Apollo 5: The Lunar Module's First Orbital Test Flight

Apollo 5 was the lunar module's first uncrewed spaceflight, using vehicle LM-1 to test its descent engine, ascent engine and staging during an aborted descent. The lunar programme required a spacecraft that could both decelerate toward the surface and, if landing could not continue, discard the descent stage and carry astronauts away from danger in the ascent stage. This flight tested those propulsion and separation processes in Earth orbit.

LM-1 was housed within the spacecraft–lunar module adapter and launched on a Saturn IB. The combination had no command module, service module or launch escape tower, making it shorter than the crewed configuration. The lunar module also lacked landing legs: the mission included neither landing nor recovery, and the vehicle had no re-entry heat shield.

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Manufacturing difficulties and changes after the Apollo crew-cabin fire delayed delivery of LM-1. Originally due at the launch base in November 1966, it actually arrived in Florida on 27 June 1967. John Williams led a Kennedy Space Center spacecraft-operations workforce of approximately 400, checking the contractor's delivery condition and supervising testing, maintenance and modifications.

The Saturn IB numbered 204 had originally been prepared for Apollo 1. Ground teams dismantled it at Launch Complex 34, inspected it for corrosion and damage from prolonged storage, and moved it to Complex 37. Rocco Petrone's launch team also dealt with propellant-loading procedures, clogged filters and ground-equipment problems. In Houston, flight director John Hodge and his principal assistant Eugene Kranz prepared to take command after liftoff.

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The rocket lifted off from Cape Kennedy's Pad 37B at 22:48:08 Greenwich Mean Time on 22 January 1968, equivalent to 17:48:08 Eastern Standard Time that day. Approximately ten minutes and three seconds later, the lunar module and S-IVB final stage entered Earth orbit. Telemetry showed that ascent loads and vibrations remained within structural design capability. After final-stage shutdown, the cooling system began regulating equipment temperatures.

The lunar module used its attitude-control thrusters to move away from the final stage, then maintained a prescribed cold-soak attitude to observe operation in low temperatures. Almost four hours into the flight, the descent engine ignited for a planned 38-second burn, but the guidance computer shut it down after only about four seconds.

The problem lay in system coordination. The test deliberately used lower-than-normal propellant-tank pressure, slowing thrust buildup, but the computer's criterion for velocity increase over time had not been adapted to that condition. Guidance and propulsion equipment each followed its own logic, yet together produced premature shutdown.

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Ground controllers switched to a prepared minimum-mission plan. The scheduled 734-second burn simulating changing thrust during lunar descent was cancelled, and subsequent ignitions were controlled by the mission programmer's automatic sequence. At approximately six hours and ten minutes into flight, the descent engine fired again for 33 seconds. Another descent-engine burn followed, along with abort staging and the first ascent-engine ignition.

This test, commonly called fire-in-the-hole staging, examined the thermal effects, structural loads and separation stability produced when the ascent engine ignited before the two stages had fully separated. The mission report judged the actual systems operation and vehicle dynamics during staging satisfactory for crewed operation. The descent engine also responded smoothly to full-thrust commands, and its gimbal mechanism worked properly.

The shortened test carried a corresponding cost: the three descent-engine burns were collectively too brief to evaluate fully the thermal characteristics of the supercritical-helium pressurization system. That limitation remained in the mission report.

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After staging, the reactivated primary guidance and control system did not promptly account for the vehicle's reduced mass. It continued calculating attitude-thruster firing times for the combined two-stage spacecraft, consuming control propellant heavily until it was exhausted. Early in the second ascent-engine burn, the attitude system temporarily drew propellant from the ascent propulsion system. The sequencer then closed the connecting valves, and the spacecraft lost attitude control and tumbled. Tracking was lost approximately two minutes after engine thrust decayed, ending the mission.

NASA nevertheless judged the three principal objectives, ascent propulsion, descent propulsion and abort staging, achieved. A March 1968 review consequently decided against a second uncrewed lunar-module flight, and delivery of the intended LM-2 and its rocket was deferred. Issues including the ascent-engine injector still required ground tests and improvements, however; successful orbital testing did not remove the need for further engineering validation before crewed flight.

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