Author Topic: Pragmatic artificial gravity  (Read 16816 times)

Offline Russel

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Pragmatic artificial gravity
« on: 11/11/2018 11:42 am »
I'm not sure if this qualifies as an "advanced concept" or not, but the subject of artificial gravity does come up often in these forums. My point of view is that artificial gravity can be rendered easily, provided you are not encumbered with the idea that you must replicate Earth gravity. I won't go fully into the arguments about how much artificial gravity is enough.

What I note however is that small levels of gravity are very useful in a practical sense. it is good to have an "up" and "down" for practical reasons such as eating or working or using a shower. Lots of activities become easier when you can predict which way things will fall. To provide this level of gravity can mean as little as 0.02g.

Then there is the usefulness of gravity for the maintenance of musculoskeletal health. We don't know whether we actually need 1g here. What can be surmised is that to some extent, more is better.

Another point I'll add is that achieving Mars gravity (0.4g) or something close to that is probably going to be a useful target on missions to Mars. So, what does it take to provide a pragmatic artificial gravity environment without resorting to large structures?

I'll give a concrete example of what can be built. Take a space habitat module - a cylinder 6m tall and 5m in diameter for the sake of the argument. Standard docking ports on either end. Now take two of these modules and connect them to either end of a tube with a docking port on each end. Make the tube 20m in length and sufficient in width to climb through. Its an access way between the two modules. Spun end over end the whole assembly has a radius of 16m to the far ends.

Spun at 1rpm the gravity produced is 0.018g or 17cm/s/s. Even at this low rate of acceleration, there are quite useful practical effects. Now at 4.7rpm we have 0.4g which is equivalent to Mars gravity. In fact to get to 0.8g you need 6.7rpm. All with a very modest vehicle. And I don't know if it is actually necessary to go this far.

So its possible to do with a relatively modest vehicle. There are of course other benefits to building a multi-module vehicle with duplicated hab sections. Those include redundancy and robustness. Being able to survive failures (like those caused by micrometeors) that that would otherwise risk loss of crew on a singular space habitat. The connecting module can also serve other functions like propulsion and tankage.

One other point I've brought up elsewhere. It is probably a good idea to wear some of you radiation shielding, particularly for the torso. This adds mass and in a lower gravity environment this means more work being done by the musculoskeletal system. So it in part compensates for the lower gravity.

I've often wondered why people when thinking of artificial gravity have either defended going to 1g, often with large and non rigid systems (cables etc) when you can do this easily with a rigid structure that isn't out of Hollywood either. On the other hand I wonder why people have defended zero gravity environments when a pragmatic level of gravity is easy to create.

What it does do is mitigate against space habitats that are also mars landing and ascent vehicles. And I happen to think that the two should be kept quite separate for other reasons. But having a pragmatic level of gravity does seem to be a good thing and its easy to assemble a rigid structure capable of doing so that also makes sense as an Earth orbit to Mars orbit (and return) vehicle.

Offline brainbit

Re: Pragmatic artificial gravity
« Reply #1 on: 11/11/2018 01:15 pm »
Unfortunately nothing like gravity is so simple. supplies would have to be loaded from the axle and this really shouldn't be turning, but could with difficulty. moving anything around would cause balancing problems which must be compensated for, even walking from one end of a tube to the other would create dynamic loads which would then create other loads etc, until the station starts to tumble if not corrected. For a simple central-fugal machine to work it must be attached to something solid at its axis. To simulate Mars gravity on the moon is a definite possibility. This is not to say it cannot be done, but it is very complex. As a test you could build a spinning top on earth and try to keep it balanced with moving parts unbalancing it. Hmmm anyone want to try that?

Offline Russel

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Re: Pragmatic artificial gravity
« Reply #2 on: 11/11/2018 01:36 pm »
Unfortunately nothing like gravity is so simple. supplies would have to be loaded from the axle and this really shouldn't be turning, but could with difficulty. moving anything around would cause balancing problems which must be compensated for, even walking from one end of a tube to the other would create dynamic loads which would then create other loads etc, until the station starts to tumble if not corrected. For a simple central-fugal machine to work it must be attached to something solid at its axis. To simulate Mars gravity on the moon is a definite possibility. This is not to say it cannot be done, but it is very complex. As a test you could build a spinning top on earth and try to keep it balanced with moving parts unbalancing it. Hmmm anyone want to try that?

I wouldn't propose a vehicle that is spinning at all stages of the flight.
Rather it would be stationary where necessary, including propulsive phases.
You're quite right that there would have to be dynamic adjustments to the center of mass. Most likely this would be transfer of fluids. Nothing a computer can't handle though.

