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How do I do a realistic Juno mission?


MAFman
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What should I know about planning for a Juno mission to Jool including all of the same gravity assists that the real thing did, in terms of the launch window? I have the spacecraft itself built, and it works well (probably too much delta-V...)

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Posted (edited)

Looking up the Juno mission, I see it did only one Earth gravity assist to get to Jupiter. I've done something similar before to get to Jool for ~1380m/s dV from LKO, but I used two Kerbin gravity assists instead of one.  I first ejected to a roughly 3:2-resonant orbit, lowered my PE  from the AP of that by burning retrograde for about 80 m/s,  then did another another ~37 m/s burn at the second Kerbin orbit intersect of that orbit to set up my first Kerbin gravity assist 1.5 years later.  I used that to put me up into a 2:1 resonant orbit, from which another AP DSM of ~207 m/s allowed me to set up my second Kerbin gravity assist, which was more than enough to get me to Jool. The process is reasonably well-documented in the first few images of my Retrosolar Rescue challenge album here: https://imgur.com/a/5URWb.  If you want to do it that way, you should launch around 5.2 years before a regular Jool transfer window. The first of these which you can use for this happens on Y6, d413 by my calculation, and then another will occur about every 472.5 days after that. So your first launch date should be somewhere around Y1, d 327, or any integral multiple of 472.5 days after that. If you really want to do it with just one assist, then you'll have no choice but to boost directly to the Kerbin 2:1-resonant orbit (i.e. a period of ~852 days), and then do what was my second DSM from the AP of that.  For that you will want to launch around 2.2 rather than 5.2 years before a regular Jool launch window, since you'll only be doing the one orbit before your gravity assist. For that you could actually use the third Jool transfer window in the game, which happens on Y3, d275, for a launch date around Y1, d190.  These numbers may not be 100% accurate, because my estimate of an extra 0.2 years is unfortunately just that, but there will be a decent amount of wiggle room on your second gravity assist if you find yourself coming in ahead of or behind Jool. Anyway, that was probably a fairly bewildering explanation, but if you look at my pix and then fiddle about some, I think you'll find its not as hard to work out as all that made it sound.  Just make sure to fly to LKO like 40 days ahead of the launch dates I suggested and quicksave as soon as you get there, in case you need to move your ejection burn forward or back.  You probably won;t need to, but if you do, whatever the phase angle is between your Jool orbit intersect and the target position marker from your best gravity assist, just move your ejection burn that many degrees forward/back in Kerbin's orbit on the next attempt and it should work fine. Good luck!

Edited by herbal space program
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On 7/1/2021 at 7:41 AM, The Aziz said:

https://forum.kerbalspaceprogram.com/index.php?/topic/33568-winmaclinux-ksp-trajectory-optimization-tool-v167-lvd-kos-integration/ try this tool. Maneuver flyby sequencer, it will calculate a launch window and every correction burn on your way.

Does this actually tell you how to plan these Juno/MESSENGER-type DSM's to get  one or more assists from the body you just ejected from? If it does, I want it!

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7 minutes ago, herbal space program said:

Does this actually tell you how to plan these Juno/MESSENGER-type DSM's to get  one or more assists from the body you just ejected from? If it does, I want it!

It's a VERY thorough tool. Last I checked, Arrowstar is a professional astrodynamicist and works for NASA.

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4 minutes ago, FleshJeb said:

It's a VERY thorough tool. Last I checked, Arrowstar is a professional astrodynamicist and works for NASA.

I'll have to check it out then. I'm able to manage these types of one-body resonant ladder ascents/descents with  Excel, a calculator, and a scratch pad, but I have no clue how to really optimize them.

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Posted (edited)
4 hours ago, herbal space program said:

Does this actually tell you how to plan these Juno/MESSENGER-type DSM's to get  one or more assists from the body you just ejected from? If it does, I want it!

Few cases it helped me create a mission:

Kerbin->Sarnus through Jool, extended mission time by 4 years but saved a lot of dV.

Kerbin->Moho through Eve, still ongoing but saves a bit of fuel on Moho approach

Comet encounters, two times already, both objects on inclined, very eccentric orbits, it set up all maneuvers and can even place a node in the game.

The tool is almost too powerful for its own good, but then it uses a lot of cpu power toto calculate those things, and it can take a while. Though you need to enter the bodies manually, you need to know which planets you're going to use for assists beforehand.

Edited by The Aziz
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On 7/2/2021 at 6:16 PM, herbal space program said:

Looking up the Juno mission, I see it did only one Earth gravity assist to get to Jupiter. I've done something similar before to get to Jool for ~1380m/s dV from LKO, but I used two Kerbin gravity assists instead of one.  I first ejected to a roughly 3:2-resonant orbit, lowered my PE  from the AP of that by burning retrograde for about 80 m/s,  then did another another ~37 m/s burn at the second Kerbin orbit intersect of that orbit to set up my first Kerbin gravity assist 1.5 years later.  I used that to put me up into a 2:1 resonant orbit, from which another AP DSM of ~207 m/s allowed me to set up my second Kerbin gravity assist, which was more than enough to get me to Jool. The process is reasonably well-documented in the first few images of my Retrosolar Rescue challenge album here: https://imgur.com/a/5URWb.  If you want to do it that way, you should launch around 5.2 years before a regular Jool transfer window. The first of these which you can use for this happens on Y6, d413 by my calculation, and then another will occur about every 472.5 days after that. So your first launch date should be somewhere around Y1, d 327, or any integral multiple of 472.5 days after that. If you really want to do it with just one assist, then you'll have no choice but to boost directly to the Kerbin 2:1-resonant orbit (i.e. a period of ~852 days), and then do what was my second