Artificial Gravity Testbeds

[LMT]

I wanted to check how long it might reasonably take to spin up/down...

Modern GNC can avoid spin-down.  1 2 

Docking is feasible with uncooperative tumbling targets.  Cooperative targets are easier to work with, easier still when docking near CoG and without tumble.

For those who don't feel like going down (yet another) insufferable self-link rabbit hole, the actual papers being cited are Oestreich 2021, Silvestrini and Lavagna 2022, and Huang et al 2022.

Such "emotion".  You might post instead a useful comparison of those methods, or better methods.

[Twark_Main]

I wanted to check how long it might reasonably take to spin up/down...

Modern GNC can avoid spin-down.  1 2 

Docking is feasible with uncooperative tumbling targets.  Cooperative targets are easier to work with, easier still when docking near CoG and without tumble.

For those who don't feel like going down (yet another) insufferable self-link rabbit hole, the actual papers being cited are Oestreich 2021, Silvestrini and Lavagna 2022, and Huang et al 2022.

Such "emotion".

How is making inaccessible links accessible.....  emotion?   

You might focus on why your link rabbit holes are so inaccessible, and how to fix that.

You might post instead a useful comparison of those methods, or better methods.

I've already helped enough, thanks.

But even without reading any papers, rim docking on a non-despun station is of course rather trivial. Just "drop" the vehicle off the bottom of the station, then time-reverse that trajectory. 

[mikelepage]

This is also why I wanted to check how long it might reasonably take to spin up/down: If spin-down takes around half an hour as per the spreadsheet, then it's possible to wait until the launch of the incoming starship is successful - then in the 6-24 hours before rendezvous, you have time to spin down the station and undock the departing starship. Expected total time the station spends in zero-G hopefully much less than 1 sleep/wake cycle for those staying onboard.

I was ambivalent at best when you first proposed this due to what I thought the energy requirements would be, but then your spreadsheet analysis provided a result that surprised me, so now I'm thinking there could be one or more use cases where this would make sense.

As you stated originally this would be for a station in LEO, but that is likely where artificial gravity testbeds will be located anyways, so all the more reason to keep looking into this concept. Nice!

Thanks. I'm really starting to think that if we accept a paradigm of regular spin-up/spin-down operations - this might be key to moving forward for advocates of spin-G space stations. Everything becomes so much simpler, and uses higher TRL technologies. Which is epecially important if we're assuming in-orbit assembly by SpaceX, with its demonstrated preference for radical simplification. I've yet to design any central docking system that maintains the spin rate of the main station, that I thought would pass muster with various spacecraft engineers that I've met. There's enough complexity in the spin-G environment that we need to tackle first. Maybe we can leave those elements for the second generation of space stations.

A paradigm of regular spin-up/spin-down operations allows a station where the rotation axis is oriented periodically to point at the sun, which further simplifies solar PV and radiator placement. It also greatly simplifies any EVA activities that might be required. Lastly, it lends itself to a space hotel/cruise-ship model of regular resupply, which should help close the business case.

[QuantumG]

I haven't looked but I would be surprised if someone hasn't filed an SBIR for bone-density testing of astronauts on a spinning Spaceship. SpinCalc with 8.9m diameter gives 3 RPM for 0.1g, 10 RPM for 1g, etc. Perhaps it's just too early?

[rsdavis9]

Another use for spin up and spin down.

Fuel transfer from depot to ship.

Dock 2 (or more) starships, spin up, and then transfer takes place without waste of continuous ullage thrust. Also if there are people aboard then the wait is more enjoyable(tolerable) with some amount of artificial gravity. Ideally the people could be in lounge area with coffee etc.

[LMT]

Another use for spin up and spin down.

Fuel transfer from depot to ship.

Dock 2 (or more) starships, spin up, and then transfer takes place without waste of continuous ullage thrust.

Here a change of spin isn't really needed for ullage, or propellant.  Electrodynamic cables can serve.  In one example, geomagnetic field vertical component gives axial ullage thrust, per (3.).

[Coastal Ron]

I haven't looked but I would be surprised if someone hasn't filed an SBIR for bone-density testing of astronauts on a spinning Spaceship. SpinCalc with 8.9m diameter gives 3 RPM for 0.1g, 10 RPM for 1g, etc. Perhaps it's just too early?

