Do manned spacecraft use reaction wheels?

[Maarek_Stele]

Nice necro!

Generally, the reaction wheels themselves stay in the same orientation in space, and the spacecraft is rotated around them, so this wouldn't actually work.

Well that was what I was leaving out.

[K^2]

That said, if the reason you are accumulating momentum is due to asymmetry of the craft, flipping it around every once in a while might do the trick. Having some RCS is still a good idea, though.

[cpast]

Question: I know spacecraft sometimes use gyroscopes to maintain attitude. If torque is applied to the spacecraft, does that then slow the gyroscope, or what happens?

[K^2]

It can, depending on orientation of the torque. Gyro is literally an angular momentum storage. If applied torque changes magnitude of angular momentum, the speed of the gyro will change to compensate.

[Accelerando]

I'm curious whether Magsail- or electrodynamic tether-style "thrusters" can be used as an RCS system to dump angular momentum into a celestial body through its magnetic field. Could this be used as an unlimited-∆L source of thrust, so long as the power stays on and you're within range of a suitable celestial?

[peadar1987]

I'm curious whether Magsail- or electrodynamic tether-style "thrusters" can be used as an RCS system to dump angular momentum into a celestial body through its magnetic field. Could this be used as an unlimited-∆L source of thrust, so long as the power stays on and you're within range of a suitable celestial?

Yep, this is completely possible. Most ideas revolve around using a relatively massive rotating tether to "throw" smaller payloads into the desired orbit, and then gradually boosting the tether's orbit back to its initial position using electrodynamic propulsion. This means that you're doing all your "thrusting" as deep in the gravity well as possible, so Oberth effect, and also stronger magnetic field.

[Maarek_Stele]

I would think any thrusters that are oriented in the right direction could dump the buildup of angular momentum.. Look at these real Hall Effect, arcjet and plasma thrusters that actually exist today... https://www.rocket.com/files/aerojet/documents/Capabilities/PDFs/Electric%20Propulsion%20Data%20Sheets.pdf

[John FX]

I'm a fan of the idea that your torque wheels can become saturated through use but for gameplay reasons this dissipates slowly over time. More advanced torque wheels could have a faster dissipation rate and a higher saturation level.

[Jovus]

One possibility I've been mulling over is desaturation through thermal dumping, i.e. using mild friction to dump angular momentum. Yes, this would inevitably impart some back to the craft, but then you spin the wheel up again.

Is this possible, and simply not done because it's not worth the extra weight in brake pads, or because thermal management would be too much of a problem? Or am I missing something?

[cpast]

One possibility I've been mulling over is desaturation through thermal dumping, i.e. using mild friction to dump angular momentum. Yes, this would inevitably impart some back to the craft, but then you spin the wheel up again.

Is this possible, and simply not done because it's not worth the extra weight in brake pads, or because thermal management would be too much of a problem? Or am I missing something?

You're missing conservation of angular momentum, unless your scheme is more complicated than you're mentioning. The spacecraft as a whole, including its reaction wheels, is a closed system. Reaction wheels don't change its angular momentum, they just shift momentum around within that system (they remove angular momentum from that part of the craft that isn't the reaction wheels, but that's because they move it to the reaction wheels). You can't convert angular momentum to heat any more than you can convert linear momentum to heat (what's actually being converted to head is kinetic energy associated with the rotation). Trying to use friction between wheel and spacecraft will just give the rest of the spacecraft all the angular momentum you'd transferred to the wheel; it'll spin, and quickly (unless it's much, much larger than the wheel). If you then spin the wheel up to stop the craft, the wheel's going just as fast as it was before we started this exercise.

Because angular momentum is conserved, the only way to desaturate reaction wheels that doesn't just spin the spacecraft a lot is to interact with things outside the spacecraft. RCS does it by ejecting things from the craft; the angular momentum of (ship + exhaust) is the same, but we don't care about the exhaust. If you channeled the heat correctly, you might be able to radiate it in a way that produces a small torque on the craft (if radiation pressure from the Sun can cause a torque, I'd imagine you can radiate heat in such a way as to cause a tiny torque, essentially using it to power a photon drive).