Offline Bob Shaw

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Re: Pragmatic artificial gravity
« Reply #3 on: 11/11/2018 01:52 pm »
There are lots of robust structures with which we are very familiar, and which are made from traditional materials - buildings, bridges and the rest - and all tend to be over-strong. Some of these even move. The true cost of AG in space is simply the launch cost - if we can build massive objects in space at moderate cost then we don't fall down the over-design hole - better is always the enemy of good enough.

Offline Bob Shaw

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Re: Pragmatic artificial gravity
« Reply #4 on: 11/11/2018 01:55 pm »
For a moving, strong object capable of rotating in a 1g field and also pressure-tight, see the US RP FLIP oceanic research ship (well, platform).

https://en.wikipedia.org/wiki/RP_FLIP



Online Hobbes-22

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Re: Pragmatic artificial gravity
« Reply #5 on: 11/11/2018 03:51 pm »

I wouldn't propose a vehicle that is spinning at all stages of the flight.

Current experience suggests it takes a while (several days) for an astronaut to get used to 0 G. Regular exposure to that switch may improve it, or they may end up in a state of permanent disorientation...

Offline Russel

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Re: Pragmatic artificial gravity
« Reply #6 on: 11/12/2018 04:41 am »
There are lots of robust structures with which we are very familiar, and which are made from traditional materials - buildings, bridges and the rest - and all tend to be over-strong. Some of these even move. The true cost of AG in space is simply the launch cost - if we can build massive objects in space at moderate cost then we don't fall down the over-design hole - better is always the enemy of good enough.

My point is that you don't need to build a truly massive object in space in order to get a useful amount of gravity. Nor do you need special construction - just standard docking.

Just to make it clear also. What I'm proposing is a simple vehicle that contains 3 sections. It cannot be spinning when you dock to it. Nor can it be spinning during propulsive phases. However the periods of zero go between such procedures will be relatively short.
« Last Edit: 11/12/2018 04:53 am by Russel »

Offline Coastal Ron

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Re: Pragmatic artificial gravity
« Reply #7 on: 11/12/2018 06:14 am »
My point is that you don't need to build a truly massive object in space in order to get a useful amount of gravity. Nor do you need special construction - just standard docking.

These issues are already being discussed and debated on an existing thread - Realistic, near-term, rotating Space Station

I suggest moving this discussion over there.
If we don't continuously lower the cost to access space, how are we ever going to afford to expand humanity out into space?

Offline KelvinZero

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Re: Pragmatic artificial gravity
« Reply #8 on: 11/12/2018 06:57 am »
My point is that you don't need to build a truly massive object in space in order to get a useful amount of gravity. Nor do you need special construction - just standard docking.

These issues are already being discussed and debated on an existing thread - Realistic, near-term, rotating Space Station

I suggest moving this discussion over there.
Two examples of pragmatic artificial gravity that don't really fit in the realistic rotating space station:

(1) I have always thought that the tiny radius AG shown in 2001 might in fact be reasonable for bone health for dedicated astronauts. Rather than actually having a convincing gravity environment, it could aim for say merely moon gravity, but it can also be used as a jogging track. Running in the right direction increases gravity. Dizziness from turning your head is largely avoided by.. not turning your head. It is more like a piece of gym equipment like a running track than an actual zero-g environment.

(2) Im also a big fan of expansive VR environments for passengers of long distance space travel. Not only can it give you the impression of much more space than any rotating ship and goals that actually motivate you to walk around, you could spend all your time in what is effectively the solar storm shelter. Again it is more like a piece of exercise equipment that you can stay in for many hours of the day. It can have elastic bands instead of gravity, similar to the current ISS treadmills. Unlike the ISS, you don't have microgravity chores. Your real-world movement is even more limited than (1) so possibly you could also use small radius spin instead of/as well as elastic bands, without the disorientation you would get from free motion.

Offline vulture4

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Re: Pragmatic artificial gravity
« Reply #9 on: 11/12/2018 11:41 am »
Bone loss in spinal cord injury is more severe than has ever been seen in space, so there is a substantial basis of experience here on earth.

Bone density is maintained by physiological feedback mechanisms based not on gravity as an external force but rather on loading by muscles. This load can be reproduced by resistance exercise, which was one of the motivations behind various resistance exercise devices that have been flown. However some of the Russian long duration crewmen did not exercise at all and did have considerable bone loss. In one case one of the Russians resumed an intense regimen of running shortly after return and was reported to have suffered a stress fracture. The same thing has happened to paraplegics placed abruptly in full weight bearing in some early experimental rehab programs. Bone requires three to six months to gain significant strength when loading in increased, it responds m ore slowly than muscle.

However bone loss is of significance only if it results in fracture. The risk of fracture after return is low even with long duration spaceflight without countermeasures, and moderate resistance exercise during spaceflight can limit it to levels that do not pose a significant risk. Artificial gravity may well be useful for control of floating objects or passenger comfort, but it is not essential for crew health.

