MechJeb Smart A.S.S. flips my rocket on ascent

[Gnawer]

I have a rocket that's fairly complex, but flies fine with regular SAS. What I want to do, is launch it straight up to certain altitude (I don't want to do the gravity turn for the purpose of this launch). Regular SAS just keeps it straight the entire way, and I am confident that there's nothing wrong with the rocket, because I launched this design like 50 times already.

Now, I tried to use MechJeb (2.5.1, game is 1.0.2). I added the external control box and enabled Smart A.S.S., then set it to SURF with default parameters (hdg 90 pitch 90 roll 0) and hit execute. It started rotating right away on launch, which I guess is fine, because it was getting to heading 90. Then it ascended to about 7000m and for some reason started deviating from zenith towards heading 90. I don't know why, but it was definitely intentional on its part. Anyway it tipped the rocket over after deviating for about 3-5 degrees (which is pretty normal for my rockets), did some kind of crazy loop, and eventually managed to return to the zenith course. I repeated the launch about 5 times, it does that every time.

Does anyone have any idea what's up with that?

[cicatrix]

Try Surf - UP or Orbit - Prograde.

This might be because you placed the control pod / computer core not facing the prograde direction. MechJeb always thinks that the pilot cabin or computer probe is oriented into the prograde direction. You can easily check that at the launch pad - your nav ball should be all blue, no brown (as your ship pointing up). If this is not the case you need to enter different numbers depending on the orientation of your cabin / computer probe.

[A_name]

I think your problem might actually be the Mechjeb box causing asymmetrical drag on your rocket. Try putting it in a utility bay or inside a fairing, or under something or at least put it on with symmetry.

[Gnawer]

Surf-UP makes the rocket mad unstable, almost like there's no SAS at all. Or rather, it tries to steer, but I don't understand where exactly.

Orbit-Prograde just tips me over right away, because orbital prograde is way more than 5% from zenith. In fact I don't understand how it is possible to launch like that with simple enough rocket (PS: maybe with like beefy fins, but I just avoid this kind of stuff).

The core is in prograde direction, but is off-center (as it's the box, not the pod). Here's the screenshot if it helps. http://imgur.com/cDQXm79

- - - Updated - - -

I think your problem might actually be the Mechjeb box causing asymmetrical drag on your rocket. Try putting it in a utility bay or inside a fairing, or under something or at least put it on with symmetry.

Unfortunately, not that. I tried to put it in the service bay, but now it does point in incorrect direction. So I just made a totally symmetrical rocket and installed two boxes. It does exactly the same thing. Plus, SAS can control it in both cases, so it couldn't be that serious.

Edited May 26, 2015 by Gnawer

[clivman]

Try using surface velocity + instead of orbit prograde, if you are using it.

[sarbian]

SURF UP is what you want. If it does not work for you you either :

- modified the values in the Attitude adjustment window

- use FAR and control surfaces (not handled ATM)

- use non stock gimbal

[Gnawer]

Ok, so noone else has this issue? Must be something with my system then. Maybe it doesn't like Windows XP or something.

[Tontow]

It would help if we had your rocket.

[Gnawer]

I think I figured it out. I had a setup with central booster and several identical radial boosters (asparagus). Turns out, it wasn't completely symmetrical. Some radial boosters had struts missing on one side, which made that side kinda wobbly. Apparently, SmartASS has a real issue with that. While stock SAS just keeps heading without so much as noticing this wobble, SmartASS tries to do something that unbalances it way more and eventually flips the rocket over. It's pretty easy to reproduce: create a rocket with 1+6 large boosters (I use double T800 tanks), and put struts just on one side.

[Jansn]

snipped

Edited May 27, 2015 by Jansn