Why isn't a centrifuge used for astronauts on the space station?

Question

One thing I've always wondered is why don't astronauts sleep in a kind of rotating bed that spins creating force? This would allow them to sleep and would be able to simulate earths gravity. Why don't they do this considering the impacts zero g has on the human body?

Answer 1

The short answer is it would cost a lot of money.

In order to get a 1G force, you'd either need something really big, or rotating very fast. For example, the reference design for the space colonies I'm working on calls for a structure with a 900 meter radius rotating once a minute. For something the size of the ISS, it would have to be rotating much faster. (I will get the actual numbers in a bit, when I'm not in the middle of another project.)

In addition to the rotational speed problem, you have to also take into account that the structure would have a lot of mass in order to be strong enough to support all that (centrifugal) weight - and the more mass you put into orbit, the more it costs.

On top of that, since you probably don't want to make the entire ISS rotating that fast (to keep the mass - and costs - down), you'd need to have a set of bearings between the rotating and the non-rotating parts of the station, preferably one that's big enough to provide a passageway for the crew to go through (so they don't have to put on their space suits to get to bed) - and that bearing is going to - guess what - have a lot of mass that needs to be launched - which means it would cost more money.

Oh, and you'd also have to make sure that bearing isn't leaking, or you'd have to send up more air to replace what got lost - which would cost more money.

There are a host of other issues, but I'm guessing the list I've given already made the ISS designers realize a centrifugal gravity sleeping chamber was probably not something that would fit the project's budget.

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EDIT

OK, I did some calculations. If your centrifuge is 5 meters in diameter, it's got to be rotating at 18.9 RPM for a 1G acceleration at the rim which will be moving at 17.82 kph (11 mph).

Since you don't want the centrifuge to be torquing the station around it, you're actually going to need two counter-rotating centrifuges of equal mass, and both arms of each centrifuge must have the same mass being rotated so everything is in balance. That's not impossible, you could, for example, have a system that pumps a balancing amount of water into each of the four ends - but that adds system complexity, weight and cost. I'm open to suggestions for a better solution.

As Russell Borogove has pointed out, this could be done in an enclosing compartment to eliminate the seal issue, but now you've got to build a vessel 5.5 meters or so in diameter that's twice the width of a centrifuge pod, plus clearances, in length, figure 3 meters. That's a larger diameter but about half the length of the Unity module (4.57 m dia x 5.47 m long), so it's not entirely out of the question. The noise of the equipment and the pods passing each other at 22 mph relative speed would be pretty substantial.

Speaking of the equipment, the centrifuges are going to need motors to start and stop them every time an astronaut goes to bed or gets up. If you don't want to spend all night getting up to speed, you will need a bigger motor, along with a beefier power system to run it. Then, when you're slowing the centrifuge down so the astronauts can get in or out, you don't want to throw away all the energy that was used to accelerate it, so you need an energy storage system. Batteries may come to mind first, but rapid-cycle batteries that could repeatedly store and release enough energy across many cycles would be really heavy and expensive. An alternative would be winding up a flywheel for energy storage, but again, that's going to be heavy and expensive.

Oh, and if you're going to have more than one of those four sleeping pods occupied at a time, make sure the astronauts all have the same sleep cycles: We wouldn't want an early riser having to lie in bed awake waiting for the other guy to get back from dreamland, or astronauts cranky from having been awakened too soon because the centrifuge stopped to let the other one off.

...and make sure there aren't any emergencies that require getting out of bed on short notice - yeah, you could jump out of a pod moving at 11 mph without too much danger of hurting yourself - but make sure you get out of the way before the next one comes along a second and a half later and bops you in the head!

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The math:

$$ \begin{align} a &= v^2 / r = 1G = 9.8\:\mathrm{m/s^2}\\ d &= 5\:\mathrm m\\ r &= 2.5\:\mathrm m\\ \end{align} $$

$$ \begin{align} v^2 &= 9.8\:\mathrm{m/s^2} \cdot 2.5\:\mathrm{m} = 24.5\:\mathrm{m^2/s^2} \\ v &= \sqrt{24.5\:\mathrm{m^2/s^2}} = 4.95\:\mathrm{m/s} = 17.82\:\mathrm{kph} = 11\:\mathrm{mph} \end{align} $$

$$ \text{circumference} = \pi \cdot d = \pi \cdot 5\:\mathrm{m} = 15.71\:\mathrm{m} $$

$$ {15.71\:\mathrm{m} \over 4.95\:\mathrm{m/s}} = 3.17\:\text{sec per rotation} = 18.9\:\text{RPM} $$

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Centrifugal Acceleration

Answer 2

The point of the ISS is to study 0G. 1G sleeping bags defeat the purpose... The humans are experiment subjects too:)

Answer 3

OK let's build a hypothetical cylindrical sleeper system that could fit inside the crewed area of the current ISS for example, and look at some of the issues you would need to address. We'll name it after the famous Bill Haley and the Comets song: Shake, Rattle and Roll.

