Artificial Gravity

[Lampyridae]

Zero-gee is an unhealthy environment, plain and simple. Rigorous exercise and pressure pants can counteract some of the effects but is not even close to 100% effective. It also affects individuals differently. There are no real data on non-exercising astronauts, but bed-ridden patients can reach the 33% genetic baseline bone density level after only a few years. 50-66% is the easy fracture zone. Cosmic radiation also screws up bone structure. With rigorous exercise in the right individual, the bone loss can be as little as 1% a year. But if an astronaut is seriously injured and can't exercise... odds are they won't survive the 10gee Apollo-style re-entry.

Quite possibly even low gravity environments are unhealthy as well. Short doses of hypergravity may be able to mostly counteract this.

[GI-Thruster]

On a related issue, it's instructive that rotation generates artificial gravity at all.  Why is that?  If we believe gravity and inertia are separate forces, how can we mimic gravity with rotation/inertia?

No, I don't need someone to chime in about how centrifugal force works.  I'd have to respond that it is by definition a "false force".

Point being, the fact we can simulate gravity with inertia through rotation says that there is a real connection between gravity and inertia.  We can't after all simulate gravity with electromagnetism.  But inertia does the trick.  Why?

Well, probably because Ernst Mach was right and that it is gravity that causes inertia.  Not virtual particles in the ZPF.  Not an intrinsic property of matter.  Gravity, acting on all the Universe's mass, causes it's constituent parts which are causally connected by gravity, to have inertia.

If that's true, Dr. Woodward's thrusters ought to work as well as all the rest of the gravinertial technology that will one day hopefully come from the science.

[Lampyridae]

GR chooses inertial frames, it doesn't say how they're created.

So, despite Einstein’s hopes, general relativity does not in any way explain or obviate the principle of inertia. Granted, if the field equations didn’t include the trace term (so that the covariant divergence didn’t vanish), the resulting theory would have many problems and be subject to many objections, but this goes without saying. No one disputes that the principle of inertia is extremely well-founded in observation.  It is an extremely well-justified postulate – but it is still a postulate. General relativity does not explain inertia, nor does it dispense with the need to organize our spatio-temporal theories on the topology and morphology implicit in the principle of inertia and the associated distinguished coordinate systems.

http://www.mathpages.com/HOME/kmath588/kmath588.htm

So, just like the critics of the time were saying, you can't make everything covariant. You need some kind of coordinate system after all!

And there's also this:

Furthermore, once we recognize that the inertial and gravitational field are one and the same, the twins paradox becomes even more acute, because we must then acknowledge that within the theory of relativity it's possible to contrive a situation in which two identical clocks in identical local circumstances (i.e., without comparing their positions to any external reference) can nevertheless exhibit different lapses in proper time between two given events. The simplest example is to place the twins in intersecting orbits, one circular and the other highly elliptical. Each twin is in freefall continuously between their periodic meetings, and yet they experience different lapses of proper time. Thus the difference between the twins is not a consequence of local effects; it is a global effect. At any point along those two geodesic paths the local physics is identical, but the paths are embedded differently within the global manifold, and it is the different embedding within the manifold that accounts for the difference in proper length. (The same point can be made by referring to a flat cylindrical spacetime.) This more general form of the twins paradox compels us to abandon the view that physical phenomena are governed solely by locally sensible influences. (Notice, however, that we are forced to this conclusion not by logical contradiction, but only by our philosophical devotion to the principle of sufficient cause, which requires us to assign like physical causes to like physical effects.) Likewise the identification of gravity with local spacetime curvature is untenable, as shown by the fact that a suitable arrangement of gravitating masses can produce an extended region of flat spacetime in which the metrical field is nevertheless accelerating in the global sense, and we surely would not regard such a region as free of gravitation.

This sounds a LOT like the mm/s errors we're getting with gravity slingshots. And also the nanosecond errors in GPS, despite GR correction for all effects. It also bakes my noodle. Time could be running at different rates elsewhere in the universe, even with zero relative velocity and gravitational effect. Explains a lot too.

[GI-Thruster]

Bakes my noodle too.  But I'm not willing to grant all the conclusions in the second quote as they seemed to be jumping a bit.  Where did that one come form?  Math Pages both?

