When NASA first began launching rockets into space – barely half a century after the first airplane took flight – it encountered what it calls the “pogo problem”.
During the launch of Saturn V rocket’s first stage burn, part of the uncrewed Apollo 6 mission, the rocket experienced oscillations along its length, like springs going up and down on a pogo stick.
“Pogo occurred when a partial vacuum in the fuel and oxidizer feed lines reached the engine firing chamber causing the engine to skip,” John Uri of NASA’s Johnson Space Center explains. “These oscillations then traveled up the axis of the launch vehicle resulting in intense vibration in the Command Module and causing some superficial structural damage to the Spacecraft Lunar Module Adaptor (SLA). Had a crew been onboard, they would have experienced severe vibrations and even possible injury.”
The problem had occurred in previous rocket launches, including Titan II used for the Gemini program. Though it might not sound like a huge deal, and crews carried in the Command Module could have been perfectly safe, it led Marshall Space Flight Center Director Wernher von Braun to conclude that unless it was solved “we just cannot go to the Moon.”
NASA created a Pogo Working Group to examine the problem and come up with solutions, concluding that they should “detune” the engine and change the frequency of the vibration by filling prevalve cavities on the liquid oxygen feed lines with helium.
“Injecting helium into those lines prior to ignition would effectively work as a shock absorber to prevent the oscillations from traveling up and down fuel and oxidizer feed lines,” Uri explains.
This worked well enough, and soon NASA was able to send astronauts to the Moon. However, the pogo effect continued to pose problems for the space agency, notably during the near-catastrophic Apollo 13 mission.
“During the second stage burn, two episodes of pogo occurred on the center J-2 engine as expected from previous missions, but the third occurrence diverged severely and acceleration at the engine attachment reached an estimated 34 g’s (the accelerometer went out of range) before the engine’s combustion chamber low-level pressure sensor commanded a shut down,” NASA explains in a report on pogo in spaceflight. “It was estimated in the post-flight investigation that only one more cycle of amplitude growth could have been sustained without catastrophic structural failure.”
Following this second incident, the least of Apollo 13’s problems, NASA installed a pogo suppressor on all subsequent Apollo missions. After this, pogo was not experienced, though a small buzz occurred for a few seconds during second-stage burns.
Source Link: How NASA Solved The "Pogo Problem" And Got Humans To The Moon