Apollo command and service module

Before Apollo

Before President Kennedy set the Moon landing as a national goal, NASA was already planning a new three-person spacecraft. This vehicle was designed mainly for Earth orbit, where a large pressurized module would let the crew live and work for weeks, performing station-like tasks or later carrying cargo. Its primary job was to support Project Olympus, a foldable rotating space station launched on a single Saturn V. Future versions were intended for flights around the Moon, for a direct ascent lunar mission, and for interplanetary travel. In late 1960, NASA asked industry for design proposals. Kennedy’s May 25, 1961 announcement of a lunar landing before 1970 made the Olympus station plans unnecessary.

Development history

When NASA awarded the initial Apollo contract to North American Aviation on November 28, 1961, the lunar landing was still assumed to be by direct ascent, so design proceeded without a docking means for a lunar excursion module. The change to lunar orbit rendezvous and technical obstacles led NASA in 1963 to develop two versions: Block I for early low Earth orbit test flights, and Block II as the lunar-capable version with a docking hatch. By January 1964, North American started presenting Block II design details.

Block I spacecraft were used for all uncrewed Saturn 1B and Saturn V test flights. One crewed Block I flight, Apollo 1, was planned for February 21, 1967, but during a dress rehearsal on January 27, a cabin fire killed the crew and revealed serious design shortcomings. After investigation, the crewed Block I phase was terminated, and Block II was redefined to incorporate the review board's recommendations, including a revised CM heat shield design tested on Apollo 4 and Apollo 6. The first all-up Block II spacecraft flew on Apollo 7.

Modifications for Saturn IB missions

The Saturn IB launch vehicle could not handle the fully fueled CSM mass of 66900 lb, so for low Earth orbit missions the spacecraft were launched with less than half the full SPS propellant load, filling only the sump tanks. CSM masses ranged from 32558 lb (Apollo–Soyuz) to 46000 lb (Skylab 4). The high-gain S-band antenna was omitted from Apollo 1, Apollo 7, and the three Skylab flights, but restored for Apollo–Soyuz to communicate through the ATS-6 satellite. On Skylab and Apollo–Soyuz missions, empty fuel and oxidizer storage tanks and one helium pressurant tank were removed, allowing extra RCS propellant as backup for the deorbit burn. The command module could be modified with jump seat couches; CM-119 was fitted with two jump seats as a Skylab Rescue vehicle, never used.

Major differences between Block I and Block II

Block II used a one-piece, quick-release, outward opening hatch instead of the two-piece plug hatch of Block I, which required unbolting the inner piece. The Block II forward access tunnel was larger, with a flat removable hatch beneath a docking ring and probe mechanism. Block II had a shorter forward heat shield with a shiny aluminized PET film layer, while Block I exposed light gray epoxy resin. Block II removed the VHF scimitar antenna strakes found on Block I, moving the antennas to the service module. The CM/SM umbilical connector on Block II was larger, located nearly 180 degrees from the crew hatch, with a hinged arm mounted on the service module separating at the CM sidewall, unlike the smaller Block I connector near the hatch. The two negative pitch RCS engines in the forward compartment were arranged horizontally on Block II, vertically on Block I.