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 was required to carry enough propellant for a 10.5-year mission; at the end of commissioning the actual capacity was estimated as supporting more than 20 years of mission life.
Orbit correction maneuvers, also referred to as Delta-V maneuvers, were used to augment the launch vehicle injection velocity, to maintain a transfer trajectory into orbit about L2, and 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 2 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 were used for the first Delta-V maneuvers, which executed before the sun shield was 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 were used for the orbit insertion Delta-V maneuvers after the observatory was fully deployed, and are also used for station-keeping maneuvers.
The MRE-1 thrusters use hydrazine as a propellant. There are 8 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.