Quick question on centrifuges.

[SpaceMouse]

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.

[sevenperforce]

Aerodynamic drag on the spinning part within the pressurized chamber will slow the spin rapidly.

[p1t1o]

Also: maaaaaaaass.

[SpaceMouse]

I figured increased mass would be a drawback. Hadn't even considered aerodynamic drag though. Still seems like they might be useful tradeoffs for more mechanical simplicity. 

I would think as a backup you'd want each chamber sealed separately, wouldn't it be reasonably easy to maintain the outer chamber at a much lower pressure? 

[sevenperforce]

1 minute ago, SpaceMouse said:

I figured increased mass would be a drawback. Hadn't even considered aerodynamic drag though. Still seems like they might be useful tradeoffs for more mechanical simplicity. 

Aerodynamic drag is the primary problem. You want the thing to keep spinning without loss; otherwise you have to constantly add energy to keep rotating. 

[Stargate525]

The other problem with aerodynamic drag is that the air will start to spin, and then the spinning air will start to spin the station...

[Kerbart]

5 minutes ago, Stargate525 said:

The other problem with aerodynamic drag is that the air will start to spin, and then the spinning air will start to spin the station...

True, but that would be offset by the torque that is generated to counter the aerodynamic losses. You can consider the centrifuge chamber a closed system where—aside from the initial torque to get it spinning—no torque is “exchanged” with the outside world.

[sevenperforce]

12 minutes ago, Kerbart said:

True, but that would be offset by the torque that is generated to counter the aerodynamic losses. You can consider the centrifuge chamber a closed system where—aside from the initial torque to get it spinning—no torque is “exchanged” with the outside world.

It would depend on how you set it up, whether you were using a reaction wheel, etc.

[StrandedonEarth]

Have the air being pumped in at an angle from the rim of the outer shell, using the incoming air to spin the centrifuge. If torque is a problem (not sure cuz this is making my head spin) add a second counter-rotating unit. 

[sevenperforce]

6 minutes ago, StrandedonEarth said:

Have the air being pumped in at an angle from the rim of the outer shell, using the incoming air to spin the centrifuge. If torque is a problem (not sure cuz this is making my head spin) add a second counter-rotating unit. 

Yeah, this will cause the outer shell to spin in the opposite direction, which will spin the whole spacecraft. Also, constant power supply required to keep pumping air through.

Edited March 7, 2016 by sevenperforce

[StrandedonEarth]

2 hours ago, sevenperforce said:

Yeah, this will cause the outer shell to spin in the opposite direction, which will spin the whole spacecraft. Also, constant power supply required to keep pumping air through.

Constant power is required anyway to keep air flowing through life support, and having multiple units counter-rotating would cancel spin. There's the possibility of having an air-bearing effect in there somewhere, but doubtful. 

[Kerbart]

3 hours ago, sevenperforce said:

It would depend on how you set it up, whether you were using a reaction wheel, etc.

I don't think so. There's a closed loop of the frame absorbing friction losses,  introducing torque, which is neutralized by the torque generated by the rotor, which on the other side of the spindle would result in the centrifuge spinning faster if it weren't for equal but opposite torque coming from the friction losses.  Draw a harpoon diagram and it's easy to see.

The whole system can be constructed in such a way that all the torque losses are maintained inside it. I'm sure, with some effort, that it can be constructed in such a way that you "leak" momentum to the outside (introducing air through tangential channels instead of axial) so that you need a reaction wheel on the outside. But why would you?

5 hours ago, 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.

On a side note: this is how it's done on the Discovery in 2001: A space odyssey. The downside is that these centrifuges need to spin quite fast at small diameters, and at large diameters they require a lot of material. A larger centrifuge, as shown in The Martian doesn't even have to be a ring, just two (or four) elements counter-balancing each other, and with less constraints on the diameter it can rotate a lot slower (making entry and exiting a lot easier)

 

[Shpaget]

You don't need a rotating seal. Use regular vacuum bearings and an airlock that is separated from both spinning and stationary sections. When you want to go from stationary to spinning section you uncouple the airlock from the stationary section and spin it up to match the rotating segment. Once the rotation is matched, you align and "dock" to the spinning segment.

[sevenperforce]

20 minutes ago, StrandedonEarth said:

Constant power is required anyway to keep air flowing through life support, and having multiple units counter-rotating would cancel spin. There's the possibility of having an air-bearing effect in there somewhere, but doubtful. 

"Constant air flow for ventilation" of orders of magnitude less of an energy drain than "constant air flow for drag compensation on a gigantic suspended centrifuge".

