THE SATURN BALKS: APOLLO 6

By the spring of 1968, with two flights completed, the Apollo programme seemed to be hitting its stride. It had demonstrated all three stages of the Saturn V worked, the command module had survived its high-speed re-entry, and an early version of the lunar module had performed satisfactorily. Before the Saturn V could be declared fit to carry astronauts, a second А-mission was required. This flight was named Apollo 6 and. once again, events unfolded that threatened to stop the programme in its tracks.

After a successful lift-off on 4 April 1968. the first problem appeared towards the end of the SIC’s flight. Rockets have always been prone to vibrations along their length, but for about ten seconds immediately before the first stage was to shut down, the longitudinal shaking of the entire vehicle (known as pogo) became alarming. Meanwhile, at the front end of the rocket, a conical aerodynamic shroud that would normally protect the lunar module (not carried on this flight) was losing chunks of its outer surface. Since this section had to support the mass of the CSM multiplied by the g-forces of acceleration, its structural integrity was of some concern.

Halfway through the flight of the S-II stage, one of its five J-2 engines began to falter, prompting the instrument unit to shut it down. As it did so. another engine that had been showing no distress also shut down, causing the thrust from the other three to be applied asymmetrically. Considering that the Saturn’s control system had been programmed only to deal with a single-engine failure, it did a remarkably good job of compensating for the off-axis thrust and burned the remaining engines for longer on the residual propellant. The first burn of the S-IVB third stage successfully pul the vehicle into orbit, but a subsequent command to restart the engine failed. Some of the flight’s objectives were met, but if the problems could not be fixed, NASA would not dare to put men on top of the next Saturn V. as was being considered instead of a third Л-mission test.

In the event, engineers managed to find solutions for all these problems. The first stage vibrations were suppressed by the addition of helium gas to cavities in the LOX feed lines, which damped out pressure oscillations. Elaborate tests on the J-2 engine discovered a design fault in a liquid hydrogen fuel line that had not only caused one of the engines on the S-II to shut down but also prevented the S-IVB from restarting. Compounding the S-II problem, a wiring error had sent the shutdown command from the Saturn’s instrument unit to the wrong engine, shutting it down unnecessarily. The aerodynamic shroud had failed because frictional atmospheric heating as the rocket went supersonic caused trapped moisture and air within its aluminium honeycomb sandwich skin to expand, in turn causing the skin to peel off in sheets. This problem was remedied by making small ventilation holes in the shroud’s skin and adding cork insulation.

The launch vehicle issues apart, the CSM-020 spacecraft successfully performed a number of remote-controlled manoeuvres and was recovered from the Pacific Ocean. Preparations for Apollo 7 continued because it would use a Saturn IB launch vehicle. It was decided that if this mission went well, the third Saturn V would indeed carry a crew.