Do you really need to add reaction wheels?

[What Zitt Tooya]

Is it necessary to add any type of reaction wheel on a ships? Some of my ships even have the "remote guidance units" which provide torque. Moreover, is it worth the weight and money to putting it into, say a probe or station expander.

[Man Of Many Wololo's]

you don't NEED them, though in vanilla kerbal, reaction wheels are little bundles of magic. If you find you can fly without them, no need for them. But they can help you a ♥♥♥♥ ton.

[Glitter Trash]

On some bigger ships where the capsule/probe core torque isn't enough anymore they can be useful, but in that case RCS is usually better and way more mass-efficient.

[Empiro]

Usually you don't need them. The crewed command capsules usually have more than enough.

I tend to only use them on unmanned probes, which have very little torque.

[GeneralVeers]

Oh ye of little faith. :)

Reaction wheels are very handy when you don't feel like ordering a pizza while waiting for your starship to flip a U for a retro burn. Also handy when you realize you were AFK too long and your mothership is two minutes away from missing a maneuver node (or crashing into something!)

And finally, reaction wheels are VERY handy for keeping a starbase in position to receive a docking ship, and/or to keep the solar panels lined up on the Sun.

[AlexMBrennan]

but in that case RCS is usually better and way more mass-efficient.

You are comparing apples and oranges here - RCS is a finite resource whereas reaction wheels are free so a fair comparision would be a space ship with reaction wheels vs a spaceship with a lander module and refinery to refill monoprop during the mission.

Edit to add: Come to think of it, the 2.5m reaction wheels is 0.2t whereas the RCS thrust block is 0.05t and the 0.625m RCS tank is 0.37t so the RCS thrusters by themselves are heavier than the a reaction wheel, never mind the monoprop.

[Azunai]

引用自 sansnipple：

On some bigger ships where the capsule/probe core torque isn't enough anymore they can be useful, but in that case RCS is usually better and way more mass-efficient.

no - RCS isn't more mass efficient. reaction wheels, batteries and solar panels are all very lightweight. RCS thrusters are also lightweight, but the MP fuel tanks are not.

the only reason to use RCS over reaction wheels is if you need the translation movement of RCS thruster for docking.

[Washell]

引用自 AlexMBrennan：

Edit to add: Come to think of it, the 2.5m reaction wheels is 0.2t whereas the RCS thrust block is 0.05t and the 0.625m RCS tank is 0.37t so the RCS thrusters by themselves are heavier than the a reaction wheel, never mind the monoprop.

You're forgetting the batteries and power generation.

[AlexMBrennan]

You're forgetting the batteries and power generation.

Correct, I did not list parts that are required by all probes because they are required by all probes and thus cannot be omitted to save mass. Your RCS probe still requires power generation if you are planning to go orbit Kerbin, and may require batteries unless you plan is to miss a manouver node on the dark side of Jool and crash.

[Rhedd]

I never use them (in over 1800 hours). So no, I suppose they aren't necessary.

[Operation40]

I usually put one on the "midstage" stack incase I make it to orbit with a giant fuel tank and big engine - so it won't take 10 minutes to turn the ship around..

other than that, I always use one for a "skylift" vehicle. Because it's going to dock to something heavy and attempt to land it, it needs to be over engineered.. I put RCS on it too and use both sometimes to steady a heavy base while descending to the surface

I also like huge reaction wheels on my beastly mining lander, cause it handles like a school bus when full of ore and fuel..
the mining rovers get it too because I do a "wheelie" and then fire the engine to go into orbit

just a few uses.. generally though if I feel the need for them, I'll upgrade the control pod to the HECS2 -- cause it's 3 parts in 1.. probe core, battery, and packs some juicy reaction wheels.

[What Zitt Tooya]

thanks for the replies everyone. I might as well put them on anyway. I can afford to spend a little bit more and add a little more weight

[Astrolibrarian™]

I always assumed they were as standard as batteries. They help me a lot.

But since I'm a noob, this seems like a good place to ask:
Are multiple reaction wheels any more effective than just one?
...For example, I've tried putting a 2nd one down on the engine, in addition to my usual one under the capsule...But can't tell if there's a difference.

[Rhedd]

引用自 Astrolibrarian™：

I always assumed they were as standard as batteries. They help me a lot.

But since I'm a noob, this seems like a good place to ask:
Are multiple reaction wheels any more effective than just one?
...For example, I've tried putting a 2nd one down on the engine, in addition to my usual one under the capsule...But can't tell if there's a difference.

Well, assuming you need them at all, more do more, since they each apply their torque force to the ship, but putting them "down on the engine" is not good.

Imagine every reaction wheel is trying to spin the ship around itself (because that's what's happening). Therefore you want to put your reaction wheels close to the vessel's center of mass. Exactly the opposite of RCS thrusters, which you want as far from the CoM as possible. This also means that if you put a reaction wheel at both ends of your ship, they're essentially fighting each other trying to twist the ship around themselves. Very inefficient.

[Illyrien]

I add them to almost all missions since they make it easier to turn the vehicles around to new headings. On the other hand, I almost never use RCS thusters, since my vehicles are so fast to rotate, so that I can just rely on the main engine

[El Rushbo]

Both have their uses. RCS enables translation for docking and can be used to augment rotation. Reaction wheels allow for rotation without expending monoprop. Generally you won't need them until you start designing larger spacecraft. I also find them useful on space stations to help keep them steady, just be cautious as multiple reaction wheels in different locations can fight each other. I almost had a station tear itself apart that way.