4 ms·

> a minimum of three Correct me if I am wrong, but don't most systems use six? It is a whole lot easier to use three pairs of wheels. Pairs can be spun up fr

by keenerd 13y ago

> a minimum of three Correct me if I am wrong, but don't most systems use six? It is a whole lot easier to use three pairs of wheels. Pairs can be spun up from a dead stop and in the event of reduced power can be used to turn in either direction by braking action alone. Or does "a reaction wheel" imply two counter-rotating flywheels by definition? For an alternative, I've heard some good things about laser ablative thrusters. Still fairly young, but they are powered by electricity and have no moving parts. They do need reaction mass, but unlike traditional engines the mass is a solid block and can't boil off, corrode, leak, freeze or whatever problems affect liquid systems.

lutusp 13y ago
> Correct me if I am wrong, but don't most systems use six? Kepler was equipped with four -- three in use, one spare. The idea is that you need one reaction wheel per dimension. > It is a whole lot easier to use three pairs of wheels. All that six wheels would provide is three spare wheels, one per dimension. > Pairs can be spun up from a dead stop and in the event of reduced power can be used to turn in either direction by braking action alone. That's not how reaction wheels are normally used. Two counter-rotating wheels cancel each other out, producing zero net rotational force on the spacecraft. > Or does "a reaction wheel" imply two counter-rotating flywheels by definition? No, just one. If you have two, and if they counter-rotate, they cancel each other out. > For an alternative, I've heard some good things about laser ablative thrusters. Still fairly young, but they are powered by electricity and have no moving parts. Not nearly enough momentum to rotate a large spacecraft.
- sigstoat 13y ago
  > Kepler was equipped with four -- three in use, one spare. The idea is that you need one reaction wheel per dimension. are you sure? i thought it was four (in a tetrahedral arrangement), but they were all constantly in use, and one failure could be tolerated.
  - lutusp 13y ago
    > are you sure? i thought it was four (in a tetrahedral arrangement), but they were all constantly in use, and one failure could be tolerated. Source: http://kepler.nasa.gov/mission/faq/ http://kepler.nasa.gov/mission/faq/ Quote: "Only 3 reaction wheels are needed to control the 3 degrees of freedom of rotation of spacecraft. But Kepler was provided with 4 reaction wheels, one extra for redundancy in case a wheel fails."
elteto 13y ago
A spacecraft in space has three rotational degrees of freedom (translation is a different deal), therefore you only need three wheels to be able to point the spacecraft in any direction that you want. Any more than three is just a spare.
sigstoat 13y ago
> For an alternative, I've heard some good things about laser ablative thrusters. kepler also had thrusters, for desaturing the reaction wheels. took at least half an hour or something every few days, i think. thrusters spew stuff into the local environment, cloudying it up (relatively speaking). so it might not have been a solution for kepler, even if you could pack on the necessary reaction mass.
mapt 13y ago
As long as their axes have some orthogonal component, three allow for a torque-cancelling solution in three dimensions. It may not be as easy an equation as three pairs of cancelled-moment wheels, but it's more weight-efficient. However... What little I can puzzle out about systems like this suggests that six active wheels (plus spares) would allow for desaturation of spin (cutting back when one flywheel approaches its maximum RPM) without either additional spacecraft rotation or thrusters.
- lutusp 13y ago
  > What little I can puzzle out about systems like this suggests that six active wheels (plus spares) would allow for desaturation of spin (cutting back when one flywheel approaches its maximum RPM) without either additional spacecraft rotation or thrusters. But in the absence of thrusters or another method, to stop one reaction wheel from spinning, you have to start another one spinning -- in other words you have to transfer the conserved momentum to another wheel. So wheels acting by themselves cannot cancel their own momentum, they can only transfer it from one wheel to another. Another approach is to stop a wheel spinning with a brake pad, which would convert the momentum into heat. But this has obvious drawbacks, among which the energy is irretrievable.

by keenerd 13y ago

lutusp 13y ago

> Correct me if I am wrong, but don't most systems use six? Kepler was equipped with four -- three in use, one spare. The idea is that you need one reaction wheel per dimension. > It is a whole lot easier to use three pairs of wheels. All that six wheels would provide is three spare wheels, one per dimension. > Pairs can be spun up from a dead stop and in the event of reduced power can be used to turn in either direction by braking action alone. That's not how reaction wheels are normally used. Two counter-rotating wheels cancel each other out, producing zero net rotational force on the spacecraft. > Or does "a reaction wheel" imply two counter-rotating flywheels by definition? No, just one. If you have two, and if they counter-rotate, they cancel each other out. > For an alternative, I've heard some good things about laser ablative thrusters. Still fairly young, but they are powered by electricity and have no moving parts. Not nearly enough momentum to rotate a large spacecraft.

sigstoat 13y ago
> Kepler was equipped with four -- three in use, one spare. The idea is that you need one reaction wheel per dimension. are you sure? i thought it was four (in a tetrahedral arrangement), but they were all constantly in use, and one failure could be tolerated.
- lutusp 13y ago
  > are you sure? i thought it was four (in a tetrahedral arrangement), but they were all constantly in use, and one failure could be tolerated. Source: http://kepler.nasa.gov/mission/faq/ http://kepler.nasa.gov/mission/faq/ Quote: "Only 3 reaction wheels are needed to control the 3 degrees of freedom of rotation of spacecraft. But Kepler was provided with 4 reaction wheels, one extra for redundancy in case a wheel fails."

elteto 13y ago

A spacecraft in space has three rotational degrees of freedom (translation is a different deal), therefore you only need three wheels to be able to point the spacecraft in any direction that you want. Any more than three is just a spare.

sigstoat 13y ago

> For an alternative, I've heard some good things about laser ablative thrusters. kepler also had thrusters, for desaturing the reaction wheels. took at least half an hour or something every few days, i think. thrusters spew stuff into the local environment, cloudying it up (relatively speaking). so it might not have been a solution for kepler, even if you could pack on the necessary reaction mass.

mapt 13y ago

As long as their axes have some orthogonal component, three allow for a torque-cancelling solution in three dimensions. It may not be as easy an equation as three pairs of cancelled-moment wheels, but it's more weight-efficient. However... What little I can puzzle out about systems like this suggests that six active wheels (plus spares) would allow for desaturation of spin (cutting back when one flywheel approaches its maximum RPM) without either additional spacecraft rotation or thrusters.

lutusp 13y ago
> What little I can puzzle out about systems like this suggests that six active wheels (plus spares) would allow for desaturation of spin (cutting back when one flywheel approaches its maximum RPM) without either additional spacecraft rotation or thrusters. But in the absence of thrusters or another method, to stop one reaction wheel from spinning, you have to start another one spinning -- in other words you have to transfer the conserved momentum to another wheel. So wheels acting by themselves cannot cancel their own momentum, they can only transfer it from one wheel to another. Another approach is to stop a wheel spinning with a brake pad, which would convert the momentum into heat. But this has obvious drawbacks, among which the energy is irretrievable.