About torque

[Kasuha]

This thing is bugging me for a while now.

KSP uses two ways to orient the ship, RCS and torque, similar to how it's done in real world.

But in my opinion, when using torque things behave differently than how it's done in KSP. As well as you cannot get translation out of gyroscopes, you shouldn't be able to pull permanent rotational moment from them too.

In my opinion, when using gyros to orient the ship, you can rotate it any way you want but it must inevitably stop rotating once you stop the gyros. There's nothing like giving it an impulse after which it would continue indefinitely without adding energy to keep the gyros rotating. Or like spinning the ship faster and faster. Maximum rotational speed of the ship achievable using gyros is given by proportion of gyros weight to the ship weight, multiplied by gyros rotational speed. Or something close to that.

Or am I wrong about it?

[Sapphire]

you shouldn't be able to pull permanent rotational moment from them too.

In my opinion, when using gyros to orient the ship, you can rotate it any way you want but it must inevitably stop rotating once you stop the gyros.

Why? This is space. If you start moving, you won't stop unless you put force in the opposite direction.

[Whackjob]

Why? This is space. If you start moving, you won't stop unless you put force in the opposite direction.

Newton says wrong. Say you got the gyro spinning. Ship spins the other way. You slam the brakes on the gyro. Ship stops too. Ship's only rotating because of the mass of the reaction wheel. Equal and opposite reaction. If there's no torque from the wheel, you get no reaction from the ship.

Net momentum is zero, even during use.

[Ralathon]

Why? This is space. If you start moving, you won't stop unless you put force in the opposite direction.

Conservation of momentum.

Gyros work by spinning a wheel in one direction, so the rest of the spaceship rotates in the opposite direction. If you want to keep rotating indefinitely you need to keep that wheel spinning, else it'll bleed velocity via drag etc and both the spaceship and the gyro will slow down.

Anyway, while gyros are rather OP right now I don't think continuous power draw and those other restrictions are a very good idea. It's a game, not a simulation. setting up gyro discharge schematics is rather tedious work and wouldn't make for very interesting gameplay. I'd simply remove gyros from command pods and lower their torque by a factor 2 or so.

[Akinesis]

I believe that the probe would spin down relative to the giro's due to friction, once power was removed. The speed the giro's (or reaction wheels - whatever you want to call them) spin down would depend on how good their bearings are. I firmly believe you are right - the rotation of the probe would cease once the giro's rotation ceases (the 'equal and opposite' the second poster is looking for would be the friction on the bearings).

So, maybe it would provide some comfort to you if you pretended that the Kerbals had invented frictionless bearings, meaning the probe (and giros) would never stop spinning Well, until opposite torque was applied.

Edit: Ninja'd x2!!

Also, as well as drag from friction on the bearings, windage is also another factor on the disc. So, also pretend that Kerbals have overcome that (impossibly perfect aerodynamics, vacuum operation, whatever!).

Plus, as mentioned - it's a game

Edited October 5, 2013 by Akinesis

[adinfinitum]

Perhaps the kerbals, with the amazing engineering prowess, have come up with frictionless bearings to use in their gyros. That way the wheels, and the ship, don't stop turning until you stop the gryo yourself.

[Whackjob]

The wheel would stop eventually. There would have to be a microscopic flaw on the wheel where one side is heavier than the other. You are near a large planet. Even that little reaction wheel would eventually succumb to tidal lock. Given the size of the wheel, and the microscopic imperfection, it would probably take longer than the lifetime of the Sun, but it would happen.

[Akinesis]

Now, that's going deep, Whackjob

[Lazro]

Whackjob goes deeper into KSP than any of us would care to

[Akinesis]

Whackjob goes deeper into KSP than any of us would care to

Care to... or dare to?

[Whackjob]

I could extrapolate all the way out to the heat death of the universe, if there were time for it. Or as long as a quantum metastability event which almost certainly has already happened got to us and obliterated all matter and redefined the fundamental forces of the universe.

Don't mistake me for an actual scientist, however. I am not. I'm more of an arm-chair enthusiast who reads more than he understands.

[adinfinitum]

However, by the time it does eventually stop, my kerbals will no longer bother with trivial reaction wheels and will need only bend the curvature of space to change their direction/orientation.

Perhaps after that quantum metastability even reaches us it will create a universe that leads to planetary bodies which are ten times as dense, and provide shelter for little green bug-eyed men.

[m4v]

I'm good with reaction wheels the way they're now except that they have too much torque, I haven't done any math but it feels like they are as good as RCS for rotating stuff.

