Flight Question Shuttle Fleet OMS gimbal

[StargazerBranden]

I've done a fairly extensive search of the forum and have found a plethora of information regarding the default Atlantis. The tutorial Space Shuttle to ISS gives a pitch up value of 15 degrees. But in the threads I have read the figure I seem to come across is 13.5 degrees pitch. Using both values with SF, I am still getting some unwanted rotation that is costing me more fuel than what I'd like to burn in the long run. This includes the actual shuttle xz offset of 15.817 (15 degrees 49 minutes) Does anyone know what the shuttle fleet offset is? Or is that slight rotation with engine burns just the nature of the beast?

 

[boogabooga]

Read the included documentation. IIRC, there is a parameter in the scenario file that addresses the gimbal.

 

[StargazerBranden]

Thanks, boogabooga, I'll double check that when I get home from work this evening. Been looking in the SF docs left and right but I bet I skimmed right by it. You know how when you're looking for something so hard and its right in front of your face but you'll miss it 5 times before you finally find it?

---------- Post added at 11:00 PM ---------- Previous post was at 01:14 PM ----------

Boogabooga, and any other orbinauts who use Shuttle Fleet, the documentation does not specifically say what the OMS gimbal offset is, however, it talks about adding the line "OMS" in the scenario config file to whatever shuttle you are using. The doc states that adding this line will allow the Shuttle Fleet OMS to work with the STS Guidance and Control/Soyuz Guidance and Control MFD. I tested this in Sim using STS Guidance MFD, Attitude MFD, and of course one of the Shuttle Fleet Orbital Vehicles. What the OMS line in the cfg does is null out the gimbal, so the thrust vector is in plane with the velocity vector, canceling out any need to have a pitch offset. Just use a P/Y/R setting of 0 degrees for HLVL prograde (attitude MFD) or use the stock prograde/retro/anti-normal/normal autopilots and burn

 

[Thorsten]

Or is that slight rotation with engine burns just the nature of the beast?

The nature of the real beast is that the OMS thrust vector control digital autopilot controls the engine gimbals such as to null all attitude rates (in the absence of control input from the burn control or the stick).

So on the real Shuttle the thrust is automatically vectored through the CoG for one or two engine burns, which means that the Shuttle nose points some 13 degrees (precise number depends on where the CoG is) away from the thrust vector, and in body coordinates accelerometers sense both x and z accelerations during a 2-engine burn and y acceleration during a single engine burn (it flies a bit 'sideways' with a single engine).

Rotations during OMS burns that need to be caught manually are not part of real Shuttle operations, nor does the crew have to fiddle with computing CoG vectors by hand.