I'm reading a public document on the powered explicit guidance algorithm used in the space shuttle.

The document suggests that PEG ascent maneuvers always involved at least three phases:

  1. Constant thrust SSME burn
  2. Constant acceleration SSME burn
  3. Constant thrust OMS burn

I expected the first two phases, but the third caught me by surprise. It seems the OMS thrusters would fire just before orbital insertion?

But why not just do the whole PEG ascent maneuver on the SSMEs? Was it for ullage control, maybe? Any idea how long the OMS thrusters would burn for in this maneuver?



1 Answer 1


You don't say how old the document is, but I suspect it's old, even for shuttle.

In the early days, shuttle missions executed an OMS-1 burn to raise the apogee, followed by an OMS-2 burn to circularize the orbit by raising the perigee.

I suspect your document refers to OMS-1.

To answer the part about burn duration, on STS-1 OMS-1 was 86.1 seconds. You can get the numbers for any flight from the Space Shuttle Missions Summary.

Later in the program, "direct insertion" ascent profiles were flown, obviating the need for OMS-1. The burn was retained in procedures and could have been executed if there was a performance problem during ascent.

Further reading:

  • $\begingroup$ Oh wow. Yeah, 1973 is the year. It's still an excellent overview of PEG, and the only one I've found that I can follow, but I see now that it made room for engine burns that wound up not being necessary. Thanks for clarifying! $\endgroup$
    – user39728
    Mar 24, 2021 at 16:21
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    $\begingroup$ @user39728 direct insertion resulted in a higher apogee and no requirement for OMS-1; the OMS-2 burn was still required to raise the perigee and circularize the orbit. Let me look for some numbers. $\endgroup$ Mar 24, 2021 at 16:37
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    $\begingroup$ @user39728 I have a document that, oddly, starts with STS-4, no idea why. It gives the orbit for that mission post OMS-1 as 130x34, post OMS-2 130x130 (nautical miles) $\endgroup$ Mar 24, 2021 at 16:47
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    $\begingroup$ Oh, I see. OMS-1 would raise the apogee and OMS-2 would circularize the orbit. So if you do direct insertion to higher apogee, you can skip OMS-1 but not OMS-2 since you still need to circularize. Thanks for clarifying! $\endgroup$
    – user39728
    Mar 24, 2021 at 17:03
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    $\begingroup$ It's given as altitude of apogee x altitude of perigee. This was a very early mission, non rendezvous. For an ISS mission like STS-130 OMS-2 might not circularize - it went 123x85 Post OMS-1, 123x109 Post NC-1, a lot of burns, working up to 187x183 for the final rndz burn. Suggest you read ntrs.nasa.gov/citations/20110023479 for super detailed info on how that all worked. $\endgroup$ Mar 24, 2021 at 17:20

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