RCS Engines

Each RCS engine contains a fuel and oxidizer valve, injector head assembly, combustion chamber, nozzle and electrical junction box.

Each primary RCS engine has one fuel and one oxidizer solenoid-operated pilot poppet valve that is energized open by an electrical thrust-on command, permitting the propellant hydraulic pressure to open the main valve poppet and allow the respective propellant to flow through the injector into the combustion chamber. When the thrust-on command is terminated, the valves are de-energized and closed by spring and pressure loads.

Each vernier RCS engine has one fuel and one oxidizer solenoid-operated poppet valve. The valves are energized open by an electrical thrust-on command. When the thrust-on command is terminated, the valves are de-energized and closed by spring and pressure loads.

The primary RCS engine injector head assembly has injector holes arranged in two concentric rings; the outer ring is fuel and the inner ring is oxidizer. They are canted toward each other to cause impingement of the fuel and oxidizer streams for combustion within the combustion chamber. Separate outer fuel injector holes provide film cooling of the combustion chamber walls.

Each of the six vernier RCS engines has a single pair of fuel and oxidizer injector holes canted to cause impingement of the fuel and oxidizer streams for combustion.

The combustion chamber of each RCS engine is constructed of columbium with a columbium disilicide coating to prevent oxidation. The nozzle of each RCS engine is tailored to match the external contour of the forward RCS module or the left and right aft RCS pods. The nozzle is radiation-cooled, and insulation around the combustion chamber and nozzle prevents the excessive heat of 2,000 to 2,400 F from radiating into the orbiter structure.

The electrical junction box for each RCS engine has electrical connections for an electrical heater, a chamber pressure transducer, a leak detection device for each valve, and the propellant valves.

Because of the possibility of random but infrequent combustion instability of the primary RCS thrusters, which could cause a burnthrough in the combustion chamber wall of a RCS primary thruster in a very few seconds, an instability protection system is incorporated into each of the 38 primary RCS thrusters. The electrical power wire of each primary RCS thruster fuel and oxidizer valve is wrapped around the outside of each primary RCS thruster combustion chamber wall. If instability occurs within a primary RCS thruster, the burnthrough would cut the electrical power wire to that primary RCS thruster's valves, remove electrical power to the valves, close the valves and render the thruster inoperative for the remainder of the mission.