[PB666]

Reaction wheels provide the advantages of virtually infinite attitude control "fuel" and more accurate attitude control over RCS. The disadvantage is that reaction wheels will become saturated, only absorbing so much kinetic energy in the motion of a spacecraft before reaching max rpms, RCS systems must be present to bleed off this energy and allow the wheels to spin down and desaturate.

In KSP "reaction wheels" or what ever it is ASAS/SAS/Pods use does not saturate, I propose that they should add code that does just that, forcing ship designs to use RCS for more then docking.

EM drives (new part). I think in career mode you start with 30 parts and then add RCS tank and RCS jets. ...........

Better yet a directional gravoli generator, momentarilty send your ships gravity through a single ships surface to a single target . . . . . . . . .Hey, if they can detect the gravoli why can't they generate them.

[StrandedonEarth]

There is a Larry Niven short story, titled Grendel (in the "Neutron Star" collection),

not realizing one of his landing legs had been damaged and didn't deploy. The gyros spun up trying to keep the ship upright, until they seized up and all Jeb broke loose

[Lukaszenko]

So I understand that in real life, reaction wheels get saturated because of minute outside effects such as magnetic fields, photon pressure, and gravity gradients. Could we just say then that in KSP it's not that the reaction wheels violate conservation of momentum, but that simply these minute magnetic/photon/gravity effects are not modeled? Would nothing be violated then?

[K^2]

So I understand that in real life, reaction wheels get saturated because of minute outside effects such as magnetic fields, photon pressure, and gravity gradients. Could we just say then that in KSP it's not that the reaction wheels violate conservation of momentum, but that simply these minute magnetic/photon/gravity effects are not modeled? Would nothing be violated then?

There are plenty of things modeled by KSP which can also saturate reaction wheels. Thrust not being perfectly colinear with center of mass, aerodynamic forces, RCS (mis)use. All of these things can change angular momentum of the craft, and so all of these things can saturate reaction wheels and/or gyros.

[Lukaszenko]

True, point taken.

Seems then it shouldn't be so hard to implement reaction wheel saturation into KSP. Make the wheels overpowered, give them extreme performance, make the "bleed off momentum" function automatic, and do whatever else needs to be done to maintain playability, but at least make it be grounded in some sort of accepted theory.

[Jovus]

You're missing conservation of angular momentum, unless your scheme is more complicated than you're mentioning. The spacecraft as a whole, including its reaction wheels, is a closed system. Reaction wheels don't change its angular momentum, they just shift momentum around within that system (they remove angular momentum from that part of the craft that isn't the reaction wheels, but that's because they move it to the reaction wheels). You can't convert angular momentum to heat any more than you can convert linear momentum to heat (what's actually being converted to head is kinetic energy associated with the rotation).

Unless I'm grievously wrong, yes you can. You're correct that technically the spacecraft considered as a closed system conserves angular momentum, but if we break it down to the (classical) particle level, that momentum is conserved by spinning up the particles on the brake pads, no? Kinetic energy and momentum in collisions are the same phenomenon considered as a scalar and as a vector respectively. (For proof, read Huygens; he treats mv as a scalar and it is not conserved in all collisions.)

Then you dump the excess heat to space using radiators. No doubt you can (classically) model this as imparting momentum to particles in the void. After all, angular momentum and linear momentum can be exchanged; that's how cars work.

[cpast]

Unless I'm grievously wrong, yes you can. You're correct that technically the spacecraft considered as a closed system conserves angular momentum, but if we break it down to the (classical) particle level, that momentum is conserved by spinning up the particles on the brake pads, no? Kinetic energy and momentum in collisions are the same phenomenon considered as a scalar and as a vector respectively. (For proof, read Huygens; he treats mv as a scalar and it is not conserved in all collisions.)

Then you dump the excess heat to space using radiators. No doubt you can (classically) model this as imparting momentum to particles in the void. After all, angular momentum and linear momentum can be exchanged; that's how cars work.

Radiators dump heat with photon drive efficiency (i.e. *not* efficiently). It isn't imparting momentum to particles in the void, it's *creating* particles, which takes the exact same amount of power as a photon drive does for the same thrust, and without a ludicrously impractical radiator moment arm won't be effective at stopping the spin of a spacecraft (if the spin were entirely due to radiation pressure it might be sort of possible, but it's not, so it really isn't).

