# What kind of thrusters will the James Webb Space Telescope use for station keeping?

Question is fairly self-explanatory, but I couldn't find an answer on Google. I would expect ion thrusters because of their efficiency.

Thanks!

According to the James Webb Space Telescope Initial Mid-Course Correction Monte Carlo Implementation using Task Parallelism, J. Petersen et al. (PDF):

### 3.1 Propulsion System Overview

Two sets of thrusters comprise the observatory’s propulsion system. The first is a set of Secondary Combustion Augmented Thrusters (SCATs) that are the main thrusters for the MCC maneuvers. The SCATs are bi-propellant thrusters and draw from two separate tanks for a hypergolic reaction. Two pairs of SCATs exist: one for MCC-1a and -1b, and one for MCC-2. The MCC-2 SCAT is also employed in station keeping throughout the life of the mission. Two pairs are required because the center of mass of the observatory changes between MCC-1b and MCC-2 because of the sun shield deployment; the thrust vector from a SCAT is directed through the center of mass at the time of that maneuver. Each pair is composed of a primary and a redundant thruster (the respective thrusters are coupled to the prime and redundant flight systems; only one system is on at a time). SCAT 1 and 2 are the primary and redundant pair for MCC-1a/b while SCAT 3 and 4 are the primary and redundant pair for MCC-2 and stationkeeping. For a given maneuver only one SCAT is on throughout that maneuver. The second set of thrusters is composed of eight Dual Thruster Modules (DTMs), each comprising of a primary and redundant Monopropellant Rocket Engine, 1 lbf, (MRE-1) thruster. A schematic of the propulsion system appears in Fig. 3. The MRE-1s consume hydrazine fuel only (and no oxidizer) from the same tank as the SCATs. The MRE-1s are subdivided into two sets: MRE-1s 1 through 4, which are directed generally along the observatory J3 axis for pitch and roll control, and MRE-1s 5 through 8, which are directed radially about the J3 axis for yaw control [3]. A picture of the spacecraft bus with the body J frame and orientation of the SCATs relative to the J frame appears in Fig. 4. Together, the MRE-1s on-pulse throughout a maneuver to control the attitude. Because of the location and alignment of MRE-1s 1 through 4, any firings to control pitch and roll during a maneuver may result in an additional ΔV to the MCC maneuver. Therefore, thrust contributions from the MRE-1s must be included in the design of the MCC maneuver.

Figure 3. Schematic of the JWST propulsion system [4].

Figure 4. An independent view of the spacecraft bus showing the orientation of the body J frame along with the orientation of the SCATs relative to the spacecraft bus. The +J1 axis points in the general direction of the OTE boresight. The +J3 axis is roughly perpendicular to the multilayer sunshield and pointing parallel to the primary mirror (Image credit: jwst.nasa.gov)

### References

[3] Gidanian, D. 7.0. ACS design & analysis: Thruster Configuration and Modeling. In James Webb Space Telescope (JWST) Spacecraft Attitude Control Subsystem Hardware Critical Design Audit. Northrop Grumman, 28 August 2013.

[4] Hammann, Jeff. JWST Propellant Budget Document, Northrup Grumman. 19 July 2013. Document Number D40258

I wanted to share this as it describes JWST stationkeeping thrusters in detail, and as you can see in text and the attached images:

• SCAT (Secondary Combustion Augmented Thrusters) use dinitrogen tetroxide (N2O4) as oxidizer and hydrazine (N2H4) as fuel components of its bipropellant, and
• DTMs (Dual Thruster Modules) each use two MREs (Monopropellant Rocket Engines) propelled only by hydrazine as low-thrust monopropellant and each providing 1 lbf (4.45 N) of thrust

Also see James Webb Space Telescope Orbit Determination Analysis, Yoon et al., NTRS 20140008865 (PDF) for explanation of JWST's three mid-course correction (MCC) maneuvers mentioned in the quote above.