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)