The main engine was used for spacecraft velocity and trajectory correction changes. To be on the safe side, there were two identical main engines: One was in use and the other was a backup. There were also 16 monopropellant hydrazine thrusters of which eight were prime and eight were backups. The thrusters were used for attitude control and also for small velocity-change maneuvers.

At a Glance

Two completely isolated systems:

  • Monopropellant System - Hydrazine (N2H4)
    • 16 Redundant ~1 Newton thrusters, 4 per “cluster”
    • Used for attitude control and small (< 0.3 m/s) trajectory control maneuvers
  • Bipropellant system- Nitrogen Tetroxide (NTO)/Monomethylhydrazine (MMH)
    • Main (445 Newton) engine for propulsive maneuvers – Burns can be blow-down or pressurized (with Helium)
    • Engines are protected from micrometeoroid particles by articulating cover
    • Redundant main engine (never used in flight)
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