Apollo 6: An Uncrewed Test Flight Completed amid Failures

Apollo 6 flew on 4 April 1968 as the second flight of the Saturn V launch vehicle. Its payload included command and service module 020 and the LTA-2R test article, simulating the lunar module's shape and mass. The plan was to enter a nearly circular Earth orbit, restart the final stage after two revolutions to simulate acceleration towards the Moon, and then use the service-module engine to establish a return trajectory.

The mission did not aim to end its flight at a point near the Moon. Besides checking coordination between launch vehicle and spacecraft, its main spacecraft test objectives included operating the emergency detection system in closed-loop mode. The command module retained the basic Block I configuration while incorporating improvements from the Block II design, including a new unified side hatch and heat-shield structure.

References: [1] [2]

The first rocket stage reached Kennedy Space Center in March 1967, followed by the second in May. On 6 February 1968, the crawler-transporter carried the complete stack from the assembly building towards Launch Complex 39. A communications-line failure caused a stop along the way, while wind and rain complicated ground operations. North American Aviation supplied the command and service module, Houston's Manned Spacecraft Center handled the spacecraft and mission planning, and Marshall Space Flight Center was responsible for launch-vehicle engineering.

As the flight approached, NASA added Earth photography. Mission planners led by John Mayer had to coordinate trajectory, attitude and photographic coverage, and prepare software for the onboard computer. Photography thus became part of flight planning, rather than simply adding a camera to an already settled route.

References: [2]

At about 12:00 Greenwich Mean Time on 4 April, the rocket lifted off from Pad 39A at Kennedy Space Center, Florida. Longitudinal oscillations at about 5 hertz developed during first-stage flight, with command-module acceleration variations reaching plus or minus 0.6 g, exceeding design standards. Ground and aircraft cameras also recorded material breaking away from the adapter surrounding the lunar-module test article.

Two second-stage J-2 engines then shut down prematurely. The remaining three burned for about a minute longer, and the third stage also operated longer than planned to reach orbit. The resulting orbit had a perigee of approximately 178 kilometres and an apogee of about 367 kilometres, rather than the planned near-circular shape.

The emergency detection system recorded one abort signal during the abnormal oscillations, but automatic abort required two voting signals. The flight therefore continued while ground control assessed which tests could still be completed.

References: [1]

Mission Director William Schneider and Flight Director Clifford Charlesworth kept the combined spacecraft in its parking orbit for two revolutions, conducting equipment checks, communications tests and attitude manoeuvres. When commanded to restart, the S-IVB final stage failed to ignite. Ground control then ordered the command and service module to separate and carry out a prepared backup mission.

The service-module engine burned for about 442 seconds, putting the spacecraft into a highly elliptical orbit with an apogee of approximately 22,200 kilometres. The command module then maintained a cold-soak attitude for around six hours. Because this acceleration used substantial propellant, a planned pre-return acceleration burn was cancelled. Re-entry speed was about 10 kilometres per second, below that needed to simulate a lunar return.

After roughly nine hours and 57 minutes of flight, the command module splashed down in the Pacific at about 21:57 Greenwich Mean Time that day, approximately 91 kilometres from its target. It initially floated upside down before the uprighting system restored it, and USS Okinawa recovered it. The parachutes, uprighting equipment and new side hatch were all tested under actual flight conditions.

References: [1] [2]

The thermal protection system withstood re-entry, but the report still classified simulation of the lunar-return thermal environment as only partially achieved. Lower speed meant lower heating intensity and heat load than planned. The long service-module engine burn, navigation and attitude operations, however, showed that the spacecraft could execute a more complex backup route after launch-vehicle failures.

The onboard 70-millimetre camera took colour stereo images of land, coasts and shallow seas, including the Gulf of California and Colorado River delta. These photographs were later used in mapping, terrain studies and geographical research. Such results were a separate measure of success from whether the rocket met every flight objective.

References: [1] [2]

NASA engineering teams investigated the longitudinal oscillations, J-2 engine problems and adapter damage. To address oscillation, Marshall and the engine manufacturer studied pressure fluctuations in the liquid-oxygen supply system through ground tests, eventually using helium-filled chambers to absorb them. Vacuum testing also exposed vibration failures in bellows sections of igniter feed lines, which were subsequently strengthened and modified.

Heating and pressure buildup in the adapter's honeycomb panels were addressed by adding vent holes and an external cork layer. Houston and Marshall organized structural tests of the payload combination, with astronaut Charles Duke helping coordinate information between the centres. Apollo 6 exposed together problems that had been difficult to identify in individual ground tests. Subsequent crewed flights had to depend on these corrections and renewed testing.

References: [2]