Author Topic: Nose tethered BFS Spaceships for artificial gravity during the coastal phase.  (Read 7736 times)

Online Peter.Colin

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Would it be beneficial and practical to create artificial gravity during the coastal phase? It could be done by connecting the two noses of two ships with a long cable, and use the side thrusters to begin spinning to the desired centrifugal force.
How would this effect the radiation shielding of the ship.
How fast would the ships need to spin?, and could this make looking trough a window still pleasant?

Online Semmel

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Obvious advantages: less medical problems from zero gravity, easier life on board like eating, toilet, washing, etc. Less problems to find zero g solutions for trivial everyday stuff like washing clothes, pumping liquids, probably easier design of the life support system.

Obvious disadvantages: more difficult solar power generation, communication with Earth and Mars, additional mass for the nose connection system, difficult or near impossible to do course corrections, spin up and spin down difficult to Orchestra without introducing oscillations. Oscillations due to mass (people) moving around in both ships.

Indifferent: I don't think radiation shielding is a thing in general, can't get much worse than it is already. Maybe a total mass gain, despite extra mass of the cable system. Would require extensive engineering and tradeoff.

Online KelvinZero

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Had a fairly over the top thought, certainly not for initial missions

Elon Musk talks about getting to a point of sending many ships in a single launch window. What if these all docked with a lightweight central hub for the flight? The central hub would probably be some sort of cycler. This cylindrical arrangement of ships with engines on the outer side would also provide a fair bit of shielding, should that prove to be an issue.


Online Peter.Colin

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Had a fairly over the top thought, certainly not for initial missions

Elon Musk talks about getting to a point of sending many ships in a single launch window. What if these all docked with a lightweight central hub for the flight? The central hub would probably be some sort of cycler. This cylindrical arrangement of ships with engines on the outer side would also provide a fair bit of shielding, should that prove to be an issue.

I think that's a very good idea!  :)

Would you point the rotational axis of the central hub towards the sun, for easier solar power generation?

Would the central hub make steering corrections or the connected ships?
« Last Edit: 08/06/2017 06:55 AM by Peter.Colin »

Online Peter.Colin

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Obvious advantages: less medical problems from zero gravity, easier life on board like eating, toilet, washing, etc. Less problems to find zero g solutions for trivial everyday stuff like washing clothes, pumping liquids, probably easier design of the life support system.

Obvious disadvantages: more difficult solar power generation, communication with Earth and Mars, additional mass for the nose connection system, difficult or near impossible to do course corrections, spin up and spin down difficult to Orchestra without introducing oscillations. Oscillations due to mass (people) moving around in both ships.

Indifferent: I don't think radiation shielding is a thing in general, can't get much worse than it is already. Maybe a total mass gain, despite extra mass of the cable system. Would require extensive engineering and tradeoff.

In the ITS animation, the tanker is put on the BFR with a crane and a cable on the nose (or at multiples sides), probably the ship has a similar nose cable system already.
« Last Edit: 08/06/2017 08:16 AM by Peter.Colin »

Offline mikelepage

Had a fairly over the top thought, certainly not for initial missions

Elon Musk talks about getting to a point of sending many ships in a single launch window. What if these all docked with a lightweight central hub for the flight? The central hub would probably be some sort of cycler. This cylindrical arrangement of ships with engines on the outer side would also provide a fair bit of shielding, should that prove to be an issue.

I think that's a very good idea!  :)

Would you point the rotational axis of the central hub towards the sun, for easier solar power generation?

Would the central hub make steering corrections or the connected ships?

Keep it simple.  Keep all the intelligence is in the ships themselves - probably much more efficient at first for ships to detach to do course corrections, since burns of a several 100m/s is huge compared to the 20-40m/s of rotation.  What is needed (and I hope Bigelow or similar does this) is a cable-reinforced, inflatable tube module that can take some load.  You cannot just use cables because of the oscillation problem (remember a bridge has both compressive and tensile elements and we're effectively building a suspension bridge in space)

The aim is to get the largest radius of rotation, because this means the rpm can be lower for a given centrifugal acceleration - its the spin which induces vertigo in most people once you get above 4rpm or so.  If you have less than 6 ships/nodes, then you connect directly, or to a dumb central node, but if you have 6 or more ships/nodes in a ring, then the angle between them decreases to 60 degrees or less, and it is a better use of tubes to dispense with the hub and attach the tubes ship to ship, so you get a larger effective radius.

Even for two ITSy ships connected nose to nose, the length of an inflatable tube connector that would fit into an ITSy payload bay could allow on the order of 20m between them, giving an effective radius of at least 30m.  That means that at 4rpm you'd be getting perhaps 0.5 G at the edges, and at least Mars G for much of the crewed area. 

Online KelvinZero

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Just saw some issues with the cycler idea. With 3 month earth-mars times it probably needs 4 synods to return home. Maybe it can't be a true cycler in which case how does it brake into mars orbit, or maybe this thing passes mars and visits the asteroid belt, and you have 4 of them. Not as simple as it first sounded.

Two ships with cables has a lot less question marks, especially if the ship is already designed to be lifted from the nose under 1g like in the animations.

(Off topic, another of my hobby horses is good VR for space travellers. Spin gravity creates a lot of design constraints and you will still be in a cramped spaceship. Good VR, a treadmill and elastic bands could let you jog though expansive fantasy worlds for hours every day.)

Online Peter.Colin

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Just saw some issues with the cycler idea. With 3 month earth-mars times it probably needs 4 synods to return home. Maybe it can't be a true cycler in which case how does it brake into mars orbit, or maybe this thing passes mars and visits the asteroid belt, and you have 4 of them. Not as simple as it first sounded.

Two ships with cables has a lot less question marks, especially if the ship is already designed to be lifted from the nose under 1g like in the animations.

(Off topic, another of my hobby horses is good VR for space travellers. Spin gravity creates a lot of design constraints and you will still be in a cramped spaceship. Good VR, a treadmill and elastic bands could let you jog though expansive fantasy worlds for hours every day.)

Assuming a cycler central hub takes to long.
I think this central rotating hub structure should brake the same way as it's accelerates, with a few Raptor engines.
I know this consumes a lot of fuel, but this amount of fuel/engines could be minimized by pumping from Earth tankers only the required fuel for accelerating.
And for braking at mars pumping fuel from the connected Spaceships.
The central rotating hub structure can also double as a Mars fluids depot in orbit for mars tankers when they're full at the surface. It makes more sense to store the bulk of the liquids in mars orbit, than on the surface.

Spin gravity begins to feel like normal gravity the longer the cable is, so VR would feel similar to VR on earth.

