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((short description|Type of vibration in a rocket engine))
'''Pogo oscillation''' is a self-excited [[vibration]] in [[liquid-propellant rocket]] engines caused by [[Rocket engine#Combustion instabilities|combustion instability]].<ref name="Vibrationdata">((cite web |format=PDF-0.96 Mb |url=http://www.vibrationdata.com/Newsletters/October2008_NL.pdf |title=Apollo 13 Pogo Oscillation |author=Tom Irvine |work=Vibrationdata Newsletter |date=October 2008 |pages=2–6 |accessdate=June 18, 2009))</ref>  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 [[Flight control surfaces#Longitudinal axis|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.<ref name="Vibrationdata" /> <!-- The following says absolutely nothing about severity or potential danger: <ref name=nasa-perils-pogo>((cite web |url=http://www.hq.nasa.gov/office/pao/History/SP-350/ch-3-5.html |title=The Perils of Pogo, Ch. 3.5 |work=Apollo Expeditions to the Moon |publisher=[[NASA]] |accessdate=June 17, 2014))</ref> -->

== Origin ==
((More citations needed section|talk=I don't understand this explanation|date=January 2021))
NASA Associate Administrator for Manned Space Flight [[George Mueller (NASA)|George Mueller]] explained [[Apollo 6]]'s pogo oscillation to a congressional hearing:

((blockquote|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 organ pipe 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.<ref name = "moonport 6a">((cite book |last1=Benson |first1=Charles D. |last2=Faherty |first2=William Barnaby |title=Moonport: A History of Apollo Launch Facilities and Operations |url=https://www.hq.nasa.gov/office/pao/History/SP-4204/contents.html |access-date=September 27, 2021 |year=1978 |publisher=NASA |id=NASA SP-4204 |chapter=Two engines out but still running |chapter-url=https://www.hq.nasa.gov/office/pao/History/SP-4204/ch20-3.html |archive-date=January 23, 2008 |archive-url=https://web.archive.org/web/20080123133438/https://www.hq.nasa.gov/office/pao/History/SP-4204/contents.html |url-status=dead )) Ch. 20-3.</ref> ))

In general, pogo oscillation occurs when a surge in combustion chamber [[pressure]] increases [[back pressure]] against the [[fuel]] coming into the engine. This reduces fuel flow and thus chamber pressure. The reduced chamber pressure in turn reduces back pressure at the pump, causing more fuel to come in and repeating the cycle. In this way, a rocket engine experiencing pogo oscillations is conceptually operating somewhat like a [[pulsejet]] or [[pulse detonation engine]]. If the pulse cycle happens to match a [[Resonance|resonance frequency]] of the rocket then dangerous [[oscillation]]s 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.<ref name="prince-lvdesign">((cite web |format=PDF-3.0 Mb |author=Robert Stengel |publisher=[[Princeton University]] |url=http://www.princeton.edu/~stengel/MAE342Lecture4.pdf |title=Launch Vehicle Design: Configurations and Structures  |accessdate=June 18, 2009))</ref><ref name="threshold-pogo">((cite journal|first=Jim |last=Fenwick |date=Spring 1992 |title=Pogo |url=http://www.engineeringatboeing.com/articles/pogo.htm |publisher=Pratt & Whitney Rocketdyne |journal=Threshold |accessdate=September 11, 2009 |url-status=unfit |archiveurl=https://web.archive.org/web/20090113180241/http://www.engineeringatboeing.com/articles/pogo.htm |archivedate=January 13, 2009 ))</ref>

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 [[Apollo abort modes|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 about two minutes earlier than planned. The loss in thrust was compensated by longer burns from the second and third stages.

== Hazard ==
If the oscillation is left unchecked, failures can result.  One case occurred in the middle [[Rocketdyne J-2|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.<ref name="Vibrationdata" />  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.<ref name="nasa-pogo-hsv">((cite web |author=Curtis E. Larsen |format=PDF |publisher=NASA |url=https://ntrs.nasa.gov/api/citations/20080018689/downloads/20080018689.pdf |title=NASA Experience with Pogo in Human Spaceflight Vehicles |accessdate=June 26, 2012))</ref>  One of the [[Soviet Union]]'s [[N1 (rocket)|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.<ref name="German">((cite web |url=http://www.bernd-leitenberger.de/hercules.shtml |title=Die russische Mondrakete N-1 (The Russian moon rocket N-1) |website=www.bernd-leitenberger.de |language=German |accessdate=June 17, 2014))</ref> 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.<ref>((cite web |author=[[Boris Chertok]] |format=PDF |publisher=NASA |url=https://history.nasa.gov/SP-4110/vol2.pdf  |title=Rockets and People, Volume 2: Creating a Rocket Industry |date=2006 |accessdate=February 18, 2021 ))</ref>((rp|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 [[RS-25|Space Shuttle main engine]]s each had a damper in the [[Liquid oxygen|LOX]] line,<ref name=threshold-pogo/> but not in the [[hydrogen]] fuel line.

== See also ==
*[[Slosh dynamics]]

== References ==
((Reflist))

== External links ==
*[http://yarchive.net/space/rocket/pogo.html Sci.space.shuttle newsgroup discussions of pogo]
*[http://articles.adsabs.harvard.edu//full/1977NASSP8123....../0000001.000.html NASA Technical Paper on Flexible Propellant Lines Including Pogo Suppressors]

((DEFAULTSORT:Pogo Oscillation))
[[Category:Spacecraft propulsion]]

[[ja:ロケットエンジン#Pogo振動]]

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