Going off-plane during prograde

[rocketman768]

So, I was in a nearly circular orbit in Atlantis at about 360km altitude and within 0.01 degrees of the ISS' plane. After turning on the prograde autopilot and burning at perigee to make my apogee higher in order to let the ISS catch up to me, the difference in planes was 0.13 degrees and was going to require a 40 second burn at one of the nodes to correct it.

Why would a prograde (in-plane) burn affect my inclination?

 

[Urwumpe]

Two questions:

1. Which Atlantis did you use:

[ ] Default Atlantis
[ ] Space Shuttle Ultra
[ ] Shuttle Fleet

2. Did you fly in orbiters typical prograde orientation?
[ ] yes
[ ] no

if no, state other orientation:
___________________________________________________


Just kidding, all three Shuttles have the OMS engines angled at 13° away from the center line for compensating CoG issues, only Shuttle Fleet allows you to add the line "OMS" to the scenario for making the OMS burn along the vessel Z axis so the default autopilot modes don't cause a plane change.

When you burn in orbiters prograde attitude with the shuttle, you would usually have 22.5% of the thrust being directed off-plane, causing the plane change you experienced.

Better do the burn manually or use better attitude autopilots (attitude MFD, UNIVPTG), when you are not flying Shuttle Fleet with OMS compensation.

 

[tblaxland]

Just kidding, all three Shuttles have the OMS engines angled at 13° away from the center line for compensating CoG issues, only Shuttle Fleet allows you to add the line "OMS" to the scenario for making the OMS burn along the vessel Z axis so the default autopilot modes don't cause a plane change.

When you burn in orbiters prograde attitude with the shuttle, you would usually have 22.5% of the thrust being directed off-plane, causing the plane change you experienced.

Better do the burn manually or use better attitude autopilots (attitude MFD, UNIVPTG), when you are not flying Shuttle Fleet with OMS compensation.

So that means if you want to do a heads-up prograde burn with AttitudeMFD you should also pitch up 13deg to align the thrust vector with the velocity vector, right?

 

[rocketman768]

So, by 'doing the burn manually', do you mean that I should turn prograde and then pitch up an additional 13 degrees to get the oms engines to an in-plane orientation? I assume you got the 22.5% from doing sin(13 deg), so this type of manual burn would make sense since sin(0 deg)=0 resulting in 100% of the thrust being directed in-plane only.

How come this is not documented in it's default pdf file that comes with Orbiter (unless I missed it)?

 

[Urwumpe]

How come this is not documented in it's default pdf file that comes with Orbiter (unless I missed it)?

I would expect by the typical problem of writing manuals: How can somebody not notice that the engines are pointing THIS way?

 

[V8Li]

Yea, the OMS is at a 15 deg angle. So if you use the prograde autopilot you will have the OMS pointing 15 deg left or right and that will affect you orbit inclination. Level the craft before burning prograde or retrograde or even better pitch it up to 15 deg pitch.

 

[polaris149Tiberius]

Very Helpfull information

Very Helpfull information

Thanks for this information. I was previously unaware of this fact. As a matter of fact you can see the angle of the oms if you look at the shuttle from the side view. I wondered if they would cause this and now I know. Thanks again.