Artificial Gravity

Antius · 2014-10-22 08:18:13

I spent a little time reading into artificial rotational gravity studies and uncovered some interesting results. I had previously read that rotational gravity required strict limits on rotation rates in order to avoid disorientation problems and sickness relating to gravity gradients and coriolis forces. I can remember reading that the practical limit was 2rpm in order to avoid disorientation effects. To produce a fairly minimal 0.1g, this would necessitate a rotational radius of 22m. This would effectively limit artificial gravity to large cycler spacecraft and habitats, putting it beyond the scope of early interplanetary transportation.

However, there is some evidence that these early criteria were over conservative. One MIT study has apparently demonstrated that the human body is capable of almost complete adaptation to a rotation rate of 23rpm after just a few days. This involved 1 hour sessions in a short rotation centrifuge. After day 4, all participants had fully adapted to the conditions. This is impressive when one notes that it implies a full 1g gravity level at a radius of just 2m and the subjects head is very close to the axis of rotation.

http://adsabs.harvard.edu/full/2002ESASP.501..151H

I also managed to obtain a copy of Giles Clement’s ‘Artificial Gravity’ which discussed in detail the results of human and animal studies to date. In spite of the importance of the topic for deep space travel, there has been surprisingly little human study, with a planned rotating module on the ISS being cancelled due to lack of funding. The Soviets did however carry out orbital animal experiments that appear to verify that rotational gravity counteracts the effects of prolonged microgravity (cardiovascular degeneration and bone demineralisation).

‘Flight Animal Experiments

The Soviet space research community expressed an early and intense interest in artificial gravity. In 1961 Soviet scientists began testing rats and mice in parabolic flights, in which 25 seconds of weightless are provided for each parabola flown. The posture and locomotion of the animals appeared normal during brief periods of 0.3 g exposure, thus setting this as a minimum gravity level requirement for locomotion (Yuganov et al. 1962, 1964).

The first animals to be centrifuged in space were flown on the 20-day Cosmos-782 mission in 1975, when fish and turtles housed in containers were centrifuged at 1 g. The center of the containers was placed at 37.5 cm from the center of a platform rotating at 52 rpm. After the flight, the physiology and behavior of the centrifuged animals was indistinguishable from their 1-g ground and 0-g flight controls. Furthermore, turtles centrifuged at levels as low as 0.3 g showed none of the muscle wasting that is typically associated with exposure to weightlessness (Ilyin and Parfenov 1979).

In 1977, a significantly more extensive investigation was executed using rats that were centrifuged during the 19-day mission of Cosmos-936. The rats were kept in individual cages and were not restrained. Their cages were placed in a centrifuge with a radius of 32 cm. An artificial gravity level of 1g was obtained by spinning the centrifuge at 53.5 rpm (Figure 3-12).

Results revealed that in-flight centrifugation had a protective effect on the myocardium and the musculo-skeletal system, as compared to the animals that were exposed to microgravity and not centrifuged. However, there were some adverse effects of the in-flight centrifugation that were noted in the visual, vestibular, and motor coordination functions, such as equilibrium, the righting reflex, and orientation disorders. These deficits may have been the result of the high rotation rate of the centrifuge and the large magnitude of the gravity gradient (Adamovich et al. 1980).

Another series of experiments involved rotating four rats on suborbital rockets during a 5-min period of free fall. The rocket was rotated about its longitudinal axis using a special motor at a rate of 45 rpm. The rotation created a variable artificial gravity field of from 0.3 to 1.5 g along the boxes that housed the rats. The movements of the rats were recorded on film and showed than one rat stayed in a position where the artificial gravity level was about 0.4 g, whereas the other three settled down where the artificial gravity level was 1 g (Lange et al. 1975).

Small radius high rotation-rate centrifuges have been flown in the Spacelab of the Space Shuttle and in the Skylab, Salyut, and Mir space stations to conduct experiments on bacteria, cells, and other biological specimens. Results indicate that microgravity effects, especially at the cellular level, may be eliminated by artificial gravity (see Clément and Slenzka 2006 for review).’

