LEO Space Station with Artificial Gravity (w/Discussion of effects on the human body)

Eric Berger: Meet the space billionaire who is interested in something other than rockets

McCaleb's space habitation company, Vast, emerged publicly last fall with a plan to build space stations that featured artificial gravity. This was significant because NASA and most other space agencies around the world have devoted little time to developing systems for artificial gravity in space, which may be important for long-term human habitation due to the deleterious effects of microgravity experienced by astronauts on the International Space Station. Vast boasted three technical advisers who were major players in the success of SpaceX—Hans Koenigsmann, Will Heltsley, and Yang Li—but did not offer too much information about its plans.

And there still isn’t much information but it’s fun to think about. I’ve been waiting over five decades now. It certainly won’t be this big, but it has to be at least several hundred meters in diameter.

 

[bxr140]

Why should the tether itself need to control the system?

It needs to enable control of the system. The two starships can provide the actual control, but synchronization of the two of them in order to maintain tenable system attitude and orientation is all but impossible when just a rope is the structural link. Tension needs to be highly controlled, impulse and gyro vectors need to be equal and exactly offset, etc. I appreciate there's a simple elegance to a rope, but it doesn't stand up to first principals scrutiny once you start contemplating actual operational conditions, let alone off-nominal operational conditions...and that's not even getting into "if this keeps going its going to kill astronauts" type of corner cases that can't simply be hand-waved away.

Of course add ons and caveats and hand-waves and operational constraints can be added onto the tether concept to make it more plausible (as you've intimated in a later post), but all that of course adds complexity and mass and risk and pretty quickly racks up to something unclosable...or...a rigid structure...

The good news is that your preferred Zylon material could just as easily be implemented in a rigid structure, and certainly NOT a multiples the mass as you suggest. A big tether acting in tension requires a certain amount of material mass. A rigid structure doesn't need any more mass to manage the same amount of tension; the mass downsides are just whatever's needed on top of that 'tension controlling mass' to control the other dynamic motions of the system as well (bending, compression, torsion, etc), and those dynamics are MUCH smaller magnitude than the tension. AND...there's also mass upside for the rigid structure, since distributing the tension controlling mass over a larger cross section (say, 7m...?) vs the tether's small cross section is more structurally efficient, especially at the connection points to the starships. When all is mathsed out we're probably talking 1.2-1.5x the mass of the ideal physics 101 tether [that doesn't need to act in any other direction] or maybe even less.

For an incalculably easier to build, model, and control system. If Starship's cost and capability and frequency are even close to the aspirational figures, that's a First Principals No Brainer.

 

[Ben W]

Very different situations: Piano strings are anchored to the piano at both ends, and the damper is attached to the piano as well. With both end points and the damper attached to a single rigid structure (the piano in this case) you can dampen vibration effectively. In the case of a tethered pair of space ships the "damper" is not a damper at all: It's just an ornament attached to the tether.

Presumably the damping structure could also be attached to Starship, or at least to the catch arms, and not just floating loose on the tether. But even floating loose, it would have some damping effect, absorbing some of the vibrational energy via friction and converting it to heat. The amount of damping required at the actual resonant frequency of the tether would probably be quite small. Larger-scale oscillations and wobbles of the entire dynamical system (not just the tether) would almost certainly be much more problematic, and of course those are unlikely to be solvable by any passive damping system, or even a lightweight (relative to the tether) rigid structure.

If Starship is successful it will reduce costs. I don't believe it will cut them enough to make a Mars mission, or even a LEO rotating space station economical. And we need the low-gravity space station operating for at least a decade to conduct the needed tests before a Mars mission. My argument is that even with Starship, a sufficiently large space station is many decades away. And that while we will probably send people to mars for bragging rights, or possibly research, a self-sustaining colony will never be economical.

To be clear, my optimistic 20-year timeline is for the first flags-and-footprints mission, not for permanent settlement. Although I do think that robotic construction missions, combined with AI advances, will make permanent settlement exponentially more achievable in a shorter timeframe than might be otherwise expected.

Well, you are more optimistic than I am about the costs of sending people to space and keeping them alive there. Note that robotic missions can be designed with a simple cost:benefit analysis wherein if sending five missions and losing three of them is cheaper than sending two with a 99.99% probability of success, you can go with the former. With manned missions you have to go with the latter. And increasing probability of success near the upper end is massively expensive.

