Difference between revisions of "Reaction wheel"
(+text about location of reaction wheels; *hide parts without reaction wheels;) |
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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 | + | 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 module]]s 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.<ref>[http://forum.kerbalspaceprogram.com/entry.php/740#comment4254 Answer from C7] in his blog entry “[http://forum.kerbalspaceprogram.com/entry.php/740 Updated Information on SAS in 0.21.1]”</ref> 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 | + | 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 systems used by | + | This is one of the key systems used by [[SAS]]. |
== Reaction wheels == | == Reaction wheels == | ||
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===Command Pods=== | ===Command Pods=== | ||
{{Stats Table Command Pods|torque=Yes}} | {{Stats Table Command Pods|torque=Yes}} | ||
| + | |||
| + | == See also == | ||
| + | * [[Reaction Control System|RCS thrusters]] | ||
| + | * [[Control surface]]s | ||
| + | |||
| + | == 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 [https://en.wikipedia.org/wiki/Control_moment_gyroscope Control moment Gyroscopes] | ||
== Notes == | == Notes == | ||
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[[Category:Control]] | [[Category:Control]] | ||
| + | [[Category:Reaction wheels| ]] | ||
Latest revision as of 09:45, 3 January 2024
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.[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.
Contents
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 |  [Note 1]
|
|
50 ⚡ 7.5  MP
|
 |
Mk1 Inline Cockpit | Small | 1 600 (1 591) |
1.03 (1.00) |
2 000 (1 100) |
40 | 50 | 10 | [Note 1]
|
|
50 ⚡ 7.5 MP
|
 |
Mk2 Cockpit | Mk2 | 3 500 (3 482) |
2.06 (2.00) |
2 500 (1 400) |
45 | 50 | 15 | [Note 1]
|
+
|
150 ⚡ 15 MP
|
 |
Mk2 Inline Cockpit | Mk2 | 3 500 (3 470) |
2.10 (2.00) |
2 500 (1 400) |
45 | 50 | 15 | [Note 1]
|
+
|
150 ⚡ 25 MP
|
 |
Mk3 Cockpit | Mk3, Small | 10 000 (9 880) |
3.90 (3.50) |
2 700 (1 500) |
50 | 50 | 40/40/20[Note 2] | [Note 1]
|
+
|
500 ⚡ 100 MP
|
 |
Mk1 Command Pod | Small, Tiny | 600 (588) |
0.84 (0.80) |
2 200 (1 200) |
14 | 50 | 5 | [Note 1]
|
|
50 ⚡ 10 MP
|
 |
Mk1-3 Command Pod | Large, Small | 3 800 (3 764) |
2.72 (2.60) |
2 400 (1 400) |
45 | 50 | 15 | [Note 1]
|
+
|
150 ⚡ 30 MP
|
 |
Mk1 Lander Can | Small | 1 500 (1 482) |
0.66 (0.60) |
2 000 (1 000) |
8 | 50 | 3 | [Note 1]
|
|
50 ⚡ 15 MP
|
 |
Mk2 Lander Can | Large | 3 250 (3 202) |
1.515 (1.355) |
2 000 (1 200) |
8 | 50 | 15 | [Note 1]
|
+
|
100 ⚡ 40 MP
|
 |
PPD-12 Cupola Module | Large, Small | 3 200 (3 188) |
1.80 (1.76) |
2 000 (1 000) |
8 | 50 | 9 | [Note 1]
|
|
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[Note 2] | 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 LF55 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
|
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 In the "Career" mode in manned command modules, the performance and functionality of the SAS is determined by the presence of at least one kerbonaut with the specialization "Pilot" and the level of his specialization. In the "Science" and "Sandbox" modes, a kerbonaut with any specialization can use all the functionality of the SAS on board the manned command module without restrictions.
- ↑ 2.0 2.1 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
- ↑ Answer from C7 in his blog entry “Updated Information on SAS in 0.21.1”