From these studies, I believe that several important conclusions can be drawn:

1. Human beings are capable of full adaptation to rotation rates up to 23rpm, sufficient to generate 1g at a radius of 2m. This occurs surprisingly quickly and may be achieved by preconditioning on the ground.

2. A reduced gravity environment (0.3>1g) is effective in mitigating the physiological effects of microgravity. The Cosmos-782 results in particular tend to suggest that lower levels of gravity down to 0.3g provide most or all of the physiological benefits that would be gained from 1g.

3. Martian gravity at 0.38g would appear to be sufficient to prevent muscle loss in smaller animals and there would appear to be no reason to suspect any difference in larger mammals, i.e. humans.

4. A gravity level of 0.38g at a radius of 3m would require a rotation rate of 10.65 rev/min. At 4m, the rotation rate drops to 9.2rev/min. These are 46% and 40%, respectively, of the rotation rate at which MIT researchers demonstrated full adaptation. The original Mars direct hab had diameter of 8m. Hence, producing centrifugal modules would not appear to present engineering challenges.

5. US experiments noted that astronauts had difficulty perceiving gravity at levels less than 0.2g.

On this basis, there would appear to be no physiological barriers to the adoption of artificial gravity for near future deep space flight. There would also appear to be no reason why centrifugal modules cannot be mounted on asteroids, where gravity levels are beneath human perception (0.001g or less). This would allow manufacturing equipment designed for 1g conditions to operate without redesign.

Decimator · 2014-10-22 13:26:07

Excellent work, Antius. Up till now, I was under the impression that we had zero data on mammalian response to Martian gravity. Or anything between 1 and zero gee, for that matter.

Terraformer · 2014-10-22 13:45:20

So, providing sufficient gravity in ships of radius 10m should not be difficult..?

Tom Kalbfus · 2014-10-23 06:33:31

A 10 meter radius will get you one tenth of a g if you rotate it at 3 times per minute, ideally at a 100 meter radius, you can get a full g of centrifugal force.

Antius · 2014-10-23 07:16:43

To fit within the payload bay of planned heavy lift rockets, the radius of a cylindrical module should be no more than 4m. At that radius, 0.3g acceleration would require a spin rate of 8.2rev/m. All indications are that human beings can adapt to rotation at this angular velocity, although it may take a few days to fully adjust. Most of the acclimatisation can be achieved in ground based facilities.

SpaceNut · 2014-10-23 18:24:19

There are a few threads on the topic here on newmars.... Of course the sites crashed took along with it some very good discussions and data researched by many that are still here Much work was done with this by MIT iirc Mars Gravity Biosatellite» Where does this stand now? but as time wore on its work was turned over to MarsDrive and while MarsDrive was going strong at the time it has now faded into the past much the same as MarsDrive has.
The topic also intertwines with Artificial Magnetosphere - Electromagnetic Induction ....

You can goto any Fair and board the Gravatron or Round up and pratice what gravity would be like but it would push you flat against the wall and you would find it difficult to stand and walk along the wall....

GW Johnson · 2014-10-24 15:53:34

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.

That says you can do this artificial gravity "thing" quite easily at R=56 m for 1 gee at 4 rpm (or 0.33 gee at 4 rpm and 19 m radius), for which the gradient is da/dR = 2rw. Again, gee is proportional to rw^2. So, if you think you can get away with 0.33 gee, you can use a lower radius. But only a little lower, by the gee ratio you think might be applicable.

0.33 gee says R= 56m /3 = 19m. For the same gradient. For the same human blackout susceptibility due to blood pooling in the legs.

You don't get 19 m radii from a 4 m shroud diameter for a one-rocket / one launch scenario. Period. End of issue. You need to give up on that concept. Build your vehicle in LEO from smaller components docked together, using the rockets we already have.

And forget cable-connected designs. There are simply too many failure modes.

