Service Module

The Command/Service Module (CSM) was a spacecraft built for NASA by North American Aviation. It was one of the two spacecraft that were utilized for the Apollo program, along with the Lunar Module, to land astronauts on the Moon. Together they were called the Apollo spacecraft. After the conclusion of the Apollo program, the CSM saw service as a ferry for the Skylab program and for the Apollo-Soyuz Test Project where a CSM rendezvoused in orbit with a Soviet Soyuz spacecraft.

The spacecraft, as its name suggests, consisted of 2 segments, the command module which housed the crew and the equipment needed for re-entry and splashdown, and a service module\ that provided propulsion, electrical power and storage for various consumables required during a mission. The service module would be cast off and allowed to burn up in the atmosphere before the command module re-entered and brought the crew home.

Command Module (CM)

The Command Module was a truncated cone measuring 10 feet 7 inches (3.2 m) tall and having a diameter of 12 feet 10 inches (3.9 m) across the base. The forward compartment contained two reaction control engines, the docking tunnel, and the components of the Earth Landing System. The inner pressure vessel housed the crew accommodations, equipment bays, controls and displays, and many spacecraft systems. The last section, the aft compartment, contained 10 reaction control engines and their related propellant tanks, fresh water tanks, and the CSM umbilical cables.

Construction

The command module's inner structure was an aluminum "sandwich" consisting of a welded aluminum inner skin, a thermally bonded honeycomb core, and a thin aluminium "face sheet".

The central heat shield consisted of 40 individual panels interspersed with several holes and openings for the reaction control engines and after-compartment equipment access. The central compartment structure consisted of an inner aluminium face sheet with a steel honeycomb core, a glass-phenolic ablative honeycomb heat shield, a layer of q-felt fibrous insulation, a pore seal, a moisture barrier, and a layer of Mylar thermal strips.

The aft heat shield consisted of four brazed honeycomb panels, four spot-welded sheet metal fairings, and a circumferential ring. The fairing segments were attached to the honeycomb panels and ring with conventional fasteners. The steel honeycomb core and outer face sheets were then thermally bonded to the inner skin in a giant autoclave. The aft heat shield is nearly identical to the central, with the exception of the outer Mylar layer.

Earth Landing System

The components of the ELS were housed around the forward docking tunnel. The forward compartment was separated from the central by a bulkhead and was divided into four 90-degree wedges. The ELS consists of three main parachutes, three pilot parachutes, two drogue parachute motors, three upright bags, a sea recovery cable, a dye marker, and a swimmer umbilical.

At 24,000 feet (7.3 km) the forward heat shield was jettisoned using four pressurized-gas compression springs. The drogue parachutes were then released and slowed the spacecraft to 125 miles per hour (201 km/h). At 10,700 feet (3.3 km) the drogues were jettisoned. The pilot parachutes were deployed, which pulled out the mains. These slowed the CM to 22 miles per hour (35 km/h).

Reaction Control System

The Command Module attitude control system consisted of 12 x 93 lbf (414 N) maneuvering jets. 10 were located in the aft compartment, and 2 pitch motors in the forward compartment. 122 kg of N204 fuel and MMH oxidizer were stored in four tanks. They were pressurized by 0.5 kg of helium stored at 28.6 MPa in two tanks.

Hatches

The forward docking hatch was mounted at the top of the docking tunnel. It was 30 inches (760 mm) in diameter and weighed 80 pounds (36 kg). It was constructed from two machined rings that were weld-joined to a brazed honeycomb panel. The exterior side was covered with a half-inch (12.7 mm) of insulation and a layer of aluminium foil. It was latched in six places and operated by a pump handle. At the center was a pressure equalization valve, used to equalize the pressure in the tunnel and lunar module before the hatch was removed.

The Unified Crew Hatch (UCH) measured 29 inches (737 mm) high, 34 inches (864 mm) wide, and weighed 225 pounds (102 kg). It was operated by a pump handle, which drove a ratchet mechanism to open or close 15 latches simultaneously.

Docking Assembly

The docking probe allowed the command and lunar modules to "link up" and maintain a pressure-tight seal during the mission. It was mounted to the docking tunnel in three places, and was designed to fold easily for storage.

The docking probe consisted of three shock attenuators, three tension linkages, a retractable and extendable probe, and two power umbilicals. Before docking, a crewman in the CM extended the probe. When it came in contact with the drogue, three capture latches in the probe head held the two modules together. The probe was retracted, pulling the CM and LM together. Twelve latches attached to the forward docking ring automatically activated to form an air-tight seal.

At LM separation, the probe and forward docking ring were pyrotechnically separated leaving all docking equipment with the lunar module.

