Author Topic: Realistic, near-term, rotating Space Station  (Read 945778 times)

Offline Coastal Ron

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Re: Realistic, near-term, rotating Space Station
« Reply #3760 on: 05/27/2023 05:33 am »
I have another idea for docking to a rotating station.

Visiting ship approaches the docking port and noses into a de-spun framework which has various clamps that hold the ship in place. The frame then rotates up to match the station spin rate and lines up the ship’s docking port with a crew access arm that attaches to the ship’s hatch.  Pressurise and transfer.

No rotating seals. Slight adjustments of the ship position by the clamp mechanism could fix any balance problems.

If the central hub of the station was actually cup or tube shaped then the ship would be effectively inside the station and in the plane of rotation, on the axis of spin.  Would this solve any intermediate axis problems ?

Since the visiting vehicle is not rotating, there should be no intermediate axis problems.

The challenge becomes that it only works for visiting vehicles that have a docking port on the nose, which for today likely leaves out the SpaceX Starship.

But I know I plan to have a fleet of vehicles to shuttle cargo and crew between the rotating space station and local non-rotating space stations that I think will be needed for storage and as the primary destination for visiting vehicles. So such a non-rotating port would be an option for those, and other future vehicles that would have docking ports on their nose.
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Online MickQ

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Re: Realistic, near-term, rotating Space Station
« Reply #3761 on: 05/27/2023 12:32 pm »
Ok. I apparently wasn’t clear.  The approaching ship is not rotating when clamped into the catch frame, which has been de-spun to meet the ship.  After the ship has been secured the frame starts spinning up to match the station’s rotation so the ship is now effectively not moving wrt the station but is physically attached to it.

Now it would not matter where on the ship the docking hatch was located as the transfer arm would be flexible/ articulated enough to attach to it, within reason.  There may be more than one arm if a variety of vehicles visited regularly.

Offline Coastal Ron

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Re: Realistic, near-term, rotating Space Station
« Reply #3762 on: 05/27/2023 03:09 pm »
Ok. I apparently wasn’t clear.  The approaching ship is not rotating when clamped into the catch frame, which has been de-spun to meet the ship.  After the ship has been secured the frame starts spinning up to match the station’s rotation so the ship is now effectively not moving wrt the station but is physically attached to it.

Now it would not matter where on the ship the docking hatch was located as the transfer arm would be flexible/ articulated enough to attach to it, within reason.  There may be more than one arm if a variety of vehicles visited regularly.

As long as there is a way to keep the ship from "bending" due to the rotating forces, and in case the center of mass within the visiting vehicle is no longer in the center of rotation, then it should not be be a problem. I have a similar or same mechanism that I plan to use for my rotating space stations.

There is still the challenge that the station itself may not be rotating perfectly. The center or rotation for the station may not be the physical center of the docking port, and the station could have a wobble. As I've outlined previously (been a year or two now since I last discussed this), I plan to account for this by having a movable frame that adjusts on an X and Y axis, so that after the visiting vehicle is captured then it is de-spun and centered to the center of the station docking port. And there is a mechanism to account for the wobble too.

However I don't plan to do this with large vehicle such as the Starship, I only plan to do it with my fleet of local cargo & crew transport vehicles that shuttle between the rotating space station(s) and non-rotating space stations that are the primary port for all visiting vehicles. These "shuttles" would be pretty stout, so bending forces during rotation would not be an issue.

This is the system solution I'm pursuing...
« Last Edit: 05/27/2023 05:04 pm by Coastal Ron »
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Offline mikelepage

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Re: Realistic, near-term, rotating Space Station
« Reply #3763 on: 06/01/2023 11:00 am »
Backtracking to the conversation we were having before the rabbit hole, I've been toying with ways to compare shielding mass for bola/sphere and toroid geometry space stations. If I'm allowed to float an unfinished idea for discussion without being subjected to the Spanish Inquisition, I'd be interested to hear your thoughts.

As Twark_Main has often pointed out, toruses have significantly more surface area than a sphere of equivalent volume, so the first figure is my attempt to map out the relationship for a series of toruses of different minor radii. Y axis is of the major radii those toruses have, in order to have equivalent volume to a sphere of radius indicated on the X axis.

The second figure maps out the ratio of surface areas between the torus at the equivalent point in the first plot, against that of the sphere of equivalent volume.