GW

RobertDyck · 2014-10-24 17:11:09

GW Johnson: You raise valid points. However, your criterion is contradictory. If a Mars Direct hab is 2 metre high upper floor, and the domed ceiling is 1 metre high at the centre, then that's 3 metres. A radius of 56 metres would require 53 metres of connecting arm, plus another length to the counter weight. If the spent upper stage is used as counter weight, as with Mars Direct, then that's less mass than the hab so connecting arm must be even longer on that side. A rigid connecting arm solves the problem of manoeuvring while rotating, but the weight of the arm itself becomes significant. With a 100+ metre truss (or pipe), you just added a hell of a lot of weight. That much mass just for structure kills the mission.

Tom Kalbfus · 2014-10-25 07:34:47

Antius wrote:

To fit within the payload bay of planned heavy lift rockets, the radius of a cylindrical module should be no more than 4m. At that radius, 0.3g acceleration would require a spin rate of 8.2rev/m. All indications are that human beings can adapt to rotation at this angular velocity, although it may take a few days to fully adjust. Most of the acclimatisation can be achieved in ground based facilities.

Gee, I wonder what happens if one chose to stand up in such a cylinder? the head would be in zero gee while the feet would experience 0.3g. also people standing on opposite sides of the cylinder may tend to bump heads. I think this experience is much like a merry go round that used to be found in most playgrounds before they were banned for safety reasons. I remember going on those. After a while of being spun around on those things, one would get quite dizzy, but a merry-go-round is similar to the centrifugal forces one would feel in such a cylinder. I don't think I would want to live in one, would you? It was fun playing on the merry go round, but I would not want to eat in one while it was spinning. Anyway a centrifuge can also flip end over end as well as spin along its axis.

Tom Kalbfus · 2014-10-25 07:39:28

RobertDyck wrote:

GW Johnson: You raise valid points. However, your criterion is contradictory. If a Mars Direct hab is 2 metre high upper floor, and the domed ceiling is 1 metre high at the centre, then that's 3 metres. A radius of 56 metres would require 53 metres of connecting arm, plus another length to the counter weight. If the spent upper stage is used as counter weight, as with Mars Direct, then that's less mass than the hab so connecting arm must be even longer on that side. A rigid connecting arm solves the problem of manoeuvring while rotating, but the weight of the arm itself becomes significant. With a 100+ metre truss (or pipe), you just added a hell of a lot of weight. That much mass just for structure kills the mission.

How big is the ISS?

Could you imagine a spaceship this big, starting in Earth orbit, heading for Mars?

We already have one, its called the ISS. Could we spin this end over end? Seems like such a waste to just burn it up in the atmosphere after its useful life. What if we cannibalized its central truss, placed a hab module at one end and an engine at another. We could then accelerate it towards Mars and then spin it up for gravity. What's wrong with this?

Last edited by Tom Kalbfus (2014-10-25 07:43:17)

RobertDyck · 2014-10-25 08:12:59

Better to leave ISS operational, in LEO. And do you realize how much propellant is required to launch such a heavy (massive) structure to Mars?

Tom Kalbfus · 2014-10-25 15:33:58

Probably less propellant than is required to launch an equal mass from the surface of the Earth. One could use an ion drive to get this structure into a cycling orbit for instance. I know the plan is to deorbit the ISS when its operational lifespan is up, seems like such as waste. I'm sure the central structure could still be used as part of a centrifuge to provide gravity for astronauts on interplanetary missions. Just take a look at that diagram of the ISS, the central truss is one football field long, rotate it at 3 rpm, and you could have a half gravity in the crew module, certainly better than zero-g. I'm sure we'll be orbiting better space stations than this one in a decade, we can reuse the old structures for other purposes, just strip out the solar panels, put an engine and nuclear reactor on one end, and a crew module with lander on the other end. First the engine puts the ship into a trans-mars orbit, then we rotate this spacecraft, the truss simply has to exist and be structurally sound. I think the first mission ca be asteroid mining, bring the ore back to Low Earth orbit, where it can be processed into other useful structures, such as additional trusses for more interplanetary spacecraft.

Last edited by Tom Kalbfus (2014-10-25 15:34:48)

GW Johnson · 2014-10-26 17:38:05

The "trick" with getting radius is to use stuff you have to have with you anyway. Don't add structure for this, whether truss or cable, that's just dead inert weight. Use the propellant and stores you already have to have anyway. Just dock those modules together into a "long slender baton" shape. Then spin it end over end.