Cabin interior arrangement

The central pressure vessel of the command module was its sole habitable compartment. It had an interior volume of 210 cubic feet (5.9 m³) and housed the main control panels, crew seats, guidance and navigation systems, food and equipment lockers, the waste management system, and the docking tunnel.

Dominating the forward section of the cabin was the crescent-shaped main display panel measuring nearly seven feet (2.1 m) wide and three feet (0.9 m) tall. It was arranged into three panels, each emphasizing the duties of each crew member. The mission commander’s panel (left side) included the velocity, attitude, and altitude indicators, the primary flight controls, and the main FDAI (Flight Director Attitude Indicator).

The CM pilot’s panel (middle) included the Guidance and Navigation computer controls, the caution and warning indicator panel, the event timer, the service propulsion system and RCS controls, and the environmental control system controls.

The LM pilot’s panel (left) included the fuel cell gauges and controls, the electrical and battery controls, and the communications controls.

Flanking the right side of the main panel were sets of smaller control panels. On the left side were a circuit breaker panel, audio controls, and the SCS power controls. On the right were additional circuit breakers and a redundant audio control panel, along with the environmental control switches. In total, the command module panels included 24 instruments, 566 switches, 40 event indicators, and 71 lights.

The three crew couches were constructed from hollow steel tubing and covered in a heavy, fireproof cloth, known as Armalon. The leg pans of the two outer couches could be folded in a variety of positions, while the hip pan of the center couch can be disconnected and laid on the aft bulkhead. One rotational and one translation hand controller was installed on the armrests of the commander’s couch. The LM pilot and CM pilot couches had rotational controllers only. The couches rested on eight shock attenuation struts to ease the impact of splashdown.

There are a total of six equipment bays in the cabin:

The lower equipment bay, which houses the Guidance & Control telescope, various communications beacons, the SCS gyro assemblies, the command module computer, medical stores, the audio center, the S-band power amplifier, etc.

The left-hand forward equipment bay, containing four food storage compartments, the cabin heat exchanger, pressure suit connector, potable water supply, and G&N telescope eyepieces.

The right-hand forward equipment bay, housing two survival kit containers, a data card kit, flight data books and files, and other mission documentation.

The left hand intermediate equipment bay, housing the oxygen surge tank, the water delivery system, food supplies, the cabin pressure relief valve controls, and the ECS package.

The right hand intermediate equipment bay, which contained the bio instrument kits, the waste management system, food and sanitary supplies, and a waste storage compartment.

The aft storage, compartment, behind the crew couches. This housed the 70mm camera equipment, the astronaut’s garments, tool sets, storage bags, a fire extinguisher, CO2 absorbers, sleep restraint ropes, spacesuit maintenance kits, the 16mm camera equipment, and the contingency lunar sample container.

The CM had five windows. The side windows measured 13 inches (330 mm) square and were installed to the side of the left and right-hand couches. The triangular rendezvous windows measured 8 by 13 inches (204 by 330 mm) and were used to aid in docking and rendezvous maneuvers. The hatch window was 9 inches (229 mm) in diameter and was directly over the cm pilot’s couch. Each window was constructed of 4 ¼ inch (11 mm) thick layers of tempered silica glass separated by 1/10 of an inch (2.5 mm). Each pane had an anti-reflective coating and a blue-red reflective coating on the inner surface.

Specifications

Crew: 3

Crew cabin volume: 6.17 m³

Length: 3.47 m

Diameter: 3.90 m

Mass: 5,806 kg

Structure mass: 1,567 kg

Heat Shield mass: 848 kg

RCS mass: 400 kg

Recovery equipment mass: 245 kg

Navigation equipment mass: 505 kg

Telemetry equipment mass: 200 kg

Electrical equipment mass: 700 kg

Communications Systems mass: 100 kg

Crew Couches and Provisions mass: 550 kg

Environmental Control System mass: 200 kg

Mis. Contingency mass: 200 kg

RCS thrust: 12 x 410 N

RCS propellants: N2O4/UDMH

RCS engine propellants: 122 kg

Drinking water capacity: 15 kg

Waste water capacity: 26.5 kg

Atmosphere cleanser: Lithium hydroxide

Odor absorber: Activated charcoal

Electric System Batteries: 3 40 ampere-hour silver zinc batteries, 2 0.75 ampere-hour silver zinc pyrotechnic batteries

Parachutes: 2 5 meter conical ribbon drogue parachutes, 3 2.2 meter ringshot pilot parachutes, 3 25.45 meter ringsail main parachutes

Service Module (SM)

The Service Module was an unpressurized cylindrical structure, measuring 24 feet 7 inches (7.5 m) long and 12 feet 10 inches (3.9 m) in diameter. It housed the service propulsion system and its propellants, the fuel cell power system, four maneuvering thruster quads, the S-band antenna, and storage tanks for water and hydrogen. On Apollo 15, 16 and 17 it also carried a scientific instrument package. It was connected to the Command module using three tension ties and six compression pads. The tension ties were stainless steel straps bolted to the CM's aft heat shield.

