Reduced Gravity: the 400-kilogram gorilla in the room

[JoeStrout]

Interesting essay on the need for artificial gravity research over at The Space Review.


I couldn't agree more.  After all these decades, we really should have some idea how much artificial gravity is needed for human health, and what our real rotation tolerances are.

Anybody who plays High Frontier knows that these factors greatly impact how we build livable cities in space!

[Esme]

(08-10-2015, 09:40 PM)JoeStrout Wrote: Interesting essay on the need for artificial gravity research over at The Space Review.


I couldn't agree more.  After all these decades, we really should have some idea how much artificial gravity is needed for human health, and what our real rotation tolerances are.

Anybody who plays High Frontier knows that these factors greatly impact how we build livable cities in space!

Hopefully at some point someone'll decide to try lofting a couple of Bigelow Aerospace Beam330's connected by tethers and try spinning them, and use them to get some decent data on the effects of artificial low-g environments. It's what I'd do if I had a few billion in the bank, anyway! 

I'm skeptical of whether greater-than-fractional rpms will prove to be acceptable. For small radii, you have to be mindful of the difference in apparent g between head and feet, which, if too high, could produce extremely unpleasant and possibly fatal effects, never mind coriolis effects causing problems.  And as the radius increases, the necessary spin rate to provide any given value for g decreases, of course.

So, if I had the money, I'd equip a couple of Beam330's with propulsion units so that each could serve as a spaceship and adjust its orbit, rendevous them, then run some massively over-engineered tethers between them, and use the manouvering units to put them into a very slow spin. Maybe start at 0.001g, see what difference that makes over living in 0g, then 0.01g, then 0.1g and then slowly ratchet up to as high as was deemed sensibly safe or when bad effects started showing up, whichever came first.

[JoeStrout]

The standard figure generally accepted for rotation tolerance is 2 RPM (and this is what High Frontier uses).  Above that, and some people will start to experience motion sickness, especially when they first arrive.

But Al Globus (a long-time collaborator of mine, though I met him in person for the first time just today) recently did a literature review on human tolerance to spin.  He finds that there is actually very little evidence for that 2 RPM figure — that the most careful experiments actually show the threshold is more like 4 RPM.

At 4 RPM, you can build a very small colony; in fact, probably too small to be really a "colony" at all (though it could be quite useful for hotels or other spacecraft).

But, on the other hand, your basic point is right: we won't really know for sure until we try it.

In Al's talk this morning, he pointed out that once we get a large number of space tourists going into orbit, they are not going to want to learn how to use a zero-G toilet.  It's a complicated and unpleasant business.  So there will be a strong incentive for even early hotels to provide some gravity, even when the point is to play in zero-G most of the time.  But exactly how much gravity is needed to make a toilet reasonably easy to use is another unknown!

[Esme]

Is that 2 RPM definitely with the head closer to the axis of spin than the feet? If so, I'm surprised, but if that's what the evidence thus far says, then I bow to the evidence!

I'm envious of you being at that meeting, hope it goes well! That's an excellent point about space tourism , which IIRC is one of the things that Bigelow are interested in. And if we want to construct things the size of O'Neil colonies, even low gravity for teh workers to be able to sleep and relax in between shifts is likely to be better than zero gravity.

[antred]

Didn't O'Neill write something about Zero-G-Honeymoon Suites? I wonder how quickly people will get the hang of doing the deed in zero-G. There's likely going to be more than one accident in the beginning.

[JoeStrout]

@antred, I'm not too worried about that... that's one activity that humans are very good at (and often quite creative).

@Esme, you have a point: all the rotating-room research so far has been done on Earth, where you have a 1G field at an angle to whatever pseudogravity you're adding on top.  So it's hard to be sure how those results will generalize to the real situation where you're spinning in microgravity.  But the results are encouraging, anyway!

[Pye-rate]

I have given this some thought, there is some positions that could be achieved that are impossible with gravity .

[JoeStrout]

I have no doubt. But this is a family-oriented forum, so let's not delve any further into the details.

[Esme]

I've just found the book I had mislaid, Heppenheimer's 'Colonies in Space' from 1978, very inspiring read if you can find a copy. I'm quoting bits from pages 153 to 156 here:

"When our 1975 Summer Study got under way in June, we expected our colony design would look pretty much like O'Neill's cylinders. Thus it was a matter of considerable surprise when Wink Winkler, an undergraduate who was studying for admission to medical school, proposed that the proper rotation rate would not be three rpm but one rpm"

O'Neill was about to go to a Congressional hearing, and asked Winkler "Couldn't we do it at two rpm?", and as Winkler didn't say no, that's what he went with.

"Upon O'Neill's return, Wink was outraged that his reccommendation of a one rpm limit should be treated so lightly. His careful research convinced nearly everyone that the colony really should spin at one rpm"

This was the cause of the Stanford Torus design, as a it meant that the colony would need a diameter of slightly over a mile, which ruled out the earlier cylindrical and spherical designs as being too massive.

I'd add that from the point of view of aesthetics, I just love the O'Neill cylinders, but aside from possible issues with regard to their spin stability, I'm certain we'll be building something much smaller first because of the sheer logistics of trying to get that much mass into orbit, never mind not having sorted out how best to go about zero g construction in practice yet.

But anyway, it's that reference to Winklers work on suitable spin rates that makes me dubious about higher spin rates. Granted, I haven't looked terribly hard, but I haven't come across anything that suggests spin rates greater than 1 rpm will be suitable physiologically, and would be interested to see any evidence to the contrary.

[JoeStrout]

Well, you should read the Globus & Hall paper.  It includes a lot of research from the 80s, which of course nobody would have known about in 1978. But please do share your thoughts on it!

And I'll see if I can get either of those guys to jump into the discussion here.