Artificial Gravity

RobertDyck · 2014-10-29 22:55:58

With a tether, you should be able to manoeuvre by applying thrust that's timed so it "pulls" directly on the tether. Orient rotation in the plane of the transfer orbit, so all thrust for adjustments are in that plane. Then time thrust so you always "pull" on the tether, never push. And ensure thrust is always less than centripetal force, so "artificial gravity" is lessened during thrust, never negated. Since thrust will induce oscillations in the tether, you will have to ensure the tether can dampen oscillation.

There's an engineering challenge: how do you dampen oscillation in a tether? NASA wasn't able to do so when the tether was more than 1km long. And using a long tether from the Shuttle to generate electricity had several problems: one was that it generated more current than they expected. Excess current fried the cable. But for Mars, we don't intend to generate electricity. We won't travel through Earth's magnetic field, and the tether can be non-conductive. So electrical problems are not an issue. And oscillations should be much less when the tether is taught, and when the tether is much less than 1km long.

Antius · 2014-10-30 07:29:08

GW Johnson wrote:

Centripetal acceleration is rw^2 where r is the radius and w is the angular rate (spin rate). Its derivative is the acceleration gradient per unit radius, 2rw. You DO NOT want a large gradient (gees per meter of radius), because that leads to blood pooling in the feet, and blackouts. I don't really have a good criterion for that, other than the 1 gee at extreme radius in which we evolved.
I think you really want something closer to 1 gee at 56 m radius, for which the radial gradient is closer to something humans have proven themselves capable of withstanding (crudely 0.02 gee per meter). 2% is a relatively "small" number. I'd go with that.
GW

I’m not sure why this would be the case. In normal Earth gravity (with essentially zero gradient), going from a supine (laying) to standing position results in a reduction of core blood volume of 0.7 litres. This is due to blood pooling in the legs, but has nothing to do with gravity gradient. People with low heart rate have been known to ‘faint’ due to this effect if rising suddenly. In hyper gravity (3g) the blood redistribution increases to 1.4 litres and even healthy people notice changes to peripheral vision when performing moderate exercise.

In microgravity (and bed rest) blood tends to pool towards the head, as the cardiovascular system continues to compensate for Earth force gravity (which is not there). For intermediate gravity levels, the effect would be somewhere between these two extremes.

It is not clear to me why a ‘gravity gradient’ across the human body would produce an intolerable redistribution of blood. It would complicate balance, as the body’s centre of gravity and centre of mass would no longer be equivalent and coriolis forces would cause additional disorientation, but these are separate to the blood pooling issue. Evidence suggests that adaptation is possible for angular velocity up to 23rpm. Many of the MIT test subjects reported nausea on the first day of rotation, but none of them passed out. All had adapted after 5 days. And these individuals were centrifuged at 23rpm at radii of 2m and 3.3m, respectively. This suggests to me that a rotation rate of 8.5 revs per minute and a radius of 4m should be workable. Most of the adaptation phase can be carried out on the ground.

SpaceNut · 2014-10-30 18:05:38

Pilgrim Observer
Level Dist from center Centrifugal Accel Gravity
Level 6 10.16 m 0.447 m/s 0.05g
Level 5 12.70 m 0.559 m/s 0.06g
Level 4 15.24 m 0.671 m/s 0.07g
Level 3 17.78 m 0.782 m/s 0.08g
Level 2 20.32 m 0.894 m/s 0.09g
Level 1 22.86 m 1.006 m/s 0.10g

Deck level layouts

Last edited by SpaceNut (2014-10-30 18:09:53)

GW Johnson · 2014-10-31 10:30:47

Standing vs lying down in Earth's essentially-constant surface gravity field (relative to human dimensions) induces a slight difference in blood pressure head-to-toe standing that does not happen lying down. We did evolve to cope successfully with this. That gradient is linear, and essentially models as dP = h*g*dens, the buildup of fluid pressure with depth, based on g*dens the weight density, with a constant value of g. The gradient da/dheight = 0 for this field.

