Transfer launch from Mun orbit: what altitude?

[Hotel26]

(Almost certainly discussed elsewhere but I haven't hit it with a keyword search.  Appreciate a pointer to another topic if you give it.)

I'm making my first, serious attempt at launching interplanetary ops from Mun orbit.  I tried this some time back and chose a 2-megameter orbit (near the edge of the SOI), thinking it was so close to escape velocity, it would be economical, but the orbital periods (and e.g. time to send supplies up) were too distracting, especially when planning launch windows.

I think a much lower orbit altitude (for a space station) might be more suitable.  Could it be, that maximum tolerable orbital period is the key deciding factor?  An hour or two...?

What are people's choices/opinions and why?  Please advise...

 

Edited February 18, 2020 by Hotel26

[RealKerbal3x]

@Hotel26

The Mun has a fairly low mass, but it'd still probably be best to stage your ships from low orbit to benefit from the Oberth effect.

I would build an interplanetary transfer station in LKO, though. Launching from the Mun just makes it harder, as you'll have to line up the position of the Mun with your ejection angle, while still remaining in the transfer window.

[papuchalk]

i never tried, so i only guess, but i think that on lower orbit your ship will have much higher speed, than on much larger orbit. That means higher initial speed when launching the ops.. and to leave SOI of kerbin you need speed, right?

Also you have a good point with orbital periods. You will do engine burns on different phase of the orbital circle , on different side of the mun

Simply said you need to match Mun phase and Ship orbital phase to have the best result

For going to eve you need to have Mun going backwards around Kerbin, and your ship must be on backwards period around Mun too

For going to Duna, you need to have Mun going frontwards around Kerbin, and your ship needs to be on frontwards period around Mun too

Mun direction and speed is also giving you kick, i think

 

Now thinking about that, it might be pretty hard to find the best interplanetary trasnfer window and the sweet point of Mun together. because sweet point of Mun is happening only once per month (but is probably one week long ?)

Edited February 18, 2020 by papuchalk

[N4CDLAW]

I have a refinery in Mun orbit I use to refuel interplanetary missions. Refinery has a 55k equatorial orbit, I think. That altitude is mostly based on now-obsolete rules for how fast you can time warp based on altitude, but it works well enough I still use it. Once I undock, set a maneuver node with rough prograde dV, zoom out to Kerbin system view, advance node orbit so that you're leaving the Mun when Mun is in best position (between Kerbin and sun for inner planets, opposite side of Kerbin for outer), then tweak position on that orbit to start burn from to adjust departure trajectory. Tweak dV variables and exact burn time to get planetary intercept. I've done this for every planetary body in the game. 

I have done this from Minmus orbit and the orbital period was just too great to nail launch windows. Inclination didn't help, either. Mun orbit is perfect, haven't noticed any problems even with narrow launch windows (looking at you, Moho).

Hope this helps.

[Superfluous J]

Lower is better for Oberth but really so long as you're not crazy high (Yes, your edge-of-SOI orbit was the worst possible one) it doesn't matter that much. I'd make ease of rendezvous and docking a higher priority than absolute maximum dV savings.

[Vanamonde]

Although you are talking about doing this in the game, KSP accurately models the physics involved with what you want to do. Therefore, I have moved your thread to the science sub, where knowledgeable people can tell you all about Oberth and transit burns. 

[Wjolcz]

Launching stuff from the Mun makes sense when you can build and/or refuel there (though the refuelling part really depends on the design).

IRL it actually makes sense because launching stuff off the surface can be done with minimal effort and fuel (or even no fuel if we assume maglev is possible) as you don't need to power through all the air on your way up and then care about orbital decay due to drag (though the Moon has its orbital problems too).

Whenever I do interplanetary in KSP I start building in LKO (160km). It's simply much easier that way and KSP is much more forgiving than real life when it comes to orbital speeds and fuels.

Edited February 18, 2020 by Wjolcz

[Terwin]

The optimal conditions for starting your journey would include starting with a minimal dv requirement(a high orbit helps with this, but only if you fully refuel after getting there), and a very high velocity for the burn(max velocity is at Pe, and is higher with a larger the difference between Ap and Pe)

Practically speaking however, this is not really needed in KSP.

Generally I will fuel-up at Mun, Minmus, or a LKO fuel station and from there try to figure out how to get to my destination with my available dv.

Occasionally I have used a kerbin fly-by when starting from the Mun or Minmus, but not often.(more often when traveling between Mun and Minmus, as I rarely bother with windows)

LF tanks and nerva work pretty well, boosted with ISRU and a full ore tank for landings and round trips(topped off a Mun and again at Ike/Gilly/whichever small Joolian moon is easiest to catch).

[magnemoe]

I do this with the huge base throwers from Minmus but Mun is easier. 
The important part is that you have to drop Pe down to around 100 Km above Kerbin Burning directly from Mun orbit might easy be more expensive than burning from LKO.

Set up an satelite in low Kerbin orbit, now plan the burn on this. You now plan an burn on the ship in mun orbit so it dips down to 100 km, You probably need to do an extra orbit who you adjust for time and also inclination. This saves you around 1 km/s, so you do not need an large transfer stage going to Duna or Eve, or you can go faster.