distance road trips.

Anyways, a bit of rambling here, but my bottom line is that I suspect that we don't yet have the propulsion ability to make a practical rotating spaceship - not without a massive refueling infrastructure in place out in space.

[jdon759]

Slightly Longer-term idea here, but what if you followed a sort-of real-estate approach?  Instead of one company building the whole station (expensive), the company builds a "shell" station.  This would have little more than the basics - a Heavy-duty balancing system, a spin-up/down system, and minimal life support.  The company can then sell "land" on the station and make a profit without as much capital expenditure as required to build the entire station.

[Paul451]

Slightly Longer-term idea here, but what if you followed a sort-of real-estate approach?  Instead of one company building the whole station (expensive), the company builds a "shell" station.  This would have little more than the basics - a Heavy-duty balancing system, a spin-up/down system, and minimal life support.  The company can then sell "land" on the station and make a profit without as much capital expenditure as required to build the entire station.

Effectively, that is a whole station. The minimum build before you can sell "land" is exactly what any commercial space-station would involve.

Power, thermal, lifesupport, pressure shell, MMOD/rad shielding, attitude control, altitude thrusters/OMS, and in the case of a spin station, spin-up/down system and mass distribution control.

[Coastal Ron]

Slightly Longer-term idea here, but what if you followed a sort-of real-estate approach?  Instead of one company building the whole station (expensive), the company builds a "shell" station.

I have a rotating space station design that could be progressively built like that, but I think most of the cost of a rotating space station would be for the station itself - outfitting living spaces would likely be the minority of the total cost.

This would have little more than the basics - a Heavy-duty balancing system, a spin-up/down system, and minimal life support.

Why would there need to be a "spin-up/down system"? That would be a LOT of propellant to do that.

The company can then sell "land" on the station and make a profit without as much capital expenditure as required to build the entire station.

Here on Earth land developers only build out their properties if they have a reasonable expectation that the market is there to buy/lease them. Doing things is in space is so new, that I don't see anyone building a rotating space station unless they have a close to 100% commitment from one or more customers.

[jdon759]

Slightly Longer-term idea here, but what if you followed a sort-of real-estate approach?  Instead of one company building the whole station (expensive), the company builds a "shell" station.  This would have little more than the basics - a Heavy-duty balancing system, a spin-up/down system, and minimal life support.  The company can then sell "land" on the station and make a profit without as much capital expenditure as required to build the entire station.

Effectively, that is a whole station. The minimum build before you can sell "land" is exactly what any commercial space station would involve.

Power, thermal, life-support, pressure shell, MMOD/rad shielding, attitude control, attitude thrusters/OMS, and in the case of a spin station, spin-up/down system and mass distribution control.

&

I have a rotating space station design that could be progressively built like that, but I think most of the cost of a rotating space station would be for the station itself - outfitting living spaces would likely be the minority of the total cost.

Those are good points about it being most of the station already, and it is a problem I am aware of - the most expensive part has to be done anyway. 

But I think you may be overestimating how "complete" my suggested station is.  I do not expect the initial "shell station" to have much more habitable volume than a reentry capsule. 
Modules with their own life support, pressure shell and MMOD/rad shielding would be the "buildings" that are built on the "land" of the shell-station's structure.  Power and stationkeeping would be all that is provided to these buildings by the shell-station.  This requires a heavy-duty balancing system because the station is assured to be imbalanced due to decentralised design and non-simultaneous construction. 
It's analogous to a Stanford torus where the dwellings within the torus are not built at the time the torus is "complete", but are added later by the people living there.

[Coastal Ron]

Those are good points about it being most of the station already, and it is a problem I am aware of - the most expensive part has to be done anyway.

Who is going to pay for that? What is their return on investment (ROI) projection for building a station with no paying customers?

We space enthusiasts LOVE to solve hardware problems, but in the real world it is money that determines the solutions.

But I think you may be overestimating how "complete" my suggested station is.

Of course we are, since only you know what your station looks like, and how it operates . And you don't need to show us, since we're just having a friendly conversation here.

I do not expect the initial "shell station" to have much more habitable volume than a reentry capsule. 
Modules with their own life support, pressure shell and MMOD/rad shielding would be the "buildings" that are built on the "land" of the shell-station's structure.

Modular construction is something that is done here are Earth, which makes it easier to understand the many different approaches that can be done. What you are suggesting though may not be as easy as you hope, since the advantage of building a fully functional structure is that it is easier to finish for a customer.

