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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Apollo command and service module | 6/7 | https://en.wikipedia.org/wiki/Apollo_command_and_service_module | reference | science, encyclopedia | 2026-05-05T12:35:22.001294+00:00 | kb-cron |
The service propulsion system (SPS) engine was originally designed to lift the CSM off the surface of the Moon in the direct ascent mission mode, The engine selected was the AJ10-137, which used Aerozine 50 as fuel and nitrogen tetroxide (N2O4) as oxidizer to produce 20,500 lbf (91 kN) of thrust. A contract was signed in April 1962 for the Aerojet-General company to start developing the engine, resulting in a thrust level twice what was needed to accomplish the lunar orbit rendezvous (LOR) mission mode officially chosen in July of that year. The engine was actually used for mid-course corrections between the Earth and Moon, and to place the spacecraft into and out of lunar orbit. It also served as a retrorocket to perform the deorbit burn for Earth orbital flights. The propellants were pressure-fed to the engine by 39.2 cubic feet (1.11 m3) of gaseous helium at 3,600 pounds per square inch (25 MPa), carried in two 40-inch (1.0 m) diameter spherical tanks. The exhaust nozzle measured 152.82 inches (3.882 m) long and 98.48 inches (2.501 m) wide at the base. It was mounted on two gimbals to keep the thrust vector aligned with the spacecraft's center of mass during SPS firings. The combustion chamber and pressurant tanks were housed in the central tunnel.
=== Reaction control system ===
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 spacecraft axes. Each R-4D thruster measured 12 inches (30 cm) long by 6 inches (15 cm) diameter, generated 100 pounds-force (440 N) of thrust, and used helium-fed monomethylhydrazine (MMH) as fuel and nitrogen tetroxide (NTO) as oxidizer. Each quad assembly measured 2.2 by 2.7 feet (0.67 by 0.82 m) and had its own fuel, oxidizer, and helium tanks mounted on the inside of an 8-by-2.75-foot (2.44 by 0.84 m) skin panel. The primary fuel (MMH) tank contained 69.1 pounds (31.3 kg); the secondary fuel tank contained 45.2 pounds (20.5 kg); the primary oxidizer tank contained 137.0 pounds (62.1 kg), and the secondary oxidizer tank contained 89.2 pounds (40.5 kg). The propellant tanks were pressurized from a single tank containing 1.35 pounds (0.61 kg) of liquid helium. Back flow was prevented by a series of check valves, and back flow and ullage requirements were resolved by containing the fuel and oxidizer in Teflon bladders which separated the propellants from the helium pressurant. The four completely independent RCS clusters provided redundancy; only two adjacent functioning units were needed to allow complete attitude control. The lunar module used a similar four-quad arrangement of R-4D thruster engines for its RCS.
=== Electrical power system ===
Electrical power was produced by three fuel cells, each measuring 44 inches (1.1 m) tall by 22 inches (0.56 m) in diameter and weighing 245 pounds (111 kg). These combined hydrogen and oxygen to generate electrical power, and produced drinkable water as a byproduct. The cells were fed by two hemispherical-cylindrical 31.75-inch (0.806 m) diameter tanks, each holding 29 pounds (13 kg) of liquid hydrogen, and two spherical 26-inch (0.66 m) diameter tanks, each holding 326 pounds (148 kg) of liquid oxygen (which also supplied the environmental control system). On the flight of Apollo 13, the EPS was disabled by an explosive rupture of one oxygen tank, which punctured the second tank and led to the loss of all oxygen. After the accident, a third oxygen tank was added to obviate operation below 50% tank capacity. That allowed the elimination of the tank's internal stirring-fan equipment, which had contributed to the failure. Also starting with Apollo 14, a 400 Ah auxiliary battery was added to the SM for emergency use. Apollo 13 had drawn heavily on its entry batteries in the first hours after the explosion, and while this new battery could not power the CM for more than 5–10 hours it would buy time in the event of a temporary loss of all three fuel cells. Such an event had occurred when Apollo 12 was struck twice by lightning during launch.
=== Environmental control system === Cabin atmosphere was maintained at 5 pounds per square inch (34 kPa) of pure oxygen from the same liquid oxygen tanks that fed the electrical power system's fuel cells. Potable water supplied by the fuel cells was stored for drinking and food preparation. A thermal control system using a mixture of water and ethylene glycol as coolant dumped waste heat from the CM cabin and electronics to outer space via two 30-square-foot (2.8 m2) radiators located on the lower section of the exterior walls, one covering sectors 2 and 3 and the other covering sectors 5 and 6.
=== Communications system ===
Short-range communications between the CSM and LM employed two VHF scimitar antennas mounted on the SM just above the ECS radiators. These antennas were originally located on the Block I command module and performed a double function as aerodynamic strakes to stabilize the capsule after a launch abort. The antennas were moved to the Block II service module when this function was found unnecessary. A steerable unified S-band high-gain antenna for long-range communications with Earth was mounted on the aft bulkhead. This was an array of four 31-inch (0.79 m) diameter reflectors surrounding a single 11-inch (0.28 m) square reflector. During launch it was folded down parallel to the main engine to fit inside the Spacecraft-to-LM Adapter (SLA). After CSM separation from the SLA, it deployed at a right angle to the SM. Four omnidirectional S-band antennas on the CM were used when the attitude of the CSM kept the high-gain antenna from being pointed at Earth. These antennas were also used between SM jettison and landing.