If "g" also varies with height because of a strong gradient da/dr = 2*r*w (because a = r*w^2) from using short r and high w, the variation in blood pressure head-to toe is both nonlinear and more pronounced. We did not evolve to cope with that.

Because it is a change from what we evolved in, it will have some effect on health for long exposure times (on the order of 2-3 years), just at an unknown level. That may or may not be a significant effect. My point is, no one knows, as regards 2-3 year exposure times. Short centrifuge experiments simply cannot address long-term effects.

To reduce the risk of artificial gravity gradient effects upon long-term health, the obvious choice is just reduce the gradient. If you hold a constant and choose a longer r, w will be much smaller, because of the w^2 effect. Even though r is larger in the gradient, the much smaller w effect dominates, reducing the gradient.

This is independent of the choice of a. Only the final design dimensions differ. I see some on the forums think 0.3 gee is enough, in contrast to the 1 gee I have looked at. You may be right, I do not know.

GW

Terraformer · 2014-11-01 12:14:44

Well, I think it's clear that, before we build any ship with ersatz gravity, we need to do some proper research into both long term limits on gravity levels and rotation.

I don't think we're going to need gravity for health reasons, because I'm confident that we'll develop other means of stressing the body. But it does make doing stuff a lot easier. Perhaps 0.2g will be enough, to use normal plumbing? A 5m radius centrifuge at ~6RPM would be able to provide that. It's drops to ~0.13g at the head, but the gradient shouldn't be a problem. I'm thinking of two such centrifuges being mounted in modules which are then attached together, balancing each other out. A third, zero-g module connects the two at the front, and there are a few rings which move slower to make it easy to transition between the centrifuge and the freefall area. Obviously, that leaves a volume at least 2.5m in radius in the middle of each module, which could be used for storage or sleeping quarters (but I hope to have enough room on the outer deck for that).

That way, we can have our Cool Starship.

Tom Kalbfus · 2014-11-01 18:23:47

Terraformer wrote:

Well, I think it's clear that, before we build any ship with ersatz gravity, we need to do some proper research into both long term limits on gravity levels and rotation.
I don't think we're going to need gravity for health reasons, because I'm confident that we'll develop other means of stressing the body. But it does make doing stuff a lot easier. Perhaps 0.2g will be enough, to use normal plumbing? A 5m radius centrifuge at ~6RPM would be able to provide that. It's drops to ~0.13g at the head, but the gradient shouldn't be a problem. I'm thinking of two such centrifuges being mounted in modules which are then attached together, balancing each other out. A third, zero-g module connects the two at the front, and there are a few rings which move slower to make it easy to transition between the centrifuge and the freefall area. Obviously, that leaves a volume at least 2.5m in radius in the middle of each module, which could be used for storage or sleeping quarters (but I hope to have enough room on the outer deck for that).
That way, we can have our Cool Starship.

Why would you want to stress the body in zero-gee? Humans are designed for living under one g, not zero g. Seems to me, if you want to keep them adaptable to Earth, you keep them in an Earthlike environment. floating around is no guarantee that they will be able to walk under 1-g after spending years in weightlessness, its not just a matter of muscle tone, but also with coordination. If a person hasn't used his legs for walking, how do you know he will know what to do when the opportunity finally arises. Remember how difficult it is to get a robot to walk. What if one's legs forget how to walk after spending a decade in space? A person could easily fall on his face if hi walking reflexes are not in tune.

GW Johnson · 2014-11-02 15:29:21

Stressing the body in zero-gee by using resistance exercise might work, for a little while. That's what they did on shuttle, and it's what they do on ISS. Most ISS crews are limited to 6 months, some crews apparently will be risked for up to 1 year this way. That's way short of what would be required for any sort of Mars mission, or any sort of in-situ mission to an NEO.