Offline Coastal Ron

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Re: Pragmatic artificial gravity
« Reply #10 on: 11/12/2018 03:13 pm »
My point is that you don't need to build a truly massive object in space in order to get a useful amount of gravity. Nor do you need special construction - just standard docking.

These issues are already being discussed and debated on an existing thread - Realistic, near-term, rotating Space Station

I suggest moving this discussion over there.
Two examples of pragmatic artificial gravity that don't really fit in the realistic rotating space station:

The definition of the word "pragmatic" includes being "realistic":

Quote
- dealing with things sensibly and realistically in a way that is based on practical rather than theoretical considerations.

Great topic, but this thread is redundant.
If we don't continuously lower the cost to access space, how are we ever going to afford to expand humanity out into space?

Offline Russel

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Re: Pragmatic artificial gravity
« Reply #11 on: 11/12/2018 11:04 pm »
My point is that you don't need to build a truly massive object in space in order to get a useful amount of gravity. Nor do you need special construction - just standard docking.

These issues are already being discussed and debated on an existing thread - Realistic, near-term, rotating Space Station

I suggest moving this discussion over there.

That thread started with a post that is specifically about fixed space stations. Its also mutated into building space stations into asteroid hollows. I started this thread to talk about providing some level of artificial gravity to vehicles whose main purpose is transit between Earth space and Mars space.
« Last Edit: 11/12/2018 11:14 pm by Russel »

Offline Russel

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Re: Pragmatic artificial gravity
« Reply #12 on: 11/12/2018 11:14 pm »
Bone loss in spinal cord injury is more severe than has ever been seen in space, so there is a substantial basis of experience here on earth.

Bone density is maintained by physiological feedback mechanisms based not on gravity as an external force but rather on loading by muscles. This load can be reproduced by resistance exercise, which was one of the motivations behind various resistance exercise devices that have been flown. However some of the Russian long duration crewmen did not exercise at all and did have considerable bone loss. In one case one of the Russians resumed an intense regimen of running shortly after return and was reported to have suffered a stress fracture. The same thing has happened to paraplegics placed abruptly in full weight bearing in some early experimental rehab programs. Bone requires three to six months to gain significant strength when loading in increased, it responds m ore slowly than muscle.

However bone loss is of significance only if it results in fracture. The risk of fracture after return is low even with long duration spaceflight without countermeasures, and moderate resistance exercise during spaceflight can limit it to levels that do not pose a significant risk. Artificial gravity may well be useful for control of floating objects or passenger comfort, but it is not essential for crew health.

What I've demonstrated is that a modest level of artificial gravity is relatively easy to provide. Whilst its possible to have a discussion about exactly how much gravity is useful for the health of the crew, that's not what this thread is for.

Any vehicle capable of sustaining low levels of artificial gravity (to provide the benefits of "up" and "down") is automatically capable of providing moderate levels of artificial gravity (say 0.2 to 0.4g and quite possible more). Hence the concrete example.

Yes, exercise is a good thing. But without gravity you need to basically strap people down to provide the forces needed. With a modest level of artificial gravity, exercise occurs whilst moving around and performing ordinary tasks. Not just during structured exercise regimes.

Offline DreamyPickle

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Re: Pragmatic artificial gravity
« Reply #13 on: 11/13/2018 11:25 pm »
That other thread is indeed filling with posts that are not very pragmatic and inspired more by sci-fi that current practical limitations. Artificial gravity generated by spinning habitats shouldn't be really that hard, it could be done with much less effort than Apollo or the Shuttle. It would be interesting to discuss what it would take to simulate mars or moon-like gravity in LEO for long duration experiments using only a modest number of launches, let's say max 5 Falcon Heavy launches.

One problem with a "20-meter rigid tube" is that it doesn't fit in any current fairing so it needs to be either expandable (hard) or segmented.

The main purpose of such a station would be to gather data about how life survives over months or years at intermediary G levels, something for which there is absolutely no data. This implies that the station would require continuous habitation and resupply at least at the ISS level.

Docking is problematic for a spinning stations because you're either limited to only 2 port (which is insufficient) or the rotating habitats needs to be move around a non-spinning service/docking section.

Offline Coastal Ron

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Re: Pragmatic artificial gravity
« Reply #14 on: 11/13/2018 11:38 pm »
That thread started with a post that is specifically about fixed space stations.

Specifically realistic, near-term, rotating space stations.

Quote
Its also mutated into building space stations into asteroid hollows.

Every thread wanders - you should know that by now.

But members and mods continually refocus it. Everything I've contributed has been about realistic, near-term, rotating space stations.

