Pogo oscillation
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Pogo oscillation is the term for a potentially dangerous type of oscillation found in rocket engines. This oscillation results in variations of thrust from the engines, generally caused by variations in fuel flow rate, placing stress on the frame of the vehicle. Although the term is frequently written POGO, it is not an acronym, but in fact a reference to the bouncing of a pogo stick.
There are several causes of pogo oscillation. The main one is when a surge in engine pressure increases back pressure against the fuel coming into the engine, reducing engine pressure, causing more fuel to come in and increasing engine pressure again. If the cycle happens to match a resonant frequency of the rocket then dangerous oscillations can occur through positive feedback, which can in extreme cases tear the vehicle apart.
It is possible to suppress pogo oscillation by including damping mechanisms, consisting of chambers containing baffles, in the propellant fuel lines. The Space Shuttle Main Engines each have a damper in the LOX line, but not in the hydrogen fuel line.
The most famous case occurred in the middle engine of the second stage of the Apollo 13 lunar mission. Fortunately in this case the engine shut down before the oscillations could cause damage to the vehicle. Later events in this mission overshadowed the Pogo problem. Pogo was also the cause of some of the problems experienced by Apollo 6.
External links
- Sci.space.shuttle newsgroup discussions of Pogo (http://yarchive.net/space/rocket/pogo.html)
- The Perils of Pogo (http://www.hq.nasa.gov/office/pao/History/SP-350/ch-3-5.html)de:Pogoeffekt