Apollo command and service module

• 1. Introduction

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  The Apollo command and service module flew nineteen times, nine of those voyages taking astronauts to the Moon and home again. North American Aviation began building it in November 1961, a decade before the last one left the launchpad. The program's total cost reached $36.9 billion in 2016 dollars. Every mission ended with a deliberate sacrifice. The larger of the two spacecraft sections was cut loose just before reentry and left to burn in the atmosphere. The smaller piece plunged to splashdown, carrying the crew. What kind of engineering produces a machine where half the structure is designed to be thrown away? And why did a launchpad fire force engineers to rebuild the original design almost entirely before anyone had reached the Moon? North American Aviation had initially designed the spacecraft for direct descent to the Moon, with no means of docking with a separate vehicle. Switching to a plan that used a separate lunar lander required the addition of a docking hatch and probe, hardware the original design had no room for.

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• 2. Two Sections, One Purpose

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  At 12 feet 10 inches across its base, the conical command module formed one of two halves of the Apollo CSM. The cylindrical service module beside it measured 14 feet 10 inches long, with the main engine nozzle adding height to a total of 24 feet 7 inches. These two sections served entirely different roles. The command module was the pressurized capsule the crew lived in; the service module was the unpressurized workhorse that kept them alive. It carried propulsion, electrical power, oxygen, and water for missions lasting more than a week in deep space. A single umbilical bundled all wiring and plumbing between the two sections throughout the voyage. Just before the command module hit the atmosphere, a pyrotechnic guillotine severed that connection. The service module's thrusters then fired automatically to push it clear, ensuring it broke up faster and followed a different trajectory. Three tension ties and six compression pads had held the two sections together from launch until that moment. The pressurized crew cabin inside the command module held just 210 cubic feet of habitable space.

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• 3. Inside the Cabin

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  566 switches crowded the command module's control panels, joined by 24 instruments, 40 event indicators, and 71 lights. Those controls were arranged across a crescent-shaped main display panel measuring nearly 7 feet wide and 3 feet tall. Three panels divided the controls by crew role. The commander's section on the left managed flight controls and attitude indicators. The center panel housed the guidance computer, navigation systems, and caution-warning indicators. The right-hand panel covered fuel cells, electrical systems, and communications for the systems engineer.

  Three couches built from hollow steel tubing and covered in fireproof Armalon cloth supported the crew during launch and landing. Each rested on eight shock-attenuating struts designed to absorb the force of ocean splashdown or, if needed, an emergency landing on solid ground. Six equipment bays flanking the main panels organized the cabin's storage. Food, medical supplies, survival kits, tool sets, cameras, and the contingency lunar sample container each had their designated compartment.

  Five windows punctuated the command module's hull, each a three-pane glass assembly. Two side windows measured 9 inches square. Two smaller triangular rendezvous windows at the front aided docking with the lunar module. A circular hatch window above the center couch measured 9 inches in diameter. The inner two panes of each assembly were aluminosilicate glass, forming part of the pressure vessel. The outer pane was fused silica, serving as both a debris shield and a heat shield component. The Block II spacecraft that housed this interior flew for the first time on the 11th of October, 1968, as Apollo 7. Reaching that mission required rebuilding the design after a fire killed three astronauts and severely damaged their command module during a launch rehearsal.

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• 4. Born From Fire

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  On the 27th of January, 1967, a cabin fire broke out during a launch rehearsal for the Apollo 1 mission. It killed astronauts Gus Grissom, Ed White, and Roger Chaffee. The mission had been planned for launch on the 21st of February of that year. The fire exposed a critical flaw in the Block I design. Its two-piece hatch required unbolting the inner section and stowing it inside the capsule before the crew could exit. In a fire, that sequence was fatal.

  The Apollo 204 Review Board's investigation found that many Block I flaws had already spread into Block II modules then under construction. Engineers set about redesigning Block II to address the board's findings. The new Block II hatch was a single piece operated by a pump handle that drove fifteen latches simultaneously, openable in an emergency without unbolting anything. Weight was also reduced. The Block I Apollo 1 spacecraft had weighed approximately 45,000 pounds. The redesigned Block II Apollo 7 came in at 36,400 pounds, a reduction achieved through both safety fixes and deliberate weight reduction work. By January 1964, North American had already been presenting Block II design details to NASA; the fire made those plans essential rather than optional. The revised Block II heat shield was then tested on the uncrewed Apollo 4 and Apollo 6 flights before any crew relied on it. Those tests had to confirm the shield could survive temperatures sufficient to melt most metals on the return from the Moon.

