Do manned spacecraft use reaction wheels?

[Jackissimus]

Hey there!

Question for the space veterans here - I was wondering, we do just about all the attitude control with reaction wheels in KSP. Everyone likes them, RCS is just there to help if the wheels are not enough, or for translation. How close is this to real life? I understand that satellites such as Kepler or Hubble use reaction wheels, so do stations, such as ISS. As far as I understand it these things don't use RCS, because that would mean carrying a lot of propelant with it and they have plenty of power for the SAS ... oops, reaction wheels ... anyway. But do other spacecraft have reaction wheels? For example I know that Apollo didn't have it, not in the CM, not in the LM. So if not, why?

[King Arthur]

The ISS, which is a "manned spacecraft", uses both reaction wheels (more accurately CMGs) and RCS thrusters for attitude control. So the answer to the original question is yes, we do use reaction wheels and similar control systems on manned spacecraft.

As for Apollo, I can't answer that unfortunately as I lack the information to give a proper and accurate answer. =(

[Bunsen]

Real spacecraft always need RCS, even if they have reaction wheels or CMGs, because there can be a small net torque on the craft (usually from drag or radiation pressure) that would force the wheels to keep spinning faster and faster. You have to dump angular momentum with RCS now and then.

With only RCS, you have a limited delta-L (i.e. change in angular momentum) budget. That's fine for short missions, but long missions or lots of attitude changes require a lot of propellant. Wheels give you essentially unlimited delta-L (well, they do fail eventually) as long as you can afford the electricity, but they're heavy. So it only makes sense to add wheels to the design when they save more than their weight in RCS propellant, and that payoff generally takes a long time.

They're also much better at fine pointing than chemical thrusters (which have a minimum thrust pulse size), so they're nearly indispensable for things like space telescopes.

[Anton P. Nym]

Another reason long duration missions tend to use reaction wheels instead of RCS; exhaust fumes and side spray can accumulate over time and eventually obscure instruments' fields of view. That takes many firings, but Hubble (for example) was up long enough that this would've been a problem had it been equipped with RCS. Shorter missions like Apollo wouldn't have to worry about that.

-- Steve

[K^2]

For example I know that Apollo didn't have it, not in the CM, not in the LM. So if not, why?

They be heavy. So it's all about which one's going to require less additional mass on board. Bringing more RCS monoprop or installing huge gyro wheels. For a Lunar lander, gyros would make zero sense. For the CM such a thing would probably be more viable, but CM didn't need that many precise orientation changes, so RCS was probably still an easier option.

And as somebody already mentioned, reaction wheels (or CMGs) aren't a replacement for RCS. It's not like KSP where reaction wheels can absorb any quantity of angular momentum. They can store some, but it's limited. One way or another, you have to have RCS thrusters on board. But on longer missions or for better precision, CMGs might reduce the amount of RCS monoprop you need significantly enough to be of use.

For scientific equipment there are additional considerations, of course.

[Racer319]

Real spacecraft always need RCS, even if they have reaction wheels or CMGs, because there can be a small net torque on the craft (usually from drag or radiation pressure) that would force the wheels to keep spinning faster and faster. You have to dump angular momentum with RCS now and then.

is possible with gyroscopes (or with arms and legs). Since the force from radiation pressure is extremely small, sure it might impart some minuscule amount of angular momentum over time, but could that not be easily corrected with gyroscopes?

[K^2]

You can re-orient yourself with gyroscopes, yes. But you can't correct for gaining net angular momentum, and there are any number of sources for that. Undocking, collisions (planned, or otherwise), atmospheric effects, solar effects, ablation of material on the ship, any leaks... You can't just assume that you aren't going to pick up any considerable angular momentum during the mission.

[goldenpeach]

[...]

And as somebody already mentioned, reaction wheels (or CMGs) aren't a replacement for RCS. It's not like KSP where reaction wheels can absorb any quantity of angular momentum. They can store some, but it's limited. One way or another, you have to have RCS thrusters on board. But on longer missions or for better precision, CMGs might reduce the amount of RCS monoprop you need significantly enough to be of use.

[...]

I don't understand:what do you mean by "storing" angular momentum?

[andrewas]

Simple enough. Your craft picks up some unwanted rotation, usually from a tiny but on-going force. Tidal forces from Earth, radiation pressure. To keep orientation, the reaction wheels spin up and transfer angular momentum from the craft to the wheel - but you can't spin them down again without dumping that momentum back into the craft. Since the external force is continuous, the reaction wheels have to constantly accelerate to counter is and eventually hit their safety limit and, without thrusters, you lose the ability to maintain orientation.

[RuBisCO]

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.

[Francesco]

Wheels give you essentially unlimited delta-L (well, they do fail eventually) as long as you can afford the electricity, but they're heavy..

you mean delta-v, or is that another thing altogether?

also, do real wheels work just like in KSP, consuming electricity?

and how can they give you unlimited delta-V/L, if you have to "empty" them at some point using RCS?

a lot of questions here

[K^2]

Delta-L means change in angular momentum. And it's not by any measure unlimited.

