Reaction wheel

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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. 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 then near the center of mass.[1] 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
(Fund)
Mass
(t)
Max. Temp.
(K)
Tolerance
(m/s)
Tolerance
(g)
Torque
(kN·m)
Electricity
(⚡/s)
Inline reaction wheel.png
Small Inline Reaction Wheel Tiny 600 0.05 2 000 9 50 5 0.25
(15 ⚡/min)
Inline advanced stabilizer.png
Advanced Inline Stabilizer Small 1 200 0.1 2 000 9 50 15 0.45
(27 ⚡/min)
ASAS large.png
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
(Fund)
Mass
(t)
Max. Temp.
(K)
Tolerance
(m/s)
Tolerance
(g)
Torque
(kN · m)
S.A.S.
level
Required Crew/
Power
Capacity
(⚡)
MK1COCKPIT.png
Mk1 Cockpit Small 1 250
(1 241)
1.28
(1.25)
2 000
(1 100)
40 50 10 Crew Crew 50 ⚡
7.5 units of fuel MP
Mk1 Inline Cockpit.png
Mk1 Inline Cockpit Small 1 600
(1 591)
1.03
(1.00)
2 000
(1 100)
40 50 10 Crew Crew 50 ⚡
7.5 units of fuel MP
Mk2 Cockpit.png
Mk2 Cockpit Mk2 3 500
(3 482)
2.06
(2.00)
2 500
(1 400)
45 50 15 Crew Crew + Crew 150 ⚡
15 units of fuel MP
Mk2 Inline Cockpit.png
Mk2 Inline Cockpit Mk2 3 500
(3 470)
2.10
(2.00)
2 500
(1 400)
45 50 15 Crew Crew + Crew 150 ⚡
25 units of fuel MP
Mk3 Cockpit.png
Mk3 Cockpit Mk3, Small 10 000
(9 880)
3.90
(3.50)
2 700
(1 500)
50 50 40/40/20[Note 1] Crew Crew + CrewCrewCrew 500 ⚡
100 units of fuel MP
Command Pod Mk1.png
Mk1 Command Pod Small, Tiny 600
(588)
0.84
(0.80)
2 200
(1 200)
14 50 5 Crew Crew 50 ⚡
10 units of fuel MP
Mk1-2commandpod.png
Mk1-2 Command Pod Large, Small 3 800
(3 764)
4.12
(4.00)
2 400
(1 400)
45 50 15 Crew Crew + CrewCrew 150 ⚡
30 units of fuel MP
LanderCanMK1.png
Mk1 Lander Can Small 1 500
(1 482)
0.66
(0.60)
2 000
(1 000)
8 50 3 Crew Crew 50 ⚡
15 units of fuel MP
Can.png
Mk2 Lander-can Large 3 250
(3 202)
2.66
(2.50)
2 000
(1 200)
8 50 15 Crew Crew + Crew 100 ⚡
40 units of fuel MP
Cupola.PNG
PPD-12 Cupola Module Large, Small 3 200
(3 188)
1.80
(1.76)
2 000
(1 000)
8 50 9 Crew Crew 200 ⚡
10 units of fuel MP
Probodobodyne HECS.png
Probodobodyne HECS Tiny 650 0.1 1 200 12 50 0.5 1 1.5 ⚡/min
(90 ⚡/h)
10 ⚡
Octo.png
Probodobodyne OKTO Tiny 450 0.1 1 200 12 50 0.3 0 1.2 ⚡/min
(72 ⚡/h)
10 ⚡
Probodobodyne HECS2.png
Probodobodyne HECS2 Small 7 500 0.2 2 000 8 50 10 3 3.0 ⚡/min
(180 ⚡/h)
1 000 ⚡
Probestack.png
RC-001S Remote Guidance Unit Small 2 250 0.1 2 000 9 50 0.5 3 3.0 ⚡/min
(180 ⚡/h)
15 ⚡
Probehuge.png
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.png
MK2 Drone Core Mk2 2 700 0.2 2 500 20 50 15/3/3[Note 1] 3 3.0 ⚡/min
(180 ⚡/h)
250 ⚡
  1. 1.0 1.1 Torque differs between axes. These numbers are for pitch/yaw/roll respectively.


See also

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

Notes

  1. Answer from C7 in his blog entry “Updated Information on SAS in 0.21.1