Of topic:
Does anyone have an idea about how much fuel would be left in a fully loaded Mars tanker when it reaches Mars orbit?



Offline Humuku

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How fast would the ships need to spin?, and could this make looking trough a window still pleasant?

Period of tethered movement is proportional to the square root of the radius of the tether.

Offline livingjw

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Just saw some issues with the cycler idea. With 3 month earth-mars times it probably needs 4 synods to return home. Maybe it can't be a true cycler in which case how does it brake into mars orbit, or maybe this thing passes mars and visits the asteroid belt, and you have 4 of them. Not as simple as it first sounded.

Two ships with cables has a lot less question marks, especially if the ship is already designed to be lifted from the nose under 1g like in the animations.

(Off topic, another of my hobby horses is good VR for space travellers. Spin gravity creates a lot of design constraints and you will still be in a cramped spaceship. Good VR, a treadmill and elastic bands could let you jog though expansive fantasy worlds for hours every day.)

Assuming a cycler central hub takes to long.
I think this central rotating hub structure should brake the same way as it's accelerates, with a few Raptor engines.
I know this consumes a lot of fuel, but this amount of fuel/engines could be minimized by pumping from Earth tankers only the required fuel for accelerating.
And for braking at mars pumping fuel from the connected Spaceships.
The central rotating hub structure can also double as a Mars fluids depot in orbit for mars tankers when they're full at the surface. It makes more sense to store the bulk of the liquids in mars orbit, than on the surface.

Spin gravity begins to feel like normal gravity the longer the cable is, so VR would feel similar to VR on earth.

Of topic:
Does anyone have an idea about how much fuel would be left in a fully loaded Mars tanker when it reaches Mars orbit?

Just enough to land after aerobraking.

Offline guckyfan

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Of topic:
Does anyone have an idea about how much fuel would be left in a fully loaded Mars tanker when it reaches Mars orbit?

A cargo ITS would lift 300t to LEO and can land 450t on Mars when loaded with extra cargo in LEO and going on a slow trajectory. A tanker should be able to land at least those 450t in propellant on Mars. If that would make sense. Maybe as a rescue mission for the first crew if sabatier fuel ISRU fails. Enough methane but LOX would still need to be produced from atmospheric CO2. An unlikely string of events.

Online Peter.Colin

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Of topic:
Does anyone have an idea about how much fuel would be left in a fully loaded Mars tanker when it reaches Mars orbit?

A cargo ITS would lift 300t to LEO and can land 450t on Mars when loaded with extra cargo in LEO and going on a slow trajectory. A tanker should be able to land at least those 450t in propellant on Mars. If that would make sense. Maybe as a rescue mission for the first crew if sabatier fuel ISRU fails. Enough methane but LOX would still need to be produced from atmospheric CO2. An unlikely string of events.

Let me clarify my off topic question:
A tanker holds 2500t propelant. But when when this tanker leaves Earth and reaches LEO it only contains 380t of propelant, the rest has been spent to reach orbit. The BFR was also needed to get this propelant into LEO and that contained 6700t of propellant.
So only 4.3% of the original propelant reaches Earth orbit, not much...
I was wondering what part of propelant produced on mars surface reaches mars orbit?

Storing most propellant in mars orbit minimizes the amount of storage volume needed, this could be done in the central rotating hub.
« Last Edit: 08/06/2017 10:32 PM by Peter.Colin »

Offline darkenfast

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If the ship can reach Mars in under six months, why in the world would you spend so much mass and added complexity to provide artificial gravity?  We know that crews can be walking on Earth within a day or two of return from the ISS (with some restrictions), and in an emergency can get themselves out of a Soyuz unaided after a bad landing.  The gravity on Mars is one fourth of ours.  They don't have to leap out of their couches and start heavy lifting immediately after touchdown. 

Spending years going to Saturn?  Then maybe we need something.  But not for Mars. 

Online KelvinZero

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If the ship can reach Mars in under six months, why in the world would you spend so much mass and added complexity to provide artificial gravity?  We know that crews can be walking on Earth within a day or two of return from the ISS (with some restrictions), and in an emergency can get themselves out of a Soyuz unaided after a bad landing.  The gravity on Mars is one fourth of ours.  They don't have to leap out of their couches and start heavy lifting immediately after touchdown. 

Spending years going to Saturn?  Then maybe we need something.  But not for Mars.
The nose-tethered option is not that extravagant.

The other options only make sense with much greater scale but they don't add much mass,  proportionally. They even have the possibility of saving mass if you are preaccelerating fuel ahead of time, or packing more people into your ITS like sardines because they will have more volume during the trip. The argument against them is definitely about complexity. This could grow to be a whole new vehicle that (if not a true cycler) also needs to aerobrake.

A nose tethered option for a mission to Saturn is also an interesting suggestion IMO. Maybe we should be using that as our primary justification for a discussion of two nose tethered ITS.

Online Peter.Colin

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If the ship can reach Mars in under six months, why in the world would you spend so much mass and added complexity to provide artificial gravity?  We know that crews can be walking on Earth within a day or two of return from the ISS (with some restrictions), and in an emergency can get themselves out of a Soyuz unaided after a bad landing.  The gravity on Mars is one fourth of ours.  They don't have to leap out of their couches and start heavy lifting immediately after touchdown. 

Spending years going to Saturn?  Then maybe we need something.  But not for Mars.


Because it would start to smell, washing clothes washing yourself, going to the toilet is not easy without gravity.

And I wouldn't want to drink my glass of champagne out of a bag, that can explode because of the bubbles...  ;)

If a tether can prevent these and more inconveniences, and if the ship (the tanker we know) is allready lifted on the BFR by a nose tether. Why not do it like this.


« Last Edit: 08/07/2017 04:01 PM by Peter.Colin »

Online docmordrid

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>
Because it would start to smell, washing clothes

Supercritical CO2 laundry. Dry, isolated, works & COTS.

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washing yourself, going to the toilet is not easy without gravity.

There was a shower on Skylab, and space potties are obviously on ISS.
DM

Offline intrepidpursuit

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The ship would have to spend at least several days in microgravity before such a contraption could be assembled, so there still have to be zero g facilities for everything (I think three months of laundry is still easier to carry than something that can clean the laundry in space). Three months isn't long enough to cause major damage to the passengers. They are heading to a reduced gravity environment so they won't need as long to acclimate anyway.

Artificial gravity for mars is WAY more trouble than it is worth.

I hope one day there will be a ship with centripetal gravity, but early mars settlement ships don't need that development burden.

