Going home from Minmus

[SpacedCowboy]

I currently have an outpost on Minmus (for a contract). I have just enough fuel to make it back. (I think) The sun is directly overhead and the Kerbs just finished lunch. So here's the question: Which is more fuel efficient?; 1. Launch and get into a low Minmus orbit and then wait for the right time to burn for home? or, 2. Just launch straight up, leaving the SOI of Minmus and then burn for home? I'm an "eyeballing, fly by the seat of your pants" kind of guy so, please don't throw any numbers at me 'cause I can't catch them. Just looking for a basic theory. Appreciate any help guys. And to quote Max, "I forgot I was a dumbass!":confused:

Edited July 6, 2015 by SpacedCowboy

[Grumman]

You want to leave Minmus's SOI going in the opposite direction to Minmus's orbit, to cancel out Minmus's orbital velocity. If you can do that by direct ascent, do that.

[eddiew]

I'm fairly sure the answer is the same as to "can I go straight up to Mun?" which pops up regularly

Straight up means a portion of your thrust is being cancelled by gravity all the way. On the ground at Kerbin, you need to expend 10m/s of thrust to travel upwards at a constant speed (or to hover). Once in orbit, that gravity well pulls you sideways instead of downwards, so you waste less delta-v.

That said, Minmus is very small, as is its SoI, and you won't lose a vast amount of velocity before you exit. It'd be an interesting experiment to do both and find out which comes up better ^^

[Kryxal]

I'd say launch east, then drop a node to plan the burn ... if you don't care to do that, make your burn prograde at about 60 degrees on to the near side.

[ForScience6686]

Minimus gravity is to low to make this an issue I think. Launch straight up in the retrograde direction of minimus orbit until you escape the soi. The time saved is worth more to me than the dv cost.

[Coga19000]

Well, I know for a fact that on Minmus gravity, the difference is going to be miniscule, in the order of like, fifths of meters. Now, if you want actual numbers, I'm not exactly sure if the Delta -v expended on circulation or direct ascent gravity losses are greate, but in all certainty, the difference will not leave you stranded above Minmus.

If you want to be on the safe side, try the intermediate route ; launch vertically when facing towards Kerbin, then turn retrograde. You should avoid circularisation costs, AND burn perpendicular to the gravity well.

[pincushionman]

As others have mentioned, you can leave Minmus' SoI pretty easy, but what counts is going retrograde wrt Minmus' orbit of Kerbin. So if you want to go straight up, wait until Kerbin is setting in the west, then launch straight up and go all out. The most efficient would be to launch with Kerbin directly overhead, turn east immediately, and burn out, going upward only enough to clear the mountains.

[SpacedCowboy]

OK guys thanks for the input. I kind of took advice from everyone, especially Coga's. I made it back with just a sliver of fuel left. Five Kerbs saved and mission accomplished! Have a better understanding of what to do in that situation.

[Eric S]

As some have pointed out, a straight up launch is less efficient, but Minmus has so little gravity that it doesn't really matter. Even on the Mun, the straight up launch only costs about 100 m/s more than the mostly-horizontal launch, assuming a munar TWR in the 5-6 range. Just keep in mind that as you take off from larger bodies (or similar bodies but with a lower TWR), you'll tend to have a lower TWR and the difference will become greater.

[DCWarHound]

There is a cheaty way,point your rocket towards your maneuver node for return.Go EVA and put your kerbal behind your rocket,go to map view and go forward with your kerbal.

I quite literally returned from Mun orbit with a small lander after not having enough fuel to return.Unfortunately my lander doesn't have enough drag and i can't revert to assembly so it can never slow down fast enough (i hit 260ms at about 700m even when coming in from a 60km periapsis and burning though 2x heatshields while aerobraking)