Apollo 6

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. The Saturn V's utilization during the mission qualified it for use during human spaceflight, and it was used beginning on 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, a direct return abort using the service module's main engine with a flight time totaling about 10 hours, but 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 also shut down early due to cross-wiring with the engine that had shut down. 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 of the Saturn V 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

thumb|The Lunar Module Test Article (LTA-2R) being moved for mating with the spacecraft–LM adapter

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. 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.

Kenneth S. Kleinknecht, Command and Service Module manager at the Manned Spaceflight Center in Houston, was pleased with CSM-020 when it arrived at Kennedy Space Center from North American Aviation, the manufacturer, though he was upset it arrived wrapped in flammable mylar. In contrast with Apollo 1's ill-fated CSM, which arrived with hundreds of unresolved issues, CSM-020 had only 23, mostly routine problems.

Also flown on Apollo 6 was a lunar test article: a simulated lunar module, designated as LTA-2R. It included a flight-type descent stage without landing gear, its fuel tanks filled with a water–glycol mixture and freon in its oxidizer tanks. Containing no flight systems, its ascent stage was made of ballasted aluminum and instrumented to show vibration, acoustics and structural integrity. LTA-2R remained inside the Spacecraft-Lunar Module Adapter, numbered SLA-9, throughout the flight.

Preparation

The S-IC first stage arrived by barge on March 13, 1967, and was erected in the Vehicle Assembly Building (VAB) four days later; the S-IVB third stage and Instrument Unit computer both arrived on March 17. The S-II second stage was not yet ready and so the dumbbell-shaped spacer, used in preparation for Apollo 4 (which also had a delayed S-II), was substituted so testing could proceed. The spacer had the same height and mass as the S-II along with all the electrical connections. The S-II arrived May 24 and was stacked and mated into the rocket on July 7.

Apollo 6 saw the first use of the High Bay 3 of the VAB, and it was quickly discovered that its air conditioning facilities were inadequate. Portable high-capacity units were brought in to keep equipment and workers cool. There were delays in April as personnel and equipment were busy with Apollo 4, and not available for tests on Apollo 6. The S-II second stage arrived on May 25 and was erected in one of the VAB's low bays, but work on Apollo 6 continued to be plagued by delays, many occasioned by work on Apollo 4. The vehicle was erected on Mobile Service Launcher 2, but work on the launcher's arms, which would swing back at launch, proceeded slowly. Also slow to arrive was the CSM itself; the planned late-September arrival was pushed back two months.

After the first stage was jettisoned, the S-II second stage began to experience problems with its J-2 engines. Engine number two had performance problems from 225 seconds after liftoff, abruptly worsening at T+319 seconds. At T+412 seconds the Instrument Unit shut it down altogether, and two seconds later, engine number three also shut down.

Post mission tests showed that the propellant lines feeding the ASI could fail in vacuum. The lines had a metal bellows section to allow for thermal expansion. In ground testing, the cold propellants passing through the lines would form a layer of frost on the LOX line and liquid air on the LH₂ line, damping out vibrations. In the vacuum of space, there was no such protection from vibrations. On Apollo 6, the LH₂ line to engine number 2's ASI failed, while the LOX line continued to feed liquid oxygen to the igniter. The excess oxygen injected into the hydrogen-rich combustion chamber produced a very high temperature in the igniter area, causing a burn-through of the engine dome. The resulting loss of pressure caused the Saturn Instrument Unit (IU) to send an engine shutdown signal. Due to a wiring error, the shutdown signal caused engine number 3 to shut down. Its loss of pressure initiated another shut down signal which, due to the same crosswiring, shutdown engine number 2. The remaining three engines continued to operate. A similar failure of igniter propellant lines likely prevented the single J-2 engine on the S-IVB from restarting. The Apollo 6 command module is on display at the Fernbank Science Center in Atlanta, Georgia.

Cameras

thumb|Still from footage of Apollo 6's interstage falling away (NASA)

The Saturn V had several cameras affixed to it, intended to be ejected and later recovered. Three of the four cameras on board the S-IC failed to eject and thus were destroyed, and only one of the two cameras on the S-II was recovered. Two of these cameras were intended to film the S-IC/S-II separation, and the other two were to film the liquid oxygen tank; the one that was recovered had filmed separation. The failure to eject was attributed to a lack of nitrogen pressure in the bottles that were to cause the ejection. The command module carried a motion picture camera, intended to be activated during launch and during re-entry. Because the mission took about ten minutes longer than planned, re-entry events were not filmed.

A 70 mm still camera operated in the CM during part of the mission, pointed at the Earth through the hatch window. Coverage included parts of the United States, the Atlantic Ocean, Africa, and the western Pacific Ocean. The camera had haze-penetrating film and filter combination, with better color balance and higher resolution than photographs taken on previous American crewed missions.