Why does Mechjeb roll my rockets?

[Paleodraco]

I have this persistent issue. I'm using a HECS2 probe core to launch a station part. I press E once to rotate it 90 degrees. I then build the rocket, placing my mechjeb case (mechjeb2 version 2.3.7) on the probe core opposite the VAB door. When I launch with the ascent guidance, my rocket will roll so that if it pitches down from vertical, it will be heading north. I've tried several different orientations and placements of the probe core and mechjeb case, but the same thing happens. 

Edited August 5, 2018 by sumghai
Removed empty spoiler textarea

[sumghai]

[MOD - Moved to Add-on Discussions, as this is a question regarding an add-on/mod rather than the stock game.]

[Gargamel]

I have found that MJ will roll anywhere from 90 to 270 degrees on a regular basis, and sometimes a couple rolls before leveling off.   I have tried to figure out the method behind this to anticipate the roll and have my rocket in the correct orientation, but small changes to the rocket through my work out the window. 

So the key is to design your rocket so it's neutral in any roll orientation it flies.  MJ really doesn't seem to care which way it's rolled, it will still follow it's intended flight path.   For side boosters, though this can be an issue, as sometimes decoupling when the boosters are on top/bottom vs side/side, can have the top booster crash back into the rocket.    The key there is to just use enough sepatrons to get the booster to clear the rocket in any orientation (good safe design practice anyways)

If you are building a vessel, that by it's very nature is not roll axis symmetrical, such as a shuttle, then you really don't want to be using MJ to do the launch anyways.  MJ doesn't do well, in my experience, with weird shaped things like shuttles.  A manual ascent is usually preferable. 

 

But as to why it does that. and how to stop it, I don't know.    Before we ping sarbian here, maybe somebody will know. 

[Xd the great]

Mechjeb is programmed to roll. I think its the force rolling option.

[Tonka Crash]

Make sure the Force Roll option isn't checked on the Accent Guidance tool. I think MechJeb defaults to On and 90 degrees. I have it set to On and 0 degrees.  I'm not sure why there are two windows next to Force Roll. It would really be nice if Sarbian had some official documentation on all of MechJeb's functions. I use the left one to control roll angle for booster separation.

As @Gargamel mentioned top/bottom booster may not separate cleanly. I'll flip to 90 for top/bottom separation and back to zero for left/right separation. I also find that deactivating any control fins in the roll axis and only using reaction wheels to handle roll seems to be more stable for mechjeb.

I also use VABReorienter to just build my rockets so they point downrange by default.

 

[mystifeid]

21 hours ago, Paleodraco said:

When I launch with the ascent guidance, my rocket will roll so that if it pitches down from vertical, it will be heading north

If the rocket is actually heading north make sure the orbit inclination is set to 0° in the Ascent Guidance module. This is the the box directly underneath the orbit altitude.

 

1 hour ago, Tonka Crash said:

I'm not sure why there are two windows next to Force Roll.

Counter-intuitively I think the second box is for the roll before the turn and the first box is for the roll angle after the start of the turn. Also, I think they are measured from the orbit inclination not the heading - probably because your initial heading on the launchpad depends on which way you rotated things in the VAB. But yeah, 0° in the first box will make east the pitch down direction (but only if the orbit inclination is also 0°).

Edited August 6, 2018 by mystifeid

[Gargamel]

6 hours ago, Tonka Crash said:

Make sure the Force Roll option isn't checked on the Accent Guidance tool.

That option, for me at least, does not do what I had assumed it does.   I have found it induces a continuous roll at a given rate, not rolling to a fixed angle and stopping.   If you need a roll stabilized rocket for ascent, then use this option.