Apollo command and service module

Apollo Command/Service Module
	Apollo 15 Command and Service Module in lunar orbit
Manufacturer	North American Aviation; North American Rockwell
Designer	Maxime Faget
Country of origin	United States
Operator	NASA
Applications	Manned cislunar flight and lunar orbit; Skylab crew shuttle; Apollo-Soyuz Test Project
Specifications
Launch mass	32,390 pounds (14,690 kg) Earth orbit; 63,500 pounds (28,800 kg) Lunar
Dry mass	26,300 pounds (11,900 kg)
Payload capacity	2,320 pounds (1,050 kg)
Crew capacity	3
Dimensions	36.2 feet (11.0 m) high; 12.8 feet (3.9 m) diameter
Volume	218 cubic feet (6.2 m3)
Power	Fuel cells
Regime	Low Earth orbit; Cislunar space; Lunar orbit
Design life	14 days
Production
Status	Retired
Built	35
Launched	19
Operational	19
Failed	2
Lost	1
Maiden launch	February 26, 1966
Last launch	July 15, 1975
Last retirement	July 24, 1975
Related spacecraft
Flown with	Apollo Lunar Module
	;
Configuration
← Gemini spacecraft	Orion (spacecraft) →

The Command/Service Module (CSM) was one of two principal components of the United States Apollo spacecraft, used for the Apollo program which landed astronauts on the Moon between 1969 and 1972. The CSM functioned as a mother ship which carried a crew of three astronauts and the second Apollo spacecraft, the Lunar Module, to lunar orbit, and brought the astronauts back to Earth. It consisted of two parts: the conical Command Module, a cabin that housed the crew and carried equipment needed for atmospheric reentry and splashdown; and the cylindrical Service Module which provided propulsion, electrical power and storage for various consumables required during a mission. An umbilical connection transferred power and consumables between the two modules. Just before reentry of the Command Module on the return home, the umbilical connection was severed and the Service Module was cast off and allowed to burn up in the atmosphere.

The CSM was developed and built for NASA by North American Aviation starting in November 1961. It was initially designed to land on the Moon atop a landing rocket stage, and return all three astronauts on a direct-ascent mission which would not use a separate Lunar Module, and thus had no provisions for docking with another spacecraft. This, plus other required design changes, led to the decision to design two versions of the CSM: Block I was to be used for uncrewed missions and a single crewed Earth orbit flight (Apollo 1), while the more advanced Block II was designed for use with the Lunar Module. The Apollo 1 flight was cancelled after a cabin fire killed the crew and destroyed their Command Module during a launch rehearsal test. Corrections of the problems which caused the fire were applied to the Block II spacecraft, which was used for all crewed spaceflights.

Nineteen CSMs were launched into space. Of these, nine flew humans to the Moon between 1968 and 1972, and another two performed crewed test flights in low Earth orbit, all as part of the Apollo program. Prior to these, another four CSMs had flown as uncrewed Apollo tests, of which two were suborbital flights and another two were orbital flights. Following the conclusion of the Apollo program and from 1973-1974, three CSMs ferried astronauts to the orbital Skylab space station. Finally in 1975, the last flown CSM docked with the Soviet craft Soyuz 19 as part of the international Apollo-Soyuz Test Project.

Development history

When NASA awarded the initial Apollo contract to North American Aviation on November 28, 1961, it was still assumed the lunar landing would be achieved by direct ascent rather than by lunar orbit rendezvous.^[1] Therefore, design proceeded without a means of docking the Command Module to a Lunar Excursion Module (LEM). But the change to lunar orbit rendezvous, plus several technical obstacles encountered in some subsystems (such as environmental control), soon made it clear that substantial redesign would be required. In 1963, NASA decided the most efficient way to keep the program on track was to proceed with the development in two versions:^[2]

Block I would continue the preliminary design, to be used for early low Earth orbit test flights only.
Block II would be the lunar-capable version, including a docking hatch and incorporating weight reduction and lessons learned in Block I. Detailed design of the docking capability depended on design of the LEM, which was contracted to Grumman Aircraft Engineering.

By January 1964, North American started presenting Block II design details to NASA.^[3] Block I spacecraft were used for all unmanned Saturn 1B and Saturn V test flights. Initially two manned flights were planned, but this was reduced to one in late 1966. This mission, designated AS-204 but named Apollo 1 by its flight crew, was planned for launch on February 21, 1967. But during a dress rehearsal for the launch on January 27, all three astronauts (Gus Grissom, Ed White and Roger Chaffee) were killed in a cabin fire which revealed serious design, construction and maintenance shortcomings in Block I, many of which had been carried over into Block II Command Modules being built at the time.

