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We have acronyms for main engine cutoff (MECO) and secondary engine cutoff (SECO). Do we have an acronym for secondary engine startup---SESU maybe?

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Since you tagged this with , we can simply look at the webcasts and the mission timelines SpaceX publishes. Second stage engine start is called SES if there is only one, and SES-1, SES-2, etc. if there is more than one.

The first one, however, is normally not called out. It is simply part of the stage separation sequence. So, you will never actually hear "SES" or "SES-1" called out explicitly. You will only hear "SECO" if there is only one firing of the second stage, or "SECO-1", "SES-2", and "SECO-2" if there is a relight.

So, technically speaking, the answer should really be "There is no such term for the first firing of the engine", however since the second firing of the engine is called SES-2, we can infer that the first is probably SES-1 or simply SES if there is no SES-2.

Here is an example from the live stream for the SiriusXM SXM-7 mission. I cued up the video right before SES-2 and SECO-2:

However, this only applies to SpaceX. Other launch providers use different terminology, for example BECO instead of MECO.

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    $\begingroup$ more BECO sightings: What is BECO? (Gemini) Same as MECO? and in answer(s) to Why is fuel ratio different for upper stage of a rocket? $\endgroup$ – uhoh Dec 28 '20 at 1:24
  • $\begingroup$ Thank you both! I had no idea MECO also went by BECO. Booster engine actually sounds cooler than main engine. Ha ha. Jorg, if I wanted to refer to main engine start up, would MES be right, then? And if I wanted to refer to a specific main engine, then would I call it out as MES-1, MES-2, etc? Thanks again. $\endgroup$ – user36480 Dec 28 '20 at 3:11
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    $\begingroup$ @Alex: You can call it anything you like. You asked specifically about the Falcon 9, and there the answer is: SpaceX doesn't call it anything. The only time the main engines relight is for landing, but those are not called MES-2, etc. They are called boostback burn startup, entry burn startup and landing burn startup. $\endgroup$ – Jörg W Mittag Dec 28 '20 at 9:31
  • $\begingroup$ @Jorg, The point is to call it something others will understand, not to invent my own acronyms---hence the question. I wanted to know if there was some name or acronym others in industry use to refer to these events. Sure, I can call it what I like. If I was inclined to do that, would I have bothered with the question in the first place? I probably mean to use a standard term if one exists, no? $\endgroup$ – user36480 Dec 28 '20 at 18:49

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