Online KelvinZero

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I hope one day there will be a ship with centripetal gravity, but early mars settlement ships don't need that development burden.
This is why I suggested maybe moving the goalposts to the saturn mission. (Actually someone else suggested it)

What I mean is, I think it is better to just discuss it as a technical problem. It is after you have sorted out how difficult it is that you have the basis to argue where it could be useful and where it is not worth the bother.

A lot of things we will not know for sure until we have moved a lot of people. We only have a small sample set at the moment. Same with radiation. Im convinced we have enough info on both to not be afraid to just begin, but I don't think anyone can be confident of what we will decide is optimal by the time we are shipping hundreds of thousands of people of all ages to Mars.. so just treat it as a technical problem.

Offline guckyfan

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I hope one day there will be a ship with centripetal gravity, but early mars settlement ships don't need that development burden.
This is why I suggested maybe moving the goalposts to the saturn mission. (Actually someone else suggested it)

I did. Not claiming I was the only one.

What I mean is, I think it is better to just discuss it as a technical problem. It is after you have sorted out how difficult it is that you have the basis to argue where it could be useful and where it is not worth the bother.

As such it is an interesting argument.

Offline Paul451

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If the ship can reach Mars in under six months, why in the world would you spend so much mass and added complexity to provide artificial gravity?

What "mass and added complexity"? At Mars gravity and 4-6 RPM, AG is almost free. The only reason not to do it would be out of spite.

We know that crews can be walking on Earth within a day or two of return from the ISS

We also know that that is a meaningless measure of their actual health, and that astronauts experience random orthostatic hypotension episodes for weeks after their return due to a suppressed baroreceptor reflex. According to flight surgeons and works published by long-duration astronauts, typical recovery is 1 day on the ground for 1 day in orbit.

Offline douglas100

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What "mass and added complexity"? At Mars gravity and 4-6 RPM, AG is almost free. The only reason not to do it would be out of spite.

There must be added mass and complexity to do this. How much is up for discussion.

As for a another reason not to do it, what about "it's unnecessary?" Spite has nothing to do with it.
Douglas Clark

Offline Paul451

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There must be added mass and complexity to do this.

Sounds like a religious tenet.

Offline RonM

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There must be added mass and complexity to do this.

Sounds like a religious tenet.

If you look at the ITS design from the original presentation you'll see that there is no provision for docking. Two ITS spacecraft can't dock, let alone spin together for AG. Providing that capability would add extra mass.

Are you talking about spinning an ITS around its long axis? That would require a redesign of the deck layout to function properly in flight and when landed.

Offline alexterrell

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Obvious disadvantages: ....., communication with Earth and Mars, ....
Have a small satellite in "wifi range" - perhaps 2km away, with a big dish pointed at Earth. The "satellite" could also have a telescope on the BFR to check the outside.

Offline blasphemer

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I made a mock up picture of two ITS ships connected by a 200m long cable. Enough for 1g gravity at comfortable 2 RPM.


Online Peter.Colin

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Very nice!

Now let's hope the cable is not to heavy or complex ..... lol
« Last Edit: 09/04/2017 06:33 PM by Peter.Colin »

Online AncientU

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I made a mock up picture of two ITS ships connected by a 200m long cable. Enough for 1g gravity at comfortable 2 RPM.



I believe the 200-ish meters is a radius, not a 'length' -- the exact number is closer to 225m radius or 450m 'length'
"If we shared everything [we are working on] people would think we are insane!"
-- SpaceX friend of mlindner

Offline Paul451

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If you look at the ITS design from the original presentation you'll see that there is no provision for docking. Two ITS spacecraft can't dock, let alone spin together for AG. Providing that capability would add extra mass.

The refuellers attach side-by-side to the main ships, and on Earth the ships are lifted by the nose to be re-mounted on the booster. They have attachments up the wazzoo, and clearly the necessary prox-ops systems.

However, my objection to the original statement is that it solely assumes costs, but ignores any possibility of benefits or savings. That's pre-defining the issue to failure.

I made a mock up picture of two ITS ships connected by a 200m long cable. Enough for 1g gravity at comfortable 2 RPM.

A cable is a bad idea. Picture a 200m (or 400m, for 1g/2RPM) cable hanging in still air, with an ITS-like platform hanging off the bottom, with dozens to a hundred people moving around. It will twist and oscillate like crazy. In free-space it's actually worse, the oscillations dampen very slowly (bouncing back and forth along the cable). There's a similar problem with compressive structures like trusses. The suggested alternative is a tensegrity structure, a combination of tensile and compressive structures; eg, a truss held under compression by cables. A design suggested for long tethers is to have an inflated tube (or four) providing the compressive-resistive structure, which is held in compression by the tensile cables. In simulations, that apparently dampens all axes of oscillation, including twist, better than just cables, or just trusses.

IMO, 2RPM is ridiculously low. And 1g is too high, if you expect people to permanently colonise Mars. (If 0.38g isn't sufficient, we aren't colonising Mars, so the point is moot.) 4RPM and 0.38g gives you 40m total length. Including the length of the ITS cabins, it's barely worth the cable.

It would be nice to know for sure; since radius/diameter scales with the square of RPM, so doubling the RPM quarters the length. And linearly with g-load, so double the RPM at 38% gravity gives you a ten-fold reduction in length.

Online Coastal Ron

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A cable is a bad idea. Picture a 200m (or 400m, for 1g/2RPM) cable hanging in still air, with an ITS-like platform hanging off the bottom, with dozens to a hundred people moving around. It will twist and oscillate like crazy.

I'm not sure what you envision all those people are doing, but I can't imagine anything they could be doing that would make an ITS twist and oscillate. And with the tension between the two ITS being so high, random movement of people on both ITS are likely to be very muted vibration-wise. At most all you'd need is a hydraulic damper located somewhere on the cable to smooth out minor vibrations.

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In free-space it's actually worse, the oscillations dampen very slowly (bouncing back and forth along the cable).

However because of the weight of the two ITS, and the rotation keeping the cable under tremendous compression tension* forces, it's going to be hard to get a cable vibrating in the first place, and the natural tendency would be to mute the vibrations out of the cable.

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The suggested alternative is a tensegrity structure, a combination of tensile and compressive structures; eg, a truss held under compression by cables.

Just stringing a cable between two ITS and getting them rotating without free body issues will be tricky by itself, but having to inflate things in between is going to be even harder.

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IMO, 2RPM is ridiculously low.

Low RPM's are much better than higher ones, not only for people but also for safety reasons.

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And 1g is too high, if you expect people to permanently colonise Mars. (If 0.38g isn't sufficient, we aren't colonising Mars, so the point is moot.)

