Quick question on centrifuges.

[p1t1o]

Ooh I thought of a good point.

We have rotating seals that go around propeller shafts on ships and submarines. The ones on submarines routinely have to resist pressure differentials of tens of atmospheres, it seems pretty evident that one that only has to be airtight at a differential of, at the maximum, 1 atmosphere is going to pose that much of a challenge, in the grand scheme of things.

[wumpus]

Scott Kelly and Mikhail Kornienko exercised for something like two and a half hours per day over their year in space.  I remember reading that the Skylab astronauts (who were up for much shorter lengths) hated their exercise times, largely because the sweat would pool on their skin in zero-gee.  After a little googling, it looks like the treadmill had a serious fan (that hopefully knocked some of the sweat off) but the resistance training appeared to ignore such things (hopefully you could wipe down between sets).

I have to wonder if spending such time exercising is sufficient motivation for a "gravity zone" in a space station or long term spacecraft (Mars intercept, perhaps).  It doesn't look like the problem described in Skylab though: I suspect much of the culture aspect may have changed (Dr. Ken Cooper started the modern push for "exercise=health" during the space age when he was doing research for the Air Force to discover why pilots were having heart attacks while flying.  He may have been in the perfect place to adjust astronaut attitudes).

[sgt_flyer]

3 minutes ago, p1t1o said:

Ooh I thought of a good point.

We have rotating seals that go around propeller shafts on ships and submarines. The ones on submarines routinely have to resist pressure differentials of tens of atmospheres, it seems pretty evident that one that only has to be airtight at a differential of, at the maximum, 1 atmosphere is going to pose that much of a challenge, in the grand scheme of things.

vacuum (especially in microgravity have more engineering problems than simply pressure differential. in boats and such, the can use oil and such to complete the sealing without adding too much friction - additional pressure won't change a lot. in vacuum, you'd need a special liquid that won't boil away due to low pressure (or dry sealing). on top of that, the torque has to be counteracted (as your sealing will create friction, you'll need to add more energy to keep the thing spinning, the other section of the spacecraft will start to spin the the opposite direction - in water, buoyancy and gravity are largely enough to counteract this kind of torque. 

also, if you have friction, it means you are generating heat at the friction point. dealing with heat in space is a serious problem  (you need to basically add a heat-transfer fluid loop around your mechanical seal to carry away the heat to the ship's radiators.)

 

[p1t1o]

Just now, sgt_flyer said:

vacuum (especially in microgravity have more engineering problems than simply pressure differential. in boats and such, the can use oil and such to complete the sealing without adding too much friction - additional pressure won't change a lot. in vacuum, you'd need a special liquid that won't boil away due to low pressure (or dry sealing). on top of that, the torque has to be counteracted (as your sealing will create friction, you'll need to add more energy to keep the thing spinning, the other section of the spacecraft will start to spin the the opposite direction - in water, buoyancy and gravity are largely enough to counteract this kind of torque. 

also, if you have friction, it means you are generating heat at the friction point. dealing with heat in space is a serious problem  (you need to basically add a heat-transfer fluid loop around your mechanical seal to carry away the heat to the ship's radiators.)

 

Yah but all of those problems already have solutions, such as specialist vacuum rated lubricants (used very commonly in the chemical industry, especially where you want very minimal boil-off, eg: in electron microscopes) and anti-torque rings; heat disposal is going to be a problem everywhere, not just in your rotating seal, but yes it is a bit different to submarines, but the scale of the problem is fairly gentle with our current technology. I doesn't seem necessary to me to resort to such solutions as gross modifications to the architecture of the vessel.

[sgt_flyer]

3 minutes ago, p1t1o said:

Yah but all of those problems already have solutions, such as specialist vacuum rated lubricants (used very commonly in the chemical industry, especially where you want very minimal boil-off, eg: in electron microscopes) and anti-torque rings; heat disposal is going to be a problem everywhere, not just in your rotating seal, but yes it is a bit different to submarines, but the scale of the problem is fairly gentle with our current technology. I doesn't seem necessary to me to resort to such solutions as gross modifications to the architecture of the vessel.

still, we don't need additional layers of complexity in a manned spacecraft. having either only manned section in the hab ring (with a 'classic' countertorque ring) or rotating the whole spacecraft would greatly minimize complexity (and point of failures).

keep in mind that all the equipments here on earth can be easily serviced - and are not crucial for crew survival ! (in a submarine, if a propeller shafts seal breaks, you can stop the propellers, and seal the engine room, then empty your ballasts to surface and wait for rescue.) the same kind of problem in a spacecraft during an interplanetary transfer, you'd have to isolate the section with the joint, trapping your crew (as your docking ports would be on the non rotating section. - so you'll need to have additionnal EVA airlocks (+mass +complexity) to be able to get from the hab ring to the rest of the spacecraft in case of problem.

besides, while it would be useful to have a non rotating section fot the spacecraft, why would you need habitable sections in this part ? if you want, you can even add your docking port and lander in the middle of the ring, lined up with the center of rotation of the ring - and you only need 1 airlock on the ring the be able to send someone to service the non habitable parts !

