Problem Multistage2-no attitude control

[Zatnikitelman]

I'm building my rocket using Multistage2 and got a successful Orbit insertion on Thursday. Today however, the only thing changed is the thrust of the one and only stage (it's multistage, but other stages are setup as boosters) from the hugely VASIMR like performance down to the more realistic performace of an RL-10, but now, the guidance system can't seem to pitch the rocket over, nor does laying on the manual control. I tried adding 5,000N pitch, yaw, and roll thrust to the stage, but there was no change in what either the guidance program OR manual input did. The stage isn't burning, but I tried firing the stage and it seemed to thrust, but still, no change in the control.
If anyone has encountered something similar, please help me with this.
Thanks,
Zat

 

[MajorTom]

Is this a problem mainly with the multistage2 guidance program? I find that sometimes (but not always) I won't get any pitching at all with my guidance program if there is no roll program.

Also try upping your pitch, roll and yaw thrust levels to something much closer to your main engine thrust level and see if that doesn't help...

 

[Zatnikitelman]

Upping the P,R,Y levels seems to have worked for now. I've put them at 3.5MN same as the main engines lofting the thing. It's right now about 90 seconds into launch under guidance so we'll see how the rest of the orbital insertion goes.

 

[Dr Pepper]

Have you tried simply leaving the P,R,Y thrust lines out of the .cfg all together? Multistage should automatically compute a default value if you do this.

 

[Zatnikitelman]

That's what I did to begin with as it was working, then all of a sudden, I couldn't steer the rocket, nor could the guidance program.

 

[Dr Pepper]

Hm, that's weird. I used to get this "no-pitch" problem when I created launchers in multistage, but I always solved it by either removing the P,Y,R lines or by making their thrust a very high value. The only problem with the second method was that as the launcher burned off more and more fuel (and thus weight), maneuvering became harder as the P,Y,R thrusters were overpowered for the mass of the near-empty launcher. I'm not sure how the guidance file will handle it, though; I never could figure those out.

 

[Zatnikitelman]

Ok, got a new one. Once Multistage2 guidance runs out, the vessel starts spinning in one axis uncontrollably with or without time accel, killrot, and disable commands in the guidance file. Is this just an annoying bug, or am I doing something wrong? This only happens when there are no longer any commands to process in the guidance file.

 

[MajorTom]

Ok, got a new one. Once Multistage2 guidance runs out, the vessel starts spinning in one axis uncontrollably with or without time accel, killrot, and disable commands in the guidance file. Is this just an annoying bug, or am I doing something wrong? This only happens when there are no longer any commands to process in the guidance file.

Ah, this one's terrible and I encounter it a lot. IIRC, it has to do with the fact that the time steps used in Multistage2's guidance are related to the video frame rate. (I've attempted to harness this awesome spinning to simulate an upper stage going out of control, for example, Soyuz 18. But that's another story...)

And as you say, it does seem worse at the end of the guidance program, typically when you're separating from your launcher. Seems to me that Orbiter (at least on my PC) pauses while it loads the new mesh, and that slowdown is enough to send my launcher into the start of an uncontrollable pitch program.

Here's what I do to avoid this:

(1) Make sure your guidance file has you pitched at your final angle for a good many seconds before ending the guidance commands, so your booster truly has no rotation. Say 10 seconds minimum, 20 is better. You should hear no thruster activity for a while.

(2) Watch your frame rate (hit f key). If there are things you customarily do that cause the frame rate to go down, for example, panning the camera around the outside of your ship, and if the multistage bug occurs then; or if you simply can't avoid the bug any other way, then try a lower time speed just before the bug usually occurs. For instance, go to 1/10 time (hit r key). This makes your time steps smaller, which could get you through the rough patches.