We should really have Karl wade in here. . .

[Lampyridae]

Sorry, forgot to reference that. Also from Math Pages but it was on the Twins Paradox. Agreed, though, we need brains greater than ours.

Actually, reading Sciama's paper is REALLY baking my noodle. Centrifugal force is the gravitomagnetism of the rest of the universe!?! (forehead slap)

[Spacenick]

Well, of course it's better to get to Mars in one week rather than in 6 months, whatever speed the technology available lets us use we should use. What I argue is that we don't need to design the whole ship after AG just to make it more complex and waste a lot of space if there is any way to do it differently.
Either by using smaller intergrated centrifuges on the ship for hyper gravity training (say 20 minutes at 2 G jogging per day) or by using the ships accceleration.
What we need is research to find out how much gravity do we need per day and how big does the centrifuge need to be.
What we don't want to get is either ships wasting a lot of space for AG generation and limiting the space usable by the Astonauts or even worse tetheredspinning spacecraft from which one can't even stage an EVA wihtout a lot of hazzle.
We need to conquer the vbad effects of zero g while keeping good things like how much more efficient one can use the available volume.

[KelvinZero]

Making EVAs rather less convenient during a trip to mars doesn't seem a great loss... But I certainly agree that when it comes down to it there is probably a much simpler way that doesnt put a constraint on the entire craft design.

[JohnFornaro]

So we khave two twins, one in a circular orbit, and one in a highly elleptical orbit, such that their paths intersect 30 times a second, or whatever frames per second it is that you see in a movie theatre.  The idea being that they would experience each other as if they were on TV.  One of the twins would age faster than the other, as they interacted on each orbital pass.

Is this just another way of stating the twins paradox?

[spacester]

If one accepts that we need to know the answer to the question of the effectiveness for livability and human health of spin-gravity, and one further accepts that building a Mars-bound spin-gravity ship only makes sense if that answer has been found, then shouldn't one be advocating for a spin-g lab in LEO?

[Patchouli]

I've always preferred the term "spin-g" when talking about the (sole) method available with today's technology. From my archives:

For tons of information on the realities of spin-gravity, see:
This discussion about the effects of micro-g and the issues regarding artificial gravity and this for all the math you could ever want (scroll down to the conclusions) and then this for an architect's view of what it would be like.

I'd like to see a ship built in LEO to be spun for 0.38 g to simulate Mars gravity, and 0.16 g to simulate Lunar gravity (IIRC)....

I agree, an LEO spin gravity lab would be very worthwhile. It could tell us if lunar or Mars gravity is sufficient to maintain health.

Human tolerance to angular velocity could also be tested. Some have suggested humans could become acclimated to high angular velocity just as sailors get used to sea motion. As you point out, for a given fraction of G, diameter goes down with the inverse square of angular velocity.

If we could endure 3 rpm and lunar gravity is sufficient to maintain health, this would drastically reduce mission requirements for spin gravity habs. This could make orbital settlement and/or long interplanetary trips cheaper.

A variable gravity station based off even just a single sundancer module with a counter weight would go a very long way in finding out what the artificial gravity requirements are.

If lunar gravity at 3rpm is all that is needed then you might even be able to get away with a configuration like 2001's discovery vs spinning the whole ship,having a spinning section,or being tethered to a counter weight.

The centrifuge would only have to be 32M wide it might be possible to fit it inside a large inflatable module.
It might be possible to make such module fit inside a 10 to 12M fairing.

Of course the centrifuge would have to assembled inside the expandable module after it is deployed but you would be working inside a pressurized environment.

Though if all you need is short doses of hyper gravity the centrifuge could fit inside a BA330.

But I heard bad things on small diameter centrifuges.

[Spacenick]

What if people would tolerate much higher spin rates then one could mount a very small centrifuge to the ship that is only used during the training sessions, then we could also add another one of them spinning in the other direction to counter the torque that would bring all advantages of gravity (if we find out hypergravity for 1 hour per day is enough) and we wouldn't have to spinn the whole spacecraft.
Spinning a spacecraft and a counterweight around just seems extremely awkward, it might be a short term solution for a trip to mars but imho it fails as a long term solution.