13 minutes ago, Kerbart said:

I don't think so. There's a closed loop of the frame absorbing friction losses,  introducing torque, which is neutralized by the torque generated by the rotor, which on the other side of the spindle would result in the centrifuge spinning faster if it weren't for equal but opposite torque coming from the friction losses.  Draw a harpoon diagram and it's easy to see.

The whole system can be constructed in such a way that all the torque losses are maintained inside it. I'm sure, with some effort, that it can be constructed in such a way that you "leak" momentum to the outside (introducing air through tangential channels instead of axial) so that you need a reaction wheel on the outside. But why would you?

On a side note: this is how it's done on the Discovery in 2001: A space odyssey. The downside is that these centrifuges need to spin quite fast at small diameters, and at large diameters they require a lot of material. A larger centrifuge, as shown in The Martian doesn't even have to be a ring, just two (or four) elements counter-balancing each other, and with less constraints on the diameter it can rotate a lot slower (making entry and exiting a lot easier)

 

I think his example involved air jets as the driver of the rotation, which would tend to produce net torque. But sure, you can set it up in such a way as to zero out transfer of angular momentum. 

A tethered arrangement, with the hab on one end and the counterweight on the other end, is probably the simplest arrangement.

[Kerbart]

2 minutes ago, sevenperforce said:

A tethered arrangement, with the hab on one end and the counterweight on the other end, is probably the simplest arrangement.

Given that mass will always be at a premium, a better solution is likely to split the hab in two halves and have them on either end. No need for a counter weight.

Of course, the counter weight could also be useful payload. Batteries, a power generator, life support, etc. 

[fredinno]

1 hour ago, Kerbart said:

Given that mass will always be at a premium, a better solution is likely to split the hab in two halves and have them on either end. No need for a counter weight.

Of course, the counter weight could also be useful payload. Batteries, a power generator, life support, etc. 

1 hour ago, sevenperforce said:

"Constant air flow for ventilation" of orders of magnitude less of an energy drain than "constant air flow for drag compensation on a gigantic suspended centrifuge".

I think his example involved air jets as the driver of the rotation, which would tend to produce net torque. But sure, you can set it up in such a way as to zero out transfer of angular momentum. 

A tethered arrangement, with the hab on one end and the counterweight on the other end, is probably the simplest arrangement.

 

Not this debate again...

[problemecium]

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

[sevenperforce]

1 hour ago, parameciumkid said:

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

Moving parts.

[Stargate525]

10 hours ago, Kerbart said:

True, but that would be offset by the torque that is generated to counter the aerodynamic losses. You can consider the centrifuge chamber a closed system where—aside from the initial torque to get it spinning—no torque is “exchanged” with the outside world.

If the ring inside is a rigid moving walkway, with electric engines wheeling it along the outside static habitat, the vessel would encounter a net torque, I'm fairly certain.

[sevenperforce]

6 minutes ago, Stargate525 said:

If the ring inside is a rigid moving walkway, with electric engines wheeling it along the outside static habitat, the vessel would encounter a net torque, I'm fairly certain.

Before it was spun up, yes. I think the case he's suggesting is where the ring is already in motion, and the vessel applies a reaction force only to counteract drag. In such a case, the torque on the force is only as much as is needed to offset the drag torque, so they cancel out.

[K^2]

14 hours ago, sevenperforce said:

Aerodynamic drag on the spinning part within the pressurized chamber will slow the spin rapidly.

Trivially solved with an electric motor. For a centrifuge of practical size and rotation speed, the power drain will be low enough so as not to break your power budget. And if you dual-purpose this aerodynamics for ventilation, you can drop net losses even lower.

I don't know if it's the best way to do it, but it is certainly an option.

[Nuke]

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.

[kurja]

Why would it be required for people to move to and fro between a rotating crew module and stationary parts of the vessel in the first place?

 

It's not like they could just open a door in their module to visit another part of a soyuz rocket... So why should that change if the crewed section was made to rotate.

[Jovus]

What's special about orbit that makes centrifuges difficult? Why not just use one from Earth?

[andrewas]

There are very few centrifuges on this scale on Earth, and they don't need to worry about sealing against vacuum.

Anyway. Simple solution - pressurize the hab ring, and nothing else. Also, keep your life support systems on the ring because passing pipework through a rotating joint has all the problems of sealing the hub in the first place.  Power can be passed easily enough with sliding contacts, data could be done wirelessly or with yet more sliding contacts.

If we also need hab space in the non-rotating section, it may not be desirable to EVA between the two, but an airlock which can spin up and down to engage with either section of the station may be more practical. Or a sealed joint, but with pressure doors around it so that we don't need to keep it permanently pressurized and, if it does fail, the worst that can happen is that we lose some air before it can be sealed off.