How do you go about planning gravity assists?

[xcorps]

I just had an ingame epiphany today. I was doing a Minmus science mission with a probe lander designed for Duna (which failed over and over) that I attached a capsule too with the intent of grabbing science from multiple biomes. Worked like a charm, with over 1200 science gained on a single launch, which was a first for me. But that wasn't the best part of the mission, oddly enough.

On my Minmus escape, I plotted a return to Kerbin. Just for giggles, I changed the plot to intercept Mun, and lo and behold the Mun escape gave me an orbit over Kerbin with a Peri of 1.2Mk. So for a 92.3 Dv burn to Mun, then a 435 Dv burn to bring Peri to Kerbins surface, I saved almost 500Dv on the return. I've never actually done this before, but I see a lot of potential in it for interplanetary.

How do you go about making a plan for these assists? Do you just drag the node around until it shows the orbits you want? That seems tedious as all holy hell.

Edited April 25, 2014 by xcorps

[bsalis]

Only time I have really tried to plan grav assists was using Tylo to get to Eelloo from Laythe. Done this twice, both times with lots of time warp and fiddling with maneuver nodes.

[FREEFALL1984]

I generally don't bother using gravity assists within kerbin SOI unless its climbing from the mun to minmus in the early game, I sometimes use them for interplanetary insertions but the precision required is rarely achievable so it means a fair bit of course correction. But here's a simple method for single munar gravity assist to duna. Firstly, timing is everything, Firstly you need to make sure the moon will be in the right place at the right time, and many times duna and the mun will be up to half a month out of phase so you will have to make do and best guess it but this will cost a lot moreDV than if they're properly in phase. This is quite tough to explain so bare with me.

1 - For a single body assist (which is much easier) your burn to the mun needs to be performed just before (about 15 degrees before) you hit midday on the sunny side of kerbin. Obviously this means the mun needs to be in the correct location so you hit its SOI at apoapsis, for me this is generally done be eye and as educated guesses although I'm sure someone around here could do the maths. This all needs to be timed so you reach munar periapsis in the Duna transfer window (roughly) and that the mun is located 15 degrees after kerbin midnight.

2 - Burn towards the mun but make sure your trajectory will take you prograde around its far side and before you do this though make sure your inclination matches exactly since any inclination differences prior to mun encounter will be greatly amplified due to the muns relatively small radius, and then try and get your peri as low as safely possible 9km should suffice.

3 - you can burn at peri and perform your powered gravity assist, use this to straighten out your kerbin SOI Vector,

4 - after you leave the muns SOI, make sure you'll be traveling towards kerbins prograde vector just as you exit kerbins SOI, burn normal or antinormal to compensate (if done properly this should only take less than 20m/s, or if possible burn prograde, dont burn retrograde you'll be killing speed which should only be done if your gravity assist caused you to overshoot duna. if the big yellow line (kerbols SOI trajectory) appears to pass through kerbins SOI then you're on the right track

5 - while still in kerbins SOI check your intercept with duna, tweek the course if needed by burning normal or antinormal or prograde or wait until you leave kerbin SOI if you need to burn retrograde, since burning retrograde now can mess up your kerbin exit trajectory and only burn prograde just before you exit SOI, the important thing is that your kerbol apoapsis is on the other side of kerbol from your current location.

6 - As soon as you leave Kerbins SOI perform a corrective burn, this will be a lot bigger than if you came straight from an LKO since your timing has been a lot less precise. possibly up to 100-200 m/s or more depending on the quality of the gravity assist you may then need to tweek the inclination of your orbit at AN/DN to get a dunan encounter.

Performing a double gravity assist follows the same principles but requires a lot more precision. This is generally easier in systems like jool since there are a lot of large SOIs to hop through, You first need to encounter the mun about 90 degrees earlier than with a single gravity assist, the mun need to then fling you into a minmus encounter at about 15 degrees before kerbin midnight and you need to then burn at minmus peri as before, the chances of both moons aligning exactly when duna is in its launch window is slim to none and you'll likely end up closer to jool than duna since this can save you a hella lot of DV, also your inclination changes need to be precise and early to account for minmus inclined orbit, in fact it makes a lot of sense to only try this when minmus is at an AN or DN although this only happens twice per year.

[KerikBalm]

I've often wondered about using the Mun for interplanetary launches... with an orbital period of 1 day 14 hours, most transfer windows (lets say all except Moho?) are sufficiently "wide" that you should be able to use the Mun during Kerbin escape.

Of course, the farther from the optimal time, the more dV needed... so I've heard the Mun is sort of barely worth it in most cases.

[FREEFALL1984]

Indeed while good for some nearby Bodies, the lack if accuracy causes it to be a nono for moho and eeloo, also don't forget the days on the mission counter are only 6 hours long now

[Superfluous J]

with an orbital period of 1 day 14 hours

You mean 6 days 2 hours.