The very first crewed missions will undoubtedly be quite risky. If we wait for a 99.99% probability of success before making the attempt, I agree we will be waiting a much longer time. (And would undoubtedly be scooped by others who are willing to accept a 99% or 90% or even 50% chance of success.)

 

[Ben W]

It needs to enable control of the system. The two starships can provide the actual control, but synchronization of the two of them in order to maintain tenable system attitude and orientation is all but impossible when just a rope is the structural link. Tension needs to be highly controlled, impulse and gyro vectors need to be equal and exactly offset, etc. I appreciate there's a simple elegance to a rope, but it doesn't stand up to first principals scrutiny once you start contemplating actual operational conditions, let alone off-nominal operational conditions...and that's not even getting into "if this keeps going its going to kill astronauts" type of corner cases that can't simply be hand-waved away.

Of course add ons and caveats and hand-waves and operational constraints can be added onto the tether concept to make it more plausible (as you've intimated in a later post), but all that of course adds complexity and mass and risk and pretty quickly racks up to something unclosable...or...a rigid structure...

The good news is that your preferred Zylon material could just as easily be implemented in a rigid structure, and certainly NOT a multiples the mass as you suggest. A big tether acting in tension requires a certain amount of material mass. A rigid structure doesn't need any more mass to manage the same amount of tension; the mass downsides are just whatever's needed on top of that 'tension controlling mass' to control the other dynamic motions of the system as well (bending, compression, torsion, etc), and those dynamics are MUCH smaller magnitude than the tension. AND...there's also mass upside for the rigid structure, since distributing the tension controlling mass over a larger cross section (say, 7m...?) vs the tether's small cross section is more structurally efficient, especially at the connection points to the starships. When all is mathsed out we're probably talking 1.2-1.5x the mass of the ideal physics 101 tether [that doesn't need to act in any other direction] or maybe even less.

For an incalculably easier to build, model, and control system. If Starship's cost and capability and frequency are even close to the aspirational figures, that's a First Principals No Brainer.

The appeal of the tether is that the load-bearing attachment points already exist in the Starship design, in the form of the catch arms. A "suspension" to make the tether very slightly springy would certainly be necessary; it wouldn't just be a "dumb" rope. An alternative design would be to have two parallel tethers, one for each arm, which would give an extra degree of control if they could be independently tensioned, like a stunt kite. (Or a marionette-like arrangement of four tethers, as in kite-surfing, which would give both greater control and a larger effective cross-sectional area.) I am skeptical however that a passive rigid structure of 300m length (and massing only a fraction of the tether's mass) would be able to effectively damp out "astronaut-killing" amplitude off-nominal oscillations without itself breaking. The active control system will have to work, lightweight rigid connection or not.

If small thrusters to spin-up and spin-down the tether are positioned properly (at the center of percussion, pointing in the direction of rotational motion), they would have near-zero effect on the tether's tension when applied. If you're envisioning spinning up or down the system rapidly, using Raptor- or Merlin-size thrusters, then sure, that's begging for trouble. But spinning up to e.g. Mars gravity over a period of hours or days with tiny thrusters (even ion thrusters) should be a gradual enough process that oscillations could be caught or damped while still very small. And in the event of a large failure, say a meteor impact or unplanned large thruster firing, it's not clear (to me at least) that a lightweight rigid structure would do much if any better.

 

[daniel]

The appeal of the tether is that the load-bearing attachment points already exist in the Starship design, in the form of the catch arms. A "suspension" to make the tether very slightly springy would certainly be necessary; it wouldn't just be a "dumb" rope. An alternative design would be to have two parallel tethers, one for each arm, which would give an extra degree of control if they could be independently tensioned, like a stunt kite. (Or a marionette-like arrangement of four tethers, as in kite-surfing, which would give both greater control and a larger effective cross-sectional area.) I am skeptical however that a passive rigid structure of 300m length (and massing only a fraction of the tether's mass) would be able to effectively damp out "astronaut-killing" amplitude off-nominal oscillations without itself breaking. The active control system will have to work, lightweight rigid connection or not.

If small thrusters to spin-up and spin-down the tether are positioned properly (at the center of percussion, pointing in the direction of rotational motion), they would have near-zero effect on the tether's tension when applied. If you're envisioning spinning up or down the system rapidly, using Raptor- or Merlin-size thrusters, then sure, that's begging for trouble. But spinning up to e.g. Mars gravity over a period of hours or days with tiny thrusters (even ion thrusters) should be a gradual enough process that oscillations could be caught or damped while still very small. And in the event of a large failure, say a meteor impact or unplanned large thruster firing, it's not clear (to me at least) that a lightweight rigid structure would do much if any better.

