The Mun(or Mün) ruins efficient escape burns!

[Wizard Kerbal]

    I just did a mission to gilly, and I was planning to do 2 high-velocity escape burns. 1  960 m/s to reach kerbin’s SOI, then another one I can’t remember the cost of to escape kerbin. But I had to plot these 3 orbits in advance because The mün would throw my ship off course. The same thing happened with my duna ship!

[paul_c]

Why 960m/s first one?

[camacju]

I disagree - I think the Mun helps escape burns rather than ruins them. I've used Mun assists in many missions to get somewhere for much less delta-v than it would otherwise take.

[Wizard Kerbal]

On 3/13/2021 at 7:02 AM, paul_c said:

Why 960m/s first one?

To reach the edge of Kerbin’s SOI. When you fall from that height, It saves DeltaV doing your final escape burn.

 

On 3/13/2021 at 10:44 PM, camacju said:

I disagree - I think the Mun helps escape burns rather than ruins them. I've used Mun assists in many missions to get somewhere for much less delta-v than it would otherwise take.

I’m not that good of a player. I’ve only aerobraked successfully once, I’ve only unsuccessfully used a gravity assist. Lower your expectations 

[jimmymcgoochie]

You’re not actually helping yourself by going all the way up to the edge of Kerbin’s SOI and then coming back down to complete the transfer- just raising your apoapsis half way to the Mun is usually more than enough to make an efficient escape burn at the next periapsis without waiting for many days to get all the way up and all the way back down again. You only need to do periapsis kicks if you have a very low thrust to weight ratio, e.g. using ion thrusters or nuclear engines, and/or a very high delta-V burn to get where you’re going, otherwise just take along 20% more fuel than you think you’ll need, make the burn in one go and then correct your course afterwards if necessary.

[paul_c]

2 hours ago, Wizard Kerbal said:

To reach the edge of Kerbin’s SOI. When you fall from that height, It saves DeltaV doing your final escape burn.

 

It doesn't "save" anything net; what it DOES do, is gives you the ability to do a big burn which might be more inefficient, or just inconvenient, to do over a longer burn in 1 go.

You'll notice though, that a 700 m/s burn takes you a fair way there (about 70% of the way there, if "there" is leaving Kerbin SOI and its roughly 1000m/s to do so). But doesn't make the Ap quite so huge. As the Ap gets larger, of course so does the orbital period (time taken to do 1 orbit) and the speed on the high altitude bits gets slower and slower too, so its a double effect.

So I'd advise NOT going to the edge of the SOI with a 960m/s burn, instead if it needs to be a double burn, do ~700m/s first time, then next time just keep burning and burning until the appropriate parameter is reached. This also has the advantage of not potentially encountering Mun/Minmus 6 times, as you've noticed!!