Apollo 6 (April 4, 1968), also known as AS-502, was the third and final uncrewed flight in the United States' Apollo Program and the second test of the Saturn V launch vehicle. It qualified the Saturn V for use on crewed missions, which began with Apollo 8 in December 1968.
Apollo 6 was intended to demonstrate the ability of the Saturn V's third stage, the S-IVB, to propel itself and the Apollo spacecraft to lunar distances. Its components began arriving at the Kennedy Space Center in early 1967. Testing proceeded slowly, often delayed by testing of the Saturn V intended for Apollo 4—the inaugural launch of the Saturn V. After that uncrewed mission launched in November 1967, there were fewer delays, but enough so that the flight was postponed from March to April 1968.
The flight plan called for following trans-lunar injection with a direct return abort using the service module's main engine, with a flight time totaling about 10 hours. Instead, vibrations damaged some of the Rocketdyne J-2 engines in the second and third stages by rupturing internal fuel lines causing a second-stage engine to shut down early. An additional second-stage engine shut down early due to cross wiring with the other shutdown engine. The vehicle's onboard guidance system compensated by burning the second and third stages longer, although the resulting parking orbit was more elliptical than planned. The damaged third-stage engine failed to restart for trans-lunar injection. Flight controllers elected to repeat the flight profile of the previous Apollo 4 test, achieving a high orbit and high-speed return. Despite the engine failures, the flight provided NASA with enough confidence to use the Saturn V for crewed launches; a potential third uncrewed flight was cancelled.
Objectives
Apollo 6, the second test flight of the
Saturn V launch vehicle, was intended to send a command and service module (CSM) plus a Lunar Test Article (LTA), a simulated lunar module (LM) with mounted structural vibration sensors, into a trans-lunar trajectory, with the boost from orbit to trans-lunar velocity powered by the Saturn V's third stage, the S-IVB. That trajectory, although passing beyond the orbit of the Moon, would not encounter it. The CSM was to separate from the S-IVB soon after the burn, and the SM engine would then fire to slow the craft, dropping its apogee to and causing the CSM to return to Earth, simulating a "direct-return" abort. On the return leg, the engine was to fire once more to accelerate the craft to simulate conditions that the Apollo spacecraft would encounter on its return from the Moon, with a re-entry angle of −6.5 degrees and velocity of . The entire mission was to last about 10 hours.
The mission was intended to test the Saturn V launch vehicle's ability to send the entire Apollo spacecraft to the Moon—in particular, to test the stresses on the LM and the vibration modes of the entire Saturn V with near-full loads. With the spacecraft having been qualified for crewed flight through the Apollo 4 mission (the first flight of the Saturn V), the focus was on fully qualifying the launch vehicle. Nominal completion of planned mission events through attainment of the initial parking orbit, and the restarting of the S-IVB to propel the space vehicle towards the planned distance, beyond the Moon's orbit, was deemed sufficient to fulfill Apollo 6's main objectives.
Equipment
Apollo 6's launch vehicle was designated AS-502, the second flight-capable Saturn V. Its payload included CSM-020, a Block I CSM that had some Block II modifications. The Block I CSM did not have the capability of docking with a Lunar Module, as the Block II did. Among the modifications to CSM-020 was a new crew hatch, intended to be tested under lunar return conditions. This new hatch replaced the one which was condemned by the Apollo 1 investigation board as too difficult to open in case of emergency, circumstances that had contributed to the deaths of three astronauts in the Apollo 1 fire of January 27, 1967. The command module used was CM-020; it carried a mission programmer and other equipment to allow it to be operated remotely.
The service module used was SM-014—the originally-planned SM for Apollo 6, SM-020, was used for Apollo 4 after its SM, SM-017, was damaged in an explosion and had to be scrapped. CM-014 was unavailable for flight as it was being used to aid the Apollo 1 investigation. Not all SM systems were activated for the short Apollo 6 mission: the radiators to remove excess heat from the electrical power system and the environmental control system were not connected.