Pogo oscillation is a self-excited type of vibration in liquid-propellant rocket engines caused by combustion instability.^[1] Theunstable combustion results in variations in engine thrust, causing variation in the acceleration exerted upon the vehicle's flexible structure, which inturn causes variations in engine propellant pressure and flow rate, closing the self-excitation cycle.

The name is metaphorical, comparing the longitu­dinalaxis vibration to the bouncing of a pogo stick. Pogooscillation places stress on the vehicle frame, which can be dangerous ifexcessive.‍^[1]

NASA Associate Administrator for Manned SpaceFlight George Mueller explained Apollo 6's pogooscillation to a congressional hearing:‍^[2]

Pogo arises fundamentally because you have thrust fluctuations in the engines. Those are normal characteristics of engines. All engines have what you might call noise in their output because the combustion is not quite uniform, so you have this fluctuation in thrust of the first stage as a normal characteristic of all engine burning.

Now, in turn, the engine is fed through a pipe that takes the fuel out of the tanks and feeds it into the engine. That pipe's length is something like an organ pipe so it has a certain resonance frequency of its own and it really turns out that it will oscillate just like an organpipe does.

The structure of the vehicle is much like a tuning fork, so if you strike it right, it will oscillate up and down longitudinally. In a gross sense it is the interaction between the various frequencies that causes the vehicle to oscillate.

In general, pogo oscillation occurs when a surge in combustion chamber pressure increases back pressure against the fuel coming into the engine. This reduces fuelflow and thus chamber pressure. The reduced chamber pressure in turn reduces backpressure at the fuelpump, causing more fuel to comein and repeating the cycle. Inthis way, a rocket engine experiencing pogooscillations is conceptually operating somewhat like a pulsejet or pulse detonation engine.

If the pulse cycle happens to match a resonance frequency of the rocket, 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 fuelpipes.‍^[3]‍^[4]

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 pogooscillation on the flight of Apollo 6, which damaged the S-II and S-IVB stages and likely would have triggered anabort if the flight had carried acrew. The second stage (S-II) had less-intense pogo on other flights.

The oscillations during Apollo 13's ascent caused the center engine to shutdown about twominutes earlier than planned. The resulting loss in thrust was compensated for by longer burns from the second and third stages.

If the oscillation is left unchecked, failures can result. Onecase occurred in the middle J-2 engine of the second stage (S-II) of the Apollo 13 lunar mission in1970. Inthis case, the engine shut down before the oscillations could cause damage to the vehicle.^[1] Thelater events in this mission, which forced an abort of the planned lunar landing, overshadowed the pogoproblem. Pogo also was experienced in the S-IC first stage of the uncrewed Apollo 6 test flight in1968.‍^[5]

One of the Soviet Union's N1-L3 rocket test flights suffered pogooscillations in the first stage on February21, 1969. The launch vehicle reached initial engine cutoff, but exploded 107seconds after liftoff and disintegrated.^[6] There are other cases during uncrewed launches in the1950s and 1960s where the pogoeffect caused catastrophic launch failures, such as the first Soviet lunar mission, Luna E-1 No.1, and Luna E-1 No.2, in September and October1958.‍^[7]‍^{: 440–446}

Modern vibration analysis methods can account for the pogooscillation to ensure that it is far from the vehicle's resonant frequencies. Suppression methods include damping mechanisms or bellows in propellant lines. The SpaceShuttle mainengines each had adamper in the liquid oxygen line,^[4] but not in the hydrogen fuelline.

• Damping – Influence on an oscillating physical system which reduces or prevents its oscillation
• Feedback – Process where information about current status is used to influence future status
• Resonance – Physical characteristic of oscillating systems
• Slosh dynamics – Movement of liquid inside another moving object
• Vibration analysis – Mechanical oscillations about an equilibrium pointPages displaying short descriptions of redirect targets

• sci.space.shuttle newsgroup discussions ofpogo
• NASA technical paper on flexible propellant lines including pogosuppressors
• 👁 Wiktionary logo
  The dictionary definition of pogo oscillation at Wiktionary