Interplanetary transfer from Mun

[Bobe]

Generally speaking, is it more efficient (ie. requires less ÃŽâ€v) to perform a Hohmann transfer burn from an orbit around Mun or Kerbin? Ignore fuel already burned to get to Mun, assume both transfers begin with same amount of fuel.

I would think that, if your desired ejection angle was 150 degrees for example, then if Mun was at 150 degrees relative to Kerbin prograde, that burning Mun prograde would effectively have the same result as burning at 150 degrees from Kerbin. And since you're already travelling faster relative to Kol, it would require less ÃŽâ€v, correct?

I don't know if MechJeb factors this in to its calculations, but I'm taking a gamble right now and travelling to Mun. Maybe I can see when it schedules the transfer burn. Still 4 minutes left on the burn with these LV-Ns.

[SnappingTurtle]

Yes, if you can refuel at the Mun it's better. You already have the high GPE and both the Mun's velocity, which would be the same for any object at that altitude, and your orbit around the Mun.

[Eric S]

Yes, ignoring all of how you got there and how you refueled, a transfer directly from the Mun's orbit is more efficient. However, you can ramp up the efficiency if you want to try for a maneuver with a tighter margin of error.

If instead of burning in the direction of the Mun's prograde, if you burn retrograde, dropping your periapsis down to 70-100 km, and then burn ship-prograde at that point, the Oberth effect kicks in, saving you even more. The timing can be tricky, so don't try it if you're not willing to experiment. I strongly recommend quicksaves before the leaving-Mun burn if you try this.

[Captain Sierra]

Yes, ignoring all of how you got there and how you refueled, a transfer directly from the Mun's orbit is more efficient. However, you can ramp up the efficiency if you want to try for a maneuver with a tighter margin of error.

If instead of burning in the direction of the Mun's prograde, if you burn retrograde, dropping your periapsis down to 70-100 km, and then burn ship-prograde at that point, the Oberth effect kicks in, saving you even more. The timing can be tricky, so don't try it if you're not willing to experiment. I strongly recommend quicksaves before the leaving-Mun burn if you try this.

70-100km around the Mun? Screw that! I've made ejection burns from single digits! In all seriousness, you can safely get down to about 8-10km. The lower you get the more you maximize the Oberth effect. Also, to give yourself an extra speed kick, you want to always burn to the Mun's prograde (assuming the Mun's prograde is in the correct position to reach your target planet).

[DeMatt]

70-100km around the Mun? Screw that! I've made ejection burns from single digits! In all seriousness, you can safely get down to about 8-10km. The lower you get the more you maximize the Oberth effect. Also, to give yourself an extra speed kick, you want to always burn to the Mun's prograde (assuming the Mun's prograde is in the correct position to reach your target planet).

Pretty sure Eric S is suggesting a burn that slings him around Kerbin, getting a gravity assist AND the Oberth effect. In which case anything much below 70km periapsis would have seriously negative side effects.

[Eric S]

70-100km around the Mun? Screw that!

Pretty sure Eric S is suggesting a burn that slings him around Kerbin, getting a gravity assist AND the Oberth effect. In which case anything much below 70km periapsis would have seriously negative side effects.

Correct, I really should have been more explicit, that's burn Mun-retrograde to give yourself a low Kerbin periapsis. If you're going to Duna, for example, you'll only need a 250 m/s or so burn to depart the Mun, then another 200-300 m/s burn at kerbin periapsis to get the desired ejection velocity (not certain of the numbers, when I do this I usually use Minmus). The only reason I'd orbit that high around the Mun is if I'm going for "High Orbit" research or SCANSat work.