Although flight testing the Saturn launch vehicles went remarkably well, there were problems, some of which involved the upper stages. For example, on April 4, 1968, during the launch of AS-502 (Apollo 6), there was “an all-important dress rehearsal for the first manned flight" planned for AS-503. Stage-two separation occurred, and all five J-2 engines ignited. Then, at 319 seconds after launch, there was a sudden 5,000-pound decrease in thrust, followed by a
The second (S-II) stage of the Saturn V launch vehicle being lifted onto the A-2 test stand at the Mississippi Test Facility (later the Stennis Space Center) in 1967, showing the five J-2 engines that powered this stage. (Photo courtesy of NASA)
cutoff signal to the number two J-2 engine. This signal shut down not only engine number two but number three as well (about a second apart). It turned out that signal wires to the two engines had been interchanged. This loss of the power from two engines was a severe and unexpected test for the instrument unit (IU), but it adjusted the trajectory and the time of firing (by about a minute) for the remaining three engines to achieve (in fact, exceed) the planned altitude for separation of the third stage.73
When the IU shut down the three functioning engines in the S-II and separated it from the S-IVB, that stage’s lone J-2 ignited and placed itself, the instrument unit, and the payload in an elongated parking orbit. To do this, the IU directed it to burn 29.2 seconds longer than planned to further compensate for the two J-2s that had
cut off in stage two. The achievement of this orbit demonstrated “the unusual flexibility designed into the Saturn V." However, although the vehicle performed adequately during orbital coast, the J-2 failed to restart and propel the spacecraft into a simulated translunar trajectory. After repeated failures to get the J-2 to restart, mission controllers separated the command and service modules from the S-IVB, used burns of the service module’s propulsion system to position the command module for reentry tests, and performed these tests to verify the design of the heat shield, with reentry occurring “a little short of lunar space velocity," followed by recovery. Although this is sometimes counted a successful mission (in which Phillips and von Braun both said a crew could have returned safely), von Braun also said, “With three engines out, we just cannot go to the Moon." And in fact, restart of the S-IVB’s J-2 was a primary objective of the mission, making it technically a failure.74
A team of engineers from Marshall and Rocketdyne attacked the unknown problem that had caused the J-2 engine failures. (It turned out to be a single problem for two engines that had failed, one in stage two and the one in stage three that would not restart.) The team, which included Jerry Thomson from the F-1 combustion – instability effort, examined the telemetry data from the flight and concluded that the problem had to be a rupture in a fuel line. But why had it broken?
Increasing pressures, vibrations, and flow rates on test stands, computer analyses, and other tests led engineers to suspect a bellows section in the fuel line. To allow the line to bend around various obstructions, this area had a wire-braid shielding. On the test stand it did not break from the abnormal strains to which it was subjected. (Artificially severing the line did produce measurements that duplicated those from the flight, however.) Finally, Rocketdyne test personnel tried it in a vacuum chamber simulating actual conditions in space. Eight lines tested there at rates of flow and pressures no greater than during normal operations led to failures in the bellows section of all eight lines within 100 seconds. Motion pictures of the tests quickly revealed that in the absence of atmospheric moisture in the vacuum chamber (and in space), frost did not form inside the wire braiding as it had in regular ground tests during cryogenic liquid-hydrogen flow. The frost had kept the bellows from vibrating to the point of failure, but in its absence, a destructive resonance occurred. Engineers eliminated the bellows and replaced them with a stronger design that still allowed the necessary bends. Testing of the fuel-line redesign on the J-2 at the Mississippi Test Facility in August 1968 showed that this change had solved the problem.75
The successful Apollo 8 mission around the Moon verified the success of all the modifications to the launch vehicle since AS-502, with all launch-vehicle objectives for the mission achieved. AS-504 for Apollo 9 was the first Saturn V to use five 1.522-pound-thrust engines in stage one and six 230,000-pound-thrust J-2 engines in the upper stages. It had minor problems with rough combustion but was successful. The Saturn V for AS-505 (Apollo 10) and all subsequent Apollo missions through Apollo 17 (the final lunar landing) used F-1 and J-2 engines with the same thrust ratings as AS-504. There were comparatively minor adjustments in the launch vehicles that followed AS-505—“in timing, sequences, propellant flow rates, mission parameters, trajectories." On all missions there were malfunctions and anomalies that required fine-tuning. For example, evaluations of the nearly catastrophic Apollo 13 flight showed that oscillations in the S-II’s feed system for liquid oxygen had resulted in a drop in pressure in the center engine’s plumbing to below what was necessary to prevent cavitation in the liquid-oxygen pump. Bubbles formed in the liquid oxygen, reducing pump efficiency, hence 204 thrust from the engine. This led to automatic engine shutdown.
Chapter 5 Although the oscillations remained local, and even engine shutdown did not hamper the mission, engineers at the Space Division of North American Rockwell (as the firm had become following a merger with Rockwell Standard) nevertheless developed two modifications to correct the problem. One was an accumulator. It served as a shock absorber, consisting of a “compartment or cavity located in the liquid oxygen line feeding the center engine." Filled with gaseous helium, it served to dampen or cushion the pressures in the liquid-oxygen line. This changed the frequency of any oscillation in the line so that it differed from that of the engines as a whole and the thrust structure, thus prevented coupling, which had caused the problem in Apollo 13. As a backup to the accumulator, engineers installed a “G" switch on the center engine’s mounting beam consisting of three acceleration switches that tripped in the presence of excessive low-frequency vibration and shut off the center engine. With these modifications, the J-2 and Saturn V were remarkably successful on Apollo 14 through 17.76