[p1t1o]

You might not need habitation in the non-rotating part, but you might want labs and such.

I'm also generally against rotating the whole spacecraft, mainly for the reason that it means if anything at all became somehow detached, it is lost for good. I know this is still true for a rotating hab, but at least it would be limited to that area. A rotating craft would also suffer from heavy gyroscopic effects, not a dealbreaker, but you may want to be able to re-orient your craft.

 

This is a fun problem 

Edited March 8, 2016 by p1t1o

[SpaceMouse]

Well i started a discussion. Lol. On a mostly related topic has anyone considered a centrifuge for low gravity environments/moons? I imagine something like a amusement park chair swing ride thingy would work.... thats sealed.

[fredinno]

1 hour ago, SpaceMouse said:

Well i started a discussion. Lol. On a mostly related topic has anyone considered a centrifuge for low gravity environments/moons? I imagine something like a amusement park chair swing ride thingy would work.... thats sealed.

Almost certainly yes.

10 hours ago, Nuke said:

well there are 3 ways to have a centrifuge

1 external centrifuge

2 internal centrifuge inside the pressure hull

3 spin the whole ship

i kind of think spinning the whole ship is a bad idea. a lot of people use that as their go to answer but i think its wrong. you can come up with control systems to operate from within a rotating frame of reference, and maneuver without needing to spin down first. of course all that spinning puts a lot of stress on your structural bits, so they need to be structurally beefier (more mass). energy stored as angular momentum is going to require additional rcs torque to overcome, more fuel and bigger thrusters are needed (more mass). and then you got something thats hard to dock with.

then you look at external centrifuges. they need to have some kind of pass through. i figure the best approach is to minimize travel between the spinning and non spinning areas, and depressurize the interchange compartment when it is not needed to avoid atmosphere loss. they need to have a pressure hull and rad shielding, life support needs to be built in as well (and will likely be made redundant on both sides of the interchange compartment). you get more freedom in ship design, it doesn't need to be perfectly proportioned.

where the internal centrifuge shines is in its ability to be comparatively lighter than the other two options. you can seriously strip down its hardware and keep it in the static hull. only components that absolutely need gravity to function need be placed there. it then becomes a tiny fraction of ship mass and doesn't need as much counter-torque for stability. if it breaks down you can fix it in a shirt sleeve environment. my perfered ship geometry is a double hull toroid with an outer hull, an inner hull and the space between the hulls is used as tankage for fuel and other liquid/gaseous consumables. this gives you double redundant redundant vacuum protection and good radiation protection. the inner pressurized tube houses the centrifuge, life support equipment, airlocks, zero g command deck, storage bays, and any static hull crew accessible amenities. reactor and propulsion can be in the hub, but i imagine hubless designs would be possible, with engines built right into the toroidal hull.

Why would a double hull be less massive? I still don't get it. You do realize you're carrying a whole new set of walls...

[Nuke]

you are going to need tankage no matter what you build. those tank walls add mass. if you combine the tankage into the hull design you can net some savings and at the same time protect the crew from radiation and micrometeorites.

Edited March 8, 2016 by Nuke

[SpaceMouse]

I've thought of the spinning transit section as well before. Glad i'm not the only one to have this idea.

 

Isn't ANY centrifuge designed to keep humans alive (or Kerbals) Going to have rather modest requirements for power regardless? If frictional heating is a issue of concern here, might it be worth mentioning that i imagine any spacecraft is going to have rather modest heat generation requirements to keep the astronauts nice and toasty? Seems like generating it as a byproduct might reduce power required elsewhere. This has been touched apon elsewhere but if you need something like water to shield astronauts from radiation, might a third layer in this internal centrifuge reduce some more complexity? Your radiation shield doesn't need to rotate. Only your squishy humans.I imagine that also would greatly reduce the mass you need to swing around. the T-shirt maintenance also strikes me as one of the primary pluses.

As the resistance training regimen they use was mentioned this is partly inspired by watching the recent video of the crew retrieving Scott Kelly. My understanding was there exercise plan had improved greatly, reducing the muscle and bone loss in space. I was a little sad he couldn't climb out himself.

