Flight Question Dealing with the Shuttle's crooked engines

[destinos]

So, for most of today I've been trying to launch Atlantis from Canaveral and meet up with ISS. I finally did it, but not nearly as cleanly as I would like.

The latest in a string of puzzles ive encountered is that whenever I burn prograde or retrograde at my Pe or Ap, both of those positions begin to shift and when i finally arrive at my rendevous point, I'm about 80 kilometers too high/low.

I think this must be because the engines on the Shuttle aren't oriented to thrust straight back. This was pointed out to me in an earth to ISS tutorial. It said that to burn without screwing up your RInc, you should burn level with the horizon rather than the usual prograde/retrograde positons. But this doesn't seem to suffice when burning to decrease the DTmin to 0 during the orbit synch.

I attempted to compensate instead by pitching up from the horizon a little bit to get the engines pointed roughly onto the prograde/retrograde plane, and this did seem to help somewhat, but it's pretty hard to be totally accurate.

So, finally, my question is: How do I deal with this? Seems like this must've come up before now, but I couldn't find a thread about it in the search. Is it something so simple I'm just overlooking it?

 

[Andy44]

The OMS thrust is canted up 13.5 deg (someone correct me if I'm wrong), so that's the angle you need to offset from.

The way I do this is to use [ame="http://www.orbithangar.com/searchid.php?ID=3165"]Attitude MFD [/ame]in velocity mode, with a pitch offset of 13.5 deg. Set it to hold attitude, do your burn, and then release the hold so it won't burn your RCS fuel up while you coast.

 

[destinos]

So I've got the right idea then, and that's the way it's done? Cool, I'll look for that MFD

Do you know if it's the same for the main engines? I've been getting into orbit with a few percent fuel left in the external tank that I've been using on the aligning burns. Side question: why doesn't the real shuttle do it this way. To control the de-orbit of the tank better?

 

[tblaxland]

The OMS thrust is canted up 13.5 deg (someone correct me if I'm wrong), so that's the angle you need to offset from.

I don't know what the value in Orbiter is for the stock Atlantis. In real life the null position is:

[FONT=Arial, Helvetica][SIZE=-1] The engine null position is with the engine nozzles up 15 degrees 49 seconds (as projected in the orbiter XZ plane) and outboard 6 degrees 30 seconds (measured in the 15-degree 49-second plane).[/SIZE][/FONT]

Also, TVC is +/- 6° pitch and +/-7° yaw.

Do you know if it's the same for the main engines?

I'm not sure for the stock Atlantis, but again in real life:

[FONT=Arial,helvetica][SIZE=-1]The installed null position for the left and right main engines is 10 degrees up from the X axis in a negative Z direction and 3 degrees 30 minutes outboard from an engine centerline parallel to the X axis. The center engine's installed null position is 16 degrees above the X axis for pitch and on the X axis for yaw.[/SIZE][/FONT]

Side question: why doesn't the real shuttle do it this way. To control the de-orbit of the tank better?

Yes, they want the tank to land in the Pacific and at MECO the Shuttle's periapsis is inside the atmosphere to acheive that. The OMS-2 burn (after ET sep, at about MET 35 minutes) is performed at apoapsis to raise this.

 

[destinos]

Wow you have done some research! Does that stand for Thrust Vector Control, does that mean I can angle the nozzles down as necessary for prograde burns?

 

[Urwumpe]

Do you know if it's the same for the main engines? I've been getting into orbit with a few percent fuel left in the external tank that I've been using on the aligning burns. Side question: why doesn't the real shuttle do it this way. To control the de-orbit of the tank better?

The main engines of the real space shuttle can only be started once and that only on the ground.

The main engines are also installed at this angle, because of the heavy tank below the shuttle - the center of gravity of Shuttle + tank has to be "above" the engines, otherwise the Shuttle would easily spin out of control.

---------- Post added at 09:46 AM ---------- Previous post was at 09:42 AM ----------

Wow you have done some research! Does that stand for Thrust Vector Control, does that mean I can angle the nozzles down as necessary for prograde burns?

The thrust vector is the force of the engines. Every force has not only a magnitude (the strength of the force), but also a direction. Thrust vector control allows changing the direction of the thrust vector of each engine, making it possible to control the torque produced by each engine and thus the rotational accelerations produced by each engines.

You won't rotate, if the torques produced by engines (and aerodynamics) are neutralizing each other. The torque produced by a engine is zero, when the thrust vector points directly at the center of gravity of the spacecraft.

Just rotating the engines down until they are parallel to the Z axis, will make the shuttle pitch down rapidly. Now, they are angled that the thrust vector is pointing at the CoG, and you won't rotate. You just need to take the angle into account for the burn, by pitching up by 13°, so the vector engine-CoG is pointing prograde.

See here about the torque:
http://en.wikipedia.org/wiki/Torque

 

[destinos]

Just rotating the engines down until they are parallel to the Z axis, will make the shuttle pitch down rapidly.

So you're saying that for the shuttle, the center of gravity isn't right along the z axis? It's offset down and back a little?

 

[insane_alien]

no, he's saying the engines aren't aligned with it.

 

[destinos]

no, he's saying the engines aren't aligned with it.

But their thrust vector does point at it? So, they're canted the way that they are precisely so the craft DOESN'T rotate when they're fired?

 

[Urwumpe]

But their thrust vector does point at it? So, they're canted the way that they are precisely so the craft DOESN'T rotate when they're fired?

No,they are canted so you can control how the spacecraft rotates.

 

[destinos]

If anyone else happens upon this thread with the same issue, one tactic I've found very useful when dealing with the Shuttle is to rendezvous with the ISS at one of the orbit intersect points that the SyncMFD points out to you. The MFD seems to update the point in real time as you're burning to decrease the DTmin. This way, you don't get all the way to the rendezvous point and find that your periapsis has shifted on you. An added bonus is that there's no keeping track of ISS alt at your periapsis etc. Obviously there has to BE an intersect point for this to work but if you've flown a proper ascent there most likely will be.