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.

 

[Nikxice]

Thanks for that. (On all the other forums I've been on, posting to a thread that's been inactive for that long is called "necroposting" and is frowned upon. Good to know that's not the case here.)

On the subject of a tethered two-module spinning space station in LEO, moving the station to avoid debris would be extremely complex. A rigid station can be pushed with a thruster at its center of mass (maybe that's not exactly right, but you get the idea). But a tether doesn't allow for that. Both modules would have to be pushed, in exactly the right way to maintain the exact right tension on the tether and keep the modules in exactly the right position, orientation, and spin.

I'm going to guess that that's one reason nobody has tried it.

Was thinking the same thing. Also, another vehicle docking with a module attached to a tether would be a seat of the pants adventure for all parties. Automated thrusters would likely be the only protection to insure survival, versus a potential 3-way disaster. That wild rotating scene involving the ISS in 'Gravity' comes to mind.

 

[daniel]

^ I think the second quote box above is your reply rather than a quote?

And you're right: Docking with a module that's spinning at the end of a tether would be massively complex.

 

[ecarfan]

On the subject of a tethered two-module spinning space station in LEO, moving the station to avoid debris would be extremely complex. A rigid station can be pushed with a thruster at its center of mass (maybe that's not exactly right, but you get the idea). But a tether doesn't allow for that. Both modules would have to be pushed, in exactly the right way to maintain the exact right tension on the tether and keep the modules in exactly the right position, orientation, and spin.

Thanks, I was wondering about that as well. I’m not an aeronautical engineer by any means, but it does seem like maneuvering a tethered two-module rotating station would be quite tricky.

I also wonder about the challenges of docking an incoming vehicle to such a station.

With all that in mind, I’m not sure what the advantages are of a tether as opposed to a rigid connection. I suppose it would be easier to construct. But if SpaceX does build the Starship “clamshell” design like Elon has shown in presentations, the connection sections could easily be lofted to orbit. I found this image; not an official SpaceX image but it shows the concept well. Assuming the diameter of the connecting structure is about 3m, I would guess that four sections could fit into the cargo bay at once. And if a folded up Canadarm type device could be mounted inside the cargo bay near the nose, it could be used for assembly. Maybe no humans need be involved!

 

[bxr140]

On the subject of a tethered two-module spinning space station in LEO, moving the station to avoid debris would be extremely complex. A rigid station can be pushed with a thruster at its center of mass (maybe that's not exactly right, but you get the idea). But a tether doesn't allow for that. Both modules would have to be pushed, in exactly the right way to maintain the exact right tension on the tether and keep the modules in exactly the right position, orientation, and spin.

Yeah. Tethered--at least as far as the definition of 'something that's only materially useful in tension' goes--is certainly a bit of an unfavorable solution. Definitely want a more rigid structure that can act in more dimensions. That said, the two starship concept is pretty ideal as a first step, and so IMO practical implementation has them coupled in the middle by some rigid three dimensional structure. For a Mars mission ideally the geometry/separation enables spinning up the system to mars gravity for a mars mission (there's really not a lot of upside in having the ability to generate more than that) but something less than mars g is still going to be ok-ish, especially for early missions where the crews are well vetted superstars. (Once you get into diaper-across-Florida territory, you might need to better accommodate the transfer mission crew...but I digress...)

IMO a hubless cylinder is the ideal solution for making artificial gravity, though obviously construction is pretty untenable for the foreseeable future. A load bearing hub (including any kind of tether or central structure) concentrates loads. The cylinder instead distributes loads, and can also benefit from preloading the structural components, kinda like a double-walled pressure vessel.

While it's possible a dedicated transfer vehicle precedes this thought, I think the big inflection point on the transfer vehicle is when propellant can be consistently and reliably manufactured on mars. At that point the "Send Starships from Earth's surface to Mars' surface" concept--which is pretty sensible at first--switches to "inefficient".

