The CSM Reaction Control System: Four Quads and 12 Jets
How the Command and Service Module's thruster system handled attitude control, translation, and ullage—from translunar coast to the final reentry roll
The Command/Service Module carried two separate Reaction Control Systems: the Service Module RCS, which operated throughout the mission from orbit insertion through CM/SM separation, and the Command Module RCS, which took over after separation for the final reentry attitude control. The SM RCS consisted of four engine clusters—“quads”—mounted 90 degrees apart around the Service Module, each containing four small hypergolic rocket engines. Sixteen engines total, organized to provide rotation and translation about all three axes. The CM RCS used twelve smaller engines in two independent rings around the CM, providing attitude control during the final minutes from separation through splashdown.
SM RCS: The Quads
Each SM RCS quad was a self-contained propulsion module bolted to the exterior of the Service Module. Each quad contained four engines (designated A, B, C, and D), a pair of propellant tanks (one fuel, one oxidizer), a helium pressurant bottle, and the associated plumbing and valves. The quads were labeled A, B, C, and D, positioned at 90-degree intervals around the SM.
Each engine in a quad produced approximately 100 pounds-force of thrust. The engines burned monomethyl hydrazine (MMH) fuel and nitrogen tetroxide (N2O4) oxidizer—hypergolic propellants that ignited on contact. Like the SPS engine, the RCS engines had no ignition system. Open the valves, propellants met, combustion occurred. Close the valves, combustion stopped.
The four engines in each quad were oriented in different directions to provide torque and translation authority about different axes. The specific orientation depended on the quad’s position on the SM, but the combined geometry of all 16 engines (four engines in each of four quads) provided full six-degree-of-freedom control: pitch, roll, yaw (rotations) and forward, lateral, vertical (translations).
Each quad’s propellant supply was independent—quad A had its own tanks, separate from quad B’s tanks. This independence meant that a propellant leak in one quad wouldn’t drain the others. The total SM RCS propellant load was approximately 1,340 pounds, distributed among the four quads. This was enough for all nominal attitude control, translation maneuvers, and ullage burns during a standard lunar mission, with reserves for contingencies.
Jet Selection and Control Modes
The SM RCS engines were commanded by the AGC’s Digital Autopilot (the CM’s DAP) or by the Stabilization and Control System (SCS), depending on the control mode selected by the crew. In AGC mode, the computer calculated which jets to fire, for how long, and in what combination to achieve the desired attitude change or translation. In SCS mode, the hardware control system—an analog electronics package—performed the same function without the computer.
The crew could also command the jets directly through the hand controllers. Moving the rotational hand controller fired the appropriate jets for the commanded rotation. Moving the translational hand controller fired jets for the commanded linear motion. The selection logic—which specific jets produced the desired motion—was handled by the DAP or SCS, not by the crew. The pilot commanded a direction; the control system selected the jets.
Minimum impulse firings—the shortest possible jet activation—were approximately 14 milliseconds. This produced the smallest controllable attitude change and was the DAP’s preferred mode during fine attitude control, such as holding a stable attitude for navigation star sightings or maintaining the barbecue roll rate.
Ullage Burns: Settling the SPS Propellant
One of the SM RCS’s critical functions was ullage—firing translation jets to settle the SPS engine’s propellants before a main engine burn. In zero gravity, the propellant in the SPS tanks floated freely, with gas and liquid randomly distributed. If the SPS engine fired with gas at the tank outlet instead of liquid, the engine would receive a gas bubble rather than propellant, potentially causing combustion instability or a momentary loss of thrust.
The ullage burn used the SM RCS translation jets to accelerate the spacecraft gently in the direction of travel, creating a small artificial gravity (typically 0.05 to 0.1 G) that settled the propellant to the bottom of the tanks—where the outlets were. The ullage burn lasted approximately 15-20 seconds before the SPS ignition command, and the RCS jets continued firing through the first seconds of the SPS burn until the SPS thrust was high enough to provide its own propellant settling.