I agree. Even if the ITS flotilla is returning to Earth it should be good enough to have Mars gravity for the trip.

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4RPM and 0.38g gives you 40m total length. Including the length of the ITS cabins, it's barely worth the cable.

It would be nice to know for sure; since radius/diameter scales with the square of RPM, so doubling the RPM quarters the length. And linearly with g-load, so double the RPM at 38% gravity gives you a ten-fold reduction in length.

Are you familiar with the website SpinCalc? It's where I do all of my "what if" simulations for spinning space station designing. Unfortunately though we lack enough real experience, so the variables and results in the calculator are only estimates based on research done here on Earth.

So if a 200m cable is used, and the required gravity is Mars normal (i.e. .38 Earth), then the RPM would be 1.8. According to SpinCalc that should provide artificial gravity with no known spin-related issues.

As for the cabling, there are Ultra-high-molecular-weight polyethylene (UHMWPE, UHMW) cables that could be used that are both very strong and lightweight.

* Corrected "compression" to "tension" as noted by Paul451 (thanks Paul451!)
« Last Edit: 09/04/2017 11:07 PM by Coastal Ron »
If we don't continuously lower the cost to access space, how are we ever going to afford to expand humanity out into space?

Offline Paul451

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I'm not sure what you envision all those people are doing, but I can't imagine anything they could be doing that would make an ITS twist and oscillate.

You've obviously never seen a large mass on the end of a long cable.

and the rotation keeping the cable under tremendous compression forces

Que?

Just stringing a cable between two ITS and getting them rotating without free body issues will be tricky by itself, but having to inflate things in between is going to be even harder.

Quite the contrary, it makes everything easier, that's why it's suggested.

Are you familiar with the website SpinCalc?

Of course. (Although I use my own spreadsheet for quick'n'dirty RPM/radius/g-load calcs.) Ted is extremely conservative in his red/yellow/green warnings, which he has acknowledged in other presentations. He does link to the more recent Lackner and DiZio paper, which is part of modern AG research (including ultra-high RPM research like the "Space Cycle" exercise platform), but doesn't actually use it in the calc.

The old Apollo-era research just doesn't hold up. Even at the time, the result were wildly variable between experiments, which should tip you off that they stumbled onto a confounding factor rather than a real effect. Based on recent research, it looks like that uncontrolled variable was the amount of movement test subjects were allowed or encouraged to make, or actively discouraged from making, during the spin-up phase. The less movement, the worse the results. It's not surprising they did that; even today, you still get people saying "High RPMs might be possible if people limit their movements". The instinct is the opposite to how we actually need to adapt.

But it's funny when people say, "We can only go by Earth research," but then very pointedly ignore everything done in the last 35 years.

[IIRC, Space Cycle runs at around 40 RPM and up to 7g.]

Online KelvinZero

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If you look at the ITS design from the original presentation you'll see that there is no provision for docking. Two ITS spacecraft can't dock, let alone spin together for AG. Providing that capability would add extra mass.
There is a spacex animation showing an ITS being picked up by a cable to it's nose in order to place it on top of the 1st stage. That is probably what inspired all this.

My personal feeling is that we will find a way to avoid this, eg vr treadmills with elastic bands for gravity, but it is still fun to think about.

Despite that nose cable art, I would favour something wider, eg multiple widely spaced cables with cross connections specifically designed to damp any oscillation. Someone must have studied this extensively somewhere.

https://www.youtube.com/watch?v=0qo78R_yYFA?t=120

(for some reason, the t=120 did not work.. go to the 2:00 minute mark to see the ITS lifted by cable)
« Last Edit: 09/04/2017 11:07 PM by KelvinZero »

Online Coastal Ron

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and the rotation keeping the cable under tremendous compression forces

Que?

Mea culpa. Should have said "tension" forces. Glad you pointed that out.

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Quite the contrary, it makes everything easier, that's why it's suggested.

If you say so. This obviously points to the need for actually DOING rotational gravity research in space instead of theorizing about it. Better to have facts than opinions.

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Of course. (Although I use my own spreadsheet for quick'n'dirty RPM/radius/g-load calcs.) Ted is extremely conservative in his red/yellow/green warnings, which he has acknowledged in other presentations. He does link to the more recent Lackner and DiZio paper, which is part of modern AG research (including ultra-high RPM research like the "Space Cycle" exercise platform), but doesn't actually use it in the calc.

OK good. For myself I'm OK with "worst case", which I'm hoping SpinCalc will be, because if I can come up with solutions that work out OK with "worst case", then I should be OK with "reality".
If we don't continuously lower the cost to access space, how are we ever going to afford to expand humanity out into space?

Offline Paul451

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For myself I'm OK with "worst case", [...] because if I can come up with solutions that work out OK with "worst case", then I should be OK with "reality".

No. This is why I keep harping on about people insisting on low RPM. It's pre-establishing that the trade will fail.

Radius increases with the inverse square of RPM. It's a huge effect. If you insist on 1g and 1 or 2RPM, then you've virtually ruled out AG in advance, because of course others are going to say "It adds too much mass/complexity", and any consideration of the idea dies. As I said, the difference between 1g/2RPM and 0.38g/4RPM is a tenfold difference in scale.

Picture any other type of engineering where you just hand-waved an order of magnitude difference in the properties or scale.

It would be like designing rockets by insisting that any rocket technology must be capable of SSTO or you won't even look at it, because "if you can solve the worst case then you should be able to solve any other configuration". That's not reasonable, and it's not honest.

Online KelvinZero

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It just occurred to me, maybe a single cable can work.

I was imagining the ship twisting back and forward, with no way to damp the oscillation, sort of like a weight on the end of a string would tend to spin frictionlessly. But the cable would be able to turn it it's socket so there is no 'winding up', also each ship would have a flywheel that would keep each one rigidly oriented. I don't think there is any possibility of the flywheels becoming saturated since it is still a closed system.

That just leaves normal non-twisting oscillation. Even if it is tight like a violin string I think that sort of oscillation can be damped nicely at the endpoints.

Offline Paul451

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It just occurred to me, maybe a single cable can work. [etc]

This is where you make a system ridiculously complicated so you can keep the original idea, because the original idea was "simple".

Offline blasphemer

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If you insist on 1g and 1 or 2RPM, then you've virtually ruled out AG in advance, because of course others are going to say "It adds too much mass/complexity", and any consideration of the idea dies.

