Interplanetary Intercept from Munar Orbit

[HeliosPh0enix]

I have heard a lot about people sending interplanetary missions from Munar/Minmus orbit after refueling at a space station there, so I made a station with a refueling depot in Munar orbit (I realize now that Minmus would have been smarter, but whatever).  What I cannot figure out, however, is how to get an interplanetary intercept to, say, Duna, from orbit of the Mun.  Where in its orbit around Kerbin does the Mun have to be?  Where in my orbit around the Mun does my ship have to be?  If I am going during a Kerbin->Duna launch window, what is the delta-v change in launching from Munar orbit vice Kerbin orbit?  Thank you for your help.

[bewing]

The angle between your moon and kerbin should be equal to your ejection angle from kerbin to your destination body.

You do a standard burn to leave your moon just like you are going back to kerbin, because you are going back to kerbin. You tweak your ejection orbit from the moon to have the lowest possible kerbin Pe without burning up or losing much speed. 67 to 70km is good. Note that it usually takes one to five days to get from your moon back to kerbin Pe, so you have to plan for that. If that's too long, you can tweak it with a radial-in burn, once you've left the moon's SOI.

Then you set up your maneuver node at your kerbin Pe and burn like hell.

 

 

[Zhetaan]

@HeliosPh0enix:

I'm not so certain about it taking five days to return to Kerbin from the Mun, but to elaborate on what @bewing said, imagine for a moment that you are beginning your interplanetary burn from extremely low Kerbin orbit rather than the Mun.  There will be an ideal time and position to execute the burn.  The trick to starting from Mun orbit is to return from the Mun such that your Kerbin periapsis for the Mun return orbit is at the same location and time as this departure point--with the caveat that the timing will likely be off by a bit.  This usually isn't any great concern but it's worth mentioning; the Mun's orbital period is only six and one half days, so although you won't often get an utterly perfect launch window, it's close enough that you'll get to your destination without trouble.  Six days (three on either side, technically) won't make you miss the window, unlike Minmus where the orbital period is almost fifty days.

Of course, that's easily solved by splitting your ejection into three parts instead of two:  return from the Mun on the orbit before your burn, when the positioning is correct, burn at Kerbin periapsis so that your next pass has you arriving at the proper time (you'll want to raise your apoapsis to get the timing correct; it won't do to waste fuel by lowering it) and then burn to eject before the Mun comes back round to capture you again.

[HeliosPh0enix]

5 hours ago, bewing said:

The angle between your moon and kerbin should be equal to your ejection angle from kerbin to your destination body.

You do a standard burn to leave your moon just like you are going back to kerbin, because you are going back to kerbin. You tweak your ejection orbit from the moon to have the lowest possible kerbin Pe without burning up or losing much speed. 67 to 70km is good. Note that it usually takes one to five days to get from your moon back to kerbin Pe, so you have to plan for that. If that's too long, you can tweak it with a radial-in burn, once you've left the moon's SOI.

Then you set up your maneuver node at your kerbin Pe and burn like hell.

 

 

 

2 minutes ago, Zhetaan said:

@HeliosPh0enix:

I'm not so certain about it taking five days to return to Kerbin from the Mun, but to elaborate on what @bewing said, imagine for a moment that you are beginning your interplanetary burn from extremely low Kerbin orbit rather than the Mun.  There will be an ideal time and position to execute the burn.  The trick to starting from Mun orbit is to return from the Mun such that your Kerbin periapsis for the Mun return orbit is at the same location and time as this departure point--with the caveat that the timing will likely be off by a bit.  This usually isn't any great concern but it's worth mentioning; the Mun's orbital period is only six and one half days, so although you won't often get an utterly perfect launch window, it's close enough that you'll get to your destination without trouble.  Six days (three on either side, technically) won't make you miss the window, unlike Minmus where the orbital period is almost fifty days.

Of course, that's easily solved by splitting your ejection into three parts instead of two:  return from the Mun on the orbit before your burn, when the positioning is correct, burn at Kerbin periapsis so that your next pass has you arriving at the proper time (you'll want to raise your apoapsis to get the timing correct; it won't do to waste fuel by lowering it) and then burn to eject before the Mun comes back round to capture you again.

Thank you everyone for the advice!  Would either of you know the approximate delta-v savings I could get by using this technique (assuming I have topped of my tanks in Munar orbit and any delta-v spent getting there is void) versus launching directly from LKO?  Again, thank you!

[bewing]

The dV savings is what you put into getting to the Mun or Minmus in the first place. 850 to 950 m/s. Plus however much extra fuel you have after your fillup. Minus the cost of your ejection burn from whichever moon.

And that 5 day number is assuming that you do this technique from Minmus, on future attempts.

 

 

[Kryxal]

Figure on not quite 580 m/s savings via this method, and an easier ejection burn ... about 850 m/s to transfer to Mun, vs about 270+ for Munar ejection.  The numbers are better at Minmus, but the timing (and the possibility of passing through the Mun's SoI) are trickier.

My preferred method to set up this sort of thing is to use a planner sat in a fairly low orbit, and just plot the ejection to have the PE at the right point, then burn such that your Kerbin escape is ALSO at the same point.  Same track means same parameters, and don't fuss how you got there...

[Laie]

First off, you down't want to make a direct Mun->Duna transfer. That's comparatively expensive. You want to drop back to a low PE around Kerbin, and transfer from there.

The PE around Kerbin will be about opposite of where you depart from the Mun. IIRC the Mun has a three-day orbit. That means that you will have to wait no more than three days before you can easily set up a Kerbin PE in the desired spot -- if you plan ahead, you will arrive at the right place within give-or-take 1.5 Kerbin days, that is, nine hours. From a mission planning point of view, refueling at and starting from the Mun is much easier than Minmus with it's much longer orbital period.

Now, how you you get to the right PE around Kerbin? The workaround I use goes such:

1. use some probe in LKO in order to plan the departure maneuver.
2. switch to vessel in orbit around moon, select said probe as target, then return from the Mun so that your PE around Kerbin concides with the probe's maneuver node.
3. plot a departure to destination on (or near) the periapsis around Kerbin.

For any destination except Moho, getting to the PE within nine hours of the originally scheduled burn will be close enough. You don't even have to nail down the position of your PE relative to the maneuver node to the last fraction of a hair -- getting close will be sufficient.