Conservation of energy and of momentum are facets of the same law (conservation of stress-energy), but it's extremely inefficient in terms of power to momentum. The kinetic energy of the rotation is nowhere near enough to stop that rotation without some reaction mass.

As for spinning up particles on brake pads: We already tried spinning up something to take the angular momentum. That was our reaction wheel. If you transfer that to the brake pads, the brake pads are now spinning at high speed - they've become a reaction wheel. If our brake pads could take more angular momentum than our reaction wheel, then we should just get rid of the reaction wheel and use the brake pads instead. The angular momentum has to go somewhere. It can't stay inside our craft, because our reaction wheels were the best way we had of having lots of angular momentum inside the craft without it flying apart. Because our reaction wheels are saturated, we have to dump more angular momentum than anywhere else in the craft can take (else that would be our reaction wheel system). I mean, sure the reaction wheel has stopped, but now your brakes are in small pieces that are flying around inside your ship, or your ship is spinning just as fast as it would be if you hadn't had reaction wheels in the first place.

[K^2]

Unless I'm grievously wrong, yes you can. You're correct that technically the spacecraft considered as a closed system conserves angular momentum, but if we break it down to the (classical) particle level, that momentum is conserved by spinning up the particles on the brake pads, no? Kinetic energy and momentum in collisions are the same phenomenon considered as a scalar and as a vector respectively. (For proof, read Huygens; he treats mv as a scalar and it is not conserved in all collisions.)

Total angular momentum is a conserved charge for a rotationally-invariant Hamiltonian. Noether's Theorem. In other words, you cannot change total angular momentum without applying external torque. You can store that angular momentum as rotation of the craft, rotation of parts (such as gyros or wheels), or even elementary particles. There's an h-bar of angular momentum in just flipping spin of each electron. But you cannot dump angular momentum without dumping matter/particles that carry it away or without applying external torque. (Earth's magnetic field would count for "external" in this case.)

Angular momentum, as well as linear momentum, are perfectly conserved in all collisions. If you think you've read otherwise, you've misread.

[Jovus]

We're not actually disagreeing here. But my question still stands.

[K^2]

In that case, so does cpast's answer. A photon of energy E carries momentum p = E/c. Doesn't matter if it came from a matter-antimatter rocket, a laser, or a radiator. You can get 1N of thrust for every 300MW of power you dump into it.

[Alchemist]

Interesting example is Soyuz spacecraft. During long cruise stages to keep its panels towards the Sun it spins around the axis pointing at the Sun. It's called ×ðúрутúð ýð áþûýцõ (Spin at the Sun? Solar spin? I'm not sure what's the best translation). The cosmonauts are advised not to sleep in the orbital module with their heads towards the docking port.

Reaction wheels are only useful for long term attitude keeping with small fluctuations and without net angular momentum or with other options to bleed accumulating angular momentum (another factor on the ISS is drag, angular effects of which can be manipulated by changing the station's attitude and panel configuration). That's why if you need maintaining precise attitude only for short durations of time (as it usually is with manned crafts), it's more efficient to pack some extra RCS propellant instead of reaction wheels

Edited February 18, 2015 by Alchemist

[K^2]

It's called ×ðúрутúð ýð áþûýцõ (Spin at the Sun? Solar spin? I'm not sure what's the best translation).

I'd go with, "Twist onto the Sun." But it is just one of many phrases that makes you realize that English is a very bad language for translating things into.

[Duxwing]

I'd go with, "Twist onto the Sun." But it is just one of many phrases that makes you realize that English is a very bad language for translating things into.

I think translation is generally-hard because poetry is said to be what is lost in translation.

-Duxwing

[K^2]

Yeah, but it's usually much easier to translate from English to other languages without losing subtleties than the other way around. There are exceptions, of course, but in general English is just about the simplest natural language there is, so almost any structure in English is going to carry over to another language. Articles is a notable exception. Anything that accents the article will be hard to translate into a language without articles. But almost everything else works the other way around.

None of this takes culture in consideration, naturally, which can make even translation from English to English difficult.