KEKKJ/KEEKJ gravity assist launch window list?

[Eddy119]

Hi,

I am wondering if someone can make a list of KEKKJ/KEEKJ gravity assist launch windows (other than year 2 day 160) to Jool. I can't get the gravity assist calculators (like KSPTOT or https://kerbal-transfer-illustrator.netlify.app/Flyby) to work (or please tell me ways to calculate this). I want to know if it is possible to do this at a regular (ish) interval in time. I guess any multi flyby that is <1200m/s is also nice. I've been wondering about this for years... I think in general Jool has to be opposite to Kerbin at launch (or you miss Jool even if you can get it to intersect Jool orbit) but I'm not sure about the specifics? Thanks...

[Zhetaan]

On 7/15/2023 at 8:31 PM, Eddy119 said:

I can't get the gravity assist calculators (like KSPTOT or https://kerbal-transfer-illustrator.netlify.app/Flyby) to work (or please tell me ways to calculate this).

I can at least help you with this part.  Here's a link to a relevant comment in the FlybyFinder thread by its author, @PLAD:

On 5/23/2014 at 1:09 PM, PLAD said:

Here is the latest version of my Lambert spreadsheet, I use it to test my algorithms, and to doublecheck that FF is giving good results. It can do double flybys. It is not made for casual use and is not user friendly, but you can see the entire process that FF uses, step by step. It now does 6 bodies and includes the OPM planets. If there is interest I can make a version for Galileo's Planet Pack.

https://www.dropbox.com/s/5i2foga1f2j9kq5/LambertHOPMrel.zip?dl=0

-PLAD

The link still works, so there's that.

The thing to note is that most Lambert solvers (which would be the algorithmic wizardry that makes porkchop plots and calculates flybys, among other things), cannot handle transfers that start and end at the same point.  However, this spreadsheet manages to do that by adding a way to track double-flybys through the use of orbit ratios.

It involves a lot of trial-and-error and is not very user-friendly, but in fairness, this very high-level stuff, even for KSP.

[king of nowhere]

while it's very difficult stuff, it's not as difficult as it may seem. first target is eve. from eve you take a trajectory for kerbin; the key is, it must not be an hohmann transfer - in fact, you don't want a hohmann transfer, you want to speed up compared to kerbin.

And if you really can't find the trajectory you want? easy enough: nobody forces you to meet kerbin immediately. you can do another orbit and make a flyby later. To look for one such trajectory, create a maneuver node, leave it at 0 m/s, and right click on it to select next orbit. so you'll find intercepts on the next orbit. click again next orbit, you'll find intercepts two orbits in the future. eventually, you're bound to find a close approach that's close enough you can adjust your trajectory to take it.

the kerbin-kerbin flyby is even easier: the key for a kerbin-kerbin passage is to eject into an orbit that lasts an exact number of years. if you eject into a orbit lasting 3 years, you are guaranteed that in 3 years your ship will come back in the same spot, kerbin will be in the same spot, so you'll have another encounter. you can measure the time of a planned orbit by making a maneuver node in that orbit and selecting it, it will be displayed in the lower left corner. this same trick is used to pass twice on any planet, you need an orbital time multiple of that of the target planet. or perhaps you can take a fractional resonance; for example you eject into a orbit lasting one year and a half exactly, so in 3 years your ship will have completed 2 orbits and kerbin will have completed 3 orbits and you will be in the same spot.

As for jool, same thing as I said for the first kerbin flyby. if jool is close enough when you pass near apoapsis, you can make a slight correction in your gravity assist to get an encounter. otherwise, you will have to complete an orbit, and possibly more orbits, until jool will be close enough that a small correction will get you there.  if you don't have issues with time, then you are guaranteed to find encounters.

 

As a general rule, I found that the key to chaining multiple gravity assists is to not plan them in advance. you cannot plan them in advance, because the game is not accurate after a while. you take the first flyby, then you wait a few orbits if necessary. and if you can find real spacecraft trajectories, you may notice actual orbital engineers also use this tactic. rosetta didn't meet its comet at its first apoapsis. parker solar probe is not meeting venus at every orbit. also, getting immediate multiple assists, like the voyager probes did, requires some very special planetary alignments. making multiple passages delayed by multiple orbits does not require any timing at all. I don't even bother checking transfer windows when using those trajectories.

even when time is a factor, it's generally not a problem. most of my missions use life support. but the thing is, when you are close to the sun maneuvers are expensive, but orbits are short, so you can afford to wait many orbits to find that moho intercept. when you are far from the sun, you may need to burn extra fuel to speed up an encounter, but maneuvers are cheap so you can afford the extra fuel.

 

so I suggest, first thing go to eve. try to get a trajectory that intersects kerbin's orbit, and worry about meeting kerbin afterwards. it's good practice

Edited July 17, 2023 by king of nowhere

[Eddy119]

Thanks for this guys... I forgot about this thread