What THAT says is "it's all about the artificial gravity, stupid!"

That's the very thing we have never looked at on ISS, and the very thing that never appears in any of NASA's mission plans involving SLS and Orion. Now, do you understand why they wants robots to tow an asteroid (a very small one) to the moon, so they can go back to the moon with nothing but a 2-week moon mission, and call it an "asteroid mission"?

There are groups within NASA that are looking at manned Mars missions, yes. But the things that are seriously funded have NOTHING to do with sending men to Mars. THAT tells you what the REAL priorities are, no matter who says what about them otherwise. These priorities are set by Congress, which is about as competent to decide space program priorities as your average lab rat. Or maybe just your average cockroach.

It is no different in any of the other countries. We all do this completely wrong, around the globe.

Now, I don't know what level of gee might be therapeutic for maintaining the health of crews that must return to Earth. No one does, for sure. Those experiments have never been directly done. We evolved at 1 gee, but I saw some rather-credible stuff earlier in this thread that said 0.3 might be enough. So, we have bounded the answer as 0.3 gee < req'd gee < 1.0 gee. We know FOR SURE 1 gee will work, but higher gee is more expensive. Period. Hard fact of life.

I consider the gee gradient issue unresolved. All we know is that faster spin rates inherently cause higher gradients. There is as yet no answer on this that is in any way credible. Other than "mimimize the gradient". Those experiments MUST be run.

There are short term answers that say large rpm will be OK, but zero long-term experiments have been done. There's a lot of 1950-1970's experience suggesting that 3-4 rpm is about the long-term max for untrained civilians. There's no proof of anything, though. Those experiments still need to be run. Period. End of issue. We have not yet even bounded that problem.

Assuming that 4 rpm is acceptable for multi-year exposures, then 1 gee requires a 56 m radius, and 0.3 gee needs a 19 m radius. You either do that with cables, a truss, or a docked-module baton shape. What other choices are there? I see none.

Go with the one with the least vehicle inert fraction for whatever ship design you are looking at. Simple enough. There's probably different selections that are "best" for the different possible ship design approaches. "Surprise, surprise", as Gomer Pyle would say.

Any space agency or private corporation looking to send men to Mars using chemical (or even nuclear thermal) rocketry should be doing the experiments to resolve these issues. To make electric propulsion feasible for faster trips, to circumvent the microgravity problem, will require technological breakthroughs on power supplies, breakthroughs that have not happened yet.

A rule of thumb regarding technology development, which is always a proper thing to be doing: attempt vehicle designs only with existing ready-to-apply technology. If you make technology development success a requirement for your vehicle program, you WILL NEVER fly.

Welcome to the real world. Sorry to be the bearer of bad tidings. But at least I can point toward the most fruitful way forward: go find out (best possible speed) how much gee is enough, AND how much spin rate can we stand, both answers for very long term exposures(years).

GW

Terraformer · 2014-11-02 16:00:16

Alas, the Mars Gravity Biosatellite never got off the ground. Perhaps Musk would be willing to fund and fly such an experiment, if any of the coming flights have payload left over? He needs to know, after all. It won't measure the effect of gradient or long term centrifuge effects, but the latter can be done on the ground. Anyone up for keeping (and breeding?) rats in a centrifuge? Maybe The Mars Society could do this?

Or maybe it could fly on the ISS, if Dragon has space left over on it's manifest. That would make it easier to do the analysis, since they wouldn't need a reentry vehicle. I don't know where the experiment could be mounted - the end of a module, perhaps? If it's a metre in radius, spinning at 20RPM with multiple decks for variable gees...

I don't know how NASA could oppose it, unless they are actively opposed to deep space travel.