I think it will be unlikely we'll ever see this attempted, because of the issues with the issues highlighted by the intermediate axis theorem.

A Starship with a crew compartment means that the total mass distribution of the Starship will be butt heavy, with the LOX and LMG tanks, plus the engines, providing some significant percentage of the total mass on one end of the vehicle. Add the ability for the propellant to move around on its own, and you have a configuration that becomes unstable pretty quickly once initial rotation starts.

And sure, you could use the propellant using engines to counteract the instability, but there is a LOT of mass on the butt end of the Starship that will want to change rotation direction - change from rotating through the long center of the ship, to rotating end over end. It would take large engines to counteract that tendency.

In other words, if you want to rotate a Starship to provide artificial gravity, just rotate it end over end, and it will be in a stable rotation that doesn't require propellant to keep it in that rotation plane.

However I think building small rotating stations seems like the more stable choice ( ), and provides more experiment space, which is why propellant costs for spinning up and spinning down such a station becomes interesting.

[Paul451]

I haven't looked but I would be surprised if someone hasn't filed an SBIR for bone-density testing of astronauts on a spinning Spaceship. SpinCalc with 8.9m diameter gives 3 RPM for 0.1g, 10 RPM for 1g, etc. Perhaps it's just too early?

I think it will be unlikely we'll ever see this attempted, because of the issues with the issues highlighted by the intermediate axis theorem.
[...]
And sure, you could use the propellant using engines to counteract the instability, but there is a LOT of mass on the butt end of the Starship that will want to change rotation direction - change from rotating through the long center of the ship, to rotating end over end. It would take large engines to counteract that tendency.
In other words, if you want to rotate a Starship to provide artificial gravity, just rotate it end over end, and it will be in a stable rotation that doesn't require propellant to keep it in that rotation plane.

This is exactly backwards. You are confusing intermediate axis instability with major axis instability.

If you rotate a uniform cylinder end-over-end, it is prone to intermediate axis instability. However, in the case of Starship, the fore and aft flaps put more mass/inertia in the rotational plane, creating a definitive, and stable, tertiary axis.

Long axis (major axis) rotation can also be unstable, drifting slowly into end-over-end (tertiary axis) rotation. That's not intermediate axis instability. And there are (apparently) passive measures that can counter that slow trend. No propulsive effort is required.

But even if you did want to use propulsion to counter it, only a small amount of correction would be required. It's not a significant instability (compared to the intermediate axis instability), it's just a slow drift.

[LMT]

even without reading any papers

Bgd:  the first autonomous 6 df docking with an uncooperative tumbling target in space was accomplished in 2006, with SPHERES.  3:30.

[Twark_Main]

Long axis (major axis) rotation can also be unstable, drifting slowly into end-over-end (tertiary axis) rotation. That's not intermediate axis instability. And there are (apparently) passive measures that can counter that slow trend. No propulsive effort is required.

Is there somewhere I can read more on this?  Because that seems....  surprising.

Con Hathy (an excellent and underappreciated Youtube channel) did a drop test with a water bottle spun up by a drill. The results were not encouraging. I would love to see what measures can restore stability to such a system.

[InterestedEngineer]

Long axis (major axis) rotation can also be unstable, drifting slowly into end-over-end (tertiary axis) rotation. That's not intermediate axis instability. And there are (apparently) passive measures that can counter that slow trend. No propulsive effort is required.

Is there somewhere I can read more on this?  Because that seems....  surprising.

Con Hathy (an excellent and underappreciated Youtube channel) did a drop test with a water bottle spun up by a drill. The results were not encouraging. I would love to see what measures can restore stability to such a system.

I thought the rotating space station thread was full of examples

[Twark_Main]

Long axis (major axis) rotation can also be unstable, drifting slowly into end-over-end (tertiary axis) rotation. That's not intermediate axis instability. And there are (apparently) passive measures that can counter that slow trend. No propulsive effort is required.

Is there somewhere I can read more on this?  Because that seems....  surprising.

Con Hathy (an excellent and underappreciated Youtube channel) did a drop test with a water bottle spun up by a drill. The results were not encouraging. I would love to see what measures can restore stability to such a system.

I thought the rotating space station thread was full of examples

There's approximately nine of those threads, and they're each dozens of pages long.   

Can you give, say, one example?

