Small Inline Reaction Wheel

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Small Inline Reaction Wheel
Part image
Stability Augmentation System by
Found lying by the side of the road
Radial size Tiny
Cost (total) 600.00 Funds
Mass (total) 0.05 t
Drag 0.2
Max. Temp. 2000 K
Impact Tolerance 9 m/s
Research Tech tree advanced flight control.png Flight Control
Unlock cost 3 200 Funds
Since version 0.7.3
Part configuration inlineReactionWheel
Pitch torque 5.0 kN·m
Yaw torque 5.0 kN·m
Roll torque 5.0 kN·m
Electricity required for torque
SAS level × None
For the equivalent part in the Demo, see S.A.S Module

The Small Inline Reaction Wheel (previously named Inline Reaction Wheel) is a toggleable module that provides torque and SAS functionality, a sort of autopilot to stabilize and help keep a rocket on course. Since virtually every command pod in the game already provides SAS, the torque is what will interest most players. Extra torque means that the player's spaceship will rotate faster when ordered to, resulting in a more nimble feel to the craft. The Inline Reaction Wheel is the entry model reaction wheel. It is cheap, relatively light, and provides a small amount of torque compared to the other reaction wheels.


The placement does matter for reaction wheels. Generally speaking they can cause some problems if placed far from the center of mass. Imagine you are grabbing that point and rotating it. That is what the reaction wheels will try to do. You'll get offcenter rotation anywhere other then near the COM.

comment by C7, in his blog entry “Updated Information on SAS in 0.21.1

They appear to function on a stability:mass ratio, and the effect can be enhanced by adding additional Inline Reaction Wheels. Often a single Inline Reaction Wheel isn't enough.

Each Inline Reaction Wheel adds 0.12 tons to the mass of your ship; if a lot of Inline Reaction Wheels are required to get a large ship off the ground, it should be considered to drop them with earlier stages when they are no longer needed.

Product description

The inline reaction wheel system uses a series of spinning discs that are going "Very Fast", to generate the torque necessary to control a spacecraft without wasting any propellant. Please do not attempt to service this device while it is running.


  • Renamed “Inline Reaction Wheel” into “Small Inline Reaction Wheel”
  • Resized model and radial size from small to tiny
  • Mass decreased from 0.12 to 0.05
  • Torque decreased from 8 to 5
  • Required electricity decreased from 24.0/min to 15.0/min
  • Mass decreased from 0.3 to 0.12
  • Torque decreased from 20 to 8
  • Increased torque (3 → 20)
  • Mass decreased from 0.5 to 0.3
  • Renamed from “S.A.S Module” into “Inline Reaction Wheel”
  • Mass increased from 0.07 to 0.5
  • Costs electric charge
  • New model and texture
  • (Undocumented) Mass reduced from 0.8 to 0.07, cost reduced from 2300 to 600
  • (Undocumented) Torque increased from 10 to 20, Ki/Kp/Kd changed from 1.0 / 3.0 / 5.0 to 0.0 / 5.0 / 3.5
  • (undocumented) SAS now use angular momentum instead of angular velocity - takes into account inertia momentum of the ship [1]
  • (undocumented) Tweaked Kd, Ki and Kp parameters
  • Exposed Kd, Ki and Kp parameters for tuning the SAS module
  • SAS toggle with T key
  • Initial Release