Interplanetary Transfer

[Evrion]

Am I doing this wrong?

When I read, or see videos of people going form Kerbin to other planets it seems they wait for the right time to leave LKO and do a single burn to the other planet.

ie:

LKO

wait for right time (or even do the waiting before launch)

burn to a Solar orbit with other planet rendezvous in it.

I've been using a different method:

LKO

Wait for right time to get a push or pull from Mun

burn to slighshot off Mun into Solar orbit.

Align my orbit plane with the other planet

Wait for the right time

Burn to get encounter with the other planet.

So I have two burns: 1 to solar orbit using a gravity assist from Mun. 1 to change solar orbit so I get the encounter desired.

Other people have 1 burn: straight from LKO to a solar orbit with the desired encounter.

I know the "other people" method is more like the NASA version of events and requires WAY less time warping. But I feel like I save on fuel (and thinking) using my method. I also understand the notion of: "do whatever you want, it's your game after all."

Question: who is "right"?

[alexmun]

I think it's going to depend on how much of a boost you're getting from the Mun and how close to the right direction your final ejection vector is.

Ignoring the gravity assist for a second, if you're in a 100km orbit your orbital velocity is ~2246m/s. Escape velocity at that altitude is ~3176m/s giving you a delta-v of 930m/s for a minimal escape from Kerbin's SOI. Using my calculator (http://alexmoon.github.io/ksp) to calculate the solar orbit delta-v gives 854m/s (set initial orbit to 0 to get this value). So your total delta-v would be 1784m/s minus whatever boost you get from the Mun.

Doing a straight transfer from LKO to Duna costs 1046m/s so you'd need to be saving 742m/s from your gravity assist for it to be cheaper.

Transferring to Jool requires 2802m/s from solar orbit vs 1996m/s from LKO. Adding in the escape velocity delta-v gives 3732m/s meaning you'd need to save 3732 - 1996 = 1736m/s. The largest boost you can get from the Mun is roughly double its orbital velocity or 1083m/s so that's probably never going to be more efficient.

Of course, if you could incorporate a Munar gravity assist into a single ejection burn from LKO and end up on the correct transfer trajectory that would be the most efficient, but calculating that would be a challenge!

[Evrion]

Hmmm, I largely ignore numbers while playing, but I am interested in what is more efficient.

It seems here that if direct shot from LKO is more efficient, then it's the orbital velocity from being in LKO that is helping here. The question then becomes comparing the speed boost you can get from Mun to the speed boost you get from LKO.

FYI: My Mun assist is usually done in such a way as to put me in a solar orbit that is either larger or smaller than Kerbin's, depending on my intended destination.

I do not have the patience to wait for the Mun AND the intended destination to line up, though i know that would be the theoretical optimum.

So, I may just get less lazy and do some calculations (I'm thinking energy methods: Eg + Ek stuff, I haven't yet figured out how this delta-V stuff fits with the physics I know, so maybe that would be best) Or even better, run an experiment and try both methods.