It may add mass, but complexity? I dont see why. In fact higher RPMs mean astronauts are more likely to be sick, experience Coriolis forces and any oscillations/instabilities are going to be higher frequency so if anything I would say that high RPM adds complexity. Unless you plan to make your connection rigid, you have to deal with oscillations no matter what.

As for mass, how much does a 100m of cables weight? Lets say it must be sufficiently strong to not break under the weight of two partially fueled ITS spaceships at 0.38g.
« Last Edit: 09/05/2017 12:25 PM by blasphemer »

Offline Paul451

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Offline blasphemer

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Fully fueled 12m ITS weights 2400 tons, unfueled ITS weights 450 tons. Artificial gravity makes sense during coasting so the strength of the cables depends also on how much fuel will remain in reserve after Mars transfer burn. As a lower bound, assuming there is no fuel left and we want 0.38g then cable(s) will have to be strong enough to support at least 450*2*0.38= 342 tons of weight. Anyone know how much kg per length this roughly could be?
« Last Edit: 09/05/2017 12:37 PM by blasphemer »

Offline blasphemer

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I have changed my concept pic by reversing the spaceships so that radius is increased by length of the engine section. Note that this is opposite hanging direction as in ITS video so this may not be an improvement.


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https://www.reddit.com/r/spacex/comments/3ogkfa/artifical_gravity/cvxe4yu/?st=j77ls2q8&sh=1fabfcd4

I found an older reddit thread discussing this idea. Some relevant info:

Quote
We end up with a radius of only about 3500 meters.

Assume MCT weighs 200 tonnes during cruise, and you want to simulate Mars gravity. Using three redundant Dyneema fibers, 50% engineering margin (NASA standard), 50% live load allowance, and 100% overhead for space environment protective coatings, I get a mass of 7.5 tonnes (or 3.8 tonnes per MCT). So it's totally doable.

The other thing is spin/despin fuel. That's another 8 tonnes per MCT.

These are some weird assumptions (3500m radius? 200 tons only?) but in the end cable mass is 7.5 tons and is actually less than half of spin/despin fuel mass. So maybe any AG solution will be dominated by fuel mass instead of mass of the mechanism itself?
« Last Edit: 09/05/2017 01:11 PM by blasphemer »

Offline Paul451

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https://www.reddit.com/r/spacex/comments/3ogkfa/artifical_gravity/cvxe4yu/?st=j77ls2q8&sh=1fabfcd4
Quote
We end up with a radius of only about 3500 meters.
and you want to simulate Mars gravity.

Less than one third of 1 RPM.

{sigh}

Why does talking about AG seem to turn people's brains to chicken soup?

Offline RonM

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I have changed my concept pic by reversing the spaceships so that radius is increased by length of the engine section. Note that this is opposite hanging direction as in ITS video so this may not be an improvement.

This won't work because AG direction will be 180 degrees from the landed configuration. The crew will be walking on the ceilings.

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Fully fueled 12m ITS weights 2400 tons, unfueled ITS weights 450 tons. Artificial gravity makes sense during coasting so the strength of the cables depends also on how much fuel will remain in reserve after Mars transfer burn. As a lower bound, assuming there is no fuel left and we want 0.38g then cable(s) will have to be strong enough to support at least 450*2*0.38= 342 tons of weight. Anyone know how much kg per length this roughly could be?

I'm pretty sure this is off by a factor of 2.  I think the cable only has to be strong enough to hold up a single BFS (with appropriate safety factor, of course).

Offline envy887

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As for mass, how much does a 100m of cables weight? Lets say it must be sufficiently strong to not break under the weight of two partially fueled ITS spaceships at 0.38g.

Let's assume it is Zylon, density 1540 kg/m^3 and tensile strength 5.8 GPa. An ITS craft will mass up to 720 tonnes in cruise configuration, based on the fueled mass and delta-v given in the 2016 IAC presentation. At 0.38 g that is 2.68 million N of force on the cable.

Assuming a factor of safety of a very conservative 3x, that's 8.05 MN required strength, which Zylon can provide with only 13.9 cm^2 area of cable, which masses only 2.15 kg per meter. So a 100 m cable is only 215 kg.

But with a 100 m cable, the delta-v to spin up and down is 5250 kg per ship. The mass of the cable is pretty trivial compared to the mass of spin up/down propellant. Both propellant and cable mass increase with a longer cable, so the shortest possible cable is mass-optimal.

There doesn't appear to be any reason to spin at less than 3 RPM, which can be done with a 50 m cable and 2 ITS ships nose to nose, yielding .38 g on the lowest deck of the passenger cabin, and .27 g on the highest deck.

Online Coastal Ron

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But with a 100 m cable, the delta-v to spin up and down is 5250 kg per ship. The mass of the cable is pretty trivial compared to the mass of spin up/down propellant. Both propellant and cable mass increase with a longer cable, so the shortest possible cable is mass-optimal.

Sure, the shortest possible cable is mass-optimal, but it may not provide the optimal conditions for humans.

Quote
There doesn't appear to be any reason to spin at less than 3 RPM, which can be done with a 50 m cable and 2 ITS ships nose to nose, yielding .38 g on the lowest deck of the passenger cabin, and .27 g on the highest deck.

If the weight of the cable is minimal, and if human comfort can be increased by increasing the radius of the rotation, then it only costs a small amount of cable weight to provide better human comfort. I'm assuming the amount of fuel required to spin up/down does not change given the radius as long as the resulting gravity is the same.
If we don't continuously lower the cost to access space, how are we ever going to afford to expand humanity out into space?

Online Coastal Ron

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For myself I'm OK with "worst case", [...] because if I can come up with solutions that work out OK with "worst case", then I should be OK with "reality".

No. This is why I keep harping on about people insisting on low RPM. It's pre-establishing that the trade will fail.

Radius increases with the inverse square of RPM. It's a huge effect. If you insist on 1g and 1 or 2RPM, then you've virtually ruled out AG in advance, because of course others are going to say "It adds too much mass/complexity", and any consideration of the idea dies. As I said, the difference between 1g/2RPM and 0.38g/4RPM is a tenfold difference in scale.

I understand your concern. Previously I didn't, but now I do, so thanks for explaining it. However there are some situations where your concerns should not be a problem, such as:

A. The example of two ITS joined by cable nose-to-nose, where the weight of the cable is negligible, so the radius of rotation can be lengthened without the inverse square problem.

B. If having a large vehicle/station is a benefit. A rotating space station design I'm looking at requires a large amount of habitable area to be useful, so having a longer radius with lower RPM can provide that.

Quote
It would be like designing rockets by insisting that any rocket technology must be capable of SSTO or you won't even look at it, because "if you can solve the worst case then you should be able to solve any other configuration". That's not reasonable, and it's not honest.

