CSM

The Command and Service Module was the crewed spacecraft and engine block of the Apollo program, distinct from the Lunar Module. It consisted of two sections: the Command Module (CM), a conical capsule where three astronauts lived and worked in Earth orbit and during trans-lunar coast; and the Service Module (SM), a cylindrical structure housing the main engine, fuel cells, life support consumables, and the Service Propulsion System (SPS). The CM was about 3.3 meters in diameter and 3.9 meters long; the SM extended behind it, making the stacked assembly roughly 11 meters long and weighing around 5,900 kilograms dry.

The SPS engine, a 45 kilonewton restartable hypergolic motor, performed the Trans-Lunar Injection burn, lunar orbit insertion, and the critical Trans-Earth Injection that brought crews home. This engine could be fired multiple times and carried the vehicle from 200 kilometers Earth altitude all the way to the Moon and back. Redundancy was built into nearly every system: two oxygen tanks, two hydrogen tanks, duplicate guidance platforms, and quad-redundant reaction control thrusters for attitude changes during coasting flight.

The Command Module had its own smaller thrusters for fine attitude adjustments and was the only section designed to survive reentry. Its ablative heat shield was 6 centimeters thick and protected the crew compartment during reentry heating rates exceeding 5,600 kelvin. The Service Module, by contrast, was jettisoned 15 minutes before reentry and burned up in the atmosphere. This division of labor meant the Command Module had to be precisely aligned before separation, or reentry heating would destroy unprotected areas.

Design trade-offs and operational constraints

The CSM was fundamentally limited by the 110 kilogram payload capacity of the Saturn V to lunar orbit once the Lunar Module was deployed. This drove every design decision: fuel tanks were thin-walled aluminum; water was used as both coolant and drinking water to save mass; and the cabin atmosphere was pure oxygen at 10.3 kilopascals rather than an air mixture at higher pressure. The oxygen-only choice reduced weight but created fire hazard during preflight operations, as shown in the Apollo 1 cabin fire of 1967.

Crews spent up to eight days inside the CSM during an Apollo mission. The Command Module cabin volume was only 6.2 cubic meters for three people, smaller than a modern family car. Thermal management was critical: the Service Module's radiators had to dump waste heat during low-power coast phases and at lunar orbit. The fuel cells generated electrical power and water but also heat. Temperature control failures during the Apollo 13 mission forced the use of the Lunar Module as a lifeboat and shelter, demonstrating both the redundancy that saved the crew and the tight margins by which the CSM was designed.