The Bradford Reaction Wheel Unit (RWU) channel consists of a Reaction Wheel Assembly (RWA) and one Wheel Drive Electronics Box (WDE). The RWA is a rotating inertial mass, driven by a brushless DC electric motor. When power is applied to the motor, the wheel accelerates, causing the satellite body to which the motor housing is attached to rotate in the opposite direction due to the induced counter torque.

A minimum of three sets of RWA are needed per satellite to allow rotational control around the 3 axes. One extra RWA is generally present for redundancy purposes.

Key features

  • High torque level over full speed range
  • Fixed torque rise time allowing precise and repeatable control over full speed/torque range
  • Analogue or Digital interface
  • Accurate wheel speed signal
  • Proven zero crossings and low RPM performance with excellent micro-vibration performance
  • Lifetest and in-orbit heritage (SOHO, RadarSat, Seastar, Olympus, Integral, XMM, Rosetta, Aeolus, BepiColombo)

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Last updated: 2023-05-22

Reaction Wheel Unit W18ES




momentum storage
25 N m s
maximum operational speed
4000 rpm
maximum gross torque
0.265 N m
static imbalance
0.5 g cm
dynamic imbalance
5.0 g cm^2
295 mm (RWA)
125 mm (RWA)
258 mm (2-channel WDE)
6.02 kg (RWA)
4.67 kg (2-channel WDE)
181 mm (2-channel WDE)
143 mm (2-channel WDE)
peak power consumption at max. torque and speed
168 W
steady state consumption at max. constant speed
29 W
quiescent power consumption
7.6 W
power bus interface
data interface
qualification operational temperature
-15°C to +60°C (RWA)
-30°C to +60°C (WDE)


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