Flight Question Cannot get relative inclination down to 0 during ascent.

[Lawgeenoo]

Hello, I'm new here.
I've been playing Orbiter on/off for some time now. I'm no professional by any means, but I pretty much know my way around, except for one thing, which drives me nuts.
I can't get the RInc down to 0, using the default Atlantis. My heading is ok ~138 or ~43 degrees for an ISS rendezvous. It goes down ok, but it always stops at a relatively small value. Best i got was around ~1.7 degrees, which is way too much. The required plane alignment would use more than half of the OMS's engines fuel.
Really don't understand why this happens. When i have the smallest RInc, i tried to yaw the shuttle towards Normal or Antinormal, but it doesn't work. It drops a bit, then comes back up, doesn't matter if it's pointed for AN or DN.(this being while i still use the main engines) I'm always launching when the ISS will pass over KSC.
However, i never had a problem like this using the XR2.
Maybe you could shed some light, please? And, English is not my first language, sorry if it's bad.
Thanks.

 

[garyw]

I'm always launching when the ISS will pass over KSC.

You don't need to launch when the ISS passes over KSC, you just need to launch when it's path is over KSC.

Do you use launch MFD at all? That can really help to get the inc down and will help you to launch at the right time, I always find it best to launch a few seconds before the path of the ISS is over the launch site as the launch itself takes a short time to start converging on the ISS.

If you launch at the exact moment the path is over the launch site then by the time you drop the SRB's the path of the ISS has already moved past the launch site and your rINC is increasing.

 

[Urwumpe]

I really recommend launch MFD for that.

The goal is not to have R.Inc = 0 all the time during launch, but R.Inc = 0 when you reach your target orbit.

Thus, it reality, you launch a bit ahead of the ground track of the ISS, let the ground track catch you while you accelerate to orbital speed.

But calculating the when to launch and keeping the most efficient trajectory is something that LaunchMFD does best, much better than you can do it yourself or with the basic Orbiter tools.

 

[garyw]

And, as usual, Urwumpe explains it much better than I can! :thumbup:

 

[Lawgeenoo]

Thanks for your respones. I finally did it, earlier today.
Things i've changed: I let the ISS go slightly past KSC, then launch. Immediatly after SRB separation, yawing the ship in the right direction brought my RInc down to 0.27 degrees, which i'm very happy with. The hardest part was the ISS approach, really had a hard time properly slowing down and coming to a stop relative to ISS. Those OMS engines have the power of a leaf blower. Joke, but seriously, they lack oomph.

 

[Urwumpe]

Thanks for your respones. I finally did it, earlier today.
Things i've changed: I let the ISS go slightly past KSC, then launch. Immediatly after SRB separation, yawing the ship in the right direction brought my RInc down to 0.27 degrees, which i'm very happy with. The hardest part was the ISS approach, really had a hard time properly slowing down and coming to a stop relative to ISS. Those OMS engines have the power of a leaf blower. Joke, but seriously, they lack oomph.

They are pretty powerful in comparison to unmanned spacecraft. :lol: Just imagine you would be doing these maneuvers with RCS only, because the OMS failed.

The trick with the Space Shuttle in comparison to the unrealistic DG is to approach the ISS slowly and carefully, in small steps. Not one big large burn, but a serious of smaller burns to synchronize with the ISS, reduce inclination and approach velocity and let gravity do most of the work.

Rendezvous MFD could be helpful there.

 

[boogabooga]

Use the techniques here to help set up a "realistic" approach:
https://www.youtube.com/watch?v=M2_NeFbFcSw

 

[garyw]

I don't bother with slowing down, I just tweak the orbit to modify the speed. What I'd like is an easier way to work out the relative differences in speed between me at the ISS and a way to find out the closure rate per orbit.

 

[Urwumpe]

I don't bother with slowing down, I just tweak the orbit to modify the speed. What I'd like is an easier way to work out the relative differences in speed between me at the ISS and a way to find out the closure rate per orbit.

Can't Rendezvous MFD help there?

 

[boogabooga]

What I'd like is an easier way to work out the relative differences in speed between me at the ISS and a way to find out the closure rate per orbit.

This should help a bit assuming that your phasing orbit and the target' orbit are circular.

Set "1st transfer orbit apoapsis height" to something just below the "Target orbit altitude" until you are happy with "Relative intercept velocity."

Starting from a circular phasing orbit, when your phase angle is about equal to that listed in the spreadsheet, burn to raise apogee to "1st transfer orbit apoapsis height." When you get to apogee, burn prograde again until your your orbit to touches the ISS orbit. In practice, Rendezvous MFD will help you pull it off accurately.