Again, I understand your point, but I would ask you to consider how SpaceX was able to create reusable rockets when everyone else assumed it could not be done. Sometimes you just need to ignore "the rules" to see if there are alternative solutions that still result in the desired outcome.
If we don't continuously lower the cost to access space, how are we ever going to afford to expand humanity out into space?

Online KelvinZero

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It just occurred to me, maybe a single cable can work. [etc]

This is where you make a system ridiculously complicated so you can keep the original idea, because the original idea was "simple".
No I don't think so, because the ships will (Im guessing) already have these flywheels and the software to deal with small corrections in orientation.

What was naive was the idea of the spaceship freely exchanging rotational energy with the cable, like a weight tied to the end of a rope, winding up, then spinning the other way.

Offline Paul451

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I'm assuming the amount of fuel required to spin up/down does not change given the radius as long as the resulting gravity is the same.

Not even close. Angular momentum (and hence fuel requirement) increases with radius and velocity, both of which increase with reduced RPM for the same g-load.

Eg, at the same 0.38g, angular momentum per kg of ship mass at 4, 3, 2, 1 RPM is, approx. 190, 450, 1500 and 12,000 mē/s respectively. Hence, halve the RPM and you increase the angular momentum approximately 8-fold. Cubic increase.

Sometimes you just need to ignore "the rules" to see if there are alternative solutions that still result in the desired outcome.

I find it funny that you're saying this when you are the one fixating on SpinCalc's red/yellow/green indicators as if they are the word-of-god, and I'm trying to say "No, there's been 35 years of research that says otherwise. The old assumptions are wildly out of date."
« Last Edit: 09/05/2017 10:23 PM by Paul451 »

Online Coastal Ron

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I'm assuming the amount of fuel required to spin up/down does not change given the radius as long as the resulting gravity is the same.

Not even close. Angular momentum (and hence fuel requirement) increases with radius and velocity, both of which increase with reduced RPM for the same g-load.

Eg, at the same 0.38g, angular momentum per kg of ship mass at 4, 3, 2, 1 RPM is, approx. 190, 450, 1500 and 12,000 mē/s respectively. Hence, halve the RPM and you increase the angular momentum approximately 8-fold. Cubic increase.

OK, it's good we're having this conversation, as I had the wrong assumption about the ice skater spinning with arms open and arms closed.

Quote
Sometimes you just need to ignore "the rules" to see if there are alternative solutions that still result in the desired outcome.

I find it funny that you're saying this when you are the one fixating on SpinCalc's red/yellow/green indicators as if they are the word-of-god, and I'm trying to say "No, there's been 35 years of research that says otherwise. The old assumptions are wildly out of date."

Well, 35 years of research here on Earth. What we need is a long-duration testing platform in space to validate everyone's assumptions - which I think I may have a valid concept for. Not ready to share yet, but it was inspired by a suggestion on a different topic thread. Specifically it would provide .38G at 3 RPM, while simultaneously providing levels of occupation that would be closer to the center of rotation, meaning less simulated gravity but more angular and tangential velocities (which may be detrimental at some upper limit, which needs to be determined thru actual testing).
If we don't continuously lower the cost to access space, how are we ever going to afford to expand humanity out into space?

Offline envy887

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But with a 100 m cable, the delta-v to spin up and down is 5250 kg per ship. The mass of the cable is pretty trivial compared to the mass of spin up/down propellant. Both propellant and cable mass increase with a longer cable, so the shortest possible cable is mass-optimal.

Sure, the shortest possible cable is mass-optimal, but it may not provide the optimal conditions for humans.

Quote
There doesn't appear to be any reason to spin at less than 3 RPM, which can be done with a 50 m cable and 2 ITS ships nose to nose, yielding .38 g on the lowest deck of the passenger cabin, and .27 g on the highest deck.

If the weight of the cable is minimal, and if human comfort can be increased by increasing the radius of the rotation, then it only costs a small amount of cable weight to provide better human comfort. I'm assuming the amount of fuel required to spin up/down does not change given the radius as long as the resulting gravity is the same.

Propellant requirements are roughly proportional to tangential velocity, which increases with the square root of radius.

However, the difference between 2 and 3 RPM is only 6 m/s delta-v per ship, and 44 m (less than 100 kg) of cable. If the difference is worthwhile (which I doubt), then adding that capability is hardly difficult.

Offline biosehnsucht

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There doesn't appear to be any reason to spin at less than 3 RPM, which can be done with a 50 m cable and 2 ITS ships nose to nose, yielding .38 g on the lowest deck of the passenger cabin, and .27 g on the highest deck.

Are you so sure about no reason to go less than 3 RPM? Perhaps a lower gravity target will help, but does 4 RPM for 0.38g come out to less Coriolis effect than 2 RPM at 1g?

Quote
The Coriolis effect gives an apparent force that acts on objects that move relative to a rotating reference frame. This apparent force acts at right angles to the motion and the rotation axis and tends to curve the motion in the opposite sense to the habitat's spin. If an astronaut inside a rotating artificial gravity environment moves towards or away from the axis of rotation, he or she will feel a force pushing him or her towards or away from the direction of spin. These forces act on the inner ear and can cause dizziness, nausea and disorientation. Lengthening the period of rotation (slower spin rate) reduces the Coriolis force and its effects. It is generally believed that at 2 rpm or less, no adverse effects from the Coriolis forces will occur, although humans have been shown to adapt to rates as high as 23 rpm.

Offline the_other_Doug

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I'd ask y'all to recall that the only experiments that I know of that have ever been done with spinning up two vehicles around a tether happened 51 years ago, on Geminis XI and XII.  They found that, on the mass scales and speed of movement they were testing, the recoil force from the tether itself caused the tether to have completely unacceptable dynamics, and even slight movements by the crew would take what appeared to be a stable tether dynamic and push it quickly into a series of disastrous loops.

The problem seems to be that you need the tether to be at full stress before you spin up, in order for it to act as if it is a solid, unbending link between the two end-point masses.  If you at any point stop thrusting the two masses away from each other, along the tether vector, the tether will recoil, start pulling the two masses back together, and start looping like crazy.  The spin-up maneuver, involving as it would need to the continuous thrust to apply pressure along the tether, but decreasing as the spin replaces the thrust to apply tension to the tether, is of such complexity that it would need to be demonstrated several times before you could consider using it operationally.

