Direct insertion...

[psowen]

I noticed when playing STS-41C that its the first mission that skips OMS-1 and does direct insertion.

I got a question.   What was the reason for this in reality?   Why did the original STS missions not use direct insertion.

I know I've even heard of them using the OMS engines during the launch on some shuttle missions.

[uri_ba]

STS-41C was the first to do a direct insertion.

I suppose it was to save some OMS fuel that will later allow a higher orbit - but I'm just guessing on this one.

OMS assist burn during ascent (after SRB sep) was added somewhere in the 90s. it allows the shuttle to have a greater speed allowing a safer TAL in addition to and better trajectory for the ET (better clearance for african shore if jettisoned). at least that is what I understood from Mike Mullane's "Riding rockets"

[Nephi]

Indeed, the direct insertion normally makes the MPS achieve apogee, and the OMS burn only raises the perigee, saving most needed propellant for the rendez-vous with Solar Max. And indeed they really needed it (especiallly since the SRB achieved lower than normal performance on that launch), as Challenger was starting to run low on fuel after two days of trying to stop the satellite's tumbling and wobbling.

[psowen]

Why not do direct insertion on STS-1 is what I'm asking?   Why not have made the oms-2 oms-1 and done the direct insertion from the get go?

[christra]

Why not do direct insertion on STS-1 is what I'm asking?   Why not have made the oms-2 oms-1 and done the direct insertion from the get go?

During the first years they were very conservative in using the OMS during ascent. The idea was to keep as much propellant as possible for the orbit maneuvres.
Then the RSO's (Range Safety Officers) came up with the idea because they were afraid that in some aborts the External Tank could come down in Africa. To prevent this, an extra push with the OMS would be helpful. But it was declined.
Later the trajectory planners did some own studies and they found out that using the OMS pre-MECO would improve the nominal and launch abort performance. And that finally convinced the guys in charge to do it.