After a thorough investigation by the Apollo 204 Review Board, it was decided to terminate the manned Block I phase and redefine Block II to incorporate the review board's recommendations. Block II incorporated a revised CM heat shield design, which was tested on the unmanned Apollo 4 and Apollo 6 flights, so the first all-up Block II spacecraft flew on the first manned mission, Apollo 7.

The two blocks were essentially similar in overall dimensions, but several design improvements resulted in weight reduction in Block II. Also, the Block I Service Module propellant tanks were slightly larger than in Block II. The Apollo 1 spacecraft weighed approximately 45,000 pounds (20,000 kg), while the Block II Apollo 7 weighed 36,400 lb (16,500 kg). (These two Earth orbital craft were lighter than the craft which later went to the Moon, as they carried propellant in only one set of tanks, and did not carry the high-gain S-band antenna.) In the specifications given below, unless otherwise noted, all weights given are for the Block II spacecraft.

The total cost of the CSM for development and the units produced was $36.9B in 2016 dollars, adjusted from a nominal total of $3.7B^[4] using the NASA New Start Inflation Indices.^[5]

Command Module (CM)

The Command Module was a truncated cone (frustum) 10 feet 7 inches (3.23 m) tall with a diameter of 12 feet 10 inches (3.91 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 (CM) consisted of two basic structures joined together: the inner structure (pressure shell) and the outer structure.

The inner structure was an aluminum sandwich construction which consisted of a welded aluminum inner skin, adhesively bonded aluminum honeycomb core, and outer face sheet. The thickness of the honeycomb varied from about 1.5 inches (3.8 cm) at the base, to about 0.25 inches (0.64 cm) at the forward access tunnel. This inner structure was the pressurized crew compartment.

The outer structure was made of stainless steel brazed honeycomb brazed between steel alloy face sheets. It varied in thickness from 0.5 inch to 2.5 inches. Part of the area between the inner and outer shells was filled with a layer of fiberglass insulation as additional heat protection.^[6]

Thermal Protection (Heat Shield)

An ablative heat shield on the outside of the CM protected the capsule from the heat of reentry, which is sufficient to melt most metals. This heat shield was composed of phenolic epoxy resin, a type of reinforced plastic. During reentry, this material charred and melted away, absorbing and carrying away the intense heat in the process. The heat shield has several outer coverings: a pore seal, a moisture barrier (a white reflective coating), and a silver Mylar thermal coating that looks like aluminum foil.

The heat shield varied in thickness from 2 inches (5.1 cm) in the aft portion (the base of the capsule, which faced forward during reentry) to 0.5 inches (1.3 cm) in the crew compartment and forward portions. Total weight of the shield was about 3,000 pounds (1,400 kg).^[6]

Forward Compartment

The forward compartment was the area outside the inner pressure shell in the nose of the capsule, located around the forward docking tunnel and covered by the forward heat shield. The compartment was divided into four 90-degree segments which contained Earth Ianding equipment (all the parachutes, recovery antennas and beacon light, and sea recovery sling), two reaction control engines, and the forward heat shield release mechanism.

At about 25,000 feet (7,600 m) during reentry, the forward heat shield was jettisoned to expose the Earth landing equipment and permit deployment of the parachutes.^[6]

Aft Compartment

The aft compartment was located around the periphery of the command module at its widest part, just forward of (above) the aft heat shield. The compartment was divided into 24 bays containing 10 reaction control engines; the fuel, oxidizer, and helium tanks for the CM reaction control subsystem; water tanks; the crushable ribs of the impact attenuation system; and a number of instruments. The CM-SM umbilical, the point where wiring and plumbing ran from one module to the other, was also in the aft compartment. The panels of the heat shield covering the aft compartment were removable for maintenance of the equipment before flight.^[6]

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 consisted of two drogue parachutes with mortars, three main parachutes, three pilot parachutes to deploy the mains, three inflation bags for uprighting the capsule if necessary, a sea recovery cable, a dye marker, and a swimmer umbilical.

The Command Module's center of mass was offset a foot or so from the center of pressure (along the symmetry axis). This provided a rotational moment during reentry, angling the capsule and providing some lift (a lift to drag ratio of about 0.368^[7]). The capsule was then steered by rotating the capsule using thrusters; when no steering was required, the capsule was spun slowly, and the lift effects cancelled out. This system greatly reduced the g-force experienced by the astronauts, permitted a reasonable amount of directional control and allowed the capsule's splashdown point to be targeted within a few miles.

