MechJeb issue with Mk-55 "Thud" engines.

[jbdenney]

Not sure if the is the corrected forum for this.

I am running 1.11.2.3077  (windows Player x64) KSP with MechJeb2-2.12.0.0

I am having an issue with MechJeb  controlling Mk-55 "Thud" engines.

I have a Asteroid miner/grabber that pulls asteroids behind it so the engines cannot be in the back. I placed 4 Thuds on the top sides of 4 sets of fuel tanks that surround the the Apollo Style Pod. I use a heavy booster that can get the "Tug" into orbit without using the Thuds but once there I drop the Standard engines leaving the Thuds as my sole propulsion (except some RCS thrusters). Maneuvering is slow but otherwise works fine.

If I manually control the Thuds I have no issues with control. But if I use MechJeb, when it tries to run the Thuds they "Sputter" with short blasts of thrusts - almost like a real slow machine gun. It seems that MechJeb is trying to use them as RCS thrusters to control orientation instead of just pushing the Tug. They are unusable in MechJeb. I also noted that when I was designing the lift boosters and I was using MechJeb Assent Assist that the Thuds were kicking in as the last stage during boost the thuds would fire continuously but the Tug would start to Roll and could not be controlled.   If I launch manually, or disengaged MechJeb beore the Thud stage fired, the Thuds they worked find in the last stage of launch - except for not having enough power to get the PE into orbit.

Is there an issue with MechJeb and the Thuds?

 

Thanks in advance

[Zhetaan]

1 hour ago, jbdenney said:

Is there an issue with MechJeb and the Thuds?

It may be that MechJeb is expecting the Thuds to be behind the centre of mass.  You might try testing with an otherwise-identical vessel with Thuds at the back and seeing whether MechJeb has a problem with it.  If you know how to use the cheat menu, then you can put the test rocket in orbit without needing to figure out the aerodynamics and just see whether it works.

[camacju]

Do you have differential throttle enabled? If so then Mechjeb will try to do exactly that - use the engines as large RCS. It's really only useful as a substitute for engine gimbal, like if you are using Nervs. The Thuds should have enough gimbal that you can safely turn off differential throttle.

[jbdenney]

Actually, I found that moving back the Thuds so the center of mass is behind the center of mass fixes the issue. I have to be careful on how the center of mass changes as the fuel tanks empty and/or the Ore storage fills.