--- title: "Apollo command and service module" chunk: 2/7 source: "https://en.wikipedia.org/wiki/Apollo_command_and_service_module" category: "reference" tags: "science, encyclopedia" date_saved: "2026-05-05T12:35:22.001294+00:00" instance: "kb-cron" --- 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. Block I spacecraft were used for all uncrewed Saturn 1B and Saturn V test flights. Initially two crewed 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. 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 crewed 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 uncrewed Apollo 4 and Apollo 6 flights, so the first all-up Block II spacecraft flew on the first crewed 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 $36.9 billion in 2016 dollars, adjusted from a nominal total of $3.7 billion using the NASA New Start Inflation Indices. == Command module (CM) == The command module was a truncated cone (frustum) with a diameter of 12 feet 10 inches (3.91 m) across the base, and a height of 11 feet 5 inches (3.48 m) including the docking probe and dish-shaped aft heat shield. The forward compartment contained two reaction control system thrusters, the docking tunnel, and the Earth Landing System. The inner pressure vessel housed the crew accommodation, equipment bays, controls and displays, and many spacecraft systems. The aft compartment contained 10 reaction control engines and their related propellant tanks, freshwater tanks, and the CSM umbilical cables. === Construction === The command module was built in North American's factory in Downey, California, and consisted of two basic structures joined together: the inner structure (pressure shell) and the outer structure. The inner structure was an aluminum sandwich construction consisting 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. ==== 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 formaldehyde resin. 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. The total weight of the shield was about 3,000 pounds (1,400 kg). ==== Forward compartment ==== The 1-foot-11-inch (0.58 m)-tall 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 that contained Earth landing equipment (all the parachutes, recovery antennas and beacon light, and sea recovery sling), two reaction control thrusters, 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. ==== Aft compartment ==== The 1-foot-8-inch (0.51 m)-tall 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. === Earth landing system ===