Minmus->Mun Slingshot Escape Practicality?

[COL_Tatticky]

So, I got the Kethane mod and designed a really nice mosquito to siphon it from Minmus in hefty 192 tonne chunks. Now I want to expand this into some full-blown orbital infrastructure and launch all interplanetary missions from Minmus in the future (if possible).

I was wondering if I could use a gravity assist off Mun to aid in the escape from Kerbin's gravity well. At first glance, I see two options:

1. Pass behind Mun (more than once if necessary) to increase orbital velocity directly.
   
2. Pass ahead of Mun to enter a highly eccentric orbit, then make good use of the Oberth Effect with a direct escape burn <100km above Kerbin.
   

Option B looks like it might use more delta-V, but it also looks a whole lot easier (indeed, I'm not sure that A is even possible). The two would additionally have wildly different ejection vectors to consider (although that might be a good thing to keep my options open within individual launch windows).

I've calculated (you might want to check my math) that Mun passes directly between Kerbin and Minmus every 159570.67 seconds (1 day 20 hours 19 minutes 30.7 seconds), so that's the period for launch windows. Minmus's rotation only takes it 0.8217243 rad (relative to Kerbol) in this time, though, so that may limit which interplanetary launch windows can be accessed through this method.

Other questions I have involve how best to embark from Minmus itself, including which direction to rotate around the planet, whether it would be better to start from low orbit or a high orbit resonant with the launch window, and (if I start from high orbit) whether it would be better to burn inwards first to use the Oberth Effect or to just escape Minmus directly.

Edited September 2, 2013 by COL_Tatticky

[Scarecrow88]

A possible problem of using Minmus as your starting point is that it's in an inclined orbit which may not work to your advantage.

[COL_Tatticky]

A possible problem of using Minmus as your starting point is that it's in an inclined orbit which may not work to your advantage.

Hm... I hadn't thought of that one.

Unfortunately it applies to all Minmus embarkations, Mun slingshot or no; but hopefully I should be able to correct for it. I've no idea how to calculate the delta-V needed for that, though...

[Zaeo]

Not entirely true scare. If you position your launches and wait to launch until it is aligned with the equinox, it is very viable. You just cant launch as you please.

You could even launch into low minmus orbit, then make a burn into interplanetary space once you are aligned.

[arq]

I think your best bet might actually be to drop from Minmus orbit to Kerbin Peri <100km and then boost from there out to wherever you want. The drop costs less than 200m/s from Minmus orbit and then you will have a low Kerbin orbit that is 100m/s from escape. So you should be able to escape Kerbin for <300m/s from Minmus with full benefit of the Oberth effect. It's possible Mun may shave a little bit off of that figure, but for the added complexity of lining up that maneuver with a transfer window is a lot for very little gain.

[VoidPointer]

Another possibility is to shuttle your Kethane into low Kerbin orbit and keep a fueling station there. I'm not sure how the math works out on efficiency there, though.

[polle]

just bring empty fuel tanks + RCU and push it back with a trebuchet

[chaos_forge]

Scott Manley has this to say about using Mun for gravity assists (skip to 4:00):

If you can't watch videos, what he says is that the offset from lower Oberth Effect actually makes you spend *more* fuel

[Scarecrow88]

Not entirely true scare. If you position your launches and wait to launch until it is aligned with the equinox, it is very viable. You just cant launch as you please.

You could even launch into low minmus orbit, then make a burn into interplanetary space once you are aligned.

That's why I said it's a possible problem.

[Jackissimus]

As was said here, the best way is to use Kerbin slingshots. Unfortunately it will limit your launch windows and it will become a hassle after a while. I didn't use this method much, but if you do, put a report here, I am interested!

Without using slingshots, it's actually best to launch from Kerbin to take maximum use of the Oberth effect. It's true for most planets except Eve and Duna. Read more about that here: http://forum.kerbalspaceprogram.com/showthread.php/27066-Advantages-of-high-kerbin-orbit-refueling?p=330695&viewfull=1#post330695

[COL_Tatticky]

As was said here, the best way is to use Kerbin slingshots. Unfortunately it will limit your launch windows and it will become a hassle after a while. I didn't use this method much, but if you do, put a report here, I am interested!

"Kerbin Slingshots"? I had been lead to believe that passive gravity assists were impossible when starting from inside the gravity well?

Without using slingshots, it's actually best to launch from Kerbin to take maximum use of the Oberth effect. It's true for most planets except Eve and Duna. Read more about that here: http://forum.kerbalspaceprogram.com/showthread.php/27066-Advantages-of-high-kerbin-orbit-refueling?p=330695&viewfull=1#post330695

That doesn't account for the possibility of dropping to a highly elliptical orbit first which is exactly what I was suggesting.

Scott Manley has this to say about using Mun for gravity assists (skip to 4:00):

I've seen that video; in fact that's exactly what inspired me to look at the Mun slingshot possibility in the first place: making it pay is a challenge.

If you can't watch videos, what he says is that the offset from lower Oberth Effect actually makes you spend *more* fuel

What he was talking about is a completely different thing from what I am talking about. What I am talking about here either doesn't need the Oberth Effect (the Mun is doing the hard work for us) or has a very strong Oberth Effect (the final ejection burn is done at <100km).

The most useful responce thus far has been this one:

I think your best bet might actually be to drop from Minmus orbit to Kerbin Peri <100km and then boost from there out to wherever you want. The drop costs less than 200m/s from Minmus orbit and then you will have a low Kerbin orbit that is 100m/s from escape. So you should be able to escape Kerbin for <300m/s from Minmus with full benefit of the Oberth effect. It's possible Mun may shave a little bit off of that figure, but for the added complexity of lining up that maneuver with a transfer window is a lot for very little gain.

So what I'm looking at now is setting up the capability to choose between entering HEO directly, or using Mun to break; and in either case burning near Kerbin to eject.

So, let's assume that I decide to brake using Mun to enter HEO as described in option B of the OP. Suppose I burn to escape Minmus directly from a low circular orbit. What's the delta-V cost to do this, the angles/altitude of the launch window, (this should work similar to interplanetary launch windows) how can Icalculate the orbit after the grav assist, and (most importantly) what will the escape trajectory be?

[Jackissimus]

"Kerbin Slingshots"? I had been lead to believe that passive gravity assists were impossible when starting from inside the gravity well?

That's exactly right. I was using the wrong terminology, mea culpa. I meant dropping from a moon to Kerbin first and doing the burn at periapsis. (Or dropping from highly elliptical orbit)

That doesn't account for the possibility of dropping to a highly elliptical orbit first which is exactly what I was suggesting.

Correct again. Again, I meant "without using the maneuver of dropping from a higher orbit". What should we call this maneuver anyway?