I think (?) it makes sense to assume that for points on the plot where surface area ratio of torus to equivalent sphere is equal to 2, that means that a toroidal magnetic shield would have to reduce the dose to that achievable by passive shielding, for less than half the mass, to be better than the equivalent sphere.

The key thing about a torus geometry that makes it more amenable to magnetic shielding (if I've understood the papers upthread correctly), is that aligning the superconducting cables along the full circumference of the torus allows the magnetic field produced to be much smaller for a given amount of protection, and the structural reinforcement required to restrain the cables is consequently far less massive.

A first pass at some very kludgy calcs: A 0.64T field (figure 11 of the Slough paper) seems to substantially reduce dose received, although it isn't quantified anywhere in the paper I can see. Generating that field using commercial grade YBCO superconducting cables (see wikipedia) which can carry 700-2000 amps/mm2. I assumed 1000 Amps/mm2. In this case, my BoE implies cables of 32cm2 cross section, massing ~25kg/m. Not sure how many of them we need, so for my calcs I've assumed 8 of them for the 1m minor radius torus, and scaled up linearly with minor circumference (i.e. 40 cables on the r=5m torus). More is better for protection, but has higher power requirements.

As an example, I'm looking at the 4m minor radius torus, and at the point where it has a major radius of 22.92m (so 26.92m-18.92m inner/outer), it's internal volume is equivalent to a sphere of radius 12m. To achieve 20g/cm2 of LH2 shielding (about a halving in exposure) around the sphere, we'd need ~362 ton of LH2 (excluding support structure). Alternatively, for the mass of the superconducting cables, for that torus (which I think also achieves about a halving) I get ~29 tons (excluding support structure). Not exactly apples to apples, but this fits with my intuitive understanding and why I've suggested that torus geometries (plus magnetic shielding) may be more efficient mass-wise than spheres (plus passive shielding).

« Last Edit: 06/02/2023 01:59 am by mikelepage »

Offline zubenelgenubi

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Re: Realistic, near-term, rotating Space Station
« Reply #3764 on: 06/01/2023 11:02 am »
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Offline InterestedEngineer

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Re: Realistic, near-term, rotating Space Station
« Reply #3765 on: 06/02/2023 04:51 pm »
Backtracking to the conversation we were having before the rabbit hole, I've been toying with ways to compare shielding mass for bola/sphere and toroid geometry space stations. If I'm allowed to float an unfinished idea for discussion without being subjected to the Spanish Inquisition, I'd be interested to hear your thoughts.

As Twark_Main has often pointed out, toruses have significantly more surface area than a sphere of equivalent volume, so the first figure is my attempt to map out the relationship for a series of toruses of different minor radii. Y axis is of the major radii those toruses have, in order to have equivalent volume to a sphere of radius indicated on the X axis.

The second figure maps out the ratio of surface areas between the torus at the equivalent point in the first plot, against that of the sphere of equivalent volume.

I think (?) it makes sense to assume that for points on the plot where surface area ratio of torus to equivalent sphere is equal to 2, that means that a toroidal magnetic shield would have to reduce the dose to that achievable by passive shielding, for less than half the mass, to be better than the equivalent sphere.

The key thing about a torus geometry that makes it more amenable to magnetic shielding (if I've understood the papers upthread correctly), is that aligning the superconducting cables along the full circumference of the torus allows the magnetic field produced to be much smaller for a given amount of protection, and the structural reinforcement required to restrain the cables is consequently far less massive.

A first pass at some very kludgy calcs: A 0.64T field (figure 11 of the Slough paper) seems to substantially reduce dose received, although it isn't quantified anywhere in the paper I can see. Generating that field using commercial grade YBCO superconducting cables (see wikipedia) which can carry 700-2000 amps/mm2. I assumed 1000 Amps/mm2. In this case, my BoE implies cables of 32cm2 cross section, massing ~25kg/m. Not sure how many of them we need, so for my calcs I've assumed 8 of them for the 1m minor radius torus, and scaled up linearly with minor circumference (i.e. 40 cables on the r=5m torus). More is better for protection, but has higher power requirements.

