# Flight QuestionDefault Atlantis Ascent Autopilot Behavior

#### CreponChris

##### New member
I don't think this has been covered in another thread, but if so, please excuse me. Hello, I'm new to the forums.

The built-in shuttle ascent autopilot behaves seemingly odd, at least to me based on my limited knowledge of STS operations. Particularly, when it comes to altitude. Is it normal that a shuttle launched makes it to 100-110km, and then begins to descend back down to about 75-80km to build up speed and hopefully reach about 7.6k TAS? That's what it does for me. From what I've read, ET sep should occur around 110km, but if I allow autopilot to do its thing, ET sep happens closer to 75-85km. So, other than asking whether this behavior is normal (altitudinal yo-yo-ing), I'll also ask at about what altitude in km should the highest velocity prior to ET sep be reached and in general what should that velocity be? Thank you so much. Orbiter version 2016, by the way.

#### Owenmck

I don't think this has been covered in another thread, but if so, please excuse me. Hello, I'm new to the forums.

The built-in shuttle ascent autopilot behaves seemingly odd, at least to me based on my limited knowledge of STS operations. Particularly, when it comes to altitude. Is it normal that a shuttle launched makes it to 100-110km, and then begins to descend back down to about 75-80km to build up speed and hopefully reach about 7.6k TAS? That's what it does for me. From what I've read, ET sep should occur around 110km, but if I allow autopilot to do its thing, ET sep happens closer to 75-85km. So, other than asking whether this behavior is normal (altitudinal yo-yo-ing), I'll also ask at about what altitude in km should the highest velocity prior to ET sep be reached and in general what should that velocity be? Thank you so much. Orbiter version 2016, by the way.
Welcome to the forums! I’m not sure about the altitudes in wiestion, but I think that the autopilot for the stock shuttle is just a LUA file somewhere in the Orbiter Directory. If you are familiar with the lua language, this may be of some help to understand what the shuttle is doing. I guess this file could also be edited to be a more realistic ascent...

#### CreponChris

##### New member
Thanks for pointing me in the right direction, and for the welcome! I'll have a look and browse some of the addons on orbithangar as well in case there's anything useful there. I guess it makes sense that the default ascent autopilot for the built-in shuttle (STS-101) isn't going to be perfect, or even close. And I realize with SSU and Space Shuttle Sim (2007 - PC) and in real life that ascent is automated and what actually needs to be done is following complex checklists and things of that nature so looking forward to moving on to SSU eventually (Space Shuttle Ultra). Cheers!

#### Marg

##### Active member
ET separation was never lower than 105km... I has similar problems with multistage2015 space shuttle, but editing guidance file always helped.

Replies
12
Views
1K
Replies
35
Views
3K
General Question Auto pilot for orbiter 2016
Replies
1
Views
1K
Replies
3
Views
3K
Replies
6
Views
2K