JWST's propulsion system provides maneuvering capability for orbital insertion, station keeping, and spacecraft momentum management.
The JWST propulsion subsystem is the part of the spacecraft bus that provides the means to correct JWST's orbit at the second Lagrange point (L2), to control attitude in certain ACS modes, and to unload stored momentum from the reaction wheels (when necessary). JWST nominally carries enough propellant for a 10.5-year mission, pending actual on-orbit performance.
Orbit correction maneuvers, also referred to as Delta-V maneuvers, are used to augment the launch vehicle injection velocity and to maintain a transfer trajectory into orbit about L2, and then to maintain the JWST orbit around L2 (station-keeping maneuvers) for the life of the mission. There are two types of thrusters for these functions. They are mounted on the spacecraft bus to avoid introducing contamination or heat sources near the OTE/ISIM side of the observatory. The Secondary Combustion Augmented Thrusters (SCAT) are used for orbit correction (Delta-V and station-keeping), and mono-propellant rocket engines (MRE-1) are used for attitude control and momentum unloading of the reaction wheels.
The SCATs are bi-propellant thrusters, using hydrazine (N2H4) and dinitrogen tetroxide (N2O4) as fuel and oxidizer, respectively. They operate in "blowdown mode" with one tank for each type of propellant and using gaseous helium as a pressurizing agent. There are two pairs of SCAT thrusters (paired for redundancy). One pair is located near the center of the bottom of the spacecraft bus where JWST attaches to the launch vehicle. These are used for the first Delta-V maneuvers to reach L2 with the correct velocity for the operational orbit. These maneuvers are executed before the sun shield is deployed.
The other pair of SCAT thrusters is mounted on a boom on the side of the spacecraft opposite the solar array, oriented such that their thrust direction passes through the deployed observatory's center of mass. These are used for the orbit insertion Delta-V maneuver and station-keeping maneuvers. This pair of SCAT thrusters are used after the observatory is fully is deployed.
The MRE-1 thrusters use hydrazine as a propellant. There are eight MRE-1s located on the spacecraft and are oriented so that torque can be applied in roll, pitch, or yaw control axes. For momentum unloads, these thrusters are fired so that the applied torque provides the desired change in the angular momentum of the reaction wheels.