MechJeb2

MechJeb2

Mods
4M Downloads

Principia node execution

lamont-granquist opened this issue ยท 1 comments

commented

There is an existing mechanism to do this but it requires the PEG builds of MJ:

https://github.com/lamont-granquist/MechJeb2/releases

There is a tap-dance which is required:

In the principia flight planner you need to click:

  • "instant impulse"
  • "show on navball"
  • "inertially fixed"

That does three things:

  • models the node impulsively ("instant impulse")
  • gives MJ a maneuver node to grab the trajectory off of ("show on navball")
  • fixes the node in the inertial frame of the impulse so it does not rotate around ("inertially fixed")

Then:

  • click "display patched conics"

You should see the keplerian tangent orbit coming off of the maneuver node that will closely approximate the planned trajectory (if it doesn't either something is wrong above, or you're into some N-body regime where patched conics just fails).

Then:

  • plan your maneuver.

You should do this after checking the above three boxes, otherwise you'll plan your maneuver then it will change on you when you click e.g. "instant impulse".

Then:

  • manually coast to near the maneuver node before engaging the node executor. the nearer you get the better, until you get too near. depends on how long the burn will last as to how close you need to get.

The problem here is that as you orbit your orbit is perturbed by J2 and N-body effects, and the closer you get the more accurately placed the actual maneuver node and the keplerian tangent orbit is. You want the coast before the node to be as short as possible without overshooting. A minute or two lead time before PEG actually fires the engine is going to generally be fine though and within the tolerances of everything else that is being approximated here, so you don't need to be fussy about getting it down to seconds.

This will be made better, hopefully, by phase II of #1061