 

[daniel]

Again here talking about a station in LEO, the problem with a rigid connection instead of a tether is the mass. A tether can be just a very very strong cable. An insignificant amount of mass. But moving it to avoid debris becomes almost impossibly complicated. A fully-rigid structure is trivial to move, but to be rigid it must be MUCH more massive, and now you have all that added mass to put in orbit to build it, and all that mass to move when you need to move it. And it has to be strong enough to withstand all the stresses when you start to move it.

 

[ecarfan]

I agree, but if Starship is the success it is hoped to be, the cost of mass to orbit will go way way down from current prices.

I expect Starship will radically change the economics of building an rotating station in LEO. And if such a station can be designed so that it can be mostly assembled autonomously, to minimize the amount of human EVA time, that will be a huge advantage. I can envision each section of the rotating “ring” having it’s own maneuvering thrusters so that they can dock together on their own.

 

[Ben W]

Again here talking about a station in LEO, the problem with a rigid connection instead of a tether is the mass. A tether can be just a very very strong cable. An insignificant amount of mass. But moving it to avoid debris becomes almost impossibly complicated. A fully-rigid structure is trivial to move, but to be rigid it must be MUCH more massive, and now you have all that added mass to put in orbit to build it, and all that mass to move when you need to move it. And it has to be strong enough to withstand all the stresses when you start to move it.

Agreed that mass is the primary difficulty with making a rigid connection. Even a tether is not insignificant mass; a 300m Zylon tether strong enough to support two mostly-empty Starships at 1g would mass a few metric tons. A useful rigid connection would likely add another order of magnitude, making it infeasible for Earth-to-Mars Starship pairs. For a permanent LEO station that can be docked at while spinning, rigid is certainly the way to go (not least because Zylon degrades over time), but for small-scale orbital experiments a tether is far simpler. And in the event that docking were truly needed, the system could just spin down, dock, transfer, undock, then spin back up. Especially at partial g, not much delta-v is required to do this.

I disagree however that moving the tethered structure to avoid debris would be particularly complicated (relatively speaking). Rapid evasive maneuvers, sure, but in real-world debris scenarios, just a few m/s delta-v with tens of minutes of warning would be plenty to get out of harm's way. Small synchronized burns along the spin axis (by thrusters aligned with the center of mass of each Starship) would have minimal effect on rotational dynamics perpendicular to the thrust, and small pulsed spin-up and spin-down burns by just one Starship at a time could be alternated to provide net directional thrust on the other two axes. E.g. with a 20-second rotational period, the two Starships could alternate small one-second thrusts at 10-second intervals, so each impulse would provide an inertial kick in the same direction. Using this technique, spinning up or down a Starship pair en route to Mars could also double as a course-correction maneuver, so it wouldn't even be wasted fuel.

 

[bxr140]

...the problem with a rigid connection instead of a tether is the mass. A tether can be just a very very strong cable. An insignificant amount of mass.

I think the fundamental disconnect here is the implication that a tether (and again to be overt about it, a member that only acts in tension) as part of a bolas-type system is actually a viable solution. It's kind of analogous to: "why would I need a cybertruck to move 20 sheets of plywood when a model 3 is a much lighter, cheaper, and more efficient EV?".

The issue with the tether is--and you're partially there, to give credit--it really doesn't enable control of the system. There's no torsional control, there's no off axis control, there's no vibrational control. Build a tether robust enough to provide requisite control of the system and you'd probably mass out to an equivalent rigid structure, if not more.

 

[Ben W]

I think the fundamental disconnect here is the implication that a tether (and again to be overt about it, a member that only acts in tension) as part of a bolas-type system is actually a viable solution. It's kind of analogous to: "why would I need a cybertruck to move 20 sheets of plywood when a model 3 is a much lighter, cheaper, and more efficient EV?".