Antius · 2014-11-03 13:49:11

GW Johnson wrote:

Stressing the body in zero-gee by using resistance exercise might work, for a little while. That's what they did on shuttle, and it's what they do on ISS. Most ISS crews are limited to 6 months, some crews apparently will be risked for up to 1 year this way. That's way short of what would be required for any sort of Mars mission, or any sort of in-situ mission to an NEO.
What THAT says is "it's all about the artificial gravity, stupid!"
That's the very thing we have never looked at on ISS, and the very thing that never appears in any of NASA's mission plans involving SLS and Orion. Now, do you understand why they wants robots to tow an asteroid (a very small one) to the moon, so they can go back to the moon with nothing but a 2-week moon mission, and call it an "asteroid mission"?
There are groups within NASA that are looking at manned Mars missions, yes. But the things that are seriously funded have NOTHING to do with sending men to Mars. THAT tells you what the REAL priorities are, no matter who says what about them otherwise. These priorities are set by Congress, which is about as competent to decide space program priorities as your average lab rat. Or maybe just your average cockroach.
It is no different in any of the other countries. We all do this completely wrong, around the globe.
Now, I don't know what level of gee might be therapeutic for maintaining the health of crews that must return to Earth. No one does, for sure. Those experiments have never been directly done. We evolved at 1 gee, but I saw some rather-credible stuff earlier in this thread that said 0.3 might be enough. So, we have bounded the answer as 0.3 gee < req'd gee < 1.0 gee. We know FOR SURE 1 gee will work, but higher gee is more expensive. Period. Hard fact of life.
I consider the gee gradient issue unresolved. All we know is that faster spin rates inherently cause higher gradients. There is as yet no answer on this that is in any way credible. Other than "mimimize the gradient". Those experiments MUST be run.
There are short term answers that say large rpm will be OK, but zero long-term experiments have been done. There's a lot of 1950-1970's experience suggesting that 3-4 rpm is about the long-term max for untrained civilians. There's no proof of anything, though. Those experiments still need to be run. Period. End of issue. We have not yet even bounded that problem.
Assuming that 4 rpm is acceptable for multi-year exposures, then 1 gee requires a 56 m radius, and 0.3 gee needs a 19 m radius. You either do that with cables, a truss, or a docked-module baton shape. What other choices are there? I see none.
Go with the one with the least vehicle inert fraction for whatever ship design you are looking at. Simple enough. There's probably different selections that are "best" for the different possible ship design approaches. "Surprise, surprise", as Gomer Pyle would say.
Any space agency or private corporation looking to send men to Mars using chemical (or even nuclear thermal) rocketry should be doing the experiments to resolve these issues. To make electric propulsion feasible for faster trips, to circumvent the microgravity problem, will require technological breakthroughs on power supplies, breakthroughs that have not happened yet.
A rule of thumb regarding technology development, which is always a proper thing to be doing: attempt vehicle designs only with existing ready-to-apply technology. If you make technology development success a requirement for your vehicle program, you WILL NEVER fly.
Welcome to the real world. Sorry to be the bearer of bad tidings. But at least I can point toward the most fruitful way forward: go find out (best possible speed) how much gee is enough, AND how much spin rate can we stand, both answers for very long term exposures(years).
GW

One possible solution would be to buy a second hand Gravitron, or build something similar:

http://en.wikipedia.org/wiki/Gravitron

Weld decks to the inside, fit it out with living quarters and then rotate it to produce a 1g force. The vectors of real and artificial gravity will provide a 1.4g effective gravity at the outer edge. The deck will need to curve towards the horizontal as you head towards the centre, so that pseudogravity is always perpendicular to the floor.

'The ride is completely enclosed, with 48 padded panels lining the inside wall. Riders lean against these panels, which are angled back. As the ride rotates, centrifugal force is exerted against the pads by the rider, removing the rider from the floor, due to the slant. The ride can reach a maximum speed of 24 rpm in less than 20 seconds, due to the 33 kW 3-phase motor. At this speed, the riders are experiencing centrifugal force equivalent to three times the force of gravity.'

