About reaction wheels...

[quasarrgames]

So, i've been told that reaction wheels in the game are more powerful than reaction wheels irl, but what i'd like to know, is how much more powerful they are.

I don't know much about reaction wheels, other than how they work and the fact that they break down a lot and are used to orient telescopes and the ISS. But, when compared to real reaction wheels, how much more powerful are ksp reaction wheels compared to, say, the ISS's reaction wheels?

Edited March 13, 2015 by quasarrgames

[Leszek]

Some of this I can't answer but one of the key differences to KSP reaction wheels is that they spin up. A reaction wheel can only spin so fast and as it gets faster and faster the torque output gets less and less. When the wheels reach maximum speed they are unable to provide any more torque in that direction. At that point you need to use RCS or other methods to dissipate the torque. In KSP they always provided the same amount of power and never spin up. This means you can use KSP reaction wheels for things such as holding attitude in atmosphere and to fight asymmetric thrust. If you tried to do those things IRL the wheels would quickly spin up and you would loose control. Hence any vehicle equipped with reaction wheels would also need RCS.

Another difference is how much mass they have. You could make IRL reaction wheels have the same power as KSP reaction wheels easily enough, but that would mean powerful motors attached to heavy flywheels. On the most efficient rockets we can expect to make with current fuels you will need at least 8 to 9 pounds of fuel for each pound you put in space. (And I am sure the number is higher than that since I don't think any rockets we have are near max efficiency for the fuel we have, perhaps someone can let us know.) Since the vehicle will need RCS anyway the reaction wheels can often be left out all together at considerable savings in mass.

[StrandedonEarth]

I'm not sure how much more powerful the game RW's are, but the unrealistic thing about them is that they don't saturate. That means they can continue applying torque in the same direction forever. In real life, reaction wheels trying to do that would have to keep spinning faster and faster and faster, and eventually something would fail, either the bearing would seize or the wheel itself would fly apart from centrifugal force. Or the motor would be unable to accelerate the wheel any faster (fail-safe)

[Starwhip]

I'm not sure how much more powerful the game RW's are, but the unrealistic thing about them is that they don't saturate. That means they can continue applying torque in the same direction forever. In real life, reaction wheels trying to do that would have to keep spinning faster and faster and faster, and eventually something would fail, either the bearing would seize or the wheel itself would fly apart from centrifugal force. Or the motor would be unable to accelerate the wheel any faster (fail-safe)

Yep.

Take this, for example. Can torque itself over in any direction indefinitely.

[Jouni]

I don't know much about reaction wheels, other than how they work and the fact that they break down a lot and are used to orient telescopes and the ISS. But, when compared to real reaction wheels, how much more powerful are ksp reaction wheels compared to, say, the ISS's reaction wheels?

About 150x more powerful. The CMGs at ISS weight around 270 kg each and produce up to 258 Nm of torque each. In KSP, the large ASAS module produces 30 kNm of torque and weights 200 kg.

[Deutherius]

About 150x more powerful. The CMGs at ISS weight around 270 kg each and produce up to 258 Nm of torque each. In KSP, the large ASAS module produces 30 kNm of torque and weights 200 kg.

It should also be pointed out that each CMG at ISS can only produce torque in one direction at a time, while all of the KSP's Reaction Wheels can produce torque in all 3 axes at once.

[Kryxal]

I think that what the game calls a reaction wheel is really a set of three ... watch your electricity usage sometime and you'll see (if you haven't overdone the panels and have enough batteries) that each axis in use has a certain current draw and they add up.