{{short description|Type of vibration in a rocket engine}} {{other uses|Pogo (disambiguation){{!}}Pogo}} {{Use mdy dates|date=March 2026}} {{Use American English|date=March 2026}} '''Pogo oscillation''' is a {{nowr|self-excited}} type of vibration in liquid-propellant rocket engines caused by combustion instability.<ref name="Vibrationdata">{{cite periodical |url=https://www.vibrationdata.com/Newsletters/October2008_NL.pdf |title=Apollo 13 Pogo Oscillation |first=Tom |last=Irvine |work=Vibrationdata Newsletter |date=October 2008 |pages=2–6 |accessdate=June 18, 2009 |archive-url=https://web.archive.org/web/20250103101909/www.vibrationdata.com/Newsletters/October2008_NL.pdf |archive-date=January 3, 2025 |url-status=live }}</ref> The{{nbsp}}unstable combustion results in variations in engine thrust, causing variation in the acceleration exerted upon the vehicle's flexible structure, which in{{nbsp}}turn causes variations in engine propellant pressure and flow rate, closing the {{nowr|self-excitation}} cycle.
The name is metaphorical, comparing the {{shy|longitu|dinal}}{{nbsp}}axis vibration to the bouncing of a {{nowr|pogo stick}}. Pogo{{nbsp}}oscillation places stress on the vehicle frame, which can be dangerous if{{nbsp}}excessive.{{zwj}}<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 == NASA Associate Administrator for Manned Space{{nbsp}}Flight George Mueller explained {{nowr|Apollo 6's}} pogo{{nbsp}}oscillation to a congressional hearing:{{zwj}}<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 (20-3) |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 }}</ref>
{{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 {{nowr|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{{nbsp}}pipe does.
The structure of the vehicle is much like a {{nowr|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 {{nowr|back pressure}} against the fuel coming into the engine. This reduces fuel{{nbsp}}flow and thus chamber pressure. The reduced chamber pressure in turn reduces back{{nbsp}}pressure at the fuel{{nbsp}}pump, causing more fuel to come{{nbsp}}in and repeating the cycle. In{{nbsp}}this way, a rocket engine experiencing pogo{{nbsp}}oscillations 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 fuel{{nbsp}}pipes.{{zwj}}<ref name="prince-lvdesign">{{cite web |first=Robert |last=Stengel |date=n.d. |publisher=Princeton University |url=http://www.princeton.edu/~stengel/MAE342Lecture4.pdf |title=Launch Vehicle Design: Configurations and Structures |accessdate=June 18, 2009 |archive-url=https://web.archive.org/web/20121013131406/www.princeton.edu/~stengel/MAE342Lecture4.pdf |archive-date=October 13, 2012 |url-status=deviated }}</ref>{{zwj}}<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 {{nowr|Titan II}} first stage during its development, which delayed {{nowr|man-rating}} the rocket for the Gemini program. {{nowr|The Saturn V}} first stage {{nowr|(S-IC)}} experienced severe pogo{{nbsp}}oscillation on the flight of {{nowr|Apollo 6}}, which damaged the {{nowr|S-II}} and {{nowr|S-IVB}} stages and likely would have triggered an{{nbsp}}abort if the flight had carried a{{nbsp}}crew. The second stage {{nowr|(S-II)}} had {{nowr|less-intense}} pogo on other flights.
The oscillations during {{nowr|Apollo 13's}} ascent caused the center engine to shut{{nbsp}}down about two{{nbsp}}minutes earlier than planned. The resulting loss in thrust was compensated for by longer burns from the second and third stages.
== Hazard == If the oscillation is left unchecked, failures can result. One{{nbsp}}case occurred in the middle {{nowr|J-2 engine}} of the second stage {{nowr|(S-II)}} of the {{nowr|Apollo 13}} lunar mission in{{nbsp}}1970. In{{nbsp}}this case, the engine shut down before the oscillations could cause damage to the vehicle.<ref name="Vibrationdata" /> The{{nbsp}}later events in this mission, which forced an abort of the planned lunar landing, overshadowed the pogo{{nbsp}}problem. Pogo also was experienced in the {{nowr|S-IC}} first stage of the uncrewed {{nowr|Apollo 6}} test flight in{{nbsp}}1968.{{zwj}}<ref name="nasa-pogo-hsv">{{cite report |first=Curtis E. |last=Larsen |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 |archive-url=https://web.archive.org/web/20250711104807/ntrs.nasa.gov/api/citations/20080018689/downloads/20080018689.pdf |archive-date=July 11, 2025 |url-status=live }}</ref>
One of the {{nowr|Soviet Union's}} {{nowr|N1-L3 rocket}} test flights suffered pogo{{nbsp}}oscillations in the first stage on February{{nbsp}}21, 1969. The launch vehicle reached initial engine cutoff, but exploded 107{{nbsp}}seconds after liftoff and disintegrated.<ref name="German">{{cite web |url=https://www.bernd-leitenberger.de/hercules.shtml |title=Die russische Mondrakete N-1 |trans-title=The Russian moon rocket N-1 |website=Bernd-Leitenberger |language=de |accessdate=June 17, 2014 |archive-url=https://web.archive.org/web/20150203053834/www.bernd-leitenberger.de/hercules.shtml |archive-date=February 3, 2015 |url-status=live }}</ref> There are other cases during uncrewed launches in the{{nbsp}}1950s and 1960s where the pogo{{nbsp}}effect caused catastrophic launch failures, such as the first Soviet lunar mission, {{nowr|Luna E-1 No.1}}, and {{nowr|Luna E-1 No.2}}, in September and October{{nbsp}}1958.{{zwj}}<ref>{{cite book |first=Boris |last=Chertok |author-link=Boris Chertok |publisher=NASA |url=https://www.nasa.gov/wp-content/uploads/2023/04/sp-4110-vol2.pdf |title=Rockets and People |volume=2: Creating a Rocket Industry |date=2006 |accessdate=February 18, 2021 |archive-url=https://web.archive.org/web/20250207012829/www.nasa.gov/wp-content/uploads/2023/04/sp-4110-vol2.pdf |archive-date=February 7, 2025 |url-status=live }}</ref>{{zwj}}{{rp|440–446}}
Modern vibration analysis methods can account for the pogo{{nbsp}}oscillation to ensure that it is far from the vehicle's resonant frequencies. Suppression methods include damping mechanisms or bellows in propellant lines. The Space{{nbsp}}Shuttle main{{nbsp}}engines each had a{{nbsp}}damper in the liquid oxygen line,<ref name=threshold-pogo /> but not in the hydrogen fuel{{nbsp}}line.
== See also == * {{anl|Damping}} * {{anl|Feedback}} * {{anl|Resonance}} * {{anl|Slosh dynamics}} * {{anl|Vibration analysis}}
== References == {{Reflist}}
== External links == * sci.space.shuttle [https://yarchive.net/space/rocket/pogo.html newsgroup discussions of{{nbsp}}pogo] * [https://articles.adsabs.harvard.edu//full/1977NASSP8123....../0000001.000.html NASA technical paper on flexible propellant lines including pogo{{nbsp}}suppressors] * {{wiktionary inline}}
{{DEFAULTSORT:Pogo Oscillation}} Category:Apollo program Category:Metaphors referring to objects Category:Project Gemini Category:Resonance Category:Space program of the Soviet Union Category:Space Shuttle program Category:Spacecraft propulsion Category:Titan (rocket family)