What you seem to be suggesting is something akin to an RV park, where everyone is responsible for just about everything - including air and water apparently.

It is certainly one approach - who knows, maybe even a good one. But as I mentioned earlier what determines the solutions we end up using in the real world is the money that those funding something THINK they can make in return, and what potential customers are willing to pay. So I would ask that you consider what everyone would be willing to pay for.

For instance, with the modular approach you propose the upfront cost to a user/customer is very high, as is the operating cost. Plus there is little to no redundancy between customer modules. But if you lower the cost to occupancy, maybe you would get more users/customers? Just as in engineering you have do tradeoffs, so it is in the world of money too.

My $0.02

[mikelepage]

But I think you may be overestimating how "complete" my suggested station is.  I do not expect the initial "shell station" to have much more habitable volume than a reentry capsule. 
Modules with their own life support, pressure shell and MMOD/rad shielding would be the "buildings" that are built on the "land" of the shell-station's structure.  Power and stationkeeping would be all that is provided to these buildings by the shell-station.  This requires a heavy-duty balancing system because the station is assured to be imbalanced due to decentralised design and non-simultaneous construction. 
It's analogous to a Stanford torus where the dwellings within the torus are not built at the time the torus is "complete", but are added later by the people living there.

I do think this decentralised format is a good way to approach the problem, but the real-estate metaphor implies things maybe you didn't mean. This isn't an apartment or house you're offering. If I'm reading you right, you're literally suggesting building a framework that functions as the equivalent of a new green-fields housing development site that sells empty plots of land, with standardised access to basic utilities.  That land has inherent value based on its location, (as would the spots for spin-G payloads on a spin-G structure). You probably wouldn't even be "selling" that land, so much as renting it.

Personally, I would not even be including any kind of pressurised volume, or need to include significant balancing devices since you'd just put in the spec that customers have to produce two payloads that balance each other (or maybe there would be a secondary market where smaller customers are matched together to achieve this). What you're selling is access to a standard amount of partial-gravity through spin G, and since you've got to build, assemble and power the structure anyway, you can include a power supply as part of your service.

I think it's especially valuable to delineate between the customer payload and structure since, at least for the early spin-G experiments, there's probably extra value in the ability to return the entire customer payload back to Earth for analysis.

[Paul451]

The problem I have with the real-estate model is that with an "empty" site on Earth, there's common "infrastructure" that we take for granted that doesn't exist in space. Air. (Duh.) But also everything that it allows, such easier thermal control (just circulate air through your building, or through your heat-exchanger,) and easier access to neighbouring sites and access to other infrastructure like roads. Seemingly little things like pressure automatically equalising in any system that isn't gas-tight. For example, in sewerage systems. Makes construction and use vastly easier.

Similarly, infrastructure is one-way, any "loop" is well outside the greenfield-site (possibly even environmental, like the hydrological cycle.) This allows things like fresh-water-in and waste-water-out to be treated completely independently. Therefore one can be pressurised and water-tight (although not very water-tight in practice, because it all ends up back in the rivers/seas/air or groundwater somehow), the other can be at ambient external pressure through the entire system with correspondingly lower construction/maintenance costs. (Indeed, the trick is to add vents.)

In space: You only have radiative cooling. Heating (both internal and external) is more complex than on the ground. Life-support has to be handled by each module, or connected through a common shared infrastructure which is two-way. Each module has to be gas-tight, which means anything passing in or out has to go through gas-tight (not just water-tight) lines/pipes. (And to stay gas-tight, even if something goes wrong elsewhere.) But importantly, any movement of fluids/gas in and out of the module has to be balanced to ensure you aren't messing up the internal pressure. If you bust a pipe in your home, it makes a mess. If you bust a pipe in a space-station, it might allow your air to escape back up the pipe and into space. You might die.

It's not just that "it's harder". It's "different" to the point that not only does the analogy break down, but that such a method of construction would end up costing everyone more than simply not doing it. It wouldn't even save money in the development stage of the core station (pushing costs forward to the clients, reducing buy-in for early investors.)

A closer model is the apartment/office building (and perhaps even more limiting.) Clients "furnish" their rented/leased volume with whatever specific equipment they need, but everything else is part of the "building".


