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The Apollo Command Module had a set of twelve hypergolic thrusters for attitude control, and directional reentry control similar to Gemini. The Apollo Service Module and Lunar Module each had a set of sixteen R-4D hypergolic thrusters, grouped into external clusters of four, to provide both translation and attitude control. The clusters were ...
The R-4D is a small hypergolic rocket engine, originally designed by Marquardt Corporation for use as a reaction control system thruster on vehicles of the Apollo crewed Moon landing program. Aerojet Rocketdyne manufactures and markets modern versions of the R-4D.
RCS quad containing four R-4D thrusters, as used on the Apollo Service Module. Four clusters of four reaction control system (RCS) thrusters (known as "quads") were installed around the upper section of the SM every 90°. The sixteen-thruster arrangement provided rotation and translation control in all three
The Apollo Lunar Module, used in the Moon landings, employed hypergolic fuels in both the descent and ascent rocket engines. The Apollo spacecraft used the same combination for the Service Propulsion System. Those spacecraft and the Space Shuttle (among others) used hypergolic propellants for their reaction control systems.
Ullage is often a secondary function of the reaction control system such as on the Apollo Lunar Module (LM). In his book Lost Moon , Jim Lovell recounted a description of a course-correction burn of the LEM's main descent engine to re-enter a free return trajectory to Earth during the successful recovery of the Apollo 13 capsule:
R-4D (MMH/NTO) – 100 lbf (exact thrust depends on variant) hypergolic thruster, originally developed by Marquardt as RCS thrusters for the Apollo SM and LM. Currently used as secondary engines on the Orion European Service Module, and as apogee motors on various satellite buses. MR103G — 0.2 lb hydrazine monopropellant thruster
The only remedy for this loss of control is to desaturate the CMGs by removing the excess angular momentum from the spacecraft. The simplest way of doing this is to use reaction control system (RCS) thrusters. In our example of saturation along the forward axis, the RCS will be fired to produce an anticlockwise torque about that axis.
Propulsion system thrusters are fired only occasionally to make desired changes in spin rate, or in the spin-stabilized attitude. If desired, the spinning may be stopped through the use of thrusters or by yo-yo de-spin. The Pioneer 10 and Pioneer 11 probes in the outer Solar System are examples of spin-stabilized spacecraft. [2]