[Vlk]

Hmm, now that i think of it, i think i understand. But... that means, if in real life, the spaceship starts to rotate in a direction (because of an outside force), and you would want to stop it permanently, you would have to spin the reaction wheels continously?

[doggie015]

Whenever I hear about reaction wheels I always think of

[Whackjob]

Hmm, now that i think of it, i think i understand. But... that means, if in real life, the spaceship starts to rotate in a direction (because of an outside force), and you would want to stop it permanently, you would have to spin the reaction wheels continously?

Just long enough to kill the momentum.

#EDIT: Answered too quick. Am I wrong here? Now I'm modeling the whole collision out in my head.

#EDIT2: I think you may be right here. Conservation of angular rotation. I think it's called. I'm dredging the brain-pan here, so I could easily be rectally speaking here, but I think you're right. The rotation is there, and the net rotation of ship plus reaction wheels is always zero. So rotation + 0 = rotation. You'd have to do something else to kill it, like RCS. Reaction wheel itself can never do it.

Edited October 6, 2013 by Whackjob

[Vlk]

Just long enough to kill the momentum.

#EDIT: Answered too quick. Am I wrong here? Now I'm modeling the whole collision out in my head.

I think you're wrong, but i'm not sure. In (my) theory, if the wheel stops spinning, it would give back the station exactly the original momentum.

[adinfinitum]

You would have to spin it continuously, if you stopped the reaction wheel the momentum would be transferred back to the ship.

Also, I was running through some quick math, and if kerbal reaction wheels are comparable to those such as this real life reaction wheel, then a single 1.25 meter SAS module would be able to rotate a 10 tonne ship about the z axis at a rate of about 1.9 radians a second.

[Kerbart]

Hmm, now that i think of it, i think i understand. But... that means, if in real life, the spaceship starts to rotate in a direction (because of an outside force), and you would want to stop it permanently, you would have to spin the reaction wheels continously?

Yes. So eventually you'll have to use an RCS system so you can stop the gyros. But assuming that over time those external forces cancel each other (partly) out, gyros offer a way of stabilizing the ship without burning through RCS propellant in record time.

[DMagic]

Well, one way of maintaining spin would be to drop the reaction wheels after you've started spinning. Then you wouldn't have to worry about dissipating the energy in the reaction wheels. Of course you also wouldn't be able to steer anymore.

Or they could add a way to dump energy (other than balancing it with RCS) from the reaction wheels through either gravity or magnetic fields. But that's probably not a good idea, no one wants to deal with that.

[Starwhip]

I could extrapolate all the way out to the heat death of the universe, if there were time for it. Or as long as a quantum metastability event which almost certainly has already happened got to us and obliterated all matter and redefined the fundamental forces of the universe.

Don't mistake me for an actual scientist, however. I am not. I'm more of an arm-chair enthusiast who reads more than he understands.

Aren't we all? (KSP-ers, I mean)

[Whackjob]

I am quite certain, now, that a ship equipped with reaction wheels only, being then struck obliquely and caused to spin, would have to spin the reaction wheels forever to prevent said spin in that reference frame, excepting additional external forces.

[taylorbm]

Yes. So eventually you'll have to use an RCS system so you can stop the gyros. But assuming that over time those external forces cancel each other (partly) out, gyros offer a way of stabilizing the ship without burning through RCS propellant in record time.

This is the way real spacecraft/satellites do it. They have a reaction engines of some sort aligned symmetrically on the craft that fire to maintain the current vehicle attitude while the reaction wheels (gyros) are desaturated (slowed down). If the reaction wheels just stopped the conservation of angular momentum would cause the craft to start spinning again.

[magnemoe]

The wheel would stop eventually. There would have to be a microscopic flaw on the wheel where one side is heavier than the other. You are near a large planet. Even that little reaction wheel would eventually succumb to tidal lock. Given the size of the wheel, and the microscopic imperfection, it would probably take longer than the lifetime of the Sun, but it would happen.

Think magnetic interaction with the rest of the ship will have far most effect. You can just spin the wheels a bit more to compensate.

However the wheels are used two ways: You rotate the ship 90 degree in on direction, start of rotation spins up the wheel and slow down stops it again. The result is no rotation on the wheel.

This makes them perfect for space telescopes and other stuff you want to point in different directions.

Now reaction wheels are not able to deal with constant push, in ksp this is an unbalanced ship with an ion engine or other without gimball. For real world satellites this is mostly tidal and other small effects.

[csanders]

I'm good with reaction wheels the way they're now except that they have too much torque, I haven't done any math but it feels like they are as good as RCS for rotating stuff.

Pretty sure NASA just time warps to stop spins. Saves fuel and energy.