[Twark_Main]

I'm really starting to think that if we accept a paradigm of regular spin-up/spin-down operations - this might be key to moving forward for advocates of spin-G space stations. Everything becomes so much simpler, and uses higher TRL technologies.

In this scenario, the station designs can be expected to cluster around the shortest tolerable radius and the highest tolerable RPM. For the same gravity level, the spin-up and spin-down propellant mass scales in a linear fashion with the radius and with the (inverse) RPM.

[mikelepage]

I'm really starting to think that if we accept a paradigm of regular spin-up/spin-down operations - this might be key to moving forward for advocates of spin-G space stations. Everything becomes so much simpler, and uses higher TRL technologies.

In this scenario, the station designs can be expected to cluster around the shortest tolerable radius and the highest tolerable RPM. For the same gravity level, the spin-up and spin-down propellant mass scales in a linear fashion with the radius and with the (inverse) RPM.

I agree with your math, but not your conclusion, because optimising for propellant mass isn't a strong forcing factor for anything that is likely to be built in the near-term. If we're assuming we need starship to launch it, then surely we've also got access to starship-level quantities of propellant, right? The amount of propellant required to spin up /down a 1400 ton, 50m radius torus to Mars-level gravity is less than 10% of the payload capacity of a starship flight. Playing with the spreadsheet to test smaller configurations (e.g. 12 modules, 20m radii) gives us propellant masses in the 100s of kg to a little over a ton - almost a rounding error on a starship launch - and that's not going much above 4rpm for Mars G (or 6.69 rpm for 1xG).

I think a more likely reason we will be pushing human tolerances with low-spin-radii habitats is the complexity of in-orbit assembly leading to smaller overall stations, but in the less mass-constrained context where starship is launching frequently, I think on-orbit assembly techniques are likely to develop quite quickly. So rather than clustering around the human tolerance-pushing low spin-radii, I think early designs will cluster around whatever the sweet spot is for how many modules work in a space station, where the company/funding body has to build, launch and assemble them before the space station can start making money. Really depends who is funding it and how much SpaceX launch prices come down once starship is operating.

[Paul451]

Long axis (major axis) rotation can also be unstable, drifting slowly into end-over-end (tertiary axis) rotation. That's not intermediate axis instability. And there are (apparently) passive measures that can counter that slow trend. No propulsive effort is required.

Is there somewhere I can read more on this?

Yes.

Yes there is.

Ahem.

And it would have been awesome right now if I'd bookmarked the papers that convinced me, because I can't find them any more. (I think your academic google-fu is better than mine, so (and I realise this is a massive cop-out) it might be worth your time to see if you can find stuff yourself. There was a handful from the early space era where the major/minor-axis momentum transfer mechanism is discussed, and then later papers (in the same era) that talked about mitigation. Essentially, it's creating something that is also affected by the momentum xfer but in the opposite direction, like an inverted pendulum. Weights on springs was one design I saw, and the other (which was used in actual satellites) was fluid in pipes, at which point they were talking about it as if it were a solved problem. I think the principle is using the right-hand-rule where applied torque and direction of momentum change are at 90 degrees, combined with an inherent delayed response, but I didn't understand it well enough to do maths, only enough to convince me that it exists and actual hardware flew.)

[redneck]

I am a bit uncomfortable with the idea of spin up and down as the answer to AG stations and docking. It's not the amount of propellant that is involved so much as the question of how the frequent changes in AG affect operations. For people living aboard, will it be a problem of having to secure for zero gravity every time there is a docking/undocking operation? Upthread there was mention of issues with filters and air circulation in zero that mostly didn't apply in gravity fields. How much, if any, problem will there be with making sure that all fluids and gasses are under control during the shifts?

Uncomfortable and I don't know on my part are not legitimate constraints for concepts on your part. As this is not something happening in the very near future, I am interested in the opinions on the gravity to no gravity and back effects on operations. I.E. Bathrooms are closed due to maneuvers for docking gravity for the next 4.5 hours. Go now or hold it until maneuvers are complete.