The issue with the tether is--and you're partially there, to give credit--it really doesn't enable control of the system. There's no torsional control, there's no off axis control, there's no vibrational control. Build a tether robust enough to provide requisite control of the system and you'd probably mass out to an equivalent rigid structure, if not more.

Why should the tether itself need to control the system? At each side of the tether will be a fully functional Starship with independent (and mutually synchronizable) active 6DOF maneuverability, so it's perfectly fine for the tether itself to be completely passive. You don't need an active kite string in order to fly a kite, especially not when the kite can fly itself.

 

[scaesare]

Why should the tether itself need to control the system? At each side of the tether will be a fully functional Starship with independent (and mutually synchronizable) active 6DOF maneuverability, so it's perfectly fine for the tether itself to be completely passive. You don't need an active kite string in order to fly a kite, especially not when the kite can fly itself.

You'd need to synchronize the activities of both craft, potentially on multiple axes, in order to address any needed correction. While that may be technically feasible, that feels like an order of magnitude more difficult than landing a rocket. What's more, there's no redundancy should either ship experience a failure.

I tend to agree that the issues @bxr140 brought up are significant enough that something rigid is more likely... at least initially. It wouldn't seem to have to be terribly massive, just structurally sound enough to constrain some axes of movement and exert even a small amount of corrective force would seem to be a big advantage...

 

[wwu123]

You'd need to synchronize the activities of both craft, potentially on multiple axes, in order to address any needed correction. While that may be technically feasible, that feels like an order of magnitude more difficult than landing a rocket. What's more, there's no redundancy should either ship experience a failure.

I tend to agree that the issues @bxr140 brought up are significant enough that something rigid is more likely... at least initially. It wouldn't seem to have to be terribly massive, just structurally sound enough to constrain some axes of movement and exert even a small amount of corrective force would seem to be a big advantage...

Yes, when the entire tether is in uniform tension, at least then it is easy to model (basically like a violin string or a spring) and control. But the nature of a tether, parts of it can dynamically de-tension (not sure if that's the right word) and at different times - then the dynamics become much more complicated, and extremely difficult to control the entire system.

As the discussion shifts towards light, rigid structures, it's interesting to remember that the original, early International Space Station designs were much larger than it is today, though purposefully intended to be zero-g to support micro-gravity research, so to NOT spin. It had much larger truss-like structures, but given the size and needing to be lightweight, it was less completely rigid than flexible. The size had to be scaled back significantly when we, you know, lost the space shuttle, but what I didn't realize is that the large trusses were also scaled back because the vibrations, induced by astronauts among other things, were detrimental to the micro-gravity research goals, which were perhaps the only real justification for building it.

I do remember now that there was significant university research going on then on active control and dampening of truss structures in space. Dampening those vibrations and flexing may be less of a concern for low-g research than no-g research though...

 

[Ben W]

You'd need to synchronize the activities of both craft, potentially on multiple axes, in order to address any needed correction. While that may be technically feasible, that feels like an order of magnitude more difficult than landing a rocket. What's more, there's no redundancy should either ship experience a failure.

I tend to agree that the issues @bxr140 brought up are significant enough that something rigid is more likely... at least initially. It wouldn't seem to have to be terribly massive, just structurally sound enough to constrain some axes of movement and exert even a small amount of corrective force would seem to be a big advantage...

Well, landing a rocket [on a droneship] also requires active synchronization between both systems with no redundancy, so I'd call that a similarity, not a difference. (In this context it's an inconsequential detail that the booster and droneship don't synchronize directly with each other, but rather are remotely synchronized.)

If a ship in a tether scenario experiences a failure where it can't maintain its own stability, it's almost certainly time to un-tether, which would immediately create two much simpler systems. Assuming the failing Starship could then at least null out its own rotation, the other Starship could then fly back and dock directly with it, to e.g. rescue crew if necessary. Presumably each individual Starship would have some level of redundancy where any single point of failure (e.g. a non-working thruster or gyroscope) could be temporarily compensated for by the others, for at least long enough to spin down and safely detach the tether.