So -- those who believe that there needs to be an awful lot more real-world testing, in space, of the dynamics of such tethered systems before such a thing is designed into the cruise architecture of a Mars mission are the winners of this discussion at present, IMHO... :)
-Doug  (With my shield, not yet upon it)

Online KelvinZero

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I'd ask y'all to recall that the only experiments that I know of that have ever been done with spinning up two vehicles around a tether happened 51 years ago, on Geminis XI and XII.  They found that, on the mass scales and speed of movement they were testing, the recoil force from the tether itself caused the tether to have completely unacceptable dynamics, and even slight movements by the crew would take what appeared to be a stable tether dynamic and push it quickly into a series of disastrous loops.

But since then we have things like the Colbert and the hoverslam, ie damping oscillations and control of laggy systems has advanced to an incredible art that is used for both trivial and extreme purposes.

I had assumed some sort of intelligent damping. Everything seems to have it now. Even the fact they actually physically did this experiment back then shows how much things have moved on. We are comparing biplanes to harriers

Offline the_other_Doug

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I'd ask y'all to recall that the only experiments that I know of that have ever been done with spinning up two vehicles around a tether happened 51 years ago, on Geminis XI and XII.  They found that, on the mass scales and speed of movement they were testing, the recoil force from the tether itself caused the tether to have completely unacceptable dynamics, and even slight movements by the crew would take what appeared to be a stable tether dynamic and push it quickly into a series of disastrous loops.

But since then we have things like the Colbert and the hoverslam, ie damping oscillations and control of laggy systems has advanced to an incredible art that is used for both trivial and extreme purposes.

I had assumed some sort of intelligent damping. Everything seems to have it now. Even the fact they actually physically did this experiment back then shows how much things have moved on. We are comparing biplanes to harriers

Invalid comparison.  Ever since Kitty Hawk, people have been working on VTOL systems.  The Harrier is not even the most sophisticated example -- but decades and decades of trial-and-error went into it, trying every damnfool thing you can think of, and others besides.  With the results that many different types of VTOL systems exist now - most of them some form of helicopter.

On the other hand, ISS does damping, when running microgravity experiments, by the expedient of telling the crew they can't exercise, and BTW, try not to bounce off any walls for the next couple of hours, OK?

Skylab's second and third crews were told not to run around the water tanks like Pete's crew did, because it was stuff like that that caused the CMGs to wear out way faster than anticipated.  And CMG technology is really no more advanced today than it was 45 years ago, I don't believe.  Especially enormous CMGs.

"Intelligent" damping can only accommodate so much moving and bumping around, and you can't keep a shipload of people still enough to bring the excursions down to anywhere close to the kinds of damping algorithms that are used in satellites and such.  And there has been no -- zero -- systems designed to keep two masses, with movements going on constantly within them, stable spinning around a flexible tether.  Because, unlike VTOL systems, there has never been a demand to develop such a thing.

So, once again -- as far as spinning two spacecraft around on a tether, we are still flying biplanes.  And will continue to do so until and unless someone actually addresses the extremely complex damping needed for such a dynamic and essentially chaotic system.  (How is your intelligent damping system gonna handle it when, out of pure chance, all of the people on the port side of one ship decide to flush their toilets within 15 seconds of each other?  That's gotta be a ton or more of flush-water moving around...)

I truly believe that this will end up being a lot harder than even the people who already think it will be hard really apprehend...
-Doug  (With my shield, not yet upon it)

Online KelvinZero

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On the other hand, ISS does damping, when running microgravity experiments, by the expedient of telling the crew they can't exercise, and BTW, try not to bounce off any walls for the next couple of hours, OK?
The reason I brought up the COLBERT was this:
https://en.wikipedia.org/wiki/Treadmill_with_Vibration_Isolation_Stabilization

The rest is above my pay grade. Might make a great cubesat project, though I think that today you would basically know from simulations. I have seen simulations of how badly undamped cables can behave somewhere, probably on this site.

(edit)
Hey, has anyone mentioned that although this may prove unnecessary for mars, it could be a useful experiment in LEO? I don't think ITS is going to go haring off to mars as soon as it is built. I see all sorts of shakedown runs and other work, and maybe this could include a test with people and ECLSS for some months in mars gravity.

If mars gravity is not good enough for health, there are pretty good reasons to want to know this early.
« Last Edit: 09/12/2017 02:24 AM by KelvinZero »

Offline the_other_Doug

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On the other hand, ISS does damping, when running microgravity experiments, by the expedient of telling the crew they can't exercise, and BTW, try not to bounce off any walls for the next couple of hours, OK?
The reason I brought up the COLBERT was this:
https://en.wikipedia.org/wiki/Treadmill_with_Vibration_Isolation_Stabilization

The rest is above my pay grade. Might make a great cubesat project, though I think that today you would basically know from simulations. I have seen simulations of how badly undamped cables can behave somewhere, probably on this site.

(edit)
Hey, has anyone mentioned that although this may prove unnecessary for mars, it could be a useful experiment in LEO? I don't think ITS is going to go haring off to mars as soon as it is built. I see all sorts of shakedown runs and other work, and maybe this could include a test with people and ECLSS for some months in mars gravity.

If mars gravity is not good enough for health, there are pretty good reasons to want to know this early.

I hear you -- and even COLBERT has had some issues; it's a great improvement, but even its use is not allowed during some experiment runs.

And I agree wholeheartedly with your idea of running a significant series of tests of such systems nearer Earth than in truly deep space.  I wonder if it would be better to run them outside of LEO, though, maybe at a LaGrange point.  In LEO, you get such relatively strong gravity gradients that tethers much longer than 20 feet start curling, based on one end of the tether being in a different orbit than the other.  Spinning the pair just means this dynamic would change as the assembly moved around the center point; I'd be a bit concerned that this would inject a lot of variability and instability into the system that you wouldn't see in deep space.  No need to over-engineer the system to handle problems it won't see en route to Mars, after all.

Also, tethers run through Earth's magnetic field can build up a tremendous electrical charge -- witness the demise of the Shuttle's tethered satellite experiment.  I'm pretty certain that the Sun's magnetic field strength, at least between Earth and Mars, is quite a bit weaker than Earth's is while in LEO.  So, again, that would be a complicating factor you could dispense with by testing at a LaGrange point.