I suspect that we're agreed that any system of two attached space ships, rigid or tethered, would be extremely difficult to control and maintain safe.

And I'll add that a "springy" tether, as suggested above, would result in continual, irregular rises and drops in the amount of gravity. It would be like standing on a trampoline while a dog trots around on it. You'd be constantly getting bounced. This would be disorienting, and while not as bad as microgravity, would make many people motion sick.

The classic ring-shaped sci-fi space station would probably be the only way to achieve a really comfortable artificial gravity, as its astronomical mass would indeed dampen out fluctuations, and its hundreds of meters size would reduce the head-to-toe differential. But such a space station will have a price tag proportional to its mass.

One problem with translating sci-fi to reality is that sci-fi writers don't have to answer for the cost of their imagined projects. Printing books is a lot cheaper than building space stations, and hosting e-books on a server is a lot cheaper than that.

 

[Ben W]

I suspect that we're agreed that any system of two attached space ships, rigid or tethered, would be extremely difficult to control and maintain safe.

But then again, computers are exceptionally good at "extremely difficult" control problems like this. (Such as launching and landing Falcon 9 boosters.) If all the required degrees of freedom are present, with redundancy, then it should be doable.

And I'll add that a "springy" tether, as suggested above, would result in continual, irregular rises and drops in the amount of gravity. It would be like standing on a trampoline while a dog trots around on it. You'd be constantly getting bounced. This would be disorienting, and while not as bad as microgravity, would make many people motion sick.

"Springy" was a poor choice of word. The purpose of the suspension, as it is with automobile suspensions, is to actively damp out oscillations (and to smooth out high-frequency jolts), not to magnify them. For inspiration, consider the design of the Arecibo telescope, with a 900-ton suspended observatory above the dish, that had to be resistant to earthquakes and the like. (For the years that it was properly maintained, it did just fine.) For a tether system, the purpose would be to prevent unexpected oscillations and jolts on one end from reaching the other. Airplane rides have a lot of "bouncy" movement, and most people adapt just fine to long-haul flights. But in a spinning system, tether or otherwise, Coriolis effects would almost certainly be far more problematic than high-frequency oscillations, from a motion sickness perspective.

The classic ring-shaped sci-fi space station would probably be the only way to achieve a really comfortable artificial gravity, as its astronomical mass would indeed dampen out fluctuations, and its hundreds of meters size would reduce the head-to-toe differential. But such a space station will have a price tag proportional to its mass.

As well as being impossible to implement en route to Mars. The greatest advantage of the tether is its portability and simplicity of deployment. (Assuming the control and stability issues are solvable, which I think they are.)

One problem with translating sci-fi to reality is that sci-fi writers don't have to answer for the cost of their imagined projects. Printing books is a lot cheaper than building space stations, and hosting e-books on a server is a lot cheaper than that.

It was also cheap to write in the New York Times in Oct 1903 that: "to build a flying machine would require the combined and continuous efforts of mathematicians and mechanicians from one million to ten million years." Occasionally, reality surprises and outpaces fiction.

 

[daniel]

"Springy" was a poor choice of word. The purpose of the suspension, as it is with automobile suspensions, is to actively damp out oscillations (and to smooth out high-frequency jolts), not to magnify them.

But an automobile suspension has rigid points of contact with the earth, which can be considered as motionless. Except during earthquakes where it fails spectacularly. Without that contact, I don't see a dampening system on a space tether working very well.

Airplane rides have a lot of "bouncy" movement, and most people adapt just fine to long-haul flights.

I don't know anybody who enjoys a long-haul airplane flight. I cannot read on an airplane without getting airsick!

As well as being impossible to implement en route to Mars.

Where I don't believe we're ever going to go due to the multitude of problems, the cost, and the lack of anything of value to be extracted there.

It was also cheap to write in the New York Times in Oct 1903 that: "to build a flying machine would require the combined and continuous efforts of mathematicians and mechanicians from one million to ten million years." Occasionally, reality surprises and outpaces fiction.

This is the "They Laughed at Fulton" fallacy. Or to put it another way: "One time people laughed at something that turned out to work, so my idea, whatever it may be, is just as valid as that."