As an example, I'm looking at the 4m minor radius torus, and at the point where it has a major radius of 22.92m (so 26.92m-18.92m inner/outer), it's internal volume is equivalent to a sphere of radius 12m. To achieve 20g/cm2 of LH2 shielding (about a halving in exposure) around the sphere, we'd need ~362 ton of LH2 (excluding support structure). Alternatively, for the mass of the superconducting cables, for that torus (which I think also achieves about a halving) I get ~29 tons (excluding support structure). Not exactly apples to apples, but this fits with my intuitive understanding and why I've suggested that torus geometries (plus magnetic shielding) may be more efficient mass-wise than spheres (plus passive shielding).


I know it's a long thread, so maybe I missed it, but why are y'all trying to optimize for such abstract things as volume and surface area?

If Starships costs $10M, then a tube 10-12m long and 9m in diameter costs $10M to LEO.  String about 50 of those together to make a rotating Space Station.   Net cost is probably $2B or so, running at 1/3 gravity and less than 2rpm at about 100m diameter.

Line the inside of tube with 1m of plastic for radiation and you've still got 7-8m of internal diameter for your space station, enough for 2 decks.   Make the tubes completely standard with compartments, accessways, insulation, and isolation airlocks, etc.

Not sure what to do about the engine and tanks, but someone will find a use for them.  Maybe keep 4 of them for ballast to keep things balanced.

Optimize for what's cheap, not for some abstract geometric principles.

Offline Coastal Ron

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Re: Realistic, near-term, rotating Space Station
« Reply #3766 on: 06/02/2023 06:14 pm »
I know it's a long thread, so maybe I missed it, but why are y'all trying to optimize for such abstract things as volume and surface area?

The definition of "near-term" is pretty broad, so some people are looking further into the future where size and location of the rotating space station may benefit from different designs than more near-term designs.

Quote
If Starships costs $10M, then a tube 10-12m long and 9m in diameter costs $10M to LEO.

Separate from the point you are making, as of today the Starship volume is only a max of 8m, and the length of a 8m diameter module could only be 8m long. Elon Musk has stated that they plan to extend the length of the ship, and for me I need a 10m long by 8m diameter module for one of my stations, so we'll see. But as of today 8x8m is the most we can count on.

Quote
String about 50 of those together to make a rotating Space Station.   Net cost is probably $2B or so, running at 1/3 gravity and less than 2rpm at about 100m diameter.

OK, now you are getting into the realm of design tradeoffs, such as having a station with only one pathway around it. So what happens if two separate modules have to be shut down? How safe is that design?

But assuming this was just a "what-if" question, what we are all trying to do is balance a whole bunch of known and theorized issues that we have to take into consideration, like...

Quote
Line the inside of tube with 1m of plastic for radiation and you've still got 7-8m of internal diameter for your space station, enough for 2 decks.   Make the tubes completely standard with compartments, accessways, insulation, and isolation airlocks, etc.

Regarding the standardization, yep, a number of us have been looking at that. As to the radiation protection, boy that is a BIG issue, mainly because no one really knows for sure what the requirements will be, either from a human health perspective (it changes with where the station is at), or from a use case perspective, such as whether people will be living there or just there on short work assignments.

So part of the challenge is that first you have to propose what the use case is, THEN you have to design for the use case.

Quote
Optimize for what's cheap, not for some abstract geometric principles.

I think everyone is already planning to use Starship for near-term rotating space stations. It is pretty much a given, since without Starship no rotating space station would be cheap enough to build.

But a rotating space station can't be built out of a bunch of cylinders strung together, there has to be more than that. So the number of Starship flights will likely be more like 200 for my Mars-gravity station, and bigger ones of course will need more. Which is why many proposing large stations are assuming that in-space resources will be used, like mining asteroids and such.

Hope that helps to bring you up to speed on this thread...  :D
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Offline InterestedEngineer

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Re: Realistic, near-term, rotating Space Station
« Reply #3767 on: 06/02/2023 06:32 pm »

Separate from the point you are making, as of today the Starship volume is only a max of 8m, and the length of a 8m diameter module could only be 8m long. Elon Musk has stated that they plan to extend the length of the ship, and for me I need a 10m long by 8m diameter module for one of my stations, so we'll see. But as of today 8x8m is the most we can count on.

That's internal cargo using reusable Starships.    That's a lot of logististical nightmares.  So don't do that.

I'm talking about making a batch of 50 or so custom Starships.  No heat shields. No header tanks. Separable tank and payload section.  Minimal engines.  Cost to LEO about $10M each.