Ullage was a routine but essential prelude to every SPS burn. Forgetting to perform ullage—or performing it with insufficient duration—could cause a rough SPS start or a momentary thrust interruption at the beginning of a critical burn. The ullage procedure was included in every burn checklist, and the AGC’s thrusting programs (P40, P41) commanded the ullage automatically as part of the burn sequence.
CM RCS: The Reentry System
After CM/SM separation, the Service Module with its RCS quads was jettisoned. The Command Module, now flying alone, needed its own attitude control for the reentry phase—maintaining the correct orientation as the CM plunged into the atmosphere, managing the bank angle that controlled the skip trajectory, and damping any rotation rates induced by the separation event.
The CM RCS consisted of 12 engines arranged in two independent, concentric rings around the Command Module, near the aft heat shield. Each ring contained 6 engines, and each ring had its own propellant supply (fuel and oxidizer tanks), helium pressurant, and plumbing. The two rings were designated System A and System B, and either system alone could provide full attitude control for the reentry.
Each CM RCS engine produced approximately 93 pounds-force of thrust—slightly less than the SM RCS engines—and burned the same MMH/N2O4 propellant combination. The total CM RCS propellant load was approximately 270 pounds, shared between the two systems. This was enough for the reentry attitude control maneuvers plus margin.
The CM RCS engines were commanded by the AGC’s entry guidance programs or by the SCS in backup mode. During the automated entry (the normal mode), the AGC computed the bank angle needed to fly the desired skip trajectory and commanded the appropriate jets to achieve and maintain that bank angle. During a manual entry (backup mode, using the Entry Monitoring System), the pilot commanded bank angle changes through the hand controller, and the SCS fired the jets.
The CM RCS also performed a critical function after main parachute deployment: it was used to maintain the CM’s attitude during the descent under parachutes (though the aerodynamic forces at that point largely determined the attitude), and it was deactivated before splashdown to prevent any post-landing propellant hazard. The crew armed the CM RCS propellant dump system, which vented the remaining propellant overboard before water impact.
Propellant Management: Monitoring the Budget
The SM RCS propellant budget was tracked throughout the mission by the crew and by Mission Control. The AGC maintained a running estimate of RCS propellant remaining based on jet firing history—each firing consumed a known quantity of propellant, and the cumulative consumption was subtracted from the initial load. The ground could also compute the remaining propellant from telemetry of the tank quantities.
Excessive RCS propellant consumption could become a mission constraint. If the SM RCS propellant dropped below a threshold reserve—the amount needed for contingency attitude control during an emergency return—the mission rules required evaluation of whether to continue surface operations or prepare for an early return. This threshold was never reached on any Apollo mission, but it was monitored continuously.
The largest single consumers of SM RCS propellant were the docking maneuvers (which required precise translation and rotation in close proximity to the LM) and the passive thermal control barbecue roll (which required periodic jet firings to maintain the roll rate). Active attitude hold during navigation sightings and communication antenna pointing also consumed propellant at a steady rate.
Sixteen Plus Twelve
Twenty-eight RCS engines—sixteen on the Service Module and twelve on the Command Module—controlled the attitude and translation of the CSM throughout the mission. They fired thousands of times per mission, in pulses ranging from 14 milliseconds to several seconds. They settled propellant before SPS burns, held the spacecraft steady for star sightings, rolled the vehicle for thermal control, maneuvered for docking, controlled the bank angle through reentry, and maintained attitude during parachute descent.
No SM RCS quad ever failed completely during an Apollo mission. No CM RCS system failure ever occurred. The hypergolic engines, with their zero-ignition-delay reliability, fired every time they were commanded. The independent quad architecture on the SM and the dual-ring architecture on the CM ensured that no single failure could cause loss of attitude control.
The RCS was the spacecraft’s most frequently used propulsion system—far more active than the SPS, which fired only a handful of times per mission. Every maneuver, every attitude change, every reentry roll command went through the RCS. It was the CSM’s kinesthetic system—the muscles that turned guidance commands into physical motion, one 100-pound thrust pulse at a time.