How To Go From Low Kerbin Orbit to Jool Orbit for 1051 m/s

[PLAD]

I've sent a few probes to the Jool system as warmup for an eventual Jool-5 mission. At first I used direct missions which cost about 2000 m/s from LKO to Jool aerobraking, but I've seen a few missions in these forums that use flybys to reduce that to around 1200-1300 m/s. So I wondered just how slow one can go and still get there. And I think I have the answer.

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1051m/s. This means that getting from LKO to Jool orbit takes less dV then getting from LKO to a low Munar orbit!! (which is about 1130m/s minimum) Even if you don't use the Mun flyby this is still true. I love flybys. I finally got some hard data on the value of a Mun flyby too- it is around a 60-80 m/s savings for a trip to Eve.

I made a few mistakes in my mission, like leaving 15 minutes late for Mun and not launching into an inclined orbit, so I think someone could knock 10-20m/s off of this, but I don't see a way to get to Jool for less than that.

EDIT July 4:Doh!! See 4 posts down.

To summarize for others who may wish to try this route:

-Use a node editor, Mechjeb has one and there are others. It is brutally difficult to set up a flyby without this.

-Leave Kerbin between UT Y1 day 146.0 and 148.0, with around 147.0 being best. Go outside this window and the dV to get to Eve starts climbing dramatically. Note the Z-axis, or normal velocity of -110m/s which becomes +20m/s if you use the Mun. Note I had to leave late on day 147 because I had to wait until Mun was in the right position, yet another problem with using a Munar flyby.

-Arrive at Eve as close to UT Y1 day 195 and 17 hours as possible. Try to arrive a few hours earlier if you left Kerbin early on day 146 or a few hours later if you left Kerbin late day 147. This is the tightest window in the flight (except for Mun if you do that). If you arrive at Eve on D197 the flyby will have to be in its atmosphere (i.e. impossible), and if you arrive earlier the delta V required goes way up and eventually a big maneuver will be required after the 2nd Kerbin flyby to get to Jool.

-The first Kerbin flyby is around Y1 D296 and a few hours. If you flew by Eve at the right time you will have no choice but to arrive at Kerbin around the right time, just be sure to set it up before the Eve flyby so it is cheaper. Adjust the orbit between the 2 Kerbin flybys so that it has a period exactly equal to 2 Kerbin years, and make its plane be as close to Jool's orbital plane as you can tell by eye. That will be good enough. You will have the problem that you cannot set Kerbin as a target while you are in its sphere of influence (SOI), but if you set the period of the orbit right, as soon as you leave Kerbin's SOI after the 1st flyby it will be easy to set up the 2nd Kerbin flyby.

-As always with flybys, set up the Jool encounter before the 2nd Kerbin flyby. The Jool arrival time is quite variable, it soaks up any errors you may have made earlier. Earliest arrival can be around Y3 D71, to as late as Y3 D200 if you arrived at Eve late and just skimmed its atmosphere.

Using a Mun flyby is tricky, I wouldn't try it until you have a some flyby experience.

I wouldn't have studied flybys if it hadn't been for Jasonden's 'Inspiration Kerbal' flyby challenge and then Otis' 'Lowest delta-V to Moho' challenge, if you guys read this, thanks!

Good luck!

Edited July 4, 2014 by PLAD

[spiritplumber]

Wow, awesome!

I think this is what they do IRL...

[Jasonden]

This is crazy man! I was considering entering your lowest dV to Eeloo challenge but I can see now that it's out of my league I may try anyway, but I don't know that your entry can be beat!

[Stratzenblitz75]

Very nicely done! I'm amazed at how little Delta V you used and how you planned this out so that you received a gravity assist after every orbit to arrive at Jool in less than 3 years. If I were doing this it would probably take 10 years lol.

[PLAD]

Very nicely done! I'm amazed at how little Delta V you used and how you planned this out so that you received a gravity assist after every orbit to arrive at Jool in less than 3 years. If I were doing this it would probably take 10 years lol.

Thanks! Although the sad truth is that my tools can't do multi-orbit Lambert calculations so I was forced to have a gravity assist after every orbit. One probably could knock a few m/s off by using some multi-orbit transfers in there, so I don't knock long travel times unless it's part of a challenge.

[PLAD]

So I'm trying to find a way to get to Eeloo with as little dV as possible, and I've found that such a mission has to fly by Jool on the way to Eeloo. So I've revisited this Jool mission to see if I can squeeze a little more out of it. I've tried to give some tips on how to cut your delta-V usage to the bone. I did it with 1011m/s!!