[Hop_David]

If one accepts that we need to know the answer to the question of the effectiveness for livability and human health of spin-gravity, and one further accepts that building a Mars-bound spin-gravity ship only makes sense if that answer has been found, then shouldn't one be advocating for a spin-g lab in LEO?

I certainly advocate a LEO spin-g lab. At least the need seems evident to two of us.

[Hop_David]

But I heard bad things on small diameter centrifuges.

One thing I forgot to mention is apparent gravity gradient.

Spin gravity is w^2 * r
where w is the angular velocity in radians and r is the radius.

If your height is a substantial fraction of the distance from the floor to the centrifuge center, your feet will feel more acceleration than your head. This can also be quite disorienting, I've been told.

But perhaps workers and/or tourists could become accustomed to a steep gravity gradient as well as high angular velocity. This could also be determined in a LEO spin gravity lab.

[GI-Thruster]

It might also be that we could get by with only using the spin lab for sleeping, in which case the gradiant wouldn't matter so much.

[Lampyridae]

According to current research, 5 RPM can be tolerated by most people, possibly as much as 10 RPM. 4 RPM is considered by many NASA papers as the upper limit for spin-grav design but that's based on old research.

IIRC the spin hab in Discovery rotated at 6RPM to produce 1/6 gee. When Frank Poole jogged (prograde / spinward presumably), he would experience probably 1/4-1/3 gee depending on his running speed. If he ran retrograde / antispinward he'd probably be bouncing along experiencing zero gee and kicking off the walls of a  hab that to him appeared to be rotating around him.

[Hop_David]

According to current research, 5 RPM can be tolerated by most people, possibly as much as 10 RPM. 4 RPM is considered by many NASA papers as the upper limit for spin-grav design but that's based on old research.

Very interesting! Could you provide some URLs or journal articles? I'm not being snarky. I ask because I'd like to do further reading.

[Lampyridae]

http://chamberland.blogspot.com/2006/07/dangers-of-artificial-gravity.html

http://www.graybiel.brandeis.edu/history/walthamnews.html

http://www.graybiel.brandeis.edu/history/finalfrontier.html

http://www.ncbi.nlm.nih.gov/pubmed/14501105

http://jn.physiology.org/cgi/reprint/80/2/546.pdf

DiZio and Lackner are pioneering this research. DiZio is such an appropriate name, though!

[Spacenick]

What about a completley differnt approach, why not try whether we can counter zero-g problems by shaking instead of rotation. The thing I'd imagine would basically look like to elevators oposing each other moving back and forth like the cylinders in an engine. Two people would do the gravity fittnes training at the same time each in one of the canisters. They would be suspended inside by a climbing harness that prevents them from dropping on their heads while it allows the to rotate with each cycle. It would basicaly look to them like they were doing saltis it might be a bit awkward but it's simple and guarantees strain on the muscles and acceleration towards the feet. Because of the nature of this device it could be run with g loads simulating a jump from about a meter high.

I know this sounds a little bit crazy, but at least it's antother approach and it might put just the right strain on the body.
If we agree that for the forseeable future all Astronauts will be pretty healthy and well trained people they should be able to get away without to much problems.

[Kaputnik]

The point is, would you rather spend 6 months going there and 6 months coming back and spend a few weeks on the surface, or go there in a few days and spend a whole year exploring the planet? Which is more valuable? Obviously spending the time on the planet.

That's very misleading, in fact it is incorrect.

If you spend six months going and six months coming back, then you're on a conjunction class mission and will have to spend 12-18 months at Mars, not 'a few weeks'.
If you, for some reason, want to cut your time on Mars down to that 'few weeks' then you use an opposition-class mission, but this means that one of your inbound or outbound journeys now doubles in length.

To get the mission below 2yrs total duration requires a fundamentally different approach in propulsion and any such discussions need to bear that in mind.

[GI-Thruster]

Yes well, if you have a 1 gee solution, where you are continually accelerating at 1 Earth gee, then you have artificial gravity for your trip, and a conjunction transit is 2 days while an opposition transit is 5 days.  Stay as long as you like.  All your real troubles are solved.   :-)