Reaction wheel

A reaction wheel provides torque to a craft and allows it to rotate in space with using only electric charge and is thus available as long as there is an electric power generator on board. Unlike real life reaction wheels, KSP reaction wheels can rotate a craft along all three axes simultaneously, regardless of their orientation. Those were namely added in version 0.21 but a similar unnamed system was added previously.

The command modules listed in the table below have a reaction wheel and can provide torque. Adding additional parts utilising torque allows faster change in rotation. The placement does matter for reaction wheels. Generally speaking they can cause some problems if placed far from the center of mass. They function like grabbing the point where the reaction wheel is located and rotating around that point. The rotation will get anywhere other than near the center of mass. All command pods and probe bodies provide reaction wheels, except the Probodobodyne OKTO2, Probodobodyne QBE, Probodobodyne RoveMate and the Probodobodyne Stayputnik.

To operate reaction wheels electric power needs to be available. The electric consumption is proportionate to its torque. While command modules that have reaction wheels need 0.1 E/s per kilonewton meter torque, all dedicated parts need at maximum 0.3 E/s but supplying 20 torque requiring only 0.015 E/s per kilonewton meter torque. For example the Command Pod Mk1 can generate torque up to 2.4 kNm per axis which requires up to 0.24 E/s or 0.1 E/(s·kN·m). The power usage is per axis so when the reaction wheel is applying maximum torque at two axes simultaneously it requires twice as much energy.

This is one of the key systems used by SAS.

Reaction wheels

Dedicated Units

Image	Part	Radial size	Cost ()	Mass (t)	Max. Temp. (K)	Tolerance (m/s)	Tolerance (g)	Torque (kN·m)	Electricity (⚡/s)
	Small Inline Reaction Wheel	Tiny	600	0.05	2 000	9	50	5	0.25 (15 ⚡/min)
	Advanced Inline Stabilizer	Small	1 200	0.1	2 000	9	50	15	0.45 (27 ⚡/min)
	Advanced Reaction Wheel Module, Large	Large	2 100	0.2	2 000	9	50	30	0.6 (36 ⚡/min)

Command Pods

Image	Part	Radial size	Cost ()	Mass (t)	Max. Temp. (K)	Tolerance (m/s)	Tolerance (g)	Torque (kN · m)	S.A.S. level	Required Crew/ Power	Capacity (⚡)
	Mk1 Cockpit	Small	1 250 (1 241)	1.28 (1.25)	2 000 (1 100)	40	50	10			50 ⚡ 7.5 MP
	Mk1 Inline Cockpit	Small	1 600 (1 591)	1.03 (1.00)	2 000 (1 100)	40	50	10			50 ⚡ 7.5 MP
	Mk2 Cockpit	Mk2	3 500 (3 482)	2.06 (2.00)	2 500 (1 400)	45	50	15		+	150 ⚡ 15 MP
	Mk2 Inline Cockpit	Mk2	3 500 (3 470)	2.10 (2.00)	2 500 (1 400)	45	50	15		+	150 ⚡ 25 MP
	Mk3 Cockpit	Mk3, Small	10 000 (9 880)	3.90 (3.50)	2 700 (1 500)	50	50	40/40/20		+	500 ⚡ 100 MP
	Mk1 Command Pod	Small, Tiny	600 (588)	0.84 (0.80)	2 200 (1 200)	14	50	5			50 ⚡ 10 MP
	Mk1-3 Command Pod	Large, Small	3 800 (3 764)	2.72 (2.60)	2 400 (1 400)	45	50	15		+	150 ⚡ 30 MP
	Mk1 Lander Can	Small	1 500 (1 482)	0.66 (0.60)	2 000 (1 000)	8	50	3			50 ⚡ 15 MP
	Mk2 Lander Can	Large	3 250 (3 202)	1.515 (1.355)	2 000 (1 200)	8	50	15		+	100 ⚡ 40 MP
	PPD-12 Cupola Module	Large, Small	3 200 (3 188)	1.80 (1.76)	2 000 (1 000)	8	50	9			200 ⚡ 10 MP
	Probodobodyne HECS	Tiny	650	0.1	1 200	12	50	0.5	1	1.5 ⚡/min (90 ⚡/h)	10 ⚡
	Probodobodyne OKTO	Tiny	450	0.1	1 200	12	50	0.3	0	1.2 ⚡/min (72 ⚡/h)	10 ⚡
	Probodobodyne HECS2	Small	7 500	0.2	2 000	8	50	10	3	3.0 ⚡/min (180 ⚡/h)	1 000 ⚡
	RC-001S Remote Guidance Unit	Small	2 250	0.1	2 000	9	50	0.5	3	3.0 ⚡/min (180 ⚡/h)	15 ⚡
	RC-L01 Remote Guidance Unit	Large	3 400	0.5	2 000	9	50	1.5	3	4.8 ⚡/min (288 ⚡/h)	30 ⚡
	MK2 Drone Core	Mk2	2 700	0.2	2 500	20	50	15/3/3	3	3.0 ⚡/min (180 ⚡/h)	250 ⚡
	MPO Probe	Small	9 900 (9 854)	0.895 (0.395)	2 200	9	50	6	3	3.0 ⚡/min (180 ⚡/h)	1 000 ⚡ 45 LF 55 O
	MTM Stage	Small	21 500 (6 300)	0.795 (0.415)	2 200	12	50	12	2	1.8 ⚡/min (108 ⚡/h)	4 000 ⚡ 3 800 XE

See also

• RCS thrusters
• Control surfaces

Real-World Comparisons and Physics Implications

Earth-based spacecraft use both reaction wheels and gimballed flywheels extensively. However, while KSP reaction wheels seem to be able to supply torque endlessly, real-world reaction wheels build up rotational momentum (equal and opposite to the torque they apply to the spacecraft) in flywheel speed. Friction from the flywheels also returns flywheel torque to the spacecraft. Thus, spacecraft need to intermittently perform "momentum dumps" using RCS to reset the reaction wheels.

Several KSP reaction wheel systems appear to have flywheels oriented along one fixed axis, but are able to provide torque in three dimensions. For example, the 'Advanced Reaction Wheel Module, Large' is depicted as a thin ring. How Kerbal engineers are able to use a fixed axis flywheel to obtain three dimensional control is presently beyond the ken of ape-descendant engineers.

The reaction wheels available in KSP are also remarkably powerful.

The counter-reaction speeds necessary for KSP are significant. If the 'Advanced Inline Stabilizer' includes a thin-ring flywheel of 0.6 m radius (compared to the module's 1.25 m diameter), which contains 90% of the module's 100 kg mass, it would require accelerating the flywheel's 32 kg.m^2 moment of inertia at 463 radians per second per second to supply the module's 15 kNm of torque. After 10 seconds of applied torque, the flywheel would be spinning at approximately 44,000 rpm. A spherical shell of the same mass and diameter - which would be equally efficient for 3-axis control - would require just over double the rotation speeds. However, it is not clear how a spherical shell could fit into the reaction wheel modules without significantly reducing the shell's diameter. Kerbal Engineers, for all their lack of common sense, must be remarkably clever to make such effective machines.

A possible explanation is that "reaction wheels" is used as an umbrella term for attitude control systems that use a rotating wheel, and could be used to refer to other devices like Control moment Gyroscopes