Quote
I started this thread to talk about providing some level of artificial gravity to vehicles whose main purpose is transit between Earth space and Mars space.

Then I suggest the thread title be changed to "Realistic, Near-term, Rotating Spaceships".   ;)
If we don't continuously lower the cost to access space, how are we ever going to afford to expand humanity out into space?

Offline CameronD

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Re: Pragmatic artificial gravity
« Reply #15 on: 11/13/2018 11:49 pm »
I started this thread to talk about providing some level of artificial gravity to vehicles whose main purpose is transit between Earth space and Mars space.

Then I suggest the thread title be changed to "Realistic, Near-term, Rotating Spaceships".   ;)

But.. but... that would exclude discussion on the use of constant acceleration/deceleration to provide 'some level of artificial gravity to vehicles whose main purpose is transit between Earth space and Mars space', leaving you with just "Realistic, Near-term, Spaceships" ...which is probably not  exactly an Advanced Concept. ;)
With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
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Offline Slarty1080

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Re: Pragmatic artificial gravity
« Reply #16 on: 11/14/2018 09:17 am »
Unfortunately nothing like gravity is so simple. supplies would have to be loaded from the axle and this really shouldn't be turning, but could with difficulty. moving anything around would cause balancing problems which must be compensated for, even walking from one end of a tube to the other would create dynamic loads which would then create other loads etc, until the station starts to tumble if not corrected. For a simple central-fugal machine to work it must be attached to something solid at its axis. To simulate Mars gravity on the moon is a definite possibility. This is not to say it cannot be done, but it is very complex. As a test you could build a spinning top on earth and try to keep it balanced with moving parts unbalancing it. Hmmm anyone want to try that?
A spinning station will not tumble. If it did its angular momentum would change direction with each 180 degree turn.
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Offline KelvinZero

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Re: Pragmatic artificial gravity
« Reply #17 on: 11/14/2018 08:32 pm »
spinning objects can do some weird stuff while preserving angular momentum I think.. (don't ask me to show the math! :) )

This was one example. search "spinning objects in zero gravity" on youtube for some others.

« Last Edit: 11/14/2018 08:34 pm by KelvinZero »

Offline Paul451

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Re: Pragmatic artificial gravity
« Reply #18 on: 11/14/2018 10:06 pm »
That thread [...] mutated into building space stations into asteroid hollows.

Meh, a dozen comments out of 60+ pages.

But if you want to keep this separate, I'd suggest editing the topic title to be "Minimum Useful Artificial-G for Spaceships". To make it clearer where you are differentiating from the previous AG threads.



For a simple central-fugal machine to work it must be attached to something solid at its axis.

Objects in free fall don't require a fixed axis. They are inherently self-balancing, even if 90% of the mass is at one end. Indeed trying to lock the axis just makes everything harder. The only issue for stability is that the mass must be close to either the axis of rotation or the plane of rotation. If you have that, then it's stable, even if people are moving around.

(You can replicate KZ's video demo yourself by spinning a book in the air. Throw it so it rotates around its long axis, and it's stable. Throw it so it rotates around its shortest axis (like a frisbee) and it's stable. Throw it so it rotates around its second axis, and it will tumble. See sketch below...)



I wouldn't propose a vehicle that is spinning at all stages of the flight.
Current experience suggests it takes a while (several days) for an astronaut to get used to 0 G. Regular exposure to that switch may improve it, or they may end up in a state of permanent disorientation...

Experiments in spin-tolerance suggest that adaptation is retained for days afterwards, declining slowly and returning quickly.



However bone loss is of significance only if it results in fracture.

Not so. Bones contain more than just structural material, they also have tissue that creates blood products, plays a role in nutrient balancing and endocrine function. And that tissue is lost in micro-g along with skeletal mass. The skeletal mass is the quickest to return, the other tissue taking much longer.

Offline Paul451

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Re: Pragmatic artificial gravity
« Reply #19 on: 11/14/2018 10:07 pm »
(1) I have always thought that the tiny radius AG shown in 2001 might in fact be reasonable for bone health for dedicated astronauts. Rather than actually having a convincing gravity environment, it could aim for say merely moon gravity, but it can also be used as a jogging track. Running in the right direction increases gravity.

You don't even need to spin the ship. If you have a wide enough loop/track, astronauts can generate their own gravity while running.

eg, Pete Conrad in Skylab running at >10RPM



Dizziness from turning your head is largely avoided by.. not turning your head.

Not moving your head while adapting is how you leave yourself prone to motion sickness.

Additionally, if the ship is not spinning, astronauts in free-fall adapt by switching off their inner-ear. They can't get motion sick during their running time. eg, Tim Peake on ISS spinning at I-don't-want-to-know RPM


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