The Ars Technica video Astronaut Scott Kelly teaches orbital mechanics with Kerbal Space Program (also in YouTube) is a little bit humorous.

When Scott Kelley is talking about orbit circularization he says (per the on-screen captioning):

Actually the Space Shuttle has two Ohms burns generally.

Question: What are Ohms burns, and why are two needed?

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Ars Technica's Lee Hutchinson sits down with astronaut Scott Kelly while they play Kerbal Space Program. Scott Kelly uses his experiences on the real International Space Station to give his opinion on the accuracy of the video game.

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    $\begingroup$ I'm surprised to hear Kelly say that. The OMS-1 burn was deleted for STS-41C and subsequent when the "Direct Insertion" technique became standard. During his entire astronaut career OMS-1 burns weren't performed. Perhaps spending 95.5% of a year in space affects the memory :) $\endgroup$ Commented Aug 3, 2018 at 0:19
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    $\begingroup$ wow we just made the same correction! $\endgroup$
    – uhoh
    Commented Aug 3, 2018 at 0:27
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    $\begingroup$ I wanted to watch the video! Thanks to the youtube link I found it. $\endgroup$ Commented Aug 3, 2018 at 0:38
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    $\begingroup$ @OrganicMarble, ya gotta remember that, of all the Space Shuttle Ascents Scott has "flown" over the years, well over 99% of them were in the simulator. And, since simulator ascents were virtually never nominal (normal), a goodly percentage of them involved an OMS-1 burn...sort of "sticks in the memory!" $\endgroup$
    – Digger
    Commented Aug 4, 2018 at 16:09
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    $\begingroup$ Great points @Digger! $\endgroup$ Commented Aug 4, 2018 at 16:12

2 Answers 2


Since the questioner also asks "why are two needed" and the other answer didn't address that:

Early shuttle missions flew a "standard insertion" ascent. This required two burns of the Orbital Maneuvering System after the main engines shut down and the external tank was jettisoned. The first burn (OMS-1) raised the apogee of the orbit, and the second one (OMS-2) circularized the orbit by raising the perigee.

For STS-41C and subsequent1, "direct insertion" ascents were flown. The trajectory was shaped so that a higher apogee was achieved on the main engines, and only the OMS-2 burn (it retained that name) was required.

1Vikki pointed out in a comment that some "standard insertion" missions were flown after this: STS-41D, STS-41G, STS-51A, STS-51B, STS-51F (planned, ATO instead), STS-61A, STS-61C, STS-51L (planned, never got there), STS-30, and STS-38.

These graphs from the Shuttle Crew Operations Manual show the performance advantage of direct insertion missions.

Graph of cargo weight to orbit versus orbital altitude for standard and direct insertion ascents. Graphs are shown for 28 and 57 degree inclinations.

Incidentally, "direct insertion" implied that the external tank was released into a higher orbit and it flew a lot further around the Earth before reentering, than it had for the standard insertion missions.

diagram showing the external tank impact areas for the two different inclinations and types of ascent

The OMS (and the aft Reaction Control Systems) were contained in pods on either side of the Orbiter's vertical tail.

photograph of the orbiter in space showing the OMS pods

Here is a cutaway drawing of the pod, with some pertinent info, from the 1982 Space Shuttle Press Reference.

cutaway drawing of an OMS pod showing the layout of the engines, propellant tanks, pressurization tanks, and connecting plumbing

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    $\begingroup$ There was an additional OMS burn on day 02 of STS-103. $\endgroup$
    – JdeBP
    Commented Aug 3, 2018 at 9:22
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    $\begingroup$ There were many additional OMS burns for deorbit, rendezvous, etc. STS-39 had 16 of them. space.stackexchange.com/questions/17687/… $\endgroup$ Commented Aug 3, 2018 at 13:01
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    $\begingroup$ from listening to an ascent recording, it seems to me they kept OMS-1 as a placeholder for a correction, confirming after tank separation that OMS-1 was not required. $\endgroup$
    – user20636
    Commented Aug 6, 2018 at 21:31
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    $\begingroup$ That is correct. It could always be executed if there was a performance shortfall during ascent. Which is what @Digger was reminding me about in the comments on the question. $\endgroup$ Commented Aug 6, 2018 at 22:33
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    $\begingroup$ @Sean They went to direct insertion pretty early in the program. The first three flights were in the 38-40 degree inclination range, then all the standard insertion flights were 28.4 +/- except for one to 57. ET impact zone figured heavily into flight planning so I'm sure they took care not to bomb Perth. $\endgroup$ Commented Jan 12, 2019 at 22:44

That's a mistranscription of OMS Burn, or Orbital Maneuvering System burn. The OMS system is how the shuttle changed its orbital characteristics. You can read about it here. One, two or more might have been used to fine tune the orbit, avoid space debris, rendezvous with the space station, etc.


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