At 24,000 feet (7.3 km) the forward heat shield was jettisoned using four pressurized-gas compression springs. The drogue parachutes were then deployed, slowing the spacecraft to 125 miles per hour (201 kilometres per hour). At 10,700 feet (3.3 km) the drogues were jettisoned and the pilot parachutes, which pulled out the mains, were deployed. These slowed the CM to 22 miles per hour (35 kilometres per hour) for splashdown. The portion of the capsule that first contacted the water surface contained four crushable ribs to further mitigate the force of impact. The Command Module could safely parachute to an ocean landing with only two parachutes deployed (as occurred on Apollo 15), the third parachute being a safety precaution.

Reaction Control System

The Command Module attitude control system consisted of twelve 93-pound-force (410 N) attitude control jets; ten were located in the aft compartment, and two pitch motors in the forward compartment. Four tanks stored 270 pounds (120 kg) of monomethylhydrazine fuel and nitrogen tetroxide oxidizer. They were pressurized by 1.1 pounds (0.50 kg) of helium stored at 4,150 pounds per square inch (28.6 MPa) in two tanks.^{[citation needed]}

Hatches

The forward docking hatch was mounted at the top of the docking tunnel. It was 30 inches (76 cm) 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 0.5-inch (13 mm) of insulation and a layer of aluminum foil. It was latched in six places and operated by a pump handle. The hatch contained a valve in its center, used to equalize the pressure between the tunnel and the CM so the hatch could be removed.

The Unified Crew Hatch (UCH) measured 29 inches (74 cm) high, 34 inches (86 cm) wide, and weighed 225 pounds (102 kg). It was operated by a pump handle, which drove a ratchet mechanism to open or close fifteen latches simultaneously.

Docking assembly

Apollo's mission required the LM to dock with the CSM on return from the Moon, and also in the transposition, docking, and extraction maneuver at the beginning of the translunar coast. The docking mechanism was a non-androgynous system, consisting of a probe located in the nose of the CSM, which connected to the drogue, a truncated cone located on the Lunar Module. The probe was extended like a scissor jack to capture the drogue on initial contact, known as soft docking. Then the probe was retracted to pull the vehicles together and establish a firm connection, known as "hard docking". The mechanism was specified by NASA to have the following functions:^{[citation needed]}

Allow the two vehicles to connect, and attenuate excess movement and energy caused by docking
Align and center the two vehicles and pull them together for capture
Provide a rigid structural connection between both vehicles, and be capable of removal and re-installation by a single crewman
Provide a means of remote separation of both vehicles for the return to Earth, using pyrotechnic fasteners at the circumference of the CSM docking collar
Provide redundant power and logic circuits for all electrical and pyrotechnic components.

Coupling

The probe head located in the CSM was self-centering and gimbal-mounted to the probe piston. As the probe head engaged in the opening of the drogue socket, three spring-loaded latches depressed and engaged. These latches allowed a so-called 'soft dock' state and enabled the pitch and yaw movements in the two vehicles to subside. Excess movement in the vehicles during the 'hard dock' process could cause damage to the docking ring and put stress on the upper tunnel. A depressed locking trigger link at each latch allowed a spring-loaded spool to move forward, maintaining the toggle linkage in an over-center locked position. In the upper end of the Lunar Module tunnel, the drogue, which was constructed of 1-inch-thick aluminum honeycomb core, bonded front and back to aluminum face sheets, was the receiving end of the probe head capture latches.

Retraction

After the initial capture and stabilization of the vehicles, the probe was capable of exerting a closing force of 1,000 pounds-force (4.4 kN) to draw the vehicles together. This force was generated by gas pressure acting on the center piston within the probe cylinder. Piston retraction compressed the probe and interface seals and actuated the 12 automatic ring latches which were located radially around the inner surface of the CSM docking ring. The latches were manually re-cocked in the docking tunnel by an astronaut after each hard docking event (lunar missions required two dockings).