ISS is structurally unsound to be moved anywhere of any significance. Certainly not to Mars. Plus, it's the wrong branched configuration of the docked modules to be spun up for any purpose whatsoever. See also paragraph 1, again.

If 1/3 gee works, then use 19 m at 4 rpm. That's even far easier to do than what I have been looking at for the last few years, which is 1 full gee: 56 m at 4 rpm. BTW, we may find that 1 full gee is needed for successful pregnancy. Nobody yet knows. It's a criterion for colonization ships, not exploration ships.

GW

RobertDyck · 2014-10-26 17:57:23

I respect you Mr. Johnson, but I'm going to debate this point.

You keep saying "baton", but an efficient propellant tank is as close to a sphere as possible. That reduces tank surface area, so reduces tank mass. Reducing surface area also reduce heat loss. Liquid hydrogen is colder than ambient temperature of space, so you want to reduce surface area and use good insulation. 50+ metre radius would be a very long "baton". An upper stage with cylindrical tanks with domed ends, together with a cable, would have less total mass than a baton.

But this may be a moot point. I also envision a ship with a truss for colonization. And heat shield for aerocapture. The tether is for exploration; keep the initial human mission small.

One option to resolve the manoeuvring problem: don't. De-spin at the mid point, reel in the counter weight. It would take more propellant to de-spin, but would solve the manoeuvring problem. Then re-spin and reel out the counter weight for the final leg to Mars. Upon approach to Mars, cut the cable and let the counter weight fly off into space. Would a mission work with only 3 course adjustments? After TMI, before spin. Then mid-course. Final adjustment upon approach to Mars. All three would be in zero-G, no rotation.

Tom Kalbfus · 2014-10-27 09:22:27

RobertDyck wrote:

I respect you Mr. Johnson, but I'm going to debate this point.
You keep saying "baton", but an efficient propellant tank is as close to a sphere as possible. That reduces tank surface area, so reduces tank mass. Reducing surface area also reduce heat loss. Liquid hydrogen is colder than ambient temperature of space, so you want to reduce surface area and use good insulation. 50+ metre radius would be a very long "baton". An upper stage with cylindrical tanks with domed ends, together with a cable, would have less total mass than a baton.
But this may be a moot point. I also envision a ship with a truss for colonization. And heat shield for aerocapture. The tether is for exploration; keep the initial human mission small.
One option to resolve the manoeuvring problem: don't. De-spin at the mid point, reel in the counter weight. It would take more propellant to de-spin, but would solve the manoeuvring problem. Then re-spin and reel out the counter weight for the final leg to Mars. Upon approach to Mars, cut the cable and let the counter weight fly off into space. Would a mission work with only 3 course adjustments? After TMI, before spin. Then mid-course. Final adjustment upon approach to Mars. All three would be in zero-G, no rotation.

I think one rotating cycling hab would be sufficient. You do a one way colonization mission, you have a truss, reactor fuel tanks and engine are at one end of the truss, and the Mars hab is at the other, you rotate the whole assembly after the ship has accelerated to Trans-Mars-Injection. Upon reaching the vicinity of Mars, the hab section seperates from the main craft. The hab's engines put it on an intercept course with Mars' atmosphere, heat shields, parachutes and landing rockets get the hab onto the Martian surface. The rest of the Interplanetary ship returns to Earth orbit, slows down, is refueled and another Martian hab is attached to the other end of the Truss, and another 4 people are sent over to Mars on the next trip.

GW Johnson · 2014-10-27 09:49:59

My entire point is why use a truss if you have propellant tankage anyway? Let cylindrical tank units be your "truss", and do it reusably, exactly as Tom described. That concept Tom described assumes your hab is your lander is your base. Some of the other concepts do not make that assumption, but they all work basically exactly the same way.

I know the "perfect" shape for a pressurized propellant tank is a sphere, but a very close second is cylinders with hemispherical end domes, which are more convenient for many designs, especially as the end skirts can serve many functions where these are docked together.

