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I've seen the term "burp" used, for example, in a description of the Saturn V F-1 engine start sequence (or what is perhaps the source document - see pdf p 62, source page 4-9). What does it mean? What causes it to happen?

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    $\begingroup$ When I watched the original Apollo launches on TV, I never expected I'd be downloading a Saturn V flight manual on my telephone some day. $\endgroup$ – uhoh Aug 28 '16 at 3:17
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"Burping" usually refers to a start-up failure mode in which the combustion chamber pressure rises too quickly relative to the pressure of the inflowing propellants (as the pumps spin up, for example). This can cause the feed system to backflow with undesirable effects upstream. Page 77 of this document contains an example of someone using the term in that fashion.

That said, it's not clear to me how ignition of the gas generator exhaust in the engine bell of the F1, as quoted in your link, would help to prevent that. The link seems to have quoted from and slightly changed an Apollo-era diagram found in Stages to Saturn, page 111. In the original it reads

Fuel-rich turbine combustion gas is ignited by flame from igniters.

a) Ignition of this gas prevents backfiring and burping

In this case I believe the writer is referring to the combustion chamber of the gas generator (which could, of course, be affected by "burping").

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    $\begingroup$ I added a link to what may be the source document I had also found. I agree, my first reference appears to contain a few errors when compared with the "flight manual". $\endgroup$ – Anthony X Aug 27 '16 at 22:04
  • $\begingroup$ Thanks! That's a great document. Appears to be the source for the Stages to Saturn diagram as well. I wondered if the "arrows" in the Stages to Saturn diagram were references to a schematic or timeline, and this has that diagram! $\endgroup$ – Organic Marble Aug 27 '16 at 22:09
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    $\begingroup$ This link let's you see the PDF link first without forcing the download https://dspace.mit.edu/handle/1721.1/82764#files-area. $\endgroup$ – uhoh Aug 28 '16 at 3:17
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    $\begingroup$ (WAG): Isn't the gas generator exhaust fed into the nozzle about halfway down, i.e. far downstream of the main injection point? If you leave the GG exhaust unignited, you will build up a pocket of unignited gas in the nozzle during startup. When the flame front from the main ignition hits this pocket, the GG exhaust would ignite all at once. $\endgroup$ – Hobbes Aug 28 '16 at 7:52
  • $\begingroup$ Sounds completely plausible to me! $\endgroup$ – Organic Marble Aug 28 '16 at 12:05

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