[King Arthur]

you mean delta-v, or is that another thing altogether?

also, do real wheels work just like in KSP, consuming electricity?

and how can they give you unlimited delta-V/L, if you have to "empty" them at some point using RCS?

a lot of questions here

1. I am uncertain as to what Delta-L is, Wiki's article on the subject at least seems unrelated to space manuevering.

2. As far as I know, yes reaction wheels consume electricity because they are powered by electric motors.

3. The "unlimited Delta-V" idea is on the assumption that the reaction wheels do not need desaturating; of course as you've said real reaction wheels do need to be desaturated at some point, so the whole "unlimited Delta-V" thing is more of a principle by itself.

[Bunsen]

I just came up with the term "delta-L" then. Seemed natural enough to describe what attitude control systems do -- create changes (delta) in angular momentum (L). It's not delta-v, because wheels don't change the craft's velocity. The "unlimited" thing I was talking about was meant to refer to maneuvering, which involves changing the craft's angular momentum (from zero to something, to start rotating), then changing it back (to stop). RCS would have to burn propellant to start rotating, then burn more to stop, and you'll run out if you do it enough. Reaction wheels can keep on doing that almost forever, limited only by the availability of electricity and the lifetime of their bearings (which is finite, as Kepler has so recently demonstrated).

For permanent changes in angular momentum, like counteracting external torques, you have to interact with something external to the spacecraft -- expelled RCS propellant mass and the Earth's magnetic field being the favorites, and RCS tends to be the practical option for manned craft.

I think I thought of a good metaphor: Wheels let you borrow angular momentum for a while, over and over, but with a rather low credit limit. RCS makes you buy it every time. So if you only need it for a little while (say, to change which star your telescope is pointing at, which you're going to want to do a hell of lot of times), you borrow it from a wheel. But if you need to keep it (because your solar panels are asymmetric and you need to keep that antenna pointed at Earth), you won't be able to pay it back, so you buy it through RCS and pay with propellant. If you won't be operating for long, it can be simpler to just bring a fatter wallet full of propellant than to set up a credit account with the wheels.

[K^2]

There's nothing wrong with calling it delta-L. It's clear enough in context.

And yes, so long as the net change in angular momentum is zero, the reaction wheels can account for any difference during the maneuver as many times as necessary. In that sense, sure, you have an "unlimited" resource compared to what you'd be doing with RCS thrusters. I understand what you mean now. But calling it unlimited delta-L is a bit confusing, as the net delta-L is zero.

Still, if you need limited amount of maneuvering, the clearly finite resource of RCS thrusters might still end up adding less weight to your craft than the reaction wheels.

[jwenting]

why do you think they have those exercise bikes in the ISS?

[K^2]

why do you think they have those exercise bikes in the ISS?

Somebody needs to look into this. If there are three, and they are at 90° angles to each other, I'd get really suspicious.

[Fuzzy Dunlop]

Reaction wheels are unlimited in the sense that you have unlimited pointing ability provided you have no external torque. RCS dosen't give you that.

[KSK]

Just as a quick (and probably dumb) follow up question. Are reaction wheels any good for controlling a launch vehicle? My gut instinct says no and from reading around on the internet, I've only ever found references to reaction wheels being used for attitude control of the spacecraft once in space. Just wondering if anyone on here had a better answer?

Thanks.

[Jackissimus]

You use engine thrust vectoring to maintain attitude while in the atmosphere.

[lajoswinkler]

I think launches use thrust vectoring and the careful placement of center of mass and center of pressure (has to be beneath the CoM). They also use RCS sometimes.

Controlling rockets with reaction wheels would be very cumbersome.

[KSK]

I think launches use thrust vectoring and the careful placement of center of mass and center of pressure (has to be beneath the CoM). They also use RCS sometimes.

Controlling rockets with reaction wheels would be very cumbersome.

Ok thanks. That's what I thought but I was curious to know whether it was possible in principle (as it is in KSP).

[andrewas]

In theory it is, but the wheels would have to be very heavy or very fast to provide enough control authority. Since launchers have very tight mass budgets, and there are limits on how fast a wheel can spin, its not practical in reality.

[Maarek_Stele]

Simple enough. Your craft picks up some unwanted rotation, usually from a tiny but on-going force. Tidal forces from Earth, radiation pressure. To keep orientation, the reaction wheels spin up and transfer angular momentum from the craft to the wheel - but you can't spin them down again without dumping that momentum back into the craft. Since the external force is continuous, the reaction wheels have to constantly accelerate to counter is and eventually hit their safety limit and, without thrusters, you lose the ability to maintain orientation.

I was thinking about this build up in angular momentum and the need for RCS to zero it out. Wouldn't it be possible for a gyro or reaction wheel system to cancel it's own angular momentum issues by reversing the craft's direction to start working the wheels in the opposing direction? I'm sure I'm not accounting for something in my logic so I'm hoping someone can point it out for me?

[peadar1987]

I was thinking about this build up in angular momentum and the need for RCS to zero it out. Wouldn't it be possible for a gyro or reaction wheel system to cancel it's own angular momentum issues by reversing the craft's direction to start working the wheels in the opposing direction? I'm sure I'm not accounting for something in my logic so I'm hoping someone can point it out for me?

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.