Bad Vibrations Nearly Derail the Lunar Landing Schedule | This Week In Space History

by dave dooling

Forty-five years ago this week, bad vibrations nearly derailed America’s Moon landing time table as second Saturn V rocket was launched. Two separate but similar problems, made the Apollo 6 mission less than perfect, but were solved in time to keep Apollo on track.

“For two minutes everything looked like a repeat of the first Saturn V’s textbook performance,” said Wernher von Braun, director of Marshall Space Flight Center where the Saturn V was managed. “Then a feeling of apprehension rolled through the launch control center when, around the 125th second, telemetered signals from accelerometers indicated an apparently mild Pogo vibration.”

Everyone expected a confidence-building repeat of Apollo 4 on Nov. 9, 1967, the comeback mission that revived morale after the loss of the Apollo 1 crew just 9 months earlier. Apollo 4 was a daring “all up” test of the Saturn V, leap-frogging ten incremental flights to put the program back on track for a manned Moon landing “before this decade is out,” as President Kennedy had challenged in 1961. NASA flight rules then required at least two successful missions before permitting humans to ride a new vehicle. This is important because success can be as much an accident as failure.

Apollo 6 (AS-502, in NASA’s nomenclature) not only would validate the Saturn V design but test the redesigned Apollo spacecraft with a re-entry simulating a return from the Moon.

Liftoff on April 4, 1968, was perfect. Problems appeared as the S-IC stage, powered by five F-1 engines burning liquid oxygen and a special kerosene called RP-1, neared the end of its 135-second run. In the last 10 seconds the whole rocket was pulsing up and down, five times a second, like a massive pogo stick.

Most automobile drivers have experienced a similar effect with unbalanced tires. At the right speed the imbalance resonates with part of the car, giving the impression that it will shake apart. So, too, with liquid rockets. Some oscillation is normal, George Mueller, NASA Associate Administrator for Manned Space Flight, later explained to a Congressional committee, and the stage’s five oxygen lines acted like organ pipes. Initially the full propellant load dampened the vibrations. But as propellant drained, conditions shifted so the entire vehicle oscillated, “much like a tuning fork.”

Not only were the pogo oscillations strong enough to endanger the crew, but the adapter that would cover the fragile Lunar Module shed insulation, yet held together.

Before the pogo became too rough, the first stage shut down on schedule and the S-II second stage separated and ignited.

Again, all seemed well. Four and a half minutes into their burn, one of the J-2 engines faltered and shut down. Inexplicably, a healthy J-2 shut down a second later. Still, Apollo 6 chugged on as the guidance system compensated and ran the remaining three engines overtime to compensate. At last, the S-II separated and the S-IVB third stage put itself and the Apollo spacecraft into low Earth orbit.

Two orbits later it was commanded to fire again like a restart for the Moon and send Apollo high enough to simulate a return from the Moon. Nothing happened, several times.

Finally, NASA separated the Apollo spacecraft and put it through its paces, ending in a successful re-entry and splashdown. “Had the flight been manned, the astronauts would have returned safely,” von Braun emphasized afterward, “but the flight clearly left a lot to be desired. With three engines out, we just cannot go to the Moon.”

NASA and its contractor teams soon were crawling through reams of data, a validation of why spacecraft are so heavily instrumented. It was forensic engineering on a grand scale.

Two problems were easily resolved. The prevalve for the F-1 engine was a large casting. Partly filling it with helium, already part of the stage’s plumbing, changed it into a shock absorber and detuned Mueller’s “tuning fork.” The shutdown of the No. 3 engine on the S-II stage was caused by a wiring mistake that closed its oxidizer valve at the same time as the fuel valve on engine No. 2.

Now the detective work became more challenging. Why did the No. 2 engine and the third-stage engine fail in a similar manner? Again, telemetry provided enough forensic clues. Falling temperatures outside the engine indicated that cold gas was flowing freely, followed by a burst of hot gas an instant before shutdown. This indicated that a hydrogen igniter line had leaked, ruptured, and ignited.

Engineers retested parts under gradually more rigorous conditions until they reproduced the failure. The igniter line had a bellows section to allow some flex as temperatures changed. In ground tests, air around the bellows liquefied on the bellows and dampened vibrations. In the vacuum of space the bellows resonated until it cracked. The bellows was replaced with bends in the fuel line to do the job properly.

By August 1968, the fixes were confirmed with static engine tests at Bay St. Louis, Mississippi. With the bad vibrations smoothed by two small design changes, NASA again dared to look into the distance.

“There is even a remote possibility of a spectacular swing around the Moon by the manned spacecraft,” von Braun said in the autumn. “That a mission as bold as the last is even considered, for the first Saturn V to be manned, bespeaks planners’ confidence that all about it has been set aright.”

Four months later, the crew of Apollo 8 would celebrate Christmas as they orbited the Moon after being launched by the third Saturn V.

Dave Dooling is education director at the New Mexico Museum of Space History. He is a former space journalist and past recipient of the National Space Club’s Press Award and Goddard History Essay Award.

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