You can also apply what's learned here to a futuristic, much larger structure for a higher g-force system to produce skeletal stress with hopes of reducing calcium loss.

Find a spare, or currently empty, unused module on the ISS and build a 2 meter diameter, 2 meter long rotating cylindrical "astronaut tumbler". The astronauts sleep along the inside walls, laying parallel to the cylinder's axis, around which it rotates.

Using $\mathbf{a}=-\omega^2 / \mathbf{r}$ the speed required to obtain a modest 1/6 of Earth gravity in order to provide a small but meaningful experience of "laying down" rather than floating is $\omega=1.3 \text{s}^{-1}$ which works out to one revolution every 5 seconds, or a rotation frequency of 0.2 Hz.

There may not be room for six of these, so it's going to be a shared space and astronauts will still need their cubby-holes for personal space, and a separate allotment of time to spend in it. Alternatively, they could pick up and move their personal cubby-holes and either attach them to this rotating frame, or move it back to the wall.

No matter how you look it it, it's more stuff shipped from Earth, which is OK if it offers a significant enhancement to the astronauts' well being or contribution to the science of living in space.

Balance is critical. If one astronaut wants to sleep, a "dummy astronaut" needs to be placed opposite in order not to unduly shake the ISS with 0.2 Hz mechanical oscillation. If the sleeping astronaut moves, the dummy needs to move accordingly, or a servo mechanism at each end of the cylinder must automatically and constantly translate the rotation axis of the cylinder back to the center of mass. More stuff to break and mass to ship from Earth. If there are two people arranged opposite each other and a third wants to join, one person must "re-azimuth" themselves by 60 degrees (or if they are sleeping soundly, be re-azimuthed by their fellow astronaut), or the dummy astronaut could be added opposite the third person.

If someone wants to "get on" or "get off" the whole thing must be stopped and started. That may wake up anyone already "on". Where does that angular momentum come from? If it stopped and started on a regular schedule with a fixed duty cycle, perhaps it could be balanced out by a tiny counter-rotation of the ISS, and each major stop/start cycle would alternate direction so that the net rotation of the ISS were minimal.

The alternative is to build a counter-rotating flywheel either coaxially, or at least nearby. As the load (number of real + dummy astronauts) on the astronauts' cylinder changed, the load on the flywheel would have to be adjusted as well. The flywheel could also have servos to better null out some components of the structural vibrations as long as it were spinning synchronously. You could null out angular momentum at any frequency, so you wouldn't have to change the mass, but if it's not synchronous you are now adding a second exciting frequency to your vibrations, doubling the chances you might hit a particularly dangerous one!

The ISS is in no need for a periodic source of vibration. Unless that servo-system that constantly re-aligned the rotation axis of the cylinder to pass through the instantaneous center of mass of the astronauts in the drum, a cyclic vibration will be transmitted to the ISS frame. This is a problem that must constantly be battled, and has to be dealt with every time an astronaut begins or ends a sleep period, or rolls around too much.

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Low frequency periodic vibrations are the bane of large mechanical structures not previously designed for them.

From The International Space Station (ISS) Researcher’s Guide International Space Station Acceleration Environment:

Vehicle Structural Modes

Vehicle structural modes reside at the low-frequency end of the vibratory portion of the acceleration spectrum. These vibrations fall within the frequency range from about 0.1 hertz to about 5 hertz. These vibrations arise from the excitation of natural frequencies associated with large components of the space station structure, such as the main truss, and with fundamental appendage modes, such as solar arrays. These structures are typically excited by relatively large magnitude, relatively brief impulsive events like during a reboost or by crew locomotive events like push-offs. The driving excitation of such events results in response vibrations as structural ringing damps out. Also, relatively small magnitude vibrations at just the right frequency will give rise to structural resonance. (emphasis added)

above: Cropped from Figure 4 of The International Space Station (ISS) Researcher’s Guide International Space Station Acceleration Environment. "Figure 4. Spectrogram Showing Mode One with Crew Slow Transition to Sleep." This suggests there are several structural resonances in the 0.1 to 1.0 Hz region. See original document for further discussion, and a list of about 20 different known resonant frequencies on page 12.

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A very frightening and dangerous event happened on board the ISS in 2009 when a mis-programmed servo on a booster engine started tweaking the booster engine's thrust direction at about 0.5 Hz.