In fact, people didn't think heavier-than-air flight was possible because a sufficiently powerful and lightweight motor didn't yet exist. I don't know about that NYT article, but virtually NOBODY thought that anything but a powerful enough, and lightweight enough motor stood in the way of heavier-than-air flight. And in fact, at the date you cite, flying machines did in fact exist, but they were lighter-than-air machines. Check out the excellent series on the beginnings of aviation on the Our Fake History podcast. There was a guy flying around Paris from pub to pub on a one-man steerable, pedal-powered balloon.

 

[Ben W]

But an automobile suspension has rigid points of contact with the earth, which can be considered as motionless. Except during earthquakes where it fails spectacularly. Without that contact, I don't see a dampening system on a space tether working very well.

Let's back up a bit. Rigid points of external contact are only required for damping out absolute "DC" inertia; linear or rotational. For the "AC" components of inertia (e.g. vibration, wobble), a rigid point of external contact is not needed, only a damper that converts the vibrational motion to heat via friction. The damper doesn't need to be grounded to an external object to do this; other parts of the vibrating object will do. But it works best for higher frequencies (including a tether vibrating at 15Hz), and is more difficult for low-frequency (<1Hz) higher-amplitude wobbles, for which a better solution is probably gyroscopes/flywheels or thrusters. But those don't need a rigid external connection either.

Think of Bruce McCandless' multi-hour untethered spacewalk. He did not oscillate uncontrollably during that spacewalk, despite not being rigidly connected to anything. (Or if parts of him did oscillate uncontrollably, he kept that to himself.) I suspect that the resonant modes of a tether system would not be all that difficult to identify and control, especially since it can be initially tested at smaller scale and at very low rates of spin, where the dynamic contribution of the tether is arbitrarily small.

FWIW, automobile suspension actually works quite well in a moderate earthquake. As a kid I recall carpooling to school during a magnitude 6 quake, and no one in our car felt it, though everyone on the playground did. A magnitude 9 earthquake would be a different story of course; much higher amplitude and lower frequency, which a car suspension is not equipped to block.

I don't know anybody who enjoys a long-haul airplane flight. I cannot read on an airplane without getting airsick!

It sounds like space travel is probably not for you, as it is likely not for most people. But that still leaves millions of people whom it would be perfectly fine for. I can't read either for long in the back seat of a car, but flights don't bother me. Vive la difference!

 

[bxr140]

The appeal of the tether is that the load-bearing attachment points already exist in the Starship design, in the form of the catch arms.

Is it really that appealing though? I think we can all agree SX will build starship variants for different missions; I don't think anyone would pushback on the notion that it will be inconsequential for SX to build an "AG" variant that has structurally appropriate attach points elsewhere on the vehicle.

The tether-on-arms concept also requires the vehicles to fly raptor-out. This both a) results in a much harder to control inertial environment (since the mass of the SS's is farther out) and so the control system needs to be enhanced accordingly, and b) creates a situation where you're wasting existing on orbit "structural length" for the connection (a SS has ~30m of structure between the base of the vehicle and the "base" of the payload bay/habitat). In other words, even in a situation where you actually tether two SS's vs rigidly attach them you'd want the vehicles to fly raptor-in...and thus you'd attach the tether to the base of the vehicles and not the catch arms.

As an aside, if you connect two starships ~raptor-to-raptor and spin the system at a tenable ~3rpm, that puts the habitats at ~Mars gravity. ~2rpm ends up being ~Moon gravity. (There's actually a pretty significant gravity gradient across the "height" of the habitats, but I digress...). For these kinds of early missions that go before dedicated habitats are available, that sure does seem to land squarely in the Job Done column...

(Though I get that we're talking about a hypothetical 300m separation and earth gravity.)

A "suspension" to make the tether very slightly springy would certainly be necessary; it wouldn't just be a "dumb" rope. An alternative design would be to have two parallel tethers, one for each arm, which would give an extra degree of control if they could be independently tensioned, like a stunt kite. (Or a marionette-like arrangement of four tethers, as in kite-surfing, which would give both greater control and a larger effective cross-sectional area.)

Indeed. As one contemplates top-of-the-V type mission elements one quickly get to the point where a rigid structure makes total sense over a floppy tether with all kinds of complicated designs and caveats and constraints. Damping becomes a non issue in all conditions with a rigid structure; modes analysis is Structures 101. Mass (again) likely ends up a little higher than the high-scool-physics theoretical tether, and really is all but a non-issue lifting anyway.


