Two different but similar craft, same issue. Take a single, short tank (either FL-T1000 or X200-8) and attach radial engines. Put a robotic control unit on top and an MJ remote on the side. Put it in space and try to control it with the Smart A.S.S. Instead of turning, it just produces a tiny wobble around its axis. You can't execute nodes automatically because it can't orient toward the target.

However, you can turn the ship manually just fine. If you tell it to execute next node and then line the burn up yourself, as soon as you hit the blue cross-hairs it will suddenly start warping.

I haven't seen any mention of this on the forums or here.

Thanks,
Perry Apsis

Both ships did have something else unusual that I should mention. One had docking ports at both ends and I would control from either. The other had the probe core at the bottom, rather than the top as I stated.

I had the same issue appear with a different ship now. I'm very surprised I haven't seen it mentioned. Please note that this is NOT the same as the "oscillates madly" issue that I have seen discussed. In this case, the ship should be turning, but instead there is just a tiny circular wobble around its axis.

I think that this is the same issue as in #287. I'm having the same problem. It started after I experimented with using the 'Use Stock SAS' button in the Attitude Adjustment window. Several values in there are fluctuating between -ve and +ve Infinity.

OK, after some more testing, it's only happening to some ships. They're all ships in which I turned on the 'Use Stock SAS' at some point. However, there are some ships that I did that to that are working fine, so I'm not sure if this is the cause after all.

After restarting KSP a few times and looking at things, I've noticed that the ships that are having the problem start with normal values in the Attitude Adjustment window, for everything except the inertia vector's elements. These all start out as 'NaN' for the vessels with the problem. And as soon as I turn on one of the functions in the SASS, all the other values in the Attitude Adjustment window get messed up, turning to infinity mostly.

I looked through the persistence file, but could find any reference to these inertia values, so I've not been able to fix these vessels. And yet for some reason, when I restart KSP, these vessels start (after restarting KSP completely), with inertia vector elements of NaN. The problem has come after I've already flown most of the way to the Mun, so they were working fine originally. I guess I shouldn't have started pressing random buttons during the Kerbin-Mun transfer.

I will not do any support for ship with 'Use Stock SAS' activated since they use ... the stock SAS.
So the code that make them Wobbles is part of KSP and not MJ.

@eshafto were you using Extraplanetary-Launchpads? I've just noticed that I'm only getting this problem with MechJeb on vessels that I built on that mod's orbital dock.

I was able to repeat my experience above. I built a new probe, and as usual it was launched with a locked-up MechJeb. The moment I pressed the 'C' key to enter the Probe IVA, the inertial values went from +/- infinity, to normal values.

Fair enough @sarbian, but I'm able to reproduce this bug without turning on the stock SAS option at all. Also, the stock SAS is the only one that is working - I'm having to use it when there's no other option, due to MechJeb locking up.

Interestingly, I was just now able to correct the problem in one of my probes:
My probe was (like every ship I've launched lately) glitched out in it's mechjeb, as described above. Then I switched to the Probe IVA, and opened up the MechJeb plugin on the RasterPropMonitor display, and then selected a celestial body (the Mun I think) as the current target. At this point I noticed that the inertia values had returned to normal.

I'll try it again and see if I can nail down exactly when it goes back to normal.

Is the vesselState cleanup from #319 likely to fix this? If so, what's the best way / when can we get the latest version?

Nope and Jenkins.

fair enough 😦
At least I have a workaround for now.

The only thing the cleanup should do is cost a few operations less per
frame because why grab and cache values per frame per active MJ that KSP
itself already cached for the vessel.

The wobble problem is something else, maybe an initiation bug with EL, I
don't know.

Sorry, should have mentioned that the only mods I have installed are MJ and procedural fairings. I have never clicked "Use stock SAS"

Sigh. I just launched a new ship, and as so often happens it started with a locked-up Mechjeb. Going to IVA is no longer fixing it though.

Well, it's definitely not due to some kind of conflict with Extraplanetary Launchpads. Now everything I create starts with a broken Mechjeb, even things I'm launching directly from KSC. Going IVA no longer does anything to fix it. At this point Mechjeb is completely broken for me.

I've been poking around some more and I now think the issue has to do with craft that are wider than they are long. I first saw it on ships that were a short wide can with radial engines but I've since found it with axial (?) engines and I built a ship that was about as long as it was wide and I got intermediate behavior (still swiveled in a small circle but also managed to turn successfully).

I can reproduce this if you're interested.

This issue is being closed because it appears to be more of an issue with rocket design or physics that is not directly related to MechJeb issues or bugs and is more of a general KSP issue. It may also be a better question to answer on the forum thread