But during the Jan. 14 firing, something went seriously awry. The station’s solar power wings began swaying back and forth alarmingly. More dramatically, an interior camera captured views of wall-mounted equipment and cables flopping back and forth to a two-second beat, as the camera itself swayed on its mounting bracket.

Buildup of gyrations

It was quickly apparent that some periodic force had excited the space station’s structure at one of its resonant frequencies, leading to a buildup of gyrations rather than a damping down. As with the traditional "soldiers marching across a bridge" story, and the all-too-real Tacoma Narrows bridge collapse in 1940, resonance buildup in a large structure can quickly lead to serious consequences. (emphasis added)

See also Space.com's NASA Weighs Excessive Vibrations on Space Station

Answer 4

In addition to the other answers: a small structure (like a single module on the ISS) needs to rotate really fast to create 1G. This has undesirable side effects:

• Coriolis forces make moving around inside the module non-intuitive. There's an old Soviet experiment where people lived inside a centrifuge for a while, on the film (haven't found this online, it's in the BBC documentary 'Cosmonauts: How Russia won the space race') you can see them stagger and lurch along a corridor as if they're drunk. Another segment has someone throw darts at a dartboard, with the darts flying in a 90º horizontal arc.

• in a small centrifuge, there's a significant difference in the gravity levels between your head and feet, again making movement inside this module non-intuitive.

• if you use the centrifuge module only to sleep in, the astronauts have to get used to 0 G every morning. This would mean adapting fully to 0 G (takes about 2 weeks in the current situation) takes much longer, and you lose valuable time to space sickness.

Answer 5

I'd like to add the words of Chris Hadfield about this, from the FAQ appendix of his book An astronaut's guide to life on earth:

Is it comfortable to sleep on the ISS?

It is a whole new type of comfortable to sleep in weightlessness. Even on the most expensive Earth mattress you occasionally have to roll over or adjust your pillow. In orbit, you can relax every muscle in your body. At bedtime you float into your sleeping bag (which is loosely tethered to the wall with a couple of shoelaces), do up the long zipper and shut off the light. Because there's no effect of gravity pushing you into your mattress, you are perfectly relaxed and your whole body can go deliciously limp. Your arms and leg joints bend a bit and float up, your neck slumps forward like a napping passenger on a plane; every muscle takes a rest. You can feel the slow pulse of your heartbeat, moving you slightly against the nothingness. When space travel eventually becomes inexpensive enough, it may well be the “space sleep spa” that attracts the largest crowd.

^{Chris Hadfield. An astronaut's guide to life on earth. --- Pan Books Ltd. 2015}

So astronauts would probably vote for sleeping in 0 G.

Answer 6

Offered as an addendum: a Centrifuge Module was planned for the ISS, and was partially built. Its centrifuge was for science experiments, not sleeping though. Budget problems doomed it, and it now sits in a parking lot in Japan.

Source

last image from here

Answer 7

A rotating structure big enough to accomplish that would be bulky, heavy, and very power-hungry to operate.

Space, mass, and power are all at a sharp premium on spacecraft and space stations such as the ISS, so a centrifugal bed isn't remotely within the budget.

Answer 8

There is no reason to do this for the sleeping quarters, as lying in bed at normal Earth gravity does not reduce the effects of weightlessness on the human body - in fact, it is has been used in multiple experiments to study the effects of weightlessness on bone and muscle loss:

In a recent review on bed rest studies of the past 20 years, it was concluded that head-down bed rest has proved its usefulness as a reliable simulation model for most physiological effects of spaceflight.

Simulating human space physiology with bed rest

So the difference would be between sleeping in a level bed and sleeping with your head a few degrees down. It would be better to use centrifugal gravity for zones where the astronauts where performing load bearing activities.

Answer 9

Even if a 1g sleeping compartment were feasible, which the other posts have demonstrated it is not, the health issues associate with micro gravity would not be alleviated. Just sleeping in full gravity but working and waking in micro gravity would still have significant health effects.

In particular calcium depletion would still occur. Skeletons grow in response to compression stress (on the bones), which is usually caused by weight which is the effect of gravity on body mass. In the ISS this compression stress is simulated with ample exercise which when combined with a calcium and vitamin D enriched diet provides an element of compensation.

There are many other health issues associated with microgravity and I have only focussed on one to illustrate the issue, but many of the others would also not be solved by a specially engineered sleeping chamber.

Answer 10

One reason is that rotation at the kind of speeds practical in a spacecraft would cause nausea and dizziness, if not vomiting. Not conducive to rest. Also rotating machinery would cause a large number of risks of various sorts, and the need for a maintenance program. The more I think about this the more reasons I can think of.

A rotating space station, or one with a rotating gallery, is practical, if large enough that the rotation rate doesn't cause nausea (after all that's what Earth is).