I appreciate your passion for your solution here, but you're really ramming it into the bottom of the V. Seriously, using two starships is a great idea. Take that win. Using a tether clearly is a bad idea. Don't sweat that loss.

If small thrusters to spin-up and spin-down the tether are positioned properly (at the center of percussion, pointing in the direction of rotational motion), they would have near-zero effect on the tether's tension when applied.

They'd always have to synchronously act through their own Starship's cg, coaxial with the cg's velocity vector. That's a pretty serious over-constraint on ops...

 

[Ben W]

I don't know about that NYT article, but virtually NOBODY thought that anything but a powerful enough, and lightweight enough motor stood in the way of heavier-than-air flight.

Not to put too fine a point on it, but you're saying that the _only_ concern of engineers regarding powered flight was about finding a powerful enough engine to get off the ground, and _nothing at all_ about stability or control or vibration or wobbling issues while in the air not rigidly connected to any stationary object? Interesting.

Is it really that appealing though? I think we can all agree SX will build starship variants for different missions; I don't think anyone would pushback on the notion that it will be inconsequential for SX to build an "AG" variant that has structurally appropriate attach points elsewhere on the vehicle.

No doubt there will be specialized variants, but given that Starship already has the load-bearing attachment points where they are (and presumably so will all Mars-bound Starships, because that's how they're hoisted onto the Earth launch mount and how they would be caught when returning to Earth), it would just add unnecessary mass and complexity to add additional ones elsewhere.

The tether-on-arms concept also requires the vehicles to fly raptor-out. This both a) results in a much harder to control inertial environment (since the mass of the SS's is farther out) and so the control system needs to be enhanced accordingly, and b) creates a situation where you're wasting existing on orbit "structural length" for the connection (a SS has ~30m of structure between the base of the vehicle and the "base" of the payload bay/habitat). In other words, even in a situation where you actually tether two SS's vs rigidly attach them you'd want the vehicles to fly raptor-in...and thus you'd attach the tether to the base of the vehicles and not the catch arms.

Having the mass farther out makes the system more stable, not less stable. It may be counterintuitive, but it's much easier to vertically balance a baseball bat on your hand with the barrel up (center of mass farther from the pivot) than barrel down. The center of mass of each Starship is closer to the engines than to the nose, making the nose-in spinning configuration more dynamically stable than nose-out. The extra 30m distance is not too significant anyway when you're talking about a 300m tether.

As an aside, if you connect two starships ~raptor-to-raptor and spin the system at a tenable ~3rpm, that puts the habitats at ~Mars gravity. ~2rpm ends up being ~Moon gravity. (There's actually a pretty significant gravity gradient across the "height" of the habitats, but I digress...). For these kinds of early missions that go before dedicated habitats are available, that sure does seem to land squarely in the Job Done column...

I suspect that the optimal gravitational environment for Mars transit (for the health of the crew members) will turn out to be somewhat closer to Earth gravity, and somewhat slower than 3rpm, but agreed that tail-to-tail spinning could be a potential halfway solution. One downside is that the crew compartment (and entire ship) would have to be designed to operate in three gravitational regimes: nose-up (for launch), nose-down (for spinning), and heatshield-down (for reentry). Whereas nose-inward tethered spinning would simplify the design of the crew compartment because the "floor" is always in the same place, and I'd speculate there may be additional reasons not to want fuel to be gravitationally [centripetally] pulled away from the engines.

Indeed. As one contemplates top-of-the-V type mission elements one quickly get to the point where a rigid structure makes total sense over a floppy tether with all kinds of complicated designs and caveats and constraints. Damping becomes a non issue in all conditions with a rigid structure; modes analysis is Structures 101. Mass (again) likely ends up a little higher than the high-scool-physics theoretical tether, and really is all but a non-issue lifting anyway.

It's a stretch (no pun intended) to call a tether under several hundred tons of tension "floppy", but ok. I'm still pretty sure that a 300m solid connection rigid enough not to have similar damping issues as a tether would incur a very significant mass penalty. For a point of reference, a 300m tall radio tower on Earth (which appears extremely lightweight and slender) typically masses about 220 metric tons, versus about 3 metric tons for an equivalent 300m Zylon tether capable of comfortably supporting two Starships at 1g tension. Obviously it's not apples-to-apples (e.g. the radio tower is under compression, not tension), and you may be able to make it an order of magnitude lighter than that, but probably not two orders of magnitude. Not to mention the difficulties in deploying and constructing such a thing en route to Mars. In any case, it's a starting point for napkin math.

