What is the difference "in real-life" an SAS and Reaction Wheel?

[Sirine]

SAS (Stability augmentation System) and Reaction Wheel.

Which mostly found on combat aircraft, helicopter, spacecraft and satellite.

Are they just difference name call. Or they really have something in difference?

Reference: - http://en.wikipedia.org/wiki/Autopilot

Edited August 28, 2014 by Sirine

[magnemoe]

The difference is that a Reaction Wheel keep you oriented in the direction you want, either in a satellite or a segway.

An SAS is simply an computer or other device who help you being stable.

Most does not use reaction wheels, an ship with just an command seat, or a mecjeb module would not have reaction wheels however it can use rcs or engine gimbal to stay oriented.

An satelite or a segway uses both, not much else who does in real world.

[Sirine]

So, SAS is software (System) to issues command to hardware so it maintain stable.

Reaction Wheel is a hardware that use to maintain stable?

[monophonic]

So, SAS is software (System) to issues command to hardware so it maintain stable.

Reaction Wheel is a hardware that use to maintain stable?

Yes. (10 chars)

[SkyRender]

The other big difference is that KSP's reaction wheels are absurdly effective compared to their real-world counterparts. Real-world reaction wheels generally can't send a craft from zero to "vomit comet" spin speed in 5 seconds flat the way even a single reaction wheel in KSP can.

[Roadkill]

The other big difference is that KSP's reaction wheels are absurdly effective compared to their real-world counterparts. Real-world reaction wheels generally can't send a craft from zero to "vomit comet" spin speed in 5 seconds flat the way even a single reaction wheel in KSP can.

And thank goodness that KSP is the way it is, because I couldn't think of anything funner than waiting for 30 minutes or more for my little KSP rocket to point the direction I want.

[Levelord]

Here's one explaining how reaction wheels are used in space craft.

Here's one showcasing how powerful they are, even in Earth gravity. So when I see cockpits in KSP rolling around on their own, I think it's still believable.

[Roadkill]

I'm glad that the reaction wheels in KSP are overpowered. But, they shouldn't be able to hold a craft upright on a steep slope for long periods of time. Even on Gilly with plenty of solar panels, they reaction wheels should eventually saturate.

[cantab]

Performance wise KSP's "reaction wheels" are more like control moment gyros, which work by changing the spin axis of a flywheel whereas reaction wheels change the spin speed.

Either real device saturates, but I think it's fair that KSP's don't, not least because we don't have gravitational torque to help unsaturate them.

[Brofessional]

Here's another related video

[problemecium]

I figure Kerbals just had somewhat different priorities when developing their space hardware.

For one thing, their engineers all seem to have OCD that made them insist on making every liquid engine relightable, throttleable all the way down to 0%, and with a mechanism to ensure constant thrust despite varying ISP. They also made the fuel tanks excessively strong, ruining their fuel mass ratio.

So I presume they just invested in high-tech super-powerful reaction wheels.

[tntristan12]

I'm glad that the reaction wheels in KSP are overpowered. But, they shouldn't be able to hold a craft upright on a steep slope for long periods of time. Even on Gilly with plenty of solar panels, they reaction wheels should eventually saturate.

I agree, but having spent a good portion of my master's program studying spacecraft control systems and reaction wheels I can attest to the fact that a proper implementation would be very complex and unintuitive for new players.

That said, I'd love to see a FAR or AJE type mod to come out which introduces realism to the reaction wheel system. I was actually planning to write such a mod myself but only if the game reaches its final version and nobody else has done a better job at it.

[godefroi]

Here's one showcasing how powerful they are, even in Earth gravity. So when I see cockpits in KSP rolling around on their own, I think it's still believable.

Yeah, they're not though. The Cubli is, you'll notice, essentially 100% reaction wheel. A spacecraft like that would have no room, for, you know, astronauts.

[ratchet freak]

The biggest problem with the reaction wheels in KSP is that they violate conservation of angular momentum

in actual spacecraft spinning up/moving the wheel moves the craft but when you stop the wheel the movement stops. to stop out of control spinning you need RCS

in KSP as long as you hold the button the craft will keep spinning faster and faster

[einsteiner]

That said, I'd love to see a FAR or AJE type mod to come out which introduces realism to the reaction wheel system. I was actually planning to write such a mod myself but only if the game reaches its final version and nobody else has done a better job at it.

