Realistically, any reaction wheel?

[Jestersage]

I know that all spacecraft, including and since Apollo and Soyuz, have gyro wheels. Are they basically the reaction wheel? Or should I just pack more monoprop? (note: I am aware spaceraft mean manything. in here, I actually include both Space Station and Capsule/Shuttles/Spaceplanes)

Edited March 20, 2018 by Jestersage

[Rocket In My Pocket]

Reaction Wheels are a real thing, however they are much weaker, prone to malfunction, and susceptible to "saturation" in real life as opposed to KSP.

I believe there is a mod for more realistic reaction wheels? Can't think of the name though.

Anyways if you don't go overboard with them, I wouldn't call it "unrealistic" to have some reaction wheel torque.

[fourfa]

In general, the gyro wheels present on many spacecraft are for navigation.  "All spacecraft" is going too far these days - gyro-based inertial platforms are big, heavy, expensive, hot, and prone to errors and failures, compared to modern purely-electronic MEMS-based accelerometers.  Heck, some microsats that don't have the weight or power budget for true standalone navigation just use GPS and magnetic field sensors to grab their position and orientation.

There are real-world reaction wheels, to be sure.  KSP's versions are massively simplified, and are too powerful, too lightweight, don't draw enough electrical power, don't require cooling, don't require desaturation, don't fail, and generally violate the laws of motion  

Players going for realism often do deactivate the reaction wheels built-in to the command pods, and use RCS for attitude control.  There are many mods that nerf KSP's reaction wheels to force the player to spend a lot of design work on balancing their RCS placement and fuel quantity, as we do in the real world.  I'm fond of this one:

Because it nerfs RW power when you want to rotate the ship, but it uses full RW power when you want to simply hold orientation.  This avoids a problem with KSP's stock SAS, where it's tuned to expect some reaction wheel torque.  Without that torque, SAS tends to oscillate back and forth using RCS and dump all your RCS propellant.  For full realism with no reaction wheel torque at all, I'd recommend going with mods for attitude control (MechJeb, AtmosphericAutopilot etc) and seldom using the stock SAS.

[Jestersage]

It was more of trying to figure out whether to install a reaction wheel on the Soyuz clone, since I actually need to add a lot of monoprop onto it.

I think I will use the smallest one and then go 10%, keeping it stock.

[Zeiss Ikon]

I'm not aware of any real-world crewed spacecraft that have actual reaction wheels.  Space telescopes tend to have them, because they can be used to manage pointing for rather long periods without burning RCS fuel continually (occasional observation shutdowns are required, so the RCS and desaturate the reaction wheels).  You might recall Kepler's primary mission ended when a reaction wheel failure meant it could no longer be precisely targeted on its primary field for long periods, and Hubble has required at least one repair mission to replace reaction wheels that were approaching failure (bearings were failing, as I recall).

As noted above, however, real life reaction wheels don't have the power needed for routine maneuvering, and because the flywheels have RPM limits, they can "saturate" -- that is, when the wheel can't turn any faster, it can't apply/absorb any more angular momentum in that direction.  All the stuff that goes with rapidly accelerating and decelerating heavy rotating objects applies to reaction wheels, not to mention if something goes very wrong, a wheel can fail (and a failure of a massive wheel spinning at tens of thousands of RPM strongly resembles an explosion -- not something you want in the cockpit of your Space Shuttle).

There are in-game reasons to include a wheel (system), as noted in other answers -- but if you want maximum realism for crewed craft, you'd have no reaction wheels at all (and be prepared to run out of monopropellant at an inopportune time).