In all, testing stability and damping systems for such a rotation system during a nice, long test flight of your first two Mars-sized ITS'es at L2 or L4 might be the best plan.  Heck, sell seats for the tests at cut-rate prices, for a realistic test of the ECLSS and the damping systems, seeing as how they'll need to deal with lots of people moving around and doing, well, all the things that people do... :)

If the price is right, I'd sure buy a seat.  I'll be retiring in the next few years, and I can't think of anything better to do with my "golden years" than to act as a guinea pig for Elon.  :D
-Doug  (With my shield, not yet upon it)

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I'd ask y'all to recall that the only experiments that I know of that have ever been done with spinning up two vehicles around a tether happened 51 years ago, on Geminis XI and XII.  They found that, on the mass scales and speed of movement they were testing, the recoil force from the tether itself caused the tether to have completely unacceptable dynamics, and even slight movements by the crew would take what appeared to be a stable tether dynamic and push it quickly into a series of disastrous loops.

The problem seems to be that you need the tether to be at full stress before you spin up, in order for it to act as if it is a solid, unbending link between the two end-point masses.  If you at any point stop thrusting the two masses away from each other, along the tether vector, the tether will recoil, start pulling the two masses back together, and start looping like crazy.  The spin-up maneuver, involving as it would need to the continuous thrust to apply pressure along the tether, but decreasing as the spin replaces the thrust to apply tension to the tether, is of such complexity that it would need to be demonstrated several times before you could consider using it operationally.

So -- those who believe that there needs to be an awful lot more real-world testing, in space, of the dynamics of such tethered systems before such a thing is designed into the cruise architecture of a Mars mission are the winners of this discussion at present, IMHO... :)

Please help me get my head around this.

1) Based on the Gemini Tether Experiment (GTE) it would appear that tethers don't work in the context of them being inherently stable.

2) So I went off to do some more reasearch and looked into it a bit more. According to wikipedia the GTE only got up to 0.00015g
https://en.wikipedia.org/wiki/Gemini_11#Scientific_experiments
"The passive stabilization experiment proved to be a bit troublesome. Conrad and Gordon separated the craft in a nose-down (i.e., Agena-down) position, but found that the tether would not be kept taut simply by the Earth's gravity gradient, as expected. However, they were able to generate a small amount of artificial gravity, about 0.00015 g, by firing their side thrusters to slowly rotate the combined craft like a slow-motion pair of bolas.[4]"

3) However even at this very low AG effect the NASA spiel states the following,
https://nssdc.gsfc.nasa.gov/nmc/masterCatalog.do?sc=1966-081A
"The hatch was closed at 9:57a.m. and shortly afterwards the spacecraft were undocked and Gemini 11
moved to the end of the 30 meter tether attaching the two spacecraft. At 11:55 a.m. Conrad initiated a slow rotation of the Gemini capsule about the GATV which kept the tether taut and the spacecraft a constant distance apart at the ends of the tether. Oscillations occurred initially, but damped out after about 20 minutes. The rotation rate was then increased, oscillations again occurred but damped out and the combination stabilized. The circular motion at the end of the tether imparted a slight artificial "gravitational acceleration" within Gemini 11, the first time such artificial gravity was demonstrated in space. After about three hours the tether was released and the spacecraft moved apart."

Saying that the GTE is therefore proof that tethers are unstable, given the extremely low AG effect attained, I think is unfair. (I admit this is my gut feel and is completely unsubstantiated.)

Is it wrong to say, that the greater the tension in the cable, the more stable the system will become?

Offline Eric Hedman

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Is it wrong to say, that the greater the tension in the cable, the more stable the system will become?
It would at this point be unsubstantiated to say this.  It might be true, but at this point tests with larger spacecraft at greater tension and faster rotation need to be done before that assertion can be made.  In larger craft the mass of the human body will be moving around potentially inducing oscillations.  Determining the natural frequency of such a system would be an interesting problem.  It might become necessary to put some kind of dampening system in place.  Of course we won't know unless we try.  The first thing to do is to try to create a computer simulation.

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I wonder if having a rigid pole attached to the nose of the spacecraft, with the tether attached to the end of the pole would help keep things stable?

Online Peter.Colin

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I wonder if the use of the crane to lift the nose tethered BFS, onto the BFR.
And the oscillations caused by that action are indicative for what would happen in space?
 
« Last Edit: 09/17/2017 10:50 AM by Peter.Colin »

Online Aussie_Space_Nut

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Offline corneliussulla

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I think Elon stated that the nose would be pointing towards the Sun during flight. I expect for a couple of reasons 1 so solar array could get most energy 2) to limit the effects of solar energy on fuel required for landing. Also if you spin up the ships would mean the solar arrays would then need to withstand partial G forces. Great idea but probably not realistic

Offline Paul451

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Also if you spin up the ships would mean the solar arrays would then need to withstand partial G forces.

It could make deployment easier, since the (presumably thin-film arrays) wouldn't need a stabilising structure to deploy them, they'd just unroll and "hang", then be rolled back up before despin and landing.

Offline Negan

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Also if you spin up the ships would mean the solar arrays would then need to withstand partial G forces.

I don't see how the arrays could be so flimsy as to not be able to handle this. The arrays will have to be deployed and retracted hundreds of times during the ship's lifetime.
« Last Edit: 10/16/2017 10:05 PM by Negan »

Online Coastal Ron

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I think Elon stated that the nose would be pointing towards the Sun during flight. I expect for a couple of reasons 1 so solar array could get most energy 2) to limit the effects of solar energy on fuel required for landing.

During his recent AMA on Reddit Musk said it was to keep the propellant from boiling, since the tanks are not pressurized.

Quote
Also if you spin up the ships would mean the solar arrays would then need to withstand partial G forces. Great idea but probably not realistic

As currently designed the solar panels would not operate properly if two BFS were connected via their noses and spun.

Also, I'm not sure anyone knows if there are structural attachments at the nose - which if there isn't then there isn't much to discuss...  :o
If we don't continuously lower the cost to access space, how are we ever going to afford to expand humanity out into space?

Offline envy887

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I think Elon stated that the nose would be pointing towards the Sun during flight. I expect for a couple of reasons 1 so solar array could get most energy 2) to limit the effects of solar energy on fuel required for landing.

During his recent AMA on Reddit Musk said it was to keep the propellant from boiling, since the tanks are not pressurized.

Quote
Also if you spin up the ships would mean the solar arrays would then need to withstand partial G forces. Great idea but probably not realistic

As currently designed the solar panels would not operate properly if two BFS were connected via their noses and spun.

Also, I'm not sure anyone knows if there are structural attachments at the nose - which if there isn't then there isn't much to discuss...  :o

It's a convenient place to lift for vertical integration at the pad, which is required if the booster lands on and stays on the pad.

Offline intrepidpursuit

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Apollo astronauts reported that the capsule was barely livable in long duration tests on the ground, but much more spacious in freefall. Anyone landing on mars will be in space 3-6 months and then on mars for 18+ months. Gravity during the flight will cause more harm than good imo.

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