I second this. It would be more realistic in that you would have to include RCS for every spacecraft. Since KSP doesn't include things like gravity gradient torques and solar light pressure it wouldn't pick up stray angular momentum, so it wouldn't be too bad. This could be a nice difficulty option, since Squad is working on more customization in that area.

[cantab]

If the reaction wheels can move arbitrarily then there's no issue with conservation of angular momentum. When you hit timewarp and the rotation instantly freezes, then there's an issue, but that still exists even if you only use RCS.

[ratchet freak]

If the reaction wheels can move arbitrarily then there's no issue with conservation of angular momentum. When you hit timewarp and the rotation instantly freezes, then there's an issue, but that still exists even if you only use RCS.

yeah but you can run out of electric charge and then eventually the wheels will spin down due to friction and the craft should revert to its initial rotational momentum

[Mesons]

So I presume they just invested in high-tech super-powerful reaction wheels.

*Found high-tech super-powerful reaction wheels lying by the side of the road

The biggest problem with the reaction wheels in KSP is that they violate conservation of angular momentum

in actual spacecraft spinning up/moving the wheel moves the craft but when you stop the wheel the movement stops. to stop out of control spinning you need RCS

in KSP as long as you hold the button the craft will keep spinning faster and faster

Angular momentum is be conserved, though--the spacecraft just spins in the opposite direction as the wheels inside of it. When you press the button, you are applying a torque. When you take your finger off the button, you stop changing the angular momentum, but the wheels remain spinning how they were at the moment you released.

[Renegrade]

Ugh, I'd be happy if people could learn this lesson the OP did.

SAS != reaction wheels.

I see so many streamers and posters talking about 'SAS torque'. I presume this is from older versions of KSP where Squad was a little loose on the definition? Or are people still being confused by the 2.5m Advanced SAS module? Actually..why DO the reaction wheel modules have SAS equipped?

Toning down the strength, renaming them to 'CMGs', and adding a very simplistic 'saturation' gauge (which fills up, and has to be expunged by using a little monoprop via RCS thrusters or similar) would probably align us with a decent level of HarvestR's 'authenticity' without going down the 'wheel #3 is showing 32% increased friction' route.

My own Horrible Nerf changes the advanced 1.25m wheel into a basic 0.625m wheel, and makes the strengths 0.25, 2, and 8 for 0.625m, 1.25, and 2.5m sizes. And removes 'em from all pods.

[ratchet freak]

*Found high-tech super-powerful reaction wheels lying by the side of the road

Angular momentum is be conserved, though--the spacecraft just spins in the opposite direction as the wheels inside of it. When you press the button, you are applying a torque. When you take your finger off the button, you stop changing the angular momentum, but the wheels remain spinning how they were at the moment you released.

but they will only go so fast before they can't add any more torque to the craft, you can take turns rolling left with reaction wheels and right with monoprop and still be able to get it to stop each time

also internal friction of the gyro will eventually stop the wheels (or there has to be a constant drain of electricity) and reset the rotation speed unless you neutralize the angular momentum with RCS

[Whackjob]

Ahh, reaction wheels. Reminds me of their first amusing appearance... as a sideshow. Watch the circus strongman completely fail to knock over this innocuous looking wheel of cheese!

Yeah. Serious.

[Mesons]

but they will only go so fast before they can't add any more torque to the craft, you can take turns rolling left with reaction wheels and right with monoprop and still be able to get it to stop each time

I'm pretty sure this only happens because the energy stored in the reaction wheel is proportional to the angular momentum squared, so it takes more energy the faster the wheels are spinning in one direction.

also internal friction of the gyro will eventually stop the wheels (or there has to be a constant drain of electricity) and reset the rotation speed unless you neutralize the angular momentum with RCS

They are usually designed to have extremely low friction. Rotating craft experience unrealistic rotational drag in vacuum, too--I would sooner see that eliminated, but it also helps with the gameplay.

At any rate, introducing elements of realism a la reaction wheel saturation and (more) friction would seriously harm the fun of KSP in my opinion. And this despite my adoring the realistic nature of much of the game (though of course it needs some work).