That said, I agree that the "mass balancing" requirement is probably overblown. This isn't a wheel-on-an-axle, where even slight imbalances cause vibration, free-rotating objects find their own centre-of-rotation. Beyond the intermediate-axis issue discussed {waves hand} over a bunch of threads, the system will be innately stable. The only requirement for a fixed centre-of-rotation is for docking. And IMO it's better to design a docking system that can cope with off-axis approach, rather than require the station (and approaching spacecraft) to be perfectly balanced around a (shared) fixed physical axis. It's yet-another-system-to-develop (yay!) but once solved, it eliminates so much hassle.

[Coastal Ron]

The problem I have with the real-estate model is that with an "empty" site on Earth, there's common "infrastructure" that we take for granted that doesn't exist in space. Air. (Duh.) But also everything that it allows, such easier thermal control (just circulate air through your building, or through your heat-exchanger,) and easier access to neighbouring sites and access to other infrastructure like roads. Seemingly little things like pressure automatically equalising in any system that isn't gas-tight. For example, in sewerage systems. Makes construction and use vastly easier.

I've worked on a design for a Earth-level gravity rotating space station that would have provided open space for future tenants, but the infrastructure issue, as you point out, becomes a big challenge. Bottom line is that there were too many challenges to solve with that design, and it was highly unlikely that anyone would need a 1G station anytime soon. Instead I'm focused on a Mars-level gravity rotating space station that will be fully built out, though some internal spaces could be customized, and I'm using the same approach for my rotating spaceship designs.

That said, I agree that the "mass balancing" requirement is probably overblown. This isn't a wheel-on-an-axle, where even slight imbalances cause vibration, free-rotating objects find their own centre-of-rotation. Beyond the intermediate-axis issue discussed {waves hand} over a bunch of threads, the system will be innately stable. The only requirement for a fixed centre-of-rotation is for docking. And IMO it's better to design a docking system that can cope with off-axis approach, rather than require the station (and approaching spacecraft) to be perfectly balanced around a (shared) fixed physical axis. It's yet-another-system-to-develop (yay!) but once solved, it eliminates so much hassle.

Agreed.

[mikelepage]

I took my time thinking about how to reply to this thread, since I fell into my own trap with the post above, with the real estate-model implying things I didn't mean, and Paul you assumed I was arguing a point that I wasn't arguing. Of course there is "infrastructure" present on Earth not available in space in the form of innumerable physical and environmental services. It's this leap bolded below that I disagree with.

It's not just that "it's harder". It's "different" to the point that not only does the analogy break down, but that such a method of construction would end up costing everyone more than simply not doing it. It wouldn't even save money in the development stage of the core station (pushing costs forward to the clients, reducing buy-in for early investors.)

I can agree with everything you said before that, and agree that the overall cost will be increased, but still argue that a division of function/outsourcing of components can make the whole process more cost-effective for everyone involved - and that this is a better way to approach the problem. The entities that want space on a rotating spacecraft don't necessarily want to build the spacecraft itself, but they will find value in being able to build the module/habitat that is to be placed on such a spacecraft.

Perhaps a better analogy is of launching a cubesat aboard a ride-share service (which also has a higher overall cost than if one company built the launcher and all the cubesats). It's an example of a situation where one fully functional spacecraft provides a service to a set of other (fully-functional) spacecraft. The advantage is not in the overall cost, but in making it cost-effective for a wide range of different entities interested in space for different reasons. It's because of the modular nature of cubesats that space has become accessible to small companies, universities and even schools, by reducing the significant amount of overhead involved in actually getting to space. And the rideshare business model has enabled a new class of providers that wouldn't see any particular benefit in launching a spacecraft for a particular university science project, but are happy to take money from people who do.

I think it makes sense for a spin g provider to use the same business model. I suppose you would use the same business model even if you were just offering dedicated spaces within a single, fully integrated spin g spacecraft, but then the customers don't get anywhere near as much ability to customise their setups, and the upfront cost is much larger (upfront cost being the reason that has sunk every previous spin g spacecraft proposal).

In a "spartan" spin g provider scenario, you have the provider company simply has to focus on finding a cost effective way to build a framework with the ability to produce spin gravity (a difficult enough task all by itself), and then arrange the launch of separate customer modules, which are standardised to a spec in the same way that cubesats are. This would also mean you can have the modules swapped in and out of the framework as required/desired, rather than having to deorbit a whole space station when only parts of it are reaching their end-of-life dates. Yes, you get a lot of duplication of function/cost, but that is spread among the customers and occurs later in the development process, and it also means the resulting craft has significant redundancy, which is not a terrible thing.