[Paul451]

I am a bit uncomfortable with the idea of spin up and down as the answer to AG stations and docking. It's not the amount of propellant that is involved so much as the question of how the frequent changes in AG affect operations. For people living aboard, will it be a problem of having to secure for zero gravity every time there is a docking/undocking operation? Upthread there was mention of issues with filters and air circulation in zero that mostly didn't apply in gravity fields. How much, if any, problem will there be with making sure that all fluids and gasses are under control during the shifts?
Uncomfortable and I don't know on my part are not legitimate constraints for concepts on your part. As this is not something happening in the very near future, I am interested in the opinions on the gravity to no gravity and back effects on operations. I.E. Bathrooms are closed due to maneuvers for docking gravity for the next 4.5 hours. Go now or hold it until maneuvers are complete.

But there's a big technological difference between short-term, fairly lossy or storage-based systems used by capsules (and presumably short-visit Starships), versus long-term, recycling-based systems needed for a space-station. Having the complex, long-term systems set up to operate under gravity is (or is intended to be) easier to design/maintain than having long-term zero-g systems. As a bonus, even those short-term zero-g systems can be serviced under normal operating gravity, which should also make them easier to design. Eg, you flush the contents of the storage tank of your zero-g toilet into the main recycling system once gravity is back, but you don't have to design the main recycling system to work without gravity, just to be able to seal up when despun.

You aren't wrong, but it's a trade not a show-stopper IMO. And Mike might be right that below a certain size station, it's probably easier to design a fluid/pump system that can be turned off under zero-g for a couple of days during RPOD, than to design a system that has a counter-rotating docking port that allows pass-through to a rotating station. As you get bigger, the trades shift.

The period under despin is going to be roughly the same as the period the visiting ship(s) spend under zero-g. (A day or two.) So there's a lot of overlap in the systems needed anyway. And (again IMO), every AG station must have emergency zero-g capability anyway. It's the equivalent of having battery-powered emergency lights in stairwells/exit-signs in a large building for when the power goes out. Short-term backup for the long-term system. So it's not necessarily going to be extra hardware than you already carry.

[LMT]

And Mike might be right that below a certain size station, it's probably easier to design a fluid/pump system that can be turned off under zero-g for a couple of days during RPOD, than to design a system that has a counter-rotating docking port that allows pass-through to a rotating station. As you get bigger, the trades shift.

No counter-rotating dock is on the AG roadmap, e.g., at VAST.  Not at any scale. 

Why?  Apparently, per above, and as noted before.  1 2

We should notice what engineers include and omit in modern designs.

[JohnFornaro]

Just checking in.

A station as massive as the one I propose could not be spun up and down very easily.

As to the idea of a test bed, my station is intended to empirically prove that human life can reproduce in AG.  Is there something about an adjacent mass, like the Earth, which is necessary for humans and terrestrial life to thrive?

The benefits of living and working in AG versus micro or very low gee seem self evident.  If it is proven that we can't thrive and reproduce in AG, the utility of having a space station as a waypoint to Moon, Mars and Venus remains.

[mikelepage]

I am a bit uncomfortable with the idea of spin up and down as the answer to AG stations and docking. It's not the amount of propellant that is involved so much as the question of how the frequent changes in AG affect operations. For people living aboard, will it be a problem of having to secure for zero gravity every time there is a docking/undocking operation? Upthread there was mention of issues with filters and air circulation in zero that mostly didn't apply in gravity fields. How much, if any, problem will there be with making sure that all fluids and gasses are under control during the shifts?

Uncomfortable and I don't know on my part are not legitimate constraints for concepts on your part. As this is not something happening in the very near future, I am interested in the opinions on the gravity to no gravity and back effects on operations. I.E. Bathrooms are closed due to maneuvers for docking gravity for the next 4.5 hours. Go now or hold it until maneuvers are complete.

Fair points, but there's definitely a difference between devices able to tolerate zero-G versus having to operate in zero-G. Likewise for hyper-G (i.e. launch). Maybe spin-G space stations would have emergency kits for allowing people to use the toilet in zero-G that look much like what astronauts use on the ISS today (and anyone who is at all squeamish would make a very good effort at holding it in, as you guessed). Just so long as RPOD can be kept to less than 8 hours or so, it will probably be fine for most people.

Just wondering - are the airplane toilets that operate on the "vomit comet" zero-G flights any different from regular airplane toilets? (I don't know, but I don't see why they would be). As long as theres a flap, or a hatch, or a valve, on anything that handles fluids, then it can handle temporary zero-G.

Earlier on one of the threads - I forget where - we speculated about "zero-G holidays", where brief periods of zero-G like this would actually become great sources of fun and celebration.