[sgt_flyer]

51 minutes ago, SpaceMouse said:

I've thought of the spinning transit section as well before. Glad i'm not the only one to have this idea.

 

Isn't ANY centrifuge designed to keep humans alive (or Kerbals) Going to have rather modest requirements for power regardless? If frictional heating is a issue of concern here, might it be worth mentioning that i imagine any spacecraft is going to have rather modest heat generation requirements to keep the astronauts nice and toasty? Seems like generating it as a byproduct might reduce power required elsewhere. This has been touched apon elsewhere but if you need something like water to shield astronauts from radiation, might a third layer in this internal centrifuge reduce some more complexity? Your radiation shield doesn't need to rotate. Only your squishy humans.I imagine that also would greatly reduce the mass you need to swing around. the T-shirt maintenance also strikes me as one of the primary pluses.

As the resistance training regimen they use was mentioned this is partly inspired by watching the recent video of the crew retrieving Scott Kelly. My understanding was there exercise plan had improved greatly, reducing the muscle and bone loss in space. I was a little sad he couldn't climb out himself.

frictionnal heat on mechanical parts is an issue mainly in vacuum only - not because of the heat itself, but because it would quickly ramp up and damage the mechanical seal needed between an rotating and non rotating habitable version if you don't manage this heat.

if your bearings are kept within the hab it's much less a problem, as you can circulate the air itself to carry away some of this heat, while in hard vacuum you can use bearings built for that. the problem is the seal itself . on top of that, a mechanical seal huge enough to be placed around a passage big enough to let a man get through it, is going to be a pain in the ass on tolerances if you want to limit air leakage rates  (even iss loses air continuously - and static seals are much more forgiving ! still, you can bring new air to iss easily. you'd have to pack extra air in an interplanetary starship to account for leaks)

Edited March 8, 2016 by sgt_flyer

[sevenperforce]

2 hours ago, SpaceMouse said:

Isn't ANY centrifuge designed to keep humans alive (or Kerbals) Going to have rather modest requirements for power regardless? If frictional heating is a issue of concern here, might it be worth mentioning that i imagine any spacecraft is going to have rather modest heat generation requirements to keep the astronauts nice and toasty? Seems like generating it as a byproduct might reduce power required elsewhere. 

Negative, ghostrider. 

A rotating hab doesn't need to consume any energy over time. In fact, it can serve as part of the reaction control system for the ship. And heating is not the typical problem; cooling is. Getting rid of waste heat is a much bigger problem. 

[PB666]

On 3/7/2016 at 9:56 AM, SpaceMouse said:

I want to start making some parts, and had a thought this morning that i felt adressed some of the issues of a centrifuge in space. I assume one of the most complex parts is sealing it so it can spin. What if you didnt at all? Might it be simpler to inflate a pressurized chamber and then spin a part within that? A google search didnt pull up any real concepts on this. Everyone else seems to have the idea that you either need to spin just the ring or the whole damn station. Admittedly this wouldn't look as cool, obviously. It might be heavier too, but it sounds like a more practical solution to me.

Uh on the interstellar propulsion thread I proposed just this. But you have to have a big ship and people would have to exercise in the chamber. With a generation ship this is entirely possible, you go into the centrifuge for 16 hours a day, also going to the bathroom, even bathing from time to time. But during the work shift you are zero G. I should make the point though, if the ship has a two fold axis of symmetry that is close to perfect, once you spin the whole ship up you need only make minor tweeks to keep it spinning in the direction of motion _once its sufficiently far enough away from massive objects_.

I should point out that whatever you have seen in sci-fi, its nowhere near adequate to the task.

 

omega^2*r = 9.8m/s2

if r is 4 meters then omega is 1.56 radians per second, that means roughly an 8 meter (24 foot) cylinder is spinning at 0.248. 4 meters is crowded however

if r is 8 meters then omega is 1.106 radians per second.

if r is 16 meters then omega is 0.78 radians per second. It may not seem like much but once you get something 100 feet across spinning full circule every 10 seconds. The outside of the circle is traveling at 13 meters per second so it has considerable damage force if not contained, but most important, it also is a cylinder traveling in air, and experiences drag which requires a constant force to keep it going.

[SomeGuy123]

22 hours ago, Nuke said:

i kind of think spinning the whole ship is a bad idea. a lot of people use that as their go to answer but i think its wrong.u can seriously strip down its hardware and keep it in the static hull. only components that absolutely need gravity to function need be placed there. it then becomes a tiny fraction of ship mass and doesn't need as much counter-torque for stability. if it breaks down you can fix it in a shirt sleeve environment. 