The 98 kN Service Propulsion System was used to place the Apollo spacecraft into and out of lunar orbit, and for mid-course corrections between the Earth and Moon.

The Service module remained attached to the Command Module throughout the mission. It was jettisoned just prior to re-entry into the earth's atmosphere.

The Service Module was a simple structure consisting of a center section and six pie-shaped sectors. The basic components were the forward bulkhead and fairing, six radial beams, four honeycomb panels, and the aft bulkhead and heat shield.

The forward fairing measured 2 feet 10 inches (864 mm) long and included the RCS computer, umbilical connection, power distribution block, ECS controller, separation controller, and components for the high-gain antenna. The umbilical housing contained the main electrical and plumbing connections from the SM. At SM jettison, the connections were cut using a pyrotechnic-activated guillotine assembly.

Four clusters of reaction control jets were installed around the upper section of the SM every 90 degrees. Each thruster generated approximately 441 N of thrust, and used MMH for fuel and N204 as oxidizer. Each quad assembly measures 8 by 3 feet (2.4 by 0.9 m) and has a single fuel tank, an oxidizer tank, and its associated valves and regulators.

The central tunnel housed the service propulsion engine and its two helium pressurant tanks. The engine used hydrazine/udmh fuel and nitrogen textroxide oxidizer and had a maximum thrust of 20,500 lbf (91 kN). It is 152.82 inches (3.9 m) long and 98.48 inches (2.5 m) wide at the base.

Service Module Sectors

The Service Module was divided into six sectors:

Sector 1 was usually filled with ballast to maintain the SM's center-of gravity. On Apollo 15-17, it housed a Scientific Instrument Module (SIM) for lunar study. The equipment included a panoramic camera, gamma ray spectrometer, mapping camera, laser altimeter, mass spectrometer, and lunar sub-satellite.

Sector 2 contained the oxidizer sump tank, from which oxidizer was fed to the engine. It was 51 inches (1.3 m) wide, 153.8 inches (3.9 m) wide, and contained 6,315 kilograms of oxidizer.

Sector 3 held the main oxidizer storage tank, which was 154.47 inches (3.9 m) high, 45 inches (1.1 m) wide, and held 5,118 kilograms of oxidizer.

Sector 4 contained most of the electrical power subsystem. Two oxygen tanks contained 290 kilograms of liquid oxygen, and two hydrogen tanks 25 kilograms of hydrogen. The oxygen tanks supplied the environmental control system and fuel cells, and the hydrogen tanks the fuel cells only. The fuel cells combine the two elements to generate electrical power for the spacecraft, along with a small amount of drinking water. Each fuel cell weighed 112 kilograms.

Sector 5 housed the fuel sump tank. It was the same size as the oxidizer tank and held 3,950 kilograms of propellant. Feed lines connected the tank to the SPS.

Sector 6 contained the main fuel tank, also the same size as the oxidizer tank. It held 3,201 kilograms of fuel.

Mounted on the top of the aft bulkhead was the S-band high-gain antenna. This was used for deep space communications, and was composed of 4 x 31 inch (787 mm) diameter reflectors and a single 11 inch (279 mm) square reflector. During launch, it was folded down parallel to the main engine. After launch, it deployed at a right angle to the SM.

Also on the SM exterior were a retractable forward-facing spotlight, an EVA floodlight, and a flashing rendezvous beacon visible from 100 kilometers away. Following jettison, the SM translation jets automatically fired aft to distance it from the CM. The roll jets were fired for five seconds to ensure faster break-up on re-entry.

Specifications

Length: 7.56 m

Diameter: 3.90 m

Mass: 24,523 kg

Structure mass: 1,910 kg

Electrical equipment mass: 1,200 kg

RCS thrust: 16 x 446 N

Propellants: N2O4/UDMH

Service Propulsion (SPS) engine mass: 3,000 kg

SPS engine thrust: 98 kN (throttleable)

SPS engine propellants: N2O4/UDMH

SPS engine propellants: 18,413 kg

SPS ISP: 314 lbf.s/lb (3.1 kNs/kg)

Spacecraft delta v: 2,804 m/s

Electrical System: 3 1.4 kW dc/30-volt fuel cells

CSMs produced

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