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Can it be reduced even more? I think so. For reference the next numbers all refer to starting from a 75x75km orbit and not using a Munar flyby (which we know would save 87m/s).

-This minimum start for the initial K-E in this window is 1068m/s. But you can get to Eve for as little as 1025m/s in other windows. Could we start with that? No. I found you cannot go K-E-K and arrive back at K with more Vinf than you left with if your start boost is less than 1044m/s.

-So can we start with 1044? Not exactly. When flying K-J you must leave the last K with a Vinf of at least about 2670m/s. But I have found no K-E-K-K-J window that starts with less than 1068m/s that gets up to 2670m/s by the time you head to Jool.

-I did find a route that theoretically gets to Jool with a starting dV at Kerbin of 1044m/s. It goes K-E-K-K-E-K-K-J*. So 24m/s less than 1068 but with 3 extra flybys. I budget 5m/s per flyby so your net savings would be about 9m/s over the mission shown above.

Conclusion: I hereby contend that you cannot get to a low Jool orbit from a 75x75km Kerbin orbit for less than 997m/s, except by random luck which might save a couple of m/s.

(Or maybe there is a multi-orbit solution that can do a little better? Or a small periapsis thrust at Eve? Argh!)

*Here are the start and encounter days if anyone is crazy enough to try it, enter them into LambertE to get all the mission data:

K817.4-E853.6-K932.4-(dbl.96)-K1358.5-E1394.9-K1481.9-(dbl.435)-K1694.95-J2300.

Note that you'll orbit the sun 5 times between the first two K's.

[Stratzenblitz75]

Once again, fantastic job! I'm simply astounded that you managed to shave 40 m/s Delta V off an already amazing record! I guess your next goal is to do it with less than 1 Km/s Delta V. Also THANK YOU for making the flyby calculator. I've been looking for something like this for a very long time and I haven't had much luck. I know about alex.moon's excellent launch window planner, but that really doesn't calculate for multiple flybys. I'll definitely be using it for my future missions!

[technion]

Wow I wouldn't have believed it!

But is there any way at all to do this beyond massive fluke? I can't see how you could plan an ejection to Eve with a Munar assist.

[PLAD]

Wow I wouldn't have believed it!

But is there any way at all to do this beyond massive fluke? I can't see how you could plan an ejection to Eve with a Munar assist.

The following data assumes you're using a path that will leave Kerbin's SOI at about 935m/s, which is the case for my 1011m/s Jool mission. You can get V when leaving SOI from TOT, FF, or LambertE.

I make a rough estimate by knowing that Mun will turn your path by about 29 degrees, and Mun will move about 25 degrees in the 2h 45min it takes to get to it, (empirically determined) so I set up an initial ejection-to-Eve burn when Mun is about 54 degrees retrograde from the direction a burn that doesn't use Mun leaves Kerbin's SOI at (don't execute it, just set it up and use a protractor). I then note the altitude above Mun's surface that that flight would need to go to get to Eve and use the empirically determined rule that a change in 1 minute in the boost time changes the required Mun periapsis by 1 km, at least for an hour or so before and after the optimum time. Since you can't see the Mun periapsis altiitude when it is below the surface you should shoot for a too-early initial trial. A big problem is that a low orbit around Kerbin has a period of about 30 minutes, so you can only execute your boost to Mun at a certain instant every 30 minutes. If you can live with your Mun flyby being up to 30km above the optimum possible altitude then just use the window closest to that. Otherwise you will have to change your launch time (hit F5 before you launch) to compensate.

I think of it as trial-and-error- if you're not picky then just a couple of trials will do, if you want to squeeze the last possible m/s out of the flyby then 5 or 6 might be needed. Remember each minute of error reduces the benefit of the flyby by 0.5m/s, at least for an hour or so before and after the optimal time.

Somebody ought to write something that gives you the optimal launch time for a Munar flyby if you give it the planetary transfer dates, like 'I want to leave Kerbin around day 147.0 and get to Eve at day 195.6, when is the best time to leave Kerbin and use the Mun?'. Hmmmm...

[Nich]

The 30 minute orbital period is not necessarily true if you have 45 minutes to your launch window and you are at the latitude longitude you want to launch from you cansimply do a pre-burn and extend your AP so you have a 45 minute period. This will allow you to burn exactly when you want to to get your munar flyby.