Pogo oscillation

Pogo oscillation is a self-excited vibration in liquid-propellant rocket engines caused by combustion instability.[1] The unstable combustion results in variations of engine thrust, causing variations of acceleration on the vehicle's flexible structure, which in turn cause variations in propellant pressure and flow rate, closing the self-excitation cycle. The name is a metaphor comparing the longitudinal vibration to the bouncing of a pogo stick. Pogo oscillation places stress on the frame of the vehicle, which in severe cases can be dangerous.[1]

Origin

In general, pogo oscillation occurs when a surge in combustion chamber pressure (Pc) increases back pressure against the fuel coming into the engine, reducing Pc, causing more fuel to come in and increasing Pc again. In this way, a rocket engine experiencing pogo oscillations is conceptually operating more like a pulsejet or pulse detonation engine. If the pulse cycle happens to match a resonance frequency of the rocket then dangerous oscillations can occur through positive feedback, which can, in extreme cases, tear the vehicle apart. Other situations that can induce fuel pressure fluctuations include flexing of fuel pipes.[2]

Another situation producing pogo oscillation occurs when the engine is moving longitudinally with fluctuating speed. Owing to inertia, if the speed of the vehicle suddenly increases, the fuel inside the fuel tank tends to "fall behind" and is forced into the turbopump, a situation somewhat similar to the slosh of liquid inside a tanker. This creates excess pressure to the turbopump and causes unintended excessive fuel to be delivered. This in turn creates excessive thrust and causes the vehicle to accelerate which leads to further increase in turbopump pressure and an unintended increase in fuel delivery. This can set up a vicious circle, and can result in structural failure in the vehicle.[3]

Pogo oscillation plagued the Titan II first stage during its development, which delayed man-rating the rocket for the Gemini program. The Saturn V first stage (S-IC) experienced severe pogo oscillation on the flight of Apollo 6, which damaged the S-II and S-IVB stages above and likely would have triggered an abort if the flight had carried a crew. The second stage (S-II) had less intense pogo on other flights. The oscillations during Apollo 13's ascent caused the center engine to shut down much earlier than planned.

Hazard

If the oscillation is left unchecked, failures can result. One case occurred in the middle J-2 engine of the second stage, S-II, of the Apollo 13 lunar mission in 1970. In this case, the engine shut down before the oscillations could cause damage to the vehicle.[1] Later events in this mission (an oxygen tank exploded two days later) overshadowed the pogo problem. Pogo also had been experienced in the S-IC first stage of the uncrewed Apollo 6 test flight in 1968.[4] One of the Soviet Union's N1-L3 rocket test flights suffered pogo oscillations in the first stage on February 21, 1969. The launch vehicle reached initial engine cutoff, but exploded 107 seconds after liftoff and disintegrated.[5] There are other cases during uncrewed launches in the 1950s and 1960s where the pogo effect caused catastrophic launch failures, such as the first Soviet spacecraft to the moon Luna E-1 No.1 and Luna E-1 No.2 in September and October 1958.[6]: 440–446 

Modern vibration analysis methods can account for the pogo oscillation to ensure that it is far away from the vehicle's resonant frequencies. Suppression methods include damping mechanisms or bellows in propellant lines. The Space Shuttle main engines each had a damper in the LOX line,[3] but not in the hydrogen fuel line.

See also

References
  1. Tom Irvine (October 2008). "Apollo 13 Pogo Oscillation" (PDF-0.96 Mb). Vibrationdata Newsletter. pp. 2–6. Retrieved June 18, 2009.
  2. Robert Stengel. "Launch Vehicle Design: Configurations and Structures" (PDF-3.0 Mb). Princeton University. Retrieved June 18, 2009.
  3. Fenwick, Jim (Spring 1992). "Pogo". Threshold. Pratt & Whitney Rocketdyne. Archived from the original on January 13, 2009. Retrieved September 11, 2009.{{cite journal}}: CS1 maint: unfit URL (link)
  4. Curtis E. Larsen. "NASA Experience with Pogo in Human Spaceflight Vehicles" (PDF). NASA. Retrieved June 26, 2012.
  5. "Die russische Mondrakete N-1 (The Russian moon rocket N-1)". www.bernd-leitenberger.de (in German). Retrieved June 17, 2014.
  6. Boris Chertok (2006). "Rockets and People, Volume 2: Creating a Rocket Industry" (PDF). NASA. Retrieved February 18, 2021.
This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.