Separation

An automatic extension latch attached to the probe cylinder body engaged and retained the probe center piston in the retracted position. Before vehicle separation in lunar orbit, manual cocking of the twelve ring latches was accomplished. The separating force from the internal pressure in the tunnel area was then transmitted from the ring latches to the probe and drogue. In undocking, the release of the capture latches was accomplished by electrically energizing tandem-mounted DC rotary solenoids located in the center piston. In a temperature degraded condition, a single motor release operation was done manually in the Lunar Module by depressing the locking spool through an open hole in the probe heads, while release from the CSM was done by rotating a release handle at the back of the probe to rotate the motor torque shaft manually.^[8] When the Command and Lunar Modules separated for the last time just before re-entry, the probe and forward docking ring were pyrotechnically separated, leaving all docking equipment attached to the lunar module. In the event of an abort during launch from Earth, the same system would have explosively jettisoned the docking ring and probe from the CM as it separated from the boost protective cover.

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 7 feet (2.1 m) wide and 3 feet (0.91 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 served as navigator, so his control panel (center) 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 served as systems engineer, so his control panel (right-hand side) included the fuel cell gauges and controls, the electrical and battery controls, and the communications controls.

Flanking the sides 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 could be disconnected and laid on the aft bulkhead. One rotation and one translation hand controller was installed on the armrests of the left-hand couch. The translation controller was used by the crew member performing the transposition, docking, and extraction maneuver with the LM, usually the CM Pilot. The center and right-hand couches had duplicate rotational controllers. The couches were supported by eight shock-attenuating struts, designed to ease the impact of touchdown on water or, in case of an emergency landing, on solid ground.

The contiguous cabin space was organized into six equipment bays:

The lower equipment bay, which housed the Guidance and Navigation computer, sextant, telescope, and Inertial Measurement Unit; various communications beacons; medical stores; an audio center; the S-band power amplifier; etc. There was also an extra rotation hand controller mounted on the bay wall, so the CM Pilot/navigator could rotate the spacecraft as needed while standing and looking through the telescope to find stars to take navigational measurements with the sextant. This bay provided a significant amount of room for the astronauts to move around in, unlike the cramped conditions which existed in the previous Mercury and Gemini spacecraft.
The left-hand forward equipment bay, which contained 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, which housed 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, 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, waste management system, food and sanitary supplies, and a waste storage compartment.
The aft storage bay, behind the crew couches. This housed the 70 mm camera equipment, the astronaut’s garments, tool sets, storage bags, a fire extinguisher, CO₂ absorbers, sleep restraint ropes, spacesuit maintenance kits, 16mm camera equipment, and the contingency lunar sample container.

The CM had five windows. The two side windows measured 13 inches (330 mm) square next to the left and right-hand couches. Two forward-facing triangular rendezvous windows measured 8 by 13 inches (200 by 330 millimetres), used to aid in rendezvous and docking with the LM. The circular hatch window was 10 5/8 in. diameter (27 cm) and was directly over the center couch. Each window assembly consisted of three thick panes of glass. The inner two panes, which were made of aluminosilicate, made up part of the module's pressure vessel. The fused silica outer pane served as both a debris shield and as part of the heat shield. Each pane had an anti-reflective coating and a blue-red reflective coating on the inner surface.

Specifications

^{[citation needed]}

Crew: 3
Crew cabin volume: 218 cu ft (6.2 m³) living space, pressurized 366 cu ft (10.4 m³)
Length: 11.4 ft (3.5 m)
Diameter: 12.8 ft (3.9 m)
Mass: 12,250 lb (5,560 kg)
- Structure mass: 3,450 lb (1,560 kg)
- Heat shield mass: 1,870 lb (850 kg)
- RCS engine mass: twelve x 73.3 lb (33.2 kg)
- Recovery equipment mass: 540 lb (240 kg)
- Navigation equipment mass: 1,110 lb (500 kg)
- Telemetry equipment mass: 440 lb (200 kg)
- Electrical equipment mass: 1,500 lb (680 kg)
- Communications systems mass: 220 lb (100 kg)
- Crew couches and provisions mass: 1,200 lb (540 kg)
- Environmental Control System mass: 440 lb (200 kg)
- Misc. contingency mass: 440 lb (200 kg)
RCS: twelve 93 lbf (410 N) thrusters, firing in pairs
RCS propellants: MMH/N₂O₄
RCS propellant mass: 270 lb (120 kg)
Drinking water capacity: 33 lb (15 kg)
Waste water capacity: 58 lb (26 kg)
CO₂ scrubber: lithium hydroxide
Odor absorber: activated charcoal
Electric system batteries: three 40 ampere-hour silver-zinc batteries; two 0.75 ampere-hour silver-zinc pyrotechnic batteries
Parachutes: two 16 feet (4.9 m) conical ribbon drogue parachutes; three 7.2 feet (2.2 m) ringshot pilot parachutes; three 83.5 feet (25.5 m) ringsail main parachutes