Huge vehicles, mechjeb and thrust vectoring (gimbal)

[Hoozemans]

So,

I've been messing around with a huge launch vehicle, trying to get my latest versatile asteroid hunter into orbit in one piece, rather than having to break it up. There's a problem with the design, in that there's a lot of stress on some of the key components of the structure. I got the design to work, as long as there aren't any huge lateral forces applied to the structure.

And there's a problem with that. Because each launch attempt takes almost an hour to run, at about thirty frames per minute, and it's nearly impossible to manually control the rocket under those circumstances, I've let Mechjeb do most of the work. But Mechjeb has some peculiar notions when it comes to thrust vectoring. It thinks its a good idea to start its gravity turn by making a sharp turn as soon as vertical speed exceeds 100m/s, applying lots of lateral stress. My game pauses, the program takes a couple of minutes to think about the development, and then I get to see my ship do an extremely slow unplanned disassembly.

I've tried to disable thrust vectoring in the editor - but Mechjeb doesn't seem to care. Likewise if I tie thrust vectoring to action groups: thrust snaps straight when I lock engine gimbal, and then, next frame, is vectoring again.

... O. While I was typing this, I was running an experiment with the gimbal not locked, but limited to a few percent. And Mechjeb doesn't seem to ignore gimbal limits. Currently the game is thinking about the next frame. I hope it's just staging, not a SUD.

Anyway, my original question remains of some interest, I think: is it true that Mechjeb ignores gimbal locking?

v1.7.2 64bit/Linux

[JustAToy]

I don't know, but it sounds to me that if you were to edit the launch profile which is a MJ feature you could avoid that turn until you were past max Q removing most of your issues. 

[steuben]

The other option is to forget about an efficient launch. Just go straight up until 60km and turn left.

[Nicias]

You might also want to look at the GravityTurn Mod, it is simpler for this task then MJ.

[Kryxal]

You can always limit the angle off prograde that your rocket will aim for.  Until the atmosphere thins enough, MJ will pay good attention to that ... but you'll probably want to start your turn earlier if you do.  In fact, an earlier, gentler turn should help all around...

[Hoozemans]

On 7/7/2019 at 1:24 AM, JustAToy said:

I don't know, but it sounds to me that if you were to edit the launch profile which is a MJ feature you could avoid that turn until you were past max Q removing most of your issues. 

On 7/9/2019 at 1:43 AM, Kryxal said:

You can always limit the angle off prograde that your rocket will aim for.

Unfortunately, the manoeuvre occurs at the very end of the burn to maximize apoapsis, after the vehicle has already left the atmosphere.

On 7/7/2019 at 7:07 PM, Nicias said:

You might also want to look at the GravityTurn Mod, it is simpler for this task then MJ.

I probably will, though I suspect that all automated ascent routines will seek to make tiny corrections when the trajectory deviates from the ideal.

 

[JustAToy]

Most deviation from a solid course are caused by the rocket being torqued in some manner.     Unless you stuff all that sci stuff and batteries into a cargo area or fairing then you may 

be adding torque to the rocket.    Kerbal engineer shows you the torque as you start building the rocket.   Some torque can be dealt with if it get to high expect spin or wobble to trash 

the rockets planned trajectory forcing any program to make a correction.      Of course seeing a picture of the rocket could also help make suggestions as to why your having a problem 

with it.