MechJeb and RCS efficiency

[chd]

Mostly just curious here -- when I use MechJeb's landing guidance with "use RCS for small adjustments" checked, and also sometimes with the docking autopilot, it looks like it's firing a lot more of the RCS thrusters than it actually would need to in order to translate in the desired direction. Sometimes it looks like it's firing essentially all of them, though not all at the same thrust. Sometimes it appears to cycle through them all, fairly rapidly.

What's MechJeb doing here? I usually assume that it's going to be better at this stuff than I would be doing it by hand, but it's use of RCS often at least appears to be fairly inexplicable. Is MechJeb just bad at this, or do I not appreciate it enough? :-)

 

[Anonymous49]

that never happened to me

[The Aziz]

In regards to RCS, mechjeb is far from efficient. It gets the job done, sure, but it's expensive job. That's why I learned to dock manually,

[VoidSquid]

I much prefer to dock manually, using the DPAI mod for visual aid. Way faster, more efficient, more safe.

[jimmymcgoochie]

Don’t let MechJeb use RCS unless absolutely necessary- in stock KSP reaction wheels alone should be enough to rotate your vessel in whatever direction is needed. It’s probably firing most/all of your thrusters because the mass/thrust is imbalanced when it tries to fire translation controls, resulting in a rotation which requires correction by firing more thrusters, which messes up the translation and so on in a loop.

[Spricigo]

10 hours ago, chd said:

Is MechJeb just bad at this?

yes. ( @jimmymcgoochie nailed it: imbalance and overcorrection)

Also don't help if you didn't designed the craft to minimize the issue.  

[Curveball Anders]

It's has a problem with over powered RCS.

So one way to make it work more efficient is to dial down the power of the trusters.

[chd]

Thanks all -- this makes sense. It's too bad there isn't a hybrid setting, "use RCS for translation only, use reaction wheels for rotation."

 

[VoidSquid]

Try out the DPAI mod

 

and you'll be amazed how fast and easy you can do docking.

[Spricigo]

3 hours ago, chd said:

Thanks all -- this makes sense. It's too bad there isn't a hybrid setting, "use RCS for translation only, use reaction wheels for rotation."

 

You can set the thrusters to only respond to translation inputs:

1. Enable Advanced Tweakables
2. Open the Part Action Window
3. Click Show Activation Toggles
4. Turn off  Yaw, Pitch and Roll

[VoidSquid]

This can be automated by this community MM patch written by @Snark

// Configure RCS to Translate Only by Default
// Author: Snark
@PART[*]:HAS[@MODULE[ModuleRCSFX]]
{
    @MODULE[ModuleRCSFX]
    {
        %enableYaw = false
        %enablePitch = false
        %enableRoll = false
    }
}
 

 

[antipro]

8 hours ago, chd said:

It's too bad there isn't a hybrid setting, "use RCS for translation only, use reaction wheels for rotation."

actually, there's a setting about that:
MJ - SmartRCS - uncheck "use RCS for Rotation"