A tether system would presumably include an active "suspension" mechanism to damp out longitudinal oscillations. (Either built into the catch arms themselves, or as part of the tether.) In any case, it clearly wouldn't be as simple as just two spaceships tied together with a simple rope. Torsional rigidity is trickier, but perhaps a lightweight telescoping sleeve with active torsion-adjusting rings/motors could surround the Zylon tether, which would have the convenient side-benefit of also protecting the tether from UV degradation. It depends how much torsional damping or force is needed, and of course the connector itself could only damp out relative rotation, not absolute rotation. In the Mars transit scenario, the mass of the tether system would eat directly into the payload capacity, so a fully rigid static connection (e.g. a rigid 300m truss) would almost certainly be prohibitive. Though I acknowledge that for a permanent LEO spinning station, a rigid system is likely the better choice.

 

[daniel]

I tend to agree that the issues @bxr140 brought up are significant enough that something rigid is more likely... at least initially. It wouldn't seem to have to be terribly massive, just structurally sound enough to constrain some axes of movement and exert even a small amount of corrective force would seem to be a big advantage...

I think that to structurally sound enough to keep people safe in an environment where a failure would mean almost certain death of the inhabitants, it would have to be extremely strong, and that would make it very massive.

Yes, when the entire tether is in uniform tension, at least then it is easy to model (basically like a violin string or a spring) and control. But the nature of a tether, parts of it can dynamically de-tension (not sure if that's the right word) and at different times - then the dynamics become much more complicated, and extremely difficult to control the entire system.

Very good point: A tether under tension would likely vibrate like a violin string. This probably would not be good.

... it's interesting to remember that the original, early International Space Station designs were [...] purposefully intended to be zero-g to support micro-gravity research, so to NOT spin.

Another excellent point. The ONLY reason to have people in a LEO space station is to do micro-gravity research. The only reason to have artificial gravity in such a space station would be if it were a tourist hotel. Living quarters on the rim with artificial gravity, and a zero-G area in the hub for play. But we are clearly hundreds of years, maybe very many hundreds, from that. And the energy cost of lifting people and supplies into LEO means that this would only ever be for the uber-wealthy or lottery winners. Buy a ticket for $100 and get a one chance in a thousand of a trip to the space spa.

Artificial gravity on a space journey to another world would benefit from artificial gravity, but we're very, very long way away from that.

In short, a rigid structure is the only realistic way to go for artificial gravity, and there's no use-case for a LEO space station to have artificial gravity.

If a ship in a tether scenario experiences a failure where it can't maintain its own stability, it's almost certainly time to un-tether, which would immediately create two much simpler systems.

If two ships are spinning around each other on a tether, and the tether is disconnected, two things would happen: They would go flying off away from each other at the speed at which they were formerly rotating around each other, possibly into an unstable orbit, and the tether would snap violently, hitting the ship still attached.

... Though I acknowledge that for a permanent LEO spinning station, a rigid system is likely the better choice.

Yep, we're agreed on that. Assuming someone comes up for a reason to have artificial gravity at all. Like a space spa for billionaires.

 

[Ben W]

A tether under tension would likely vibrate like a violin string. This probably would not be good.

The tether would have a "suspension" or other means to actively damp this vibration. The fundamental frequency of this vibration would be fairly high (the speed of sound in Zylon is about 9000m/s), so the fundamental vibration frequency for a 300m tether would be around 15Hz. It's not clear there would be much happening near this frequency on either Starship to amplify this resonance, but in any case it could be damped. (As could other resonant frequencies.)

The ONLY reason to have people in a LEO space station is to do micro-gravity research. The only reason to have artificial gravity in such a space station would be if it were a tourist hotel. Living quarters on the rim with artificial gravity, and a zero-G area in the hub for play. But we are clearly hundreds of years, maybe very many hundreds, from that. And the energy cost of lifting people and supplies into LEO means that this would only ever be for the uber-wealthy or lottery winners. Buy a ticket for $100 and get a one chance in a thousand of a trip to the space spa.

