Mechjeb, landings and the fear of insanity

[XrayLima]

I started writing this question below, but then managed to work it out myself.  Thing is it annoyed me for about an hour until I had a lightbulb moment.  So I thought I'd post it, and if others search for it it is already answered.

"Ok, I think I'm going mad.  About a month ago, I build a happy little lander to shuttle folks to and from my munbase.  Nothing special there, seemed normal.  Mechjeb happily put it where I wanted it, so much so I lost my second lander when I forgot to add an offset and mechjeb landed the lander ON TOP of the base, then it fell off.  The I go away for a month to work, come back and now mechjeb can't put it any closer than 1500m from target.  Annoyingly I can't remember if I added a bit of extra fuel prior to this save, but I still have a TWR of 1.70 before Mechjeb starts the de-orbit burn, but when it starts it the predicted target difference climbs away to over 3.5km before slowly coming back a small amount to land me 1500m downrange.   Is there a minimum TWR for de-orbit burns with Mechjeb?  It's almost as through it's waiting too long before starting the de-orbit burn.  Thoughts?"

The answer is starting orbital altitude.  I increased my Ap and Pe and it worked.  It is a more Dv costly, but it gave Mechjeb the time it needs to complete both the de-orbit and braking burns.  When I started lower, my TWR was to low for the Mechjeb logic to complete both tasks correctly and it wasn't able to sufficiently correct for this.  It seems to assume that the de-orbit burn and braking burn do not overlap and if they do it doesn't try and start the de-orbit sooner or burn the braking turn harder.  either way, it misses.  I'm going to test but I think the other solution is to increase TWR, but that increases mass and therefore lowers Dv across the board.

 

Anyway, just something to add to the forum pile of knowledge.