Maybe the best permutation of of the real-estate analogy, is to compare this to a caravan or trailer park, where basic services are provided to vehicles which are functional in other settings beside this one. I know "trailer park" has a specific connotation in the US, but here in Australia I'm thinking of the places where "grey nomads" on holiday will set up camp for a few days to a few weeks before moving on, at a much reduced cost as compared to staying in a dedicated motel or hotel.

[Paul451]

and Paul you assumed I was arguing a point that I wasn't arguing.

I was more replying to the originator of real-estate metaphor, jdon, and the general concept.

such a method of construction would end up costing everyone more than simply not doing it. It wouldn't even save money in the development stage of the core station

I [....] still argue that a division of function/outsourcing of components can make the whole process more cost-effective for everyone involved - and that this is a better way to approach the problem. The entities that want space on a rotating spacecraft don't necessarily want to build the spacecraft itself, but they will find value in being able to build the module/habitat that is to be placed on such a spacecraft.

I'm not sure I can explain this clearly, so apologies in advance:

I don't think you can actually divide anything in a way that saves money even for just the developer of the core alone. The division itself makes the core a harder problem to solve than a "regular" space-station. The developer of the core will still have the burden of, in effect, designing the whole spin-station and all systems, but also of designing a way to divide those systems out to customer such that they can't detrimentally affect the core (or vice versa) nor each other's modules via the shared infrastructure. And also designing the standard for hot-swappable habitat-modules.

If adding modules to a spin-station were just a matter of plugging in power, sure, why not. It's when you factor everything required, not only for supporting humans, but for supporting the systems required to support humans.

Hopefully one day, down the road, when all this spin-g & space-station stuff is already off-the-shelf and technologically mature and boring and cheap, then there might be a business model for these kinds of variants and innovations.

But jdon's suggestion was that this would make things easier for an early station. Maybe even the very first spin-g station. That's where I think it only makes things harder.

[Twark_Main]

As for the cubesat analogy....


Modularity helps, but cubesats are cheap mainly because they're small. Space station modules are not small.

Space station modules also kill people when they fail, so all the additional QA and testing adds a fair bit of cost. If a cubesat fails, nobody dies.

I think we'll see nano-racks inside a rotating space station, allowing universities to fly cheap payloads. But I doubt we'll see university-level budgets (or even single company-level budgets) launching entire habitation modules any time soon.


I think the station-to-module connection is the wrong place to put the customer interface. To borrow a phrase, "rotating space stations are not LEGO." Just like rockets, the whole thing really needs to be designed all together (structurally, infrastructurally, crew circulation and emergency egress, rotational balance, etc). This is less true for purely microgravity stations like the ISS, of course.

[Slarty1080]

The problem I have with the real-estate model is that with an "empty" site on Earth, there's common "infrastructure" that we take for granted that doesn't exist in space. Air. (Duh.) But also everything that it allows, such easier thermal control (just circulate air through your building, or through your heat-exchanger,) and easier access to neighbouring sites and access to other infrastructure like roads. Seemingly little things like pressure automatically equalising in any system that isn't gas-tight. For example, in sewerage systems. Makes construction and use vastly easier.

I've worked on a design for a Earth-level gravity rotating space station that would have provided open space for future tenants, but the infrastructure issue, as you point out, becomes a big challenge. Bottom line is that there were too many challenges to solve with that design, and it was highly unlikely that anyone would need a 1G station anytime soon. Instead I'm focused on a Mars-level gravity rotating space station that will be fully built out, though some internal spaces could be customized, and I'm using the same approach for my rotating spaceship designs.

That said, I agree that the "mass balancing" requirement is probably overblown. This isn't a wheel-on-an-axle, where even slight imbalances cause vibration, free-rotating objects find their own centre-of-rotation. Beyond the intermediate-axis issue discussed {waves hand} over a bunch of threads, the system will be innately stable. The only requirement for a fixed centre-of-rotation is for docking. And IMO it's better to design a docking system that can cope with off-axis approach, rather than require the station (and approaching spacecraft) to be perfectly balanced around a (shared) fixed physical axis. It's yet-another-system-to-develop (yay!) but once solved, it eliminates so much hassle.

Agreed.

