CM

The command module is the pressurized crew compartment of a spacecraft designed to house and protect astronauts during flight, reentry, and landing. In the Apollo program, the CM was a conical aluminum structure roughly 3.3 meters tall and 3.9 meters in diameter at its base, weighing approximately 5,800 kilograms. It contained life support systems, attitude control thrusters, and instrumentation; three crew members sat in a single cabin with no internal compartmentalization except for equipment storage bays beneath the couches.

The CM differs fundamentally from the service module, which trails behind it and houses fuel cells, cryogenic storage, and the main propulsion engine. During launch, the command module sits atop the service module; the two modules dock with the lunar module in orbit, but only the CM has a heat shield rated for atmospheric reentry at lunar return velocities around 11 kilometers per second. The blunt end of the conical design faces forward during ascent and is protected by an ablative heat shield during descent.

Design and operation variants

The interior of a CM contained five major systems: reaction control (small thrusters for maneuvering), guidance and navigation (computers and instruments), life support (oxygen, water, carbon dioxide scrubbing), electrical power (fuel cells or batteries), and environmental control (temperature and humidity regulation). Access was through a single side hatch; crews entered with spacesuits during launch and landed in the same suits. The three crew positions were arranged in a triangular layout with the commander and lunar module pilot in front couches and the command module pilot in the middle rear couch.

Soviet equivalents, called Soyuz modules, used a different architecture with three compartments (orbital module, descent module, and service module) and could land upright under parachutes rather than splashing into water. American CMs, by contrast, were designed for water recovery; a flotation collar deployed after landing to keep the capsule upright in the ocean. The term CM is also applied generically in aerospace to any crewed orbital cabin, though the Apollo design remains the most historically significant example in maintenance documentation and training curricula.

In maintenance records, the CM is treated as a single integrated system rather than a collection of subsystems, with technicians responsible for verifying hatch seals, thruster valve function, fuel cell integrity, and instrumentation calibration before flight. Heat shield ablation depth was measured and recorded after each flight; repairs to the CM structure itself were minimal because the module was not designed for reuse beyond the intended mission, though engineering data from CM examination after landing informed later spacecraft design.