I appreciate your passion for your solution here, but you're really ramming it into the bottom of the V. Seriously, using two starships is a great idea. Take that win. Using a tether clearly is a bad idea. Don't sweat that loss.

If I were taking your and daniel's technical concerns at face value, I'd be right there with you. But I don't think many of them are accurate, hence the pushback. (It's been a while since my college engineering and theoretical mechanics days, but I do have a technical career and try to keep in practice.) It's also occurred to me that Starship has an _excellent_ built-in damping mechanism for tether-induced wobbles and vibrations, which is simply the tons of onboard liquid fuel + baffles. Any oscillations in the overall system will quickly be converted to sloshing and turbulence in the fuel, which will just as quickly become non-oscillating heat energy. (Not enough heat energy to problematically warm the fuel, but enough to damp out quite a lot of oscillation.) To be fair, this helps in the rigid-connection scenario as well, but it also makes the tether scenario that much more plausible.

They'd always have to synchronously act through their own Starship's cg, coaxial with the cg's velocity vector. That's a pretty serious over-constraint on ops...

In practice it would probably be a linear combination of firings from several thrusters, so no single one of them would have to be precisely positioned or aimed. Granted, in a sci-fi world where you're envisioning Super-Draco-class thrusters spinning up the system in 30 seconds, it would have to be precisely synchronized, but with small ullage-gas or ion thrusters spinning up the system over hours or days, it wouldn't have to be. A partially-fueled Starship has a LOT of inertia; it won't turn (or even wobble much) on a dime.

 

[daniel]

Not to put too fine a point on it, but you're saying that the _only_ concern of engineers regarding powered flight was about finding a powerful enough engine to get off the ground, and _nothing at all_ about stability or control or vibration or wobbling issues while in the air not rigidly connected to any stationary object? ...

NO!!! Absolutely not!!! I am saying that the only UNSOLVABLE problem that people were concerned about was the lack of a powerful enough and light enough engine. Most people didn't even think about stability, and without engines nobody had any idea about vibration. Bicycles existed and are inherently unstable yet people could control them, and even the very first Wright flyer had control surfaces to deal with control.

If you look at the history of the development of flight, you'll see that nobody ever said "You won't be able to control that," or "That will be unstable," or "That will vibrate too much." People said "That won't fly." And until the invention of sufficiently light-weight and powerful gasoline engines, they were right, regarding heavier-than-air craft. Then, once suitable engines were developed, airplanes were inevitable. And although we in the U.S. credit the Wright brothers with creating the first heavier-than-air flying machine, there are other contenders to the title, just muddied by questions of when exactly each of them actually flew, and how far a plane had to fly to qualify for the official title of "first."

Engineers knew that stability and control were solvable problems, and the lay public wasn't aware that they were problems at all.

 

[bxr140]

Thx for the chat; it seems like we've reached the point of dancing around fundamentals so I'll just summarize instead of point-by-pointing:

--The upside of a theoretical tether is that it can mass slightly lower than other structures
--The downsides of a practical tether is that it adds significant (and potentially non-starting) system control complexity, requires significant (and potentially non-starting) operational constraints, and requires additional mass to actually work (dampers, etc.) on top of what the physics-101 equations say
--The upside of a rigid structure is that it minimizes system control complexity and maximizes operational realms.
--The downside of a rigid structure is that it masses slightly higher than a theoretical physics-101 type tether
--Starship is a launch system lauded for its mass capability and cost; if mass is the primary upside talking point around a tether for connecting two starships (and if this thread is any indication, it is), then going with a potentially incrementally heavier rigid structure is kind of a no brainer...

 

[Ben W]

NO!!! Absolutely not!!! I am saying that the only UNSOLVABLE problem that people were concerned about was the lack of a powerful enough and light enough engine. [...]
Engineers knew that stability and control were solvable problems, and the lay public wasn't aware that they were problems at all.

Right. What I was getting at was that in discussing the Starship tether you seemed hugely concerned about all of the stability and control issues, ostensibly because it wouldn't be connected to any stationary object, when all those same concerns apply just as strongly to aircraft. Yet you didn't seem to be concerned about this in the aircraft context at all, which just seemed inconsistent. I agree that stability and control are certainly real and significant problems to be solved, but I think they are eminently solvable in both domains.