The experience of the last 60 years makes it quite clear there is nothing wrong with long slender cylindrical tank designs. These can be docked end-to-end to make a really long vehicle resembling the "Discovery" as depicted in the old movie "2001". You use parallel docking as well, to be able to shed empties without losing your length. Spun as a baton for artificial gravity eliminates the need for the heavy centrifuge depicted for the movie "Discovery".

The inert fraction of tankage like that is a little higher than the "perfect" sphere, yes. But the additional vehicle inert fraction added by a truss is just higher still. The more important factor here is vehicle inerts, more so than individual component inerts.

As I have said before, you want to reuse as much of your orbital transport as possible, over multiple missions. That way you amortize costs, and no one mission has to absorb it all. That's a very important facet to selling missions to stingy congresses and agencies.

The same basic orbital transport design could serve any destination in the Solar System inside the asteroid belt, with just the technology we have ready to employ today. It does require that we quit thinking rocket-and-payload, one-launch-per-mission. But that change also eliminates the need for costly gigantic rockets that serve only one purpose, which frees up funds for the missions. Chicken-and-egg, but only because we are still refusing to jettison traditions that no longer serve.

It is also a flexible-enough design approach to switch out engines for better technologies as they come along, and even to switch out tanks (not all cryogenics are the same). Let it change and modify as time goes by, just as with naval vessels.

GW

Last edited by GW Johnson (2014-10-27 09:54:15)

RobertDyck · 2014-10-27 13:12:10

Sounds like we're converging on a single design. The only point of debate is tether vs truss vs baton.

Tom Kalbfus · 2014-10-27 13:24:26

The ship rotates when it not accelerating, and its on a free return trajectory to Earth, the engine fires exactly twice, first to get into its cycling orbit and the second time to get out of it and back to Earth orbit, to be used again. With each mission we add four people and another had to the Mars base. The nuclear reactors arrive separately and prior to each hab.

RobertDyck · 2014-10-27 13:39:59

Worst possible asteroid spacecraft:

Orion capsule, ATV service module, plus one full ISS module, plus an ISS node, plus a co-axial mounted propulsion stage, plus an off-axis addition with solar panels and radiators. Off-axis? Really? And they expect to steer with that thing.

Much better to design for Mars, then adapt for an asteroid or the Moon. Robert Zubrin designed a modification of Mars Direct for the Moon.

GW Johnson · 2014-10-29 09:37:48

Now you know why they'd prefer to capture an itty-bitty one with a robot, and relocate it near the moon. That way they can use an Apollo-on-steroids moon rocket design to reach it. They need not do anything they haven't ever done before.

The only problem with that approach is two-fold: (1) what they did before will never, ever, ever take men to Mars (or anywhere else outside cis-lunar space), and (2) after so many years, there's real doubt they can actually recreate the ability to reach cis-lunar space with men.

Too many of the mission objective and design decisions are being made by the buffoons in congress, not the engineers (who no longer have the applicable experience, since all the ones that did have died or retired).

Actually, visionary private outfits like Spacex have a better chance of actually sending crews to Mars, slim as that chance currently appears to be. Government agencies like NASA know this, and will do everything in its power bureaucratically to prevent being upstaged like that. That's just the "human nature" of bureaucrats.

GW

Last edited by GW Johnson (2014-10-29 09:40:55)

RobertDyck · 2014-10-29 14:03:34

You realize that I wanted to be an aerospace engineer since I was a pre-schooler. I went into computer science because there was no aerospace engineering in my province in Canada. And no prospects for any aerospace engineers, anyway. So I identify as an engineer.

GW Johnson · 2014-10-29 17:34:18

Robert Dyck:

I do understand the aspiration to be an aerospace engineer. I shared it, it's just that I did so decades ago. I am now too old for any "established" outfit to consider hiring me (the fatal age discrimination sets in at age 40-45, and I am now 64). That's why I am now a teacher. And an occasional consultant.

Further, like your experience, down here all across the US nobody wants anything but a young kid they can under-pay. Young kids like that have "book learning", but zero practical experience. That's been true since the mid-1970's. Quality and ability have no value anymore in the US aerospace industry, by-and-large.