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• 5. Coming Home

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  Phenolic formaldehyde resin coated the outside of the command module, bonded to a steel and aluminum honeycomb outer shell. During reentry, this resin charred and melted in a controlled process, carrying heat away from the capsule's structure beneath. The shield was thickest at the aft base: 2 inches of material at the end that faced forward during reentry. It tapered to just half an inch at the forward portions. Its total weight was roughly 3,000 pounds. The resin carried several outer coverings. A pore seal and a white moisture barrier topped the ablative layer. A silver Mylar thermal coating gave the surface an appearance similar to aluminum foil.

  Above the heat shield, the capsule's lift-to-drag ratio of about 0.368 gave the crew some directional control during descent. Astronauts steered by rotating the capsule with thrusters; when no correction was needed, they spun it slowly to cancel out the lift effects. At 24,000 feet, four pressurized-gas compression springs ejected the forward heat shield, exposing the parachute compartment. Two conical ribbon drogue parachutes deployed first, slowing the capsule to 125 miles per hour. At 10,700 feet, the drogues were jettisoned and three main ringsail parachutes opened, each 83.5 feet in diameter, slowing the capsule to 22 miles per hour for splashdown.

  On Apollo 15, only two of those three mains deployed. The system had been designed to land safely on two, with the third serving as a redundancy. Four crushable ribs built into the capsule's base provided a final layer of impact absorption. Starting with Apollo 14, the service module was fitted with an auxiliary 400 Ah battery as an emergency power reserve. On Apollo 13 in April 1970, before that backup existed, an oxygen tank rupture had nearly stranded three astronauts far from Earth.

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• 6. Power and Push

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  The AJ10-137 engine produced 20,500 pounds of thrust, a figure settled by an April 1962 contract with Aerojet-General. That contract came just months before NASA officially chose the lunar orbit rendezvous approach in July 1962. The engine turned out to be twice as powerful as the revised mission required. Its original design purpose had been to lift the entire spacecraft off the lunar surface on a direct landing concept. It ended up serving instead for mid-course corrections, for placing the spacecraft into lunar orbit, and for breaking out of it. The propellants were pressure-fed from two 40-inch-diameter spherical tanks storing gaseous helium at 3,600 pounds per square inch.

  Electrical power came from three hydrogen-oxygen fuel cells in Sector 4 of the service module. Each cell measured 44 inches tall by 22 inches in diameter and weighed 245 pounds. As a byproduct of combining hydrogen and oxygen to generate electricity, the cells produced drinkable water that was piped into the crew cabin. Apollo 12 was struck twice by lightning during its November 1969 launch, placing unusual demands on the spacecraft's power systems. On Apollo 13, one oxygen tank ruptured and punctured a second. The loss cut off all oxygen to the fuel cells, forcing the crew to rely on command module batteries for the return. After that mission, a third oxygen tank was added to the service module, allowing the tanks to never operate below half capacity. That change also allowed removal of the stirring-fan equipment inside the tanks, which had contributed to the failure. Three CSMs were then modified to carry crews to the Skylab station in 1973 and 1974. Each had one fuel cell removed. The otherwise-empty fuel and oxidizer storage tanks were stripped out as well, leaving only the partially filled sump tanks.

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• 7. After Apollo

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  CSM-116 launched to Skylab on the 25th of May, 1973, the first of three crewed missions to the orbital station. Two more followed that year: CSM-117 on the 28th of July, and CSM-118 on the 16th of November. These Earth-orbital missions needed less propellant than the lunar flights, so each spacecraft launched with less than half the full SPS propellant load. The high-gain S-band antenna was omitted from all three; omnidirectional antennas sufficed for ground communications at Earth orbital distances. Each command module was partially painted white to help manage its temperature during the extended time it would spend docked and largely unoccupied.

  CM-119 was set aside as a Skylab Rescue vehicle. Two jump-seat couches were installed in its aft equipment bay to carry stranded crew members back if their own capsule became unusable. It sat ready on the ground but was never needed.

  The final CSM to fly, CSM-111, launched on the 15th of July, 1975. It docked with the Soviet craft Soyuz 19 as part of the Apollo-Soyuz Test Project, the first international crewed spaceflight. That capsule is now on display at the California Science Center in Los Angeles. Two CSMs that were never fully completed, the vehicles intended for the canceled Apollo 19 and Apollo 20 missions, still survive. CSM-115 stands in the Saturn V display at Johnson Space Center in Houston, its SPS engine nozzle never installed. The hardware for CSM-115a was sent to Japan for an exhibition in 1978 and never returned. It remains at the Space LABO in Kitakyushu.

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