Minmus <-> Mun

[ibanix]

Anyone know what the best transfer orbit from is from Minmus to the Mun (or the reverse)?

Thanks.

Edited October 1, 2015 by ibanix

[wadedidit]

MechJeb!

[Superfluous J]

Personally, the few times I need to do it, I just leave Minmus or Mun at the An/Dn of each (you can find this by plotting an escape trajectory with your ship and then targeting the other moon). Burn to intersect the other moon's orbit, and then when at that intersect burn to raise your Pe (for Mun->Minmus) or lower your Ap (for Minmus->Mun) until you get an encounter next orbit. Do this with a Maneuver node to be sure, and don't be afraid to jump forward an orbit to get a better/cheaper encounter.

If you're in more of a hurry, look at the Transfer Window Planner mod. It will tell you when to burn to get a real Hohmann transfer from one to the other.

[nekogod]

You would want to wait until minmus was at one of the two points in it's orbit where it crosses the same plane as the mun so you don't need to do an inclination change. You'd also want to leave minmus retrograde in regard to it's orbit to bring your pe down to the muns orbit.

[ElWanderer]

According to my notebook of transfers calculated by myself (assuming 25km prograde equatorial orbit around Minmus) for playing around with kOS automated transfers:

Phase angle: -151.3 degrees (may be a bit of a wait, especially if you want to be at a node at the same time)

Delta-v: 79.7m/s

Ejection angle: 135 degrees to retrograde

Transfer time: 74 hours

Going the other way is awkward as the ejection angle calculation I use throws an error - to get to Minmus you don't need to enter a hyperbolic escape orbit of the Mun. I get a 200m/s burn when the phase angle is 90 degrees.

The online transfer planners should give you a whole range of options. I can't remember if Alexmoon's site will do Minmus-Mun transfers, but try there first.

[ibanix]

http://ksp.olex.biz/ does not

but http://alexmoon.github.io/ does!

Hm, I didn't realize that. Thanks.

Also thanks for the detailed answer.

[ibanix]

http://alexmoon.github.io/ is giving me very strange solutions, and not for Mun->Minmus either.

I'm trying to do a Kerbin->Dres transfer, and for my particular starting/ending orbits I get:

Kerbin (140.5km starting orbit)

Dres (13138km ending orbit)

UT: 21056544

Prograde ÃŽâ€v 1,517.8 m/s

Normal ÃŽâ€v 208.4 m/s

--

This does not get you ANYWHERE near Dres.

- - - Updated - - -

Yeah, something is either really messed up with the http://alexmoon.github.io/ or with my game. The Kerbal Alarm Clock is giving me transfer window dates that are so far off from optimal according to the calculator that it's scary.

[Specialist290]

http://alexmoon.github.io/ is giving me very strange solutions, and not for Mun->Minmus either.

I'm trying to do a Kerbin->Dres transfer, and for my particular starting/ending orbits I get:

Kerbin (140.5km starting orbit)

Dres (13138km ending orbit)

UT: 21056544

Prograde ÃŽâ€v 1,517.8 m/s

Normal ÃŽâ€v 208.4 m/s

--

This does not get you ANYWHERE near Dres.

- - - Updated - - -

Yeah, something is either really messed up with the http://alexmoon.github.io/ or with my game. The Kerbal Alarm Clock is giving me transfer window dates that are so far off from optimal according to the calculator that it's scary.

My first thought: I believe that there are two different models that Kerbal Alarm Clock can toggle between for transfer windows: One that uses pre-computed hard values, and one that computes transfer windows based on Hohmann equations. One of these is slightly more accurate than the other, but I forget exactly which is which.

My second thought: In the context of your craft, the exact transfer time matters less than your ejection angle at the time of the burn. It may be that the time given by the calculator is the optimal time to depart from that particular orbit, but depending on exactly when your craft reached that orbit, it may not be in the right point at the right time. Launch windows in general are broad enough that even a day or two off from the optimal time is relatively insignificant if your craft has a decent "safety margin" in its delta-v budget.

You may also have to play around a little bit with the maneuver node vectors in order for the game to register a close approach or SoI transition. As with the time, alexmun's calculator also assumes your craft is in a perfectly circular orbit, which is fairly rare.

Edited October 2, 2015 by Specialist290