It's the practical experience, not the book learning, that enables engineers to do "the impossible". Doing "the impossible" is what has to happen in order to send men to Mars, or anywhere else beyond cis-lunar space.

Exceptions to the general age discrimination are very rare, indeed. Even Spacex employs no one over age 45 (basically Musk's age). None of the things I did has proven to have any value in today's job market. No matter how sophisticated, no matter how rare the specialty. (And I had many very-deep specialties, not the least of which was ramjet propulsion.)

GW

SpaceNut · 2014-10-29 19:50:56

Yes I too have been in the bracket of age casts off as I was part of a group hired with mostly 30 year olds at my current employment myself being the youngest of 3 over 50. It was the first college level classes that these fresh out of college kids found out that I could keep up with them not only with the texts but with the use of computers as well. They not only accept me but now comes for my advanced abilities to solve problems in the work enviroment.

Now back to gravity unfortunately what we are talking about is not gravity at all but Centrifugal Force and Centripetal Force either by means to create one the other or both. The Formula Fc = mv2/r is used for both forces...

Centripetal

http://www.diffen.com/difference/Centri … etal_Force

GW Johnson · 2014-10-29 20:34:38

Hi Spacenut:

From a physics standpoint, there's no effective difference between "real gravity" and centripetal/centrifugal acceleration, excepting the gee gradient along the radius. Grossly speaking, acceleration is acceleration, it quite literally doesn't matter how it is produced.

The gradient effect leads to people blacking out, if you spin too fast at too low a radius. It's essentially a tidal effect upon blood pressure when standing or sitting-up. The gradient of which I speak is the derivative of centripetal acceleration with radius of a = r w^2, which is 2 r w, where r = radius, and w = spin rate (consistent units, of course).

Basically, you want very little difference in gee between your head and your feet, when standing. I have yet to see a trustable criterion for that, but I'm sure there already is one, somewhere out there in the biomedical field.

From a practical standpoint, there are real limits on the spin rate that can be tolerated without deranging the balance organs in the ears. People can be be "trained" (acclimatized, really) to high spin rates on the order of 10-20 rpm, yes. No one knows for how long this can be maintained, though.

Untrained "ordinary folk" can tolerate 3-4 rpm pretty readily, and for very extended periods of time, essentially indefinitely. That's just experiences since WW2 talking. That long-term experience I trust. The higher figure, I do not trust.

I would suggest from evolutionary history that we design for 1 full gee in the daily work stations, which is 56 m radius at 4 rpm. If you believe (and it may well be true) that 0.33 gees is "good enough" to be therapeutic in terms of microgravity diseases, then you can get away with 19 m radius at that same 4 rpm. Neither radius is a "killer" in terms of practical design, unless you are way-too-wedded to traditional spacecraft design concepts.

Sleeping quarters do not have to be located at that full design gee level. The bed rest studies suggest, among other things, that when people are prone, their health derives no benefit from any gee, 1 gee or lower. What that suggests is that spacecraft sleeping quarters and supply storage areas be located closer to the rotation center, while daily work stations be located farther out, at the full design gee. Simple enough.

GW

RobertDyck · 2014-10-29 22:47:19

I'm now 52 years old. I could give you my long sob story, but I've been unemployed for years. I've been a computer repair technician since October 2008, and self employed doing that job since July 2010. I still can't get a regular job. I send out job applications every week, but no response. I got a few days work as an official for the city election; city employees said I did an excellent job. One said the city needs someone like me; that I should apply. Well, I have; and I did again. But no response to my applications. At least one factor is my age.

In 1998 I was a contractor for the city; I was "Information Systems Specialist", which is one notch senior to Systems Analyst. The next position senior to that was manager of the computer department for the entire city department. I was hired because the province decided to absorb the city department of social services. They took on social workers and supervisors of social workers, but not support staff such as computer staff. I was hired in case computer staff accepted positions elsewhere in the city. They did; in fact the computer manager left before the department closed. All winter the last year I ran the computer for the city department. My job ended when the city department did. That was the contract. I did the job, and did it well. But now they won't even respond to my applications.

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