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Oct 27, 2016

CM Command Module (Part 11, Apollo Control Systems)

This article handles the Apollo Command Module. The current NASA command module is called Orion capsule or crew module.

Command Module CM with the Service Module SM connected together with an umbilical (right).  


The Command Module CM as the Service Module SM was made by North American Aviation and the history behind them was described in the previous article (see it for more details).

Command module wooden mock up


Command Module crew stations

The Command Module (CM) was the conical crew cabin, designed to carry three astronauts from launch to lunar orbit and back to an Earth ocean landing. It was the only component of the Apollo spacecraft to survive without major configuration changes as the program evolved from the early Apollo study designs.

Apollo Command Module major parts and measures

Its exterior was covered with an ablative heat shield, and had its own reaction control system (RCS) engines to control its attitude and steer its atmospheric entry path. Parachutes were carried to slow its descent to splashdown. The module was 11.42 feet (3.48 m) tall, 12.83 feet (3.91 m) in diameter, and weighed approximately 12,250 pounds (5,560 kg).

R-4D RCS jet

The forward compartment contained two reaction control engines, the docking tunnel, and the components of the Earth Landing System (ELS). 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.

Apollo command module main parts

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 aluminum "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.

Only the command module returned to the Earth after the mission

The central compartment structure consisted of an inner aluminum 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 aluminized PET film thermal strips.

1) LEB Floodlight

2) X-X Foot Attenuator Strut

3) Hand Strap

4) Translation Control

5) Rotation Control

6) Internal Viewing Mirror

7) Y-Y Attenuator Strut

8) Commander's Couch (Left)

9) CM Pilot's Couch (Center)

10) LM Pilot's couch (Right)

11) X-X Head Attenuator Strut

12) Floodlight

13) EVA Stabilizer Strut

14) Z-Z Attenuator Strut

15) L Shaped PGA Bags

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.

Apollo Command Module Fabrication Autoclave, Downey CA

The steel honeycomb core and outer face sheets were then thermally bonded to the inner skin in a giant autoclave. The aft heat shield was nearly identical to the central, except no alluminized film layer was applied.

Earth Landing Ssystem  (ELS)

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 three main parachutes, three pilot parachutes, two drogue parachute motors, three upright bags, a sea recovery cable, a dye marker, and a swimmer umbilical.

CM after landing

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). 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.

Apollo CM aerodynamics during re-entry

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.

YouTube video: "Apollo Command Module Block II Parachute Installation and Testing"

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 km/h). 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 km/h) for splashdown.

YouTube video: "Apollo 15 Splashdown"

The portion of the capsule which first contacted the water surface was built with crushable ribs to further mitigate the force of impact. The Apollo Command Module could safely parachute to an ocean landing with at least two parachutes (as occurred on Apollo 15), the third parachute being a safety precaution.

YouTube video: "Apollo Block II Parachute Fabrication Rigging Packing"

Reaction Control System (RCS)

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 mono-methyl hydrazine 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.


Command Module interior

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.

Unified Crew Hatch (UCH) It is referred to as a "unified" hatch because it incorporates both the outer heat shield hatch and inner crew compartment pressure hatch in a single hatch. This hatch was initially flown on Apollo 6.

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. 

UCH crew hatch in detail

Docking assembly

North American engineers favored probe and drogue devices to dock the command module with the lunar module. The CM probe would slip into the LM's dish-shaped drogue, and 12 latches on the docking ring would engage, to lock the spacecraft together, airtight. The astronauts could now remove a hatch, take out the docking devices, and travel between the two spacecraft. When operations were finished, they would return to the CM, reinsert the devices, install the hatch, and release the latches to disengage from the LM.

The Apollo spacecraft 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:

  • 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. 


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. 

Apollo CSM (Command Service Module) docked with the LM (Lunar Module)


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). 

Apollo CSM and LM docked


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.

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

For a general view of the CM internals look this video.

YouTube video: "James Burke takes us Inside the Apollo Command Module"

Here is a more detailed cutaway of the CM interior.

Apollo Command Module (CM) interior

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.

CM main control panel

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).

CM seats

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.

CM crew couches

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 LM docking maneuver, 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.

Apollo 16 CM stowage locations

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. 

Apollo CM windows:
1 and 5 - side windows,
2 and 4 - forward rendezvous windows,
3 - hatch window

The CM had five windows.
  •  1 and 5 - The two side windows measured 13 inches (330 mm) square next to the left and right-hand couches.
  •  2 and 4 - Two forward-facing triangular rendezvous windows measured 8 by 13 inches (204 by 330 mm), used to aid in rendezvous and docking with the LM.
  •  3 - 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.

Major differences between Block I and Block II

  • The Block II used a one-piece, quick-release, outward opening hatch instead of the two-piece plug hatch used on Block I, in which the inner piece had to be unbolted and placed inside the cabin in order to enter or exit the spacecraft (a flaw that doomed the Apollo 1 crew). The Block II hatch could be opened quickly in case of an emergency. (Both hatch versions were covered with an extra, removable section of the Boost Protective Cover which surrounded the CM to protect it in case of a launch abort.) 
  •  The Block I forward access tunnel was smaller than Block II, and intended only for emergency crew egress after splashdown in case of problems with the main hatch. It was covered with a removable plug in the nose of the forward heat shield. Block II contained a shorter forward heat shield with a flat removable hatch, beneath a docking ring and probe mechanism which captured and held the LM. 
  •  The aluminized PET film layer, which gave the Block II heat shield a shiny mirrored appearance, was absent on Block I, exposing the light gray fiberglass material, which on some flights was painted white. 
  •  The Block I VHF scimitar antennas were located in two semicircular strakes originally thought necessary to help stabilize the CM during reentry. However, the unmanned reentry tests proved these to be unnecessary for stability, and also aerodynamically ineffective at high simulated lunar reentry speeds. Therefore, the strakes were removed from Block II and the antennas were moved to the Service Module. 
  •  The Block I CM/SM umbillical connector was smaller than on Block II, located near the crew hatch instead of nearly 180 degrees away from it. The separation point was between the modules, instead of the larger hinged arm mounted on the Service Module, separating at the CM sidewall on Block II. 
  •  The two negative pitch RCS engines located in the forward compartment were arranged vertically on Block I, and horizontally on Block II.


  • Length: 11.4 ft (3.5 m) 
  • Diameter: 12.8 ft (3.9 m) 

  • 3 astronauts

Crew cabin volume:
  • 218 cu ft (6.2 m3) living space,
  • pressurized 366 cu ft (10.4 m3) 

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) 
  • RCS propellant mass: 270 lb (120 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) 

  • twelve 93 lbf (410 N) thrusters, firing in pairs 
  • RCS propellants: UDMH/N2O4 

  • Drinking water capacity: 33 lb (15 kg) 
  • Waste water capacity: 58 lb (26 kg) 

Life support

Electric system batteries:

  • 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

RCS Jets

Marquardt Company's employee Gerald R. Pfeifer remembers:

Gerald R. "Jerry" Pfeifer
"We made about 650 R-4D engines. That was 650 production units. We actually flew 469 of them during the Apollo Program, which is an astounding number of little rocket engines that actually fly in space all on one program. In all that time, and the millions of cycles that were put on during that whole program, there was not one R-4D valve or engine failure. We were really kind of tickled that we may have done something good." 

R-4D RCS jet (something good)

YouTube - "Command Module Documentary"


/1/ Wikipedia

/2/ Apollo archives, NASA

/3/ Apollo Experience Report - Command and Service Module - ECS

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