All you need then is a few spacecadet nutjobs who down spending a year inside a rotating steel can for the advancement of science.

GW Johnson · 2014-11-03 14:18:59

Why not do it right? 5 Bigelow modules connected by cables, parked not far from ISS, and spinning. Let the actual astronauts experiment with it.

The odd module is the center, and provides a safe entry point to get on and off the spinning lab.

By changing lengths and spin rates, you can investigate 4 to 40 rpm, and 0.2 to 1.1+ gee easily, and get the tolerable gee gradient information, too. We've got until at least 2020 to do this. How much could 5 Bigelow modules possibly cost, when supplied from ISS?

GW

SpaceNut · 2014-11-03 21:24:55

5 Bigelow modules connected by cables

I am not sure, could you creat an image of what you intend...

GW Johnson · 2014-11-09 13:38:46

Sorry, I can hardly figure out how to write text here. Inserting some kind of image? I have no clue. At my age it's rather unlikely I ever will.

What I had in mind was 5 modules arranged in a linear array, perhaps connected with cables (or trusses), and maybe some kind of adjustable-length personnel transfer tunnels between them. The middle one is at the spin center of what amounts to a baton-shaped object. That's the zero-gee module. The two end ones get the most a=V^2/R, the intermediate ones get less.

But with 3 radial levels like that, you can directly compare experimentally the effects of gee and spin rate from zero gee to full gee. If you rig this for various baton lengths, you can use multiple combinations of inter-module dimensions and spin rates, so that you can investigate multiple levels of spin rate as well as gee, all with the same facility, over time. It's spin rate and required gee-to-be-therapeutic that we need to know.

GW

SpaceNut · 2022-01-23 14:26:46

bump

Mars_B4_Moon · 2024-03-02 08:19:24

Groundbreaking Results from Space Station Science in 2023
https://www.nasa.gov/missions/station/i … e-in-2023/
JAXA (Japan Aerospace Exploration Agency) developed the Multiple Artificial-gravity Research System (MARS), which generates artificial gravity in space. Three JAXA investigations, MHU-1, MHU-4, and MHU-5, used the artificial-gravity system to examine the effect on skeletal muscles from different gravitation loads – microgravity, lunar gravity (1/6 g), and Earth gravity (1 g). Results show that lunar gravity protects against loss of some muscle fibers but not others. Different gravitational levels may be needed to support muscle adaptation on future missions.

Lunar gravity prevents skeletal muscle atrophy but not myofiber type shift in mice
https://www.nature.com/articles/s42003-023-04769-3

Gravity of the Situation: Time for the “G-whiz” Factor?
https://spaceref.com/space-stations/gra … rtificial/

Mars_B4_Moon · 2024-04-19 13:41:15

a question asked

Will a human mission to Mars require artificial gravity?
https://space.stackexchange.com/questio … al-gravity

New Mars Forums

Announcement

#26 2014-10-29 22:55:58

Re: Artificial Gravity

#27 2014-10-30 07:29:08

Re: Artificial Gravity

#28 2014-10-30 18:05:38

Re: Artificial Gravity

#29 2014-10-31 10:30:47

Re: Artificial Gravity

#30 2014-11-01 12:14:44

Re: Artificial Gravity

#31 2014-11-01 18:23:47

Re: Artificial Gravity

#32 2014-11-02 15:29:21

Re: Artificial Gravity

#33 2014-11-02 16:00:16

Re: Artificial Gravity

#34 2014-11-03 13:49:11

Re: Artificial Gravity

#35 2014-11-03 14:18:59

Re: Artificial Gravity

#36 2014-11-03 21:24:55

Re: Artificial Gravity

#37 2014-11-09 13:38:46

Re: Artificial Gravity

#38 2022-01-23 14:26:46

Re: Artificial Gravity

#39 2024-03-02 08:19:24

Re: Artificial Gravity

#40 2024-04-19 13:41:15

Re: Artificial Gravity

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