I'm sure that issues with rotational stability are solvable, but I wouldn't be too quick to dismiss these problems. Rotating structures often behave in very counter intuitive ways:

[mikelepage]

That said, I agree that the "mass balancing" requirement is probably overblown. This isn't a wheel-on-an-axle, where even slight imbalances cause vibration, free-rotating objects find their own centre-of-rotation. Beyond the intermediate-axis issue discussed {waves hand} over a bunch of threads, the system will be innately stable. The only requirement for a fixed centre-of-rotation is for docking. And IMO it's better to design a docking system that can cope with off-axis approach, rather than require the station (and approaching spacecraft) to be perfectly balanced around a (shared) fixed physical axis. It's yet-another-system-to-develop (yay!) but once solved, it eliminates so much hassle.

Agreed.

I'm sure that issues with rotational stability are solvable, but I wouldn't be too quick to dismiss these problems. Rotating structures often behave in very counter intuitive ways:

Yep, Slarty that's the video I often link to when talking about what the intermediate axis problem.

Also Ron, I see you mentioned the RV park analogy above my post where I said it was like a trailer park - not trying to pass off your idea as mine - just didn't see it before.

As for the cubesat analogy....

Modularity helps, but cubesats are cheap mainly because they're small. Space station modules are not small.

Space station modules also kill people when they fail, so all the additional QA and testing adds a fair bit of cost. If a cubesat fails, nobody dies.

I think we'll see nano-racks inside a rotating space station, allowing universities to fly cheap payloads. But I doubt we'll see university-level budgets (or even single company-level budgets) launching entire habitation modules any time soon.

I think the station-to-module connection is the wrong place to put the customer interface. To borrow a phrase, "rotating space stations are not LEGO." Just like rockets, the whole thing really needs to be designed all together (structurally, infrastructurally, crew circulation and emergency egress, rotational balance, etc). This is less true for purely microgravity stations like the ISS, of course.

I should clarify that my thinking on this is mainly framed by my aim to implement a small ISS-deployed spin-G free-flyer, carrying payload modules around the size of 6U (long) cubesats. Treading a bit of a fine line here because of IP concerns, but basically we're designing something where the (deployable) framework can be taken up in a Dragon2 trunk, while the pressurised modules go up (and come back) in the pressurised section of Dragon. Also working on a conops that occurs at the beginning and end of long duration crew missions, where the framework and modules can be manipulated by the station's robotic arms, so astronauts inside ISS would operate the Kibo airlock/robotic arm (JEM-RMS) and Canadarm2 in tandem to install/uninstall modules from the framework before setting it free/after grappling it.

Agreed you have to design the whole thing at the beginning, but making the modules interchangeable like cartridges would be highly desirable if it can be done, because you can then run iterative 3-6 month partial g experiments, reusing the framework component multiple times. Being able to bring those modules down from orbit after completion would be invaluable for a number of fields, so I'm pretty sure there's a saleable service there - if we can get it to be interchangeable.

That said, I also agree it becomes far more complicated once you scale up to human-sized spin-G spacecraft, and many elements obviously would not scale up. Cross that bridge when we come to it.
 

[Paul451]

I should clarify that my thinking on this is mainly framed by my aim to implement a small ISS-deployed spin-G free-flyer, carrying payload modules around the size of 6U (long) cubesats. Treading a bit of a fine line here because of IP concerns, but basically we're designing something where the (deployable) framework can be taken up in a Dragon2 trunk, while the pressurised modules go up (and come back) in the pressurised section of Dragon.
Agreed you have to design the whole thing at the beginning, but making the modules interchangeable like cartridges would be highly desirable if it can be done, because you can then run iterative 3-6 month partial g experiments, reusing the framework component multiple times. Being able to bring those modules down from orbit after completion would be invaluable for a number of fields, so I'm pretty sure there's a saleable service there - if we can get it to be interchangeable.

Deeply cool.

Most (perhaps all?) of my objections don't apply here. The modules only need power/data from the core/frame, and maybe common comms via the data-bus. (And the attachment system itself.) Limiting clients to explicit cubesat standards eliminates most of the other complexities with client-developed modules. No issues in docking while rotating (or having to despin for every dock/undock, which isn't viable for a multi-client manned station) and therefore no real balance issues, greatly simplifying the core. (Beyond the known things like the intermediate axis problem, which are solved in design, not ops.) So even moving masses, like animals or fluids/reactants in the client-modules, won't be an issue.

I genuinely hope it works.

(And not just for your sake, although that too. I think it's the kind of thing that we need more of, along with the increase in commercialisation we're seeing already. Another step making doing things in space just an extension of doing something on that scale on Earth, not a whole separate kind of activity.)