Artificial gravity on a space journey to another world would benefit from artificial gravity, but we're very, very long way away from that.

We are within two decades of that in my estimation. While preparing for such a mission, it would be highly useful to test out said artificial gravity mechanisms (and long-term effects on humans) in LEO first. And Starship will bring down the cost of launch to LEO for such testing tremendously, as well as conveniently providing a platform to test it, if the tether system proves workable.

If two ships are spinning around each other on a tether, and the tether is disconnected, two things would happen: They would go flying off away from each other at the speed at which they were formerly rotating around each other, possibly into an unstable orbit, and the tether would snap violently, hitting the ship still attached.

The Coriolis force would likely prevent the detached end of the tether from hitting the other Starship, even if it snaps back violently. At 50m/s rotational velocity (300m tether at 1G), the still-attached Starship will move "out of the way" of the inbound tether tip in about 0.1 seconds, so the tether would have to recoil at >3000m/s (about Mach 9) for the disconnected tip to hit the still-attached Starship directly. My guess is that there isn't nearly enough stored energy in the tether to accelerate it to such a velocity, and an emergency disconnect system could be designed to reduce the velocity of such recoil, or to disconnect from both ends simultaneously. Of course the ideal situation would be to spin down before disconnecting.

A 50m/s speed change would probably not be enough to make a typical LEO orbit "unstable"; e.g. deorbiting from ISS altitude requires at least 90m/s (in exactly the right direction), Hubble altitude 180m/s. Obviously artificial-gravity experiments would be done in orbits where emergency disconnects wouldn't immediately deorbit either craft, but such safe orbits are not hard to find.

 

[daniel]

The tether would have a "suspension" or other means to actively damp this vibration. The fundamental frequency of this vibration would be fairly high (the speed of sound in Zylon is about 9000m/s), so the fundamental vibration frequency for a 300m tether would be around 15Hz. It's not clear there would be much happening near this frequency on either Starship to amplify this resonance, but in any case it could be damped. (As could other resonant frequencies.)

The tether is of necessity under tension since it's holding the two space ships together. I don't think there's any way to dampen vibration.

We are within two decades of that in my estimation.

We're a decade away from NASA's own planned mission to bring back the samples. A manned mission would require sending infrastructure and a return ship before people. A manned mission will not happen in less than 100 years, and that's enormously optimistic.

The Coriolis force would likely prevent the detached end of the tether from hitting the other Starship, even if it snaps back violently. At 50m/s rotational velocity (300m tether at 1G), the still-attached Starship will move "out of the way" of the inbound tether tip in about 0.1 seconds, so the tether would have to recoil at >3000m/s (about Mach 9) for the disconnected tip to hit the still-attached Starship directly. My guess is that there isn't nearly enough stored energy in the tether to accelerate it to such a velocity, and an emergency disconnect system could be designed to reduce the velocity of such recoil, or to disconnect from both ends simultaneously. Of course the ideal situation would be to spin down before disconnecting.

The detached end may miss the other ship on the first pass, but it will continue to whip around, turning the ship this way and that, and making it extremely difficult to stabilize it.

A 50m/s speed change would probably not be enough to make a typical LEO orbit "unstable"; e.g. deorbiting from ISS altitude requires at least 90m/s (in exactly the right direction), Hubble altitude 180m/s. Obviously artificial-gravity experiments would be done in orbits where emergency disconnects wouldn't immediately deorbit either craft, but such safe orbits are not hard to find.

It may not cause it to de-orbit, but it will send it off in an unpredictable direction, possibly in the path of orbiting debris.