The payload section custom is a 12m long 9m diameter section with a separable nose cone

The first part of the nose cone contains the slight angular sections needed to join the pieces.  (12 degrees per section).

4 docking ports/airlocks per section.  On the ends, and two in the middle (for the 4 spokes).

Each section also has basic radiation protection (plastic around perimeter), basic electrical and basic plumbing.

Each section also has minimal RCS to automatically move them into position and dock with other sections.

The center is the one custom piece since there are 4 spokes.  Or maybe just have 4 mid-section airlocks on everything.

It should auto-assemble in orbit.

Legos (or more accurately TinkerToy)






Offline Coastal Ron

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Re: Realistic, near-term, rotating Space Station
« Reply #3768 on: 06/02/2023 06:56 pm »

Separate from the point you are making, as of today the Starship volume is only a max of 8m, and the length of a 8m diameter module could only be 8m long. Elon Musk has stated that they plan to extend the length of the ship, and for me I need a 10m long by 8m diameter module for one of my stations, so we'll see. But as of today 8x8m is the most we can count on.

That's internal cargo using reusable Starships.    That's a lot of logististical nightmares.  So don't do that.

Logistical nightmare? If humanity can't perfect the delivery and distribution of large cargo to and thru space, humanity won't be able to expand into space. Luckily we have already shown we have the ability to assemble large cargo in space (i.e. the 450mT ISS), so the vast number of rotating space station designs rely on cargo being delivered on reusable Starships.

Quote
I'm talking about making a batch of 50 or so custom Starships.  No heat shields. No header tanks. Separable tank and payload section.  Minimal engines.  Cost to LEO about $10M each.

LOTS of proposals for using 1-time use Starships, but none turn out to be viable once you dig into the details.

The cost for instance that you quote assumes you are getting a custom Starship at the cost it takes SpaceX to build one of those custom Starships, but you leave out the cost it took SpaceX to customize them, AND you left out the amortization cost that SpaceX has to charge to recoup their $Billions in investment.

So instead of $10M, you are likely looking at more than $100M, versus the cost of shipping a space station module on a reusable Starship for less than $10M.

I couldn't visualize your design, so I don't have any feedback on that, but who is your customer? What do they need, and why is this design the best solution?

Too many times we start engineering solutions before we understand the problem, and rotating space stations is no different. Me included. But I think many of us on this thread have finally started focusing on what the use case is, and using that as the starting point - or backing into it, same difference...  ;)

« Last Edit: 06/02/2023 08:22 pm by Coastal Ron »
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Offline InterestedEngineer

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Re: Realistic, near-term, rotating Space Station
« Reply #3769 on: 06/02/2023 10:59 pm »
Too many times we start engineering solutions before we understand the problem, and rotating space stations is no different. Me included. But I think many of us on this thread have finally started focusing on what the use case is, and using that as the starting point - or backing into it, same difference...  ;)

Alas, the thread title isn't on your side.  It says

Quote
Realistic, near-term, rotating Space Station

Which presupposes there's someone who would pay to be in a rotating something in space, presumably without getting dizzy (or we'd just rotate a Starship at 10rpm and be done)

Offline InterestedEngineer

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Re: Realistic, near-term, rotating Space Station
« Reply #3770 on: 06/02/2023 11:34 pm »
Logistical nightmare? If humanity can't perfect the delivery and distribution of large cargo to and thru space, humanity won't be able to expand into space. Luckily we have already shown we have the ability to assemble large cargo in space (i.e. the 450mT ISS), so the vast number of rotating space station designs rely on cargo being delivered on reusable Starship

That's completely glossing over the root of any space station problem.

Logistics of orbital assembly is the problem to solve esp. with the 'near term' constraint

Offline Coastal Ron

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Re: Realistic, near-term, rotating Space Station
« Reply #3771 on: 06/02/2023 11:35 pm »
Too many times we start engineering solutions before we understand the problem, and rotating space stations is no different. Me included. But I think many of us on this thread have finally started focusing on what the use case is, and using that as the starting point - or backing into it, same difference...  ;)

Alas, the thread title isn't on your side.  It says

Quote
Realistic, near-term, rotating Space Station

Which presupposes there's someone who would pay to be in a rotating something in space, presumably without getting dizzy (or we'd just rotate a Starship at 10rpm and be done)

Yes, but WHO, and WHY?

