OMS Left or Right NC Burn

[Jeff B]

Hi all, just wondering the reason for only using either left or right OMS during an NC1 / NC2 burn.  Wouldnt that cause the movement to yaw the shuttle to left/right? Is there a simple answer obviously be technical answer but intrigued as to why.

[uri_ba]

the reasons may vary.
from a faulty OMS gimble (STS-124) to minimum pulse burn (OMS engine have a minimum burn time necessary to maintain proper fuel flow, I don't remember the exact figures). if a twin OMS will require less then this amount of time, a single OMS will be used. if the time for a single OMS is less then that, the RCS would be used.

anyway, the engines are gimbled, the nozzle can be moved around a few degrees - just for these purposes.

[Pocci]

I would guess that the nozzle is gimbaled so that the thrust goes through the center of gravity. In this case no yaw force is induced.
Of course the thrust would have a Y-component and if that is not wished, the shuttle must be yawed a little before the burn, so that the thrust goes into the intended direction.
This is similar to the downward z component of each OMS burn, that must be countered by a little pitch up attitude.

Theoretically there would be a second possibility (I don't know what NASA is doing in reality):
The OMS burn could be gimbaled into the intended direction (not through the center of gravity) and the induced pitch and yaw forces could be countered by automatic RCS burns.

/Armin

[Spacewalker]

No need to guess : The OMS nozzles ARE gimballed to direct the thrust vector through the CG.

The other possibility Pocci mentioned would be too inefficient on fuel consumption. Basically, fuel would be wasted in the RCS burn to counter an unwanted part of the movement that is induced by the OMS burn.

OMS burn offsets/trims have been discussed here: http://www.space-shuttle-mission.com/forum/index.php?topic=2635.0
and here: http://www.space-shuttle-mission.com/forum/index.php?topic=3336.0

[Pocci]

Yea, I know the trim discussion but I did not want to bump it here because it was closed by Admin.

[Jeff B]

Thanks all, i think i got the info and answers i was looking for and all makes sense.

Thanks again.