Yep.  I agree 100%.

Design : there's a big cylinder.  It's made of either fiber reinforced fabric, similar to the inflatable space stations by Bigelow Aerospace, or if you were really clever you could make it by putting your propellant tanks in just the right places so the walls of the propellant tanks act as the walls of the cylinder.  (to save even moar mass!)

Inside are the rotating sections.  They are not sealed and you can look through protective mesh and see the walls of the hab module zooming past.  They are basically made as light as they can possibly be - they don't have to hold pressure, nothing that doesn't benefit from gravity is part of the centrifuge, just an open frame of high strength structural members and a floor and cubicles for the crew, you know the drill.

There are 2 rotating sections, and there are internal structural members you can just see inside the pressurized section holding the ends of the bearings.  There's a gap between the 2 rotating sections and you see this big beam thing with 2 seats at the end.  That's how you get from one rotating section to the other - the beam spins up to the speed of the section you are in, you get in the seat, press a button, and it matches with the other one.  

Similarly, at the hubs, there are these open speed matching cages.  Get in one, press a button, and the cage slows down from the hub velocity to match with the exit tunnel to the low gravity section.  

If we could build stuff in space (and manufacture it from raw materials in space as well) this is how it would be.  Absolute minimum mass used, zero air leaks, zero propellant used to get it spinning, and yeah you can spin down both centrifuges at any time - they will be designed with that as something they are meant for - and work on the whole thing in shirtsleeves.  

Think of these aluminum pieces, each one auto-cut by a CNC cutter.  The whole thing would be probably a bunch of aluminum tubes and plates and bolts and everything else.  An asteroid belt mining machine would have mined the ore, it would have been purified to ingots, then forged into the right alloy, then auto cut, and then finally it would be assembled either IKEA style by human workers in shirtsleeves or by robots.  

 

 

 

[Nuke]

you really dont need 2 centrifuges, you can have a high mass low speed centrifuge and a low mass high speed rotor to counter the torque (mass is relatively speaking, both combined are going to be a tiny fraction of the ships dry mass). the high speed rotor also can be sped up or down to work as a reaction wheel for roll control, and this will be the axis which is hardest to rotate.

ive looked at various intechange mechanisms to safely spin crew, supplies and equipment up before moving into the centrifuge from the static hull. my best idea is a small car on a track up against the centirfuge, it is loaded, spun up and used to offload its cargo and then spun down when not in use, the counter torque rotor can spin up faster durring this operation to counter the torque. there are two or more cars that operate and need to have their weights balanced to operate well using ballast or splitting up the load between them. the cars share an access channel with the anti-torque rotor, to save space.

im thinking that both the centrifuge and rotor will operate on maglev. in addition to providing low friction free travel (except air resistance) and smooth noise free operation (crew comfort mostly). as a biproduct the magnetic fields can help deflect charged particles trying to go through the centrifuge, while the neutral particles are stopped by tankage contents.

i like toroids but spheres or cylinders can also be used. spheres make for more optimal tankage, cylinders work if you need a lot of crew or passenger space (longer, or multiple centrifuges) or a big pressurized work area (mining, ship repair, etc), and both work better if you are using an orion drive to better distribute the forces created by the engine into the hull (a toroid might result in your orion drive taking off without you). a large toroid can be used with the reactor in the center, this allows a relatively small amount of shielding to protect the entire ring from core radiation, you also can exploit the inverse square law to send most of the radiation out into space. you can place the radiators as the spokes of the hub. you also have open ring designs or mount a solar truss down the center depending on propulsion used.

and thanks to the miracles of ms paint;

Edited March 9, 2016 by Nuke

[radonek]

On 8. března 2016 at 3:17 AM, parameciumkid said:

^ This. What's wrong with two counter-rotating segments now?

It flies in face of engineering principle known as  Airplane Construction Rule: Airplane with two engines will have twice as many failures as single-engined one.

Now, if those two centrifuges were independent, that would be a good thing - you'd have a backup. But this case is opposite, if any one of those breaks down, the other one have to be spun-down too. Such system is more complicated, but less reliable which is opposite of what real space engineers are after. (note that such system would be even more complex: apart from doubling the amount of machinery, you also need precise synchronization) Well there is a small advantage - you could operate in a degraded mode at cost of some fuel, so it can be considered short-term backup if you really really need gravity and hope to fix the broken one soon.