For instance, I've been designing a number of rotating space stations, and one of them is being designed as a gravity refuge for asteroid miners. It will provide Mars-level gravity, and has lots of space for temporarily housing visitors, plus it will grow some of the food that is needed. And because it is located outside of the radiation protection of a planetary body I am (hopefully) taking into account enough adding shielding for the station and everything inside of it.

Such a station will cost a lot just based on the amount of mass that needs to be moved to the orbit it is needed, but otherwise I have been focused on designing the station using modular manufacturing methods, and "bolt together" construction. Of course there will be lots of things that require humans to finish the construction, but hopefully just connecting things up and starting up operations.

So for this particular station the "WHO" is asteroid miners, and the "WHY" is that they will need/want a place nearby that has gravity that they can rest and recuperate in between long stretches of asteroid mining.

As to why Mars-gravity, I'm thinking that outside of Earth-local space, that Mars-gravity will become one of the standards of gravity that is provided, since it costs less than Earth-gravity, but is (hopefully) good enough for humans to not degrade too much while in space. And there should be a large and growing population of humans that are perfecting how to live in Mars gravity, so we can piggyback on the research and methodology.

So WHO is going to use your Starship component rotating space station, and WHY?
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Offline Coastal Ron

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Re: Realistic, near-term, rotating Space Station
« Reply #3772 on: 06/02/2023 11:37 pm »
Logistical nightmare? If humanity can't perfect the delivery and distribution of large cargo to and thru space, humanity won't be able to expand into space. Luckily we have already shown we have the ability to assemble large cargo in space (i.e. the 450mT ISS), so the vast number of rotating space station designs rely on cargo being delivered on reusable Starship

That's completely glossing over the root of any space station problem.

Logistics of orbital assembly is the problem to solve esp. with the 'near term' constraint

Why do you think it is a problem? We built the 450mT ISS using modular construction techniques, and that has worked out pretty good considering the century the ISS was designed in. With that knowledge it should only be easier to build larger structures in space.

What specific issues are you imagining?
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Offline mikelepage

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Re: Realistic, near-term, rotating Space Station
« Reply #3773 on: 06/03/2023 04:02 am »
I know it's a long thread, so maybe I missed it, but why are y'all trying to optimize for such abstract things as volume and surface area?

If Starships costs $10M, then a tube 10-12m long and 9m in diameter costs $10M to LEO.  String about 50 of those together to make a rotating Space Station.   Net cost is probably $2B or so, running at 1/3 gravity and less than 2rpm at about 100m diameter.
(Snip)
Optimize for what's cheap, not for some abstract geometric principles.

Yes, thread is long, but a common argument over the last 50 pages or so (mainly from Twark Main) was that designing toruses is inherently much less efficient (mass to volume) than baton/bola type stations either contained within a sphere, or at least with the majority of the habitat volume in spheres on each end. Ergo, those of us who think toruses are the way to go are being irrational and have been watching too much sci-fi. I’m sure Twark will have a response to my post when he next comes back to the thread.

I mean, it’s true - toruses are less efficient at containing volume than spheres - but the the dipole magnetic shielding papers have been another source of support for torus geometries, and until recently I never got around to articulating why I think this logically pushes us back to torus type architectures - especially for any vehicle with significant deltaV requirements. It might not be super-near term given the power requirements of those shields, but these first stations will be in LEO anyway.

As for cheapness, the $10m launch prices assume Starship reuse, and Vast is building ~7m modules, to fit within any possible fairing design. Plenty of ex-SpaceX’ers in that company, so I find it curious that they’re sticking to 7m instead of the 8m mentioned in the Starship payload user guide.

Lastly the thread title is one of those things where we’ve never been able to come to a consensus on what it should be called, but most of us regulars treat it as kind of a “catch-all” thread about AG ideas.

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Re: Realistic, near-term, rotating Space Station
« Reply #3774 on: 06/03/2023 01:10 pm »
Optimize for what's cheap, not for some abstract geometric principles.

While I agree that "abstract geometric principles" are sub-optimal in this discussion, your idea of connecting a number of StarShip tubes needs some work. 

Here's why:

I'm talking about making a batch of 50 or so custom Starships.  No heat shields. No header tanks. Separable tank and payload section.  Minimal engines.  Cost to LEO about $10M each.