But we're already agreed that a tethered system is a poor choice for artificial gravity. And we're clearly many decades away from a spinning space station in LEO to even begin testing reduced gravity prior to launching a Mars mission. Before we're ready to send people to Mars, we'll have sufficiently advanced artificial quasi-intelligence to do everything robotically that we'd be able to do with humans, and at a thousandth the cost. The only reason to send people will be the bragging rights.

 

[Ben W]

The tether is of necessity under tension since it's holding the two space ships together. I don't think there's any way to dampen vibration.

Piano strings are also under constant tension, yet each string has its own damper, which very effectively stops vibration when applied. This would work in space, too. (The mechanism of action of the damper is frictional, and is not dependent on gravity or atmosphere.)

We're a decade away from NASA's own planned mission to bring back the samples. A manned mission would require sending infrastructure and a return ship before people. A manned mission will not happen in less than 100 years, and that's enormously optimistic.

I think this underestimates how much of a game-changer Starship may be here, in terms of cost and volume of mass to LEO and Mars. Sure, NASA-as-we-know-it would take many decades. But if Starship pans out in terms of reliability and reusability, this will change drastically. Of course, my optimistic timeline is predicated on it panning out, and obviously there are no guarantees that it will. If Starship fails, and fully reusable orbital rockets are never developed, then your timeline is probably more accurate. We shall see, perhaps in the next year or two.

The detached end may miss the other ship on the first pass, but it will continue to whip around, turning the ship this way and that, and making it extremely difficult to stabilize it.

The ship is far more massive than the tether, and the tether will transmit most of its energy to the remaining attached ship very quickly, so it will not "whip around" for long. But it may be safer to simultaneously detach the tether from both ends, rather than have one ship try to hold onto it. If the releases can be precisely synchronized, the center of mass of the tether will remain in roughly its original position (twisting around in an interesting pattern for a while), while both ships coast safely away.

It may not cause it to de-orbit, but it will send it off in an unpredictable direction, possibly in the path of orbiting debris.

In the relevant timescale here (minutes or hours until the course can be re-corrected), debris is not an issue. The ISS has to maneuver to dodge space junk only about once a year, and Starship is significantly smaller than ISS. Obviously I hope that space junk problems don't get significantly worse between now and then, but I also hope that Starship may enable us to economically do something about it, like rendezvous with and deorbit some of the larger dead satellites.

Before we're ready to send people to Mars, we'll have sufficiently advanced artificial quasi-intelligence to do everything robotically that we'd be able to do with humans, and at a thousandth the cost. The only reason to send people will be the bragging rights.

Agreed that "intelligent" robots (e.g. Optimus) capable of building infrastructure will almost certainly precede us to Mars, and should. But I'd estimate that it is closer to one-twentieth the cost for a robotic vs. crewed mission, not one-thousandth, and with Starship (if successful) the costs will drop extremely rapidly across the board. The fact that it's merely "bragging rights" has never stopped humans from doing crazy things like this, and I don't think it will stop us this time.

 

[scaesare]

Well, landing a rocket [on a droneship] also requires active synchronization between both systems with no redundancy, so I'd call that a similarity, not a difference. (In this context it's an inconsequential detail that the booster and droneship don't synchronize directly with each other, but rather are remotely synchronized.)

If a ship in a tether scenario experiences a failure where it can't maintain its own stability, it's almost certainly time to un-tether, which would immediately create two much simpler systems. Assuming the failing Starship could then at least null out its own rotation, the other Starship could then fly back and dock directly with it, to e.g. rescue crew if necessary. Presumably each individual Starship would have some level of redundancy where any single point of failure (e.g. a non-working thruster or gyroscope) could be temporarily compensated for by the others, for at least long enough to spin down and safely detach the tether.