 

[Ben W]

Thx for the chat; it seems like we've reached the point of dancing around fundamentals so I'll just summarize instead of point-by-pointing:

Thanks for the chat as well, I've enjoyed it. Will just comment a final time on each point and where we differ; we may have to agree to disagree.

--The upside of a theoretical tether is that it can mass slightly lower than other structures

My contention is that the word should not be "slightly", but "drastically". Another not-to-be-overlooked upside would be the tether's ease of deployment, versus trying to assemble a large rigid structure in space.

--The downsides of a practical tether is that it adds significant (and potentially non-starting) system control complexity, requires significant (and potentially non-starting) operational constraints, and requires additional mass to actually work (dampers, etc.) on top of what the physics-101 equations say.

Granted that it will be more complex than physics-101. But I still contend that the extra contraints and requirements will prove to be (relatively) small and solvable.

--The upside of a rigid structure is that it minimizes system control complexity and maximizes operational realms.

Only if sufficiently rigid, in which case its mass and complexity of deployment may preclude its use in some contexts (e.g. en route to Mars).

--The downside of a rigid structure is that it masses slightly higher than a theoretical physics-101 type tether

Again, I disagree with "slightly". (I would ballpark-estimate 10x.)

--Starship is a launch system lauded for its mass capability and cost; if mass is the primary upside talking point around a tether for connecting two starships (and if this thread is any indication, it is), then going with a potentially incrementally heavier rigid structure is kind of a no brainer...

Starship may make upmass to LEO very cheap, but that is not the only constraint on a Mars mission. Each Starship will have ~100T payload capacity to Mars (even after fully refueling in LEO), much of which will presumably be taken up by crew and life support, and aspirationally the ability for each Starship to serve as a lifeboat for another in an emergency. Any en-route tether system would eat proportionally into that 100T hard limit. My guess is that SpaceX will not want to budget more than ~5T of this precious payload capacity for a not-strictly-essential artificial gravity system, at the expense of food, water, etc. A tether system should fit comfortably within this budget, or certainly a tail-to-tail spin option (cheaper but less effective), but probably not a very long rigid structure. But who knows, SpaceX has surprised me before

 

[Ben W]

Revisiting this thread again, with some high-level observations and notes. Also, some details regarding my estimates for the tether mass for connecting two Starships.

Starship has a dry mass of about 100t, and a payload capacity of 100t. I'm guessing that it would contain about 50t of fuel after achieving LEO, or after Mars-insertion burn (post-refueling), so in the scenarios where it likely to be tethered and spinning, its total mass would be around 250t. Thus a tether connecting two Starships spinning at 1g would need to support a tension of 250t-force. (Not 500t, counterintuitively.)

Dyneema is probably a better choice than Zylon for the tether material; it's slightly stronger, and is much more resistant to UV radiation. It has a tensile breaking strength of about 3.5GPa, which is about 36 tons-force per square centimeter. With a 5x safety factor (working load 20% of breaking strength), the 1g-spinning-Starship scenario would require a tether with a cross-sectiononal area of about 35 square centimeters, or a rope about 2.5 inches in diameter. A 300-meter tether of this material would mass about 1 metric ton. And if only Mars gravity is required, the tether could mass less than 400kg.

My reasons for optimism about the dynamic stability of this arrangement are twofold. First, all external energy inputs to the system would be very gradual, spinning it up slowly over hours if not days. Even course-correction maneuvers (such as dodging space junk) could be gradual. There would be no sharp impulses or shocks. Second, the fuel itself (in a tank with baffles) would provide an ideal damping mechanism to counter any oscillations or resonances. Drinking water in the crew compartment could be stored in a similar way, to provide additional damping closer to the nose. (On a Mars mission I'd guess close to a ton of the payload would be water.)

My only remaining question, really, is this: if you were in the crew compartment of such a Starship, with artificial gravity at 1g (with a period of 20 seconds), and you spun a top on a tabletop, what would happen to the top? Would it gyroscopically flip itself over, or would it adapt to the coriolis rotation quickly enough (via friction with the table) to remain "upright" and spinning? My guess is that it would likely wobble and precess quite a bit, but might manage to stay "upright" through an entire 20s revolution. An interesting physics question, coming soon to a space hotel near you!