After liftoff Apollo 6 ran into a sea of troubles. In the closing seconds of the first stage burn, the vehicle went through 30 seconds of severe longitudinal oscillation - the pogo effect, it was called, because the space vehicle vibrated up and down like a child's pogo stick. As George Mueller later explained in a congressional hearing:
Pogo arises fundamentally because you have thrust fluctuations in the engines. Those are normal characteristics of engines. All engines have what you might call noise in their output because the combustion is not quite uniform, so you have this fluctuation in thrust of the first stage as a normal characteristic of all engine burning.The pogo effect had not been significant on Apollo 4. On Apollo 6 it started about 30 seconds after maximum dynamic pressure or "Max Q" - between 110 and 140 seconds after liftoff - and produced unacceptable g loads in the spacecraft.
Now, in turn, the engine is fed through a pipe that takes the fuel out of the tanks and feeds it into the engine. That pipe's length is something like an organ pipe so it has a certain resonant frequency of its own and it really turns out that it will oscillate just like an organ pipe does.
The structure of the vehicle is much like a tuning fork, so if you strike it right, it will oscillate up and down longitudinally. In a gross sense it is the interaction between the various frequencies that causes the vehicle to oscillate.16
Simultaneously, the spacecraft lunar module adapter was experiencing trouble. Made of bonded aluminum honeycomb, the adapter not only housed the lunar module but connected the command-service module to the Saturn launch vehicle. At T + 133 seconds, sizable pieces of the outer surface, more than 3 square meters, flaked off. Telemetry data and airborne cameras verified the damage. Nevertheless, the adapter performed its function without impairment of the overall mission.17
More was to come. Despite the pogo effect, the first stage completed its task and the S-II took over. At T + 206 and T + 319 seconds, the performance of engine 2 fluctuated. At T + 412 seconds, the engine shut down. Engine 3 cut off about two seconds later. The control system kept the vehicle stable for the remainder of the burn, 427 seconds or about 58 seconds longer than normal. This resulted in a deviation from the S-II flight pattern, and the third stage had to burn 29 seconds longer.18
In a postlaunch press statement, Phillips acknowledged, "there's no question that it's less than a perfect mission." However, he took comfort in a "major unplanned accomplishment" - the ability of the second stage to lose two engines and still consume its propellants through the remaining engines.19 At the launch site Mueller described the mission as "a good job all around, an excellent launch, and, in balance, a successful mission... and we have learned a great deal... with the Apollo 6 mission."20 The flight had tested altitude control, the navigation and guidance systems in conjunction with the service module engine, and the command module's heat shield. In spite of all difficulties, Apollo 6 had gone into orbit. Nonetheless, Mueller admitted later that Apollo 6 "will have to be defined as a failure."21 The Apollo team set out to find what had gone wrong and why.
A week after the launch, Marshall issued an initial report. In relation to the malfunction of the J-2 engines, there was some speculation that the wires that carried cutoff commands to them had been interchanged. Although the basic source of the difficulties in the second stage had not yet been determined, this at least appeared to explain the premature cutoff. Later the trouble was identified as ruptures in small-diameter fuel lines that fed the engine igniters. The lines were redesigned to eliminate the flexible bellows section where the break occurred; the fix was then verified by tests at the Arnold Engineering Development Center.22
Coordinated plans for the resolution of the Apollo 6 anomalies, presented to the Apollo Program Director in a teleconference 2 May, included the fixes related to pogo. Prior to the launch of the first Saturn V, the longitudinal stability of the vehicle had been analyzed extensively. The results indicated that any pogo effect could be suppressed by detuning the natural frequencies of the propellant feed system and the vehicle structure. NASA had ruled that any modifications to existing hardware must be minimized. Now, from a screening process in which many solutions were considered, the corrective action emerged - it involved filling a series of cavities with helium gas. This required little change in hardware, but effectively changed the Saturn's resonant frequency. On 15 May a review of the oscillation problem determined that the fixes could be verified in an acceptance firing about mid-July. A final decision would be made at a planned August delta design certification review* for AS-503 (Apollo 8). All aspects of the problem were reviewed in June during a day-long teleconference among the Apollo Program Director and his staff, Marshall, Houston, KSC, and contractors. Tests and analyses had demonstrated that the modifications to 503 and subsequent vehicles had dampened the oscillations. The second of the major mechanical obstacles to man-rating had been successfully overcome.23
At the Manned Spacecraft Center, work on the spacecraft lunar module adapter's structural failure was concentrated in two areas: launch vehicle oscillation and spacecraft structures. No provision had been made to vent the honeycomb cells between the inner and outer surfaces of the adapter during launch. Pressures induced by aerodynamic heating of trapped air and free water in the cells could have ripped loose some of the adapter surface during the flight of the first stage. During the summer, North American engineers in Tulsa studied the effects of pressure on unbonded sections of the honeycomb panels. Dynamic tests at Houston verified a mathematical model of the spacecraft. At KSC the adapters for the Apollo 7 and 8 missions were inspected. Minor areas of unbonding were found and corrected. To equalize internal and external pressures during boost, holes were drilled through the adapter surface; and to reduce thermally induced stresses, a layer of thin cork was applied to all areas that had not been previously covered. The additional inspection at KSC and these two modifications were approved for subsequent missions, and as of late September no further changes were anticipated. It was generally agreed that the failure of the adapter had not been directly related to the pogo effect.24
NASA's efforts to resolve the Apollo 6 problems satisfied the Senate Committee on Aeronautical and Space Sciences, which in late April reported that NASA had analyzed the abnormalities of the flight, identified them with dispatch, and undertaken corrective action.