A tether system would presumably include an active "suspension" mechanism to damp out longitudinal oscillations. (Either built into the catch arms themselves, or as part of the tether.) In any case, it clearly wouldn't be as simple as just two spaceships tied together with a simple rope. Torsional rigidity is trickier, but perhaps a lightweight telescoping sleeve with active torsion-adjusting rings/motors could surround the Zylon tether, which would have the convenient side-benefit of also protecting the tether from UV degradation. It depends how much torsional damping or force is needed, and of course the connector itself could only damp out relative rotation, not absolute rotation. In the Mars transit scenario, the mass of the tether system would eat directly into the payload capacity, so a fully rigid static connection (e.g. a rigid 300m truss) would almost certainly be prohibitive. Though I acknowledge that for a permanent LEO spinning station, a rigid system is likely the better choice.

Yeah, suspect a number of scenarios where abort would be a better option...

 

[scaesare]

I think that to structurally sound enough to keep people safe in an environment where a failure would mean almost certain death of the inhabitants, it would have to be extremely strong, and that would make it very massive.

It seems that for both airflight and spaceflight the bar for human rated safety is "strong enough for the expected forces plus some margin and/or backup plan"

And given that strong != massive and the actual forces that need to be accommodated for this structure may be largely in tension plus a small amount of other axis rigidity, I don't necessarily think you can draw that conclusion.

 

[ecarfan]

We are within two decades of that in my estimation. While preparing for such a mission, it would be highly useful to test out said artificial gravity mechanisms (and long-term effects on humans) in LEO first. And Starship will bring down the cost of launch to LEO for such testing tremendously, as well as conveniently providing a platform to test it, if the tether system proves workable.

I agree. With Starship, building an LEO artificial gravity space station becomes much less expensive and designers will come up with creative ways to use the huge payload bay to make in-orbit assembly easier.

The driving force for building such a station will be profits. There are probably over 100,000 people who could afford to spend hundreds of thousands of dollars for a few days on such a station. There is a market for it.

Such a station could teach us a lot about how to build an artificial gravity Mars cycler which could also be used for longer missions in the Solar System.

To those who say that humans should only use robotic vehicles to explore off Earth, I respect their opinion but disagree. Humans are, at the core, curious explorers, and even today many want to experience the unknown as directly as possible.

Please don’t let that old argument take this thread off topic. Thank you.

 

[daniel]

Piano strings are also under constant tension, yet each string has its own damper, which very effectively stops vibration when applied. This would work in space, too. (The mechanism of action of the damper is frictional, and is not dependent on gravity or atmosphere.)

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.

I think this underestimates how much of a game-changer Starship may be here, in terms of cost and volume of mass to LEO and Mars.

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.

The ship is far more massive than the tether, and the tether will transmit most of its energy to the remaining attached ship very quickly, so it will not "whip around" for long. But it may be safer to simultaneously detach the tether from both ends, rather than have one ship try to hold onto it. If the releases can be precisely synchronized, the center of mass of the tether will remain in roughly its original position (twisting around in an interesting pattern for a while), while both ships coast safely away.

In the relevant timescale here (minutes or hours until the course can be re-corrected), debris is not an issue. The ISS has to maneuver to dodge space junk only about once a year, and Starship is significantly smaller than ISS. Obviously I hope that space junk problems don't get significantly worse between now and then, but I also hope that Starship may enable us to economically do something about it, like rendezvous with and deorbit some of the larger dead satellites.

Not sure what we're arguing about here, since we're agreed that a tethered pair of space ships is not the way to create a space station .

Agreed that "intelligent" robots (e.g. Optimus) capable of building infrastructure will almost certainly precede us to Mars, and should. But I'd estimate that it is closer to one-twentieth the cost for a robotic vs. crewed mission, not one-thousandth, and with Starship (if successful) the costs will drop extremely rapidly across the board.

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 fact that it's merely "bragging rights" has never stopped humans from doing crazy things like this, and I don't think it will stop us this time.

There are many orders of magnitude between the cost of sending one or two people, and creating a colony and maintaining it in perpetuity. Not to mention the politics of an obligation in perpetuity.