Interplanetary trajectories

[OTHOAB]

What is more efficient:

Leave SOI (with a gravity assist from a satellite) and then burn or burn from the orbit?

Burning from 75k orbit, 100k orbit or 2kk orbit (dV to get the exact orbit sums with an interplanetary burn)?

And.

Is a gravity assist in this game useful at all?

Edited December 13, 2014 by OTHOAB

[cantab]

It's more efficient, by a substantial margin, to do your entire ejection burn from low Kerbin orbit.

Setting up a manouvre to just escape Kerbin's SOI then another in solar orbit to get to the target planet can be a useful technique to find launch windows, but don't burn those manouvres, just timewarp to the second one then do a single burn from LKO.

Mun assists are generally not very useful for interplanetary travel, you're looking at a 10% delta-V saving at best. The Mun just isn't big enough. It can be useful for changing inclination of your orbit round Kerbin though, if for some reason you couldn't launch into the right inclination to begin with.

Planet-planet assists on the other hand are much more useful. Eve and Kerbin are both very massive bodies and with correct flying you just about anywhere by ping-ponging off them. Jool is even bigger and can fling your ship around like crazy. Duna is a bit weak but can be useful, I made two Duna assists to match planes with Dres for example. Moho is generally only useful to help you get to Moho (you can reduce your capture burn by making some flybys first), and Dres and Eeloo are too small to be much help.

[The_Rocketeer]

Gravity assists can be extremely useful, you can make very significant savings on delta-V using a string of well-constructed trajectory-modifying burns. It's also extremely satisfying to make a complicated plan and then execute it well.

As a general rule the harder a thing is to do, the more efficient it will be.

However...

The more complicated the technique to understand, arrange and figure out, the higher the chance that something will go wrong, and therefore the more punishing the game becomes. To get the most from a gravity assist from Eve, for example, you need to time your departure from Kerbin so that you'll meet Eve at 180* around the sun from where you left. In practice, Kerbin and Eve are rarely aligned suitable to do this.

In general, it's not hard to design rockets with an excess of delta-V for the intended mission and 'Kerbal' your way thru it rather than having a carefully calculated NASA-style plan that you stick to rigidly. Different players get their kicks from different styles of play, but "bodging it" is without a doubt the easier way to get results.

Edited December 13, 2014 by The_Rocketeer

[mhoram]

What is more efficient:

Leave SOI (with a gravity assist from a satellite) and then burn or burn from the orbit?

Burning from 75k orbit, 100k orbit or 2kk orbit (dV to get the exact orbit sums with an interplanetary burn)?

I made a Delta-V map that displays the needed Delta-V for your first suggestion without the gravity-assist and compared it to Low-Kerbin Departure-Burns.

The comparison showed that the margin is so large that a Mun-Gravity-Assist is by far not enough to compensate for the Oberth-Gain at LKO. (The orbital velocity of the Mun is 542 m/s and the velocity gain from a gravity-assist is at most twice that number.

Is a gravity assist in this game useful at all?

It is not only useful but also an accomplishment. I was really thrilled the first time I managed to perform a planned gravity assist.

Once I had the problem that my Minmus-Lander did not have enough fuel to return to Kerbin, but with the help of a Mun-gravity-assist he made it back safely.

Edited September 26, 2015 by mhoram
typo

[OTHOAB]

All is clear now, thank you for answers!