Getting Rid of Pogo

After a nearly flawless maiden flight in November, the Saturn 5 moon rocket ran into so many difficulties during its second mission last month that NASA officials feared yet another unmanned flight would be necessary before the rocket could be trusted to carry astronauts into space. Now, after a careful review of the troubles that cropped up in flight, NASA has decided that it can probably correct them all and make Saturn 5 safe enough to carry a manned Apollo spacecraft into orbit this November or December. By eliminating another unmanned test of the huge rocket, NASA would save about $280 million and avoid further delays in its program to place U.S. astronauts on the moon in 1969.

The first of Saturn's obvious troubles, the unexpected and early shutdown of two of the five J-2 engines powering the second stage (TIME, April 12), was traced to a fuel line that broke under the strain of liquid hydrogen flowing through it at approximately 100 m.p.h. The break set off a chain of events that lowered pressure in the engine, which automatically shut down. Because of an incorrectly wired circuit, the shutdown on the first engine sent a signal to another, perfectly operating engine, erroneously ordering it to shut down also.

Jarred Astronauts. Failure of the third-stage rocket engine to restart later in the mission was tentatively traced to a broken line that supplied hydrogen to the ignition system. Without an ignition flame, the engine could not be restarted. To reduce the possibility of future breaks in the stainless-steel fuel lines, flexible joints in the lines will be either eliminated or greatly strengthened.

NASA scientists at the same time revealed a previously undisclosed problem during the flight: the "Pogo Effect," an up-and-down, pogo-stick-like vibration that was also detected during Saturn 5's initial flight. The effect, caused by the synchronous pulsing of all five first-stage F-1 engines at their natural resonant frequency, produced a vibrational force of three-tenths Gs during the first flight--enough to jar astronauts had they been aboard, but not enough to cause any serious difficulty. On the April shot, however, the Pogo Effect reached a force of seven-tenths of a G, enough vibration so that astronauts would not be able to read instruments or flick control switches.

Helium into LOX. To rid Saturn of Pogo, scientists may inject small amounts of liquid helium into the liquid oxygen (LOX) lines of two of the engines, damping the sloshing of the LOX and thus changing the resonant frequencies of the engines. Or they may place gas-filled cavities beneath the LOX lines of two engines to act as shock absorbers. Either solution would cause two engines to pulse at different frequencies from the others, preventing a five-engine resonant buildup of the Pogo Effect.

Another worrisome problem, the apparent breaking away of two sheet-metal sections (photographed by highflying jets) early in the flight, was apparently solved when NASA investigators decided that the objects were probably thick sheets of paint that had not been properly bonded to Saturn's metal skin.

The final decision to place a manned Apollo spacecraft atop the next flight of Saturn 5 was made by NASA Administrator James Webb after he had evaluated the recommendations of top space scientists. To suggestions that his judgment might be influenced either by money, or political considerations, or the space race with Russia, Webb had a ready answer: "My instructions from President Kennedy and President Johnson have always been, and remain, to go when we were ready and not to go until we were ready."

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