Clearly these modules would be custom, but you've fogotten the cable that would tie them all together in a ring station.  If not cable, then some structural element in tension.  There would also have to be pressurized sections where astros could live while they work.  There are a number of other requirements that you haven't yet mentioned.

Point being:  $10M per Lego piece is not sufficient.
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Re: Realistic, near-term, rotating Space Station
« Reply #3775 on: 06/03/2023 01:18 pm »
Lastly the thread title is one of those things where we’ve never been able to come to a consensus on what it should be called, but most of us regulars treat it as kind of a “catch-all” thread about AG ideas.

The "catch-all" nature of the thread is a feature, not a bug.  One of the problems with the topic structure here is the overspecialization.  It would not be productive to have three topics:

Realistic, but not near-term or rotating Space Stations
Near-term, but not realistic or rotating Space Stations
Rotating, but not reaistic or near term Space Stations

And so forth.   The way the thread is organized encourages the comparison of the various features of a Space Station.
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Offline LMT

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Re: Realistic, near-term, rotating Space Station
« Reply #3776 on: 06/03/2023 07:40 pm »
A 0.64T field (figure 11 of the Slough paper) seems to substantially reduce dose received, although it isn't quantified anywhere in the paper I can see.

Quote
B00 [relates] the field strength to the shielding effectiveness. 

B00 ≥ 2.2 T [is characteristic for] scaling, current, and structural requirements.

Offline InterestedEngineer

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Re: Realistic, near-term, rotating Space Station
« Reply #3777 on: 06/04/2023 02:28 am »
Optimize for what's cheap, not for some abstract geometric principles.

While I agree that "abstract geometric principles" are sub-optimal in this discussion, your idea of connecting a number of StarShip tubes needs some work. 

Here's why:

I'm talking about making a batch of 50 or so custom Starships.  No heat shields. No header tanks. Separable tank and payload section.  Minimal engines.  Cost to LEO about $10M each.

Clearly these modules would be custom, but you've fogotten the cable that would tie them all together in a ring station.  If not cable, then some structural element in tension.  There would also have to be pressurized sections where astros could live while they work.  There are a number of other requirements that you haven't yet mentioned.

Point being:  $10M per Lego piece is not sufficient.

$10M was the launch cost.  Sorry about the confusion.

They each have airlocks which implies pressurized sections.

I also thought airlocks implied docking but it does not, sorry.  So yes 3 or 6 docking ports.

They are held together by docking.

No humans involved for basic assembly.  Humans finish the interiors.

Offline Coastal Ron

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Re: Realistic, near-term, rotating Space Station
« Reply #3778 on: 06/04/2023 03:38 am »
$10M was the launch cost.  Sorry about the confusion.

As long as you forget the cost to modify the Starships, and forget the amount that SpaceX will charge for expendable Starships - they have to recoup the $Billions they invested, so charging $100M for a custom expendable ship would not be unreasonable.

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They each have airlocks which implies pressurized sections.

I also thought airlocks implied docking but it does not, sorry.  So yes 3 or 6 docking ports.

They are held together by docking.

I was not able to visualize how these 50 or so Starships are arranged to provide Mars gravity (could be me), and I don't need to. But if each Starship is only held together by the docking mechanisms, then they are going to have to be exceedingly reinforced since each ship would be like a link in a chain, and like a chain, it is only as strong as its weakest link.

Which raise the question of what happens if there is a failure of a mechanical connection?  :o

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No humans involved for basic assembly.  Humans finish the interiors.

Considering the total cost of the ISS, the human labor cost was minuscule. You may hope that everything just clicks together, but it won't affect the overall cost if humans need to lend a hand...  ;)
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 Twark_Main

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Re: Realistic, near-term, rotating Space Station
« Reply #3779 on: 06/04/2023 07:39 am »
why are y'all trying to optimize for such abstract things as volume and surface area?

...

Line the inside of tube with 1m of plastic for radiation

Calculate the mass of your 1 meter radiation shield (and amortize that mass over the shielded volume), and you will rapidly realize why we are optimizing for the highest possible ratio of volume-to-surface-area.
« Last Edit: 06/04/2023 07:39 am by Twark_Main »
"The search for a universal design which suits all sites, people, and situations is obviously impossible. What is possible is well designed examples of the application of universal principles." ~~ David Holmgren

 

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