This idea only seems sensible to me for VERY large ships. Have, say, six centrifuges on same axis. Six times chance of failure, but also five spares to pick reverse one from and maintain 2/3 capacity. And if another one fail, you respun that reverse. (you can also try to run on five by varying relative speeds). But its a lot of engineering overhead and  you'd still need pretty reliable tech. 

[SpaceMouse]

I saw one YouTube video actually that has a internal centrifuge i think,  that was of the normal massive generation ship scale normally portrayed. Although I've generally been talking about something of a much more practical scale. I still think for normal purposes maintaining the stationary sealed chamber at a much lower pressure would make the aerodynamic drag thing a non,  or at least not major issue. It needs to hold air. That doesn't mean it needs to hold much of it, and said air can be used for heat extraction as well as  mentioned. You can still pressurize it at a normal level for maintenance purposes. 

[PB666]

If the internal centrifuge is in a electromagnetically suspended vacuum chamber then there is no friction.

Allow astronauts to enter chamber, then seal it, seal the chamber door, evacuated the chamber and spin the centrifuge, if air leaks out simply pump it back in. No friction, no air loss, if there is a breach, no problem just make the chamber differential thin, pressure equilibrates.

The only problem with this design is that is it massive. You need a craft wall to keep air from escaping into space, you need a chamber wall to keep barrier air from getting into the chamber, you need the spinner wall, and pumps inside the spinner, as well as a induced current generator (or hefty battery) to run the pump.

 

[fredinno]

21 hours ago, Nuke said:

you are going to need tankage no matter what you build. those tank walls add mass. if you combine the tankage into the hull design you can net some savings and at the same time protect the crew from radiation and micrometeorites.

How? You need a lot of insulation for the tanks to prevent boil-off.

[Kerbart]

On 3/8/2016 at 10:20 AM, sgt_flyer said:

(...) on top of that, the torque has to be counteracted (as your sealing will create friction, you'll need to add more energy to keep the thing spinning, the other section of the spacecraft will start to spin the the opposite direction - in water, buoyancy and gravity are largely enough to counteract this kind of torque. (...)

You can counteract the torque generated by friction by mounting an electric motor in the space ship that generates torque in the opposite direction. Amazingly, this "surplus" torque can then be used to drive the centrifuge so it remains a constant rotation as opposed to being slowed down by friction. This also removes the need for a "counter centrifuge".

[sgt_flyer]

1 hour ago, Kerbart said:

You can counteract the torque generated by friction by mounting an electric motor in the space ship that generates torque in the opposite direction. Amazingly, this "surplus" torque can then be used to drive the centrifuge so it remains a constant rotation as opposed to being slowed down by friction. This also removes the need for a "counter centrifuge".

with 1 single electric motor to drive 1 single hab ring, you'll end up having net torque if you start from a standstill - the fixed section will start spinning in the opposite way (at a much slower spinning rate than the hab ring due to the mass difference, but still turning - if then left unpowered, the friction would then slowly cancel out both rotations.)

so you'll need to first cancel this net torque on the 'fixed' section of the spaceship (either by firing the RCS or a counter centrifuge / flywheel), then you could maintain just enough torque in your engine to exactly counteract the friction between the rotating and non rotating section (if you use a counter centrifuge / flywheel, it'll also need to counteract it's own friction).

as for 'synchronisation' you can use accelerometers on the sides of the fixed section (even on masts to get them further away from the center of rotation), those would be able to detect the acceleration created by the spin of the fixed section. - if it happens, adjust accordingly the power sent to the electric motor  

[Nuke]

2 hours ago, fredinno said:

How? You need a lot of insulation for the tanks to prevent boil-off.

depends on fuel. i for one dont want to live in the middle of a bunch of cryofuel.

[SpaceMouse]

Just had a random idea i assume is stupid. Has anyone ever done a concept for a actual hollow sphere for a habitat? While i don't think it would be very practical or necessary, it was at least a amusing idea.CENTRIFUGE ALL THE THINGS! If it spins in all 3 axis i think it... might... work...

Edited March 9, 2016 by SpaceMouse

[NuclearNut]

Why not just spin the ship?

[Nuke]

because a spinning ship is a big massive gyroscope and gyroscopes tend to resist changes in orientation. a big ship spinning around is going to have a lot more angular momentum than a small part of a big ship spinning. so it will require additional rcs thrust to maneuver. and in turn that means more rcs propellant is required for the mission. on top of that now every part of your ship is subject to centrifugal force, so you start introducing forces that want to pull your ship apart which require additional structural strength, and that means more mass as well.

at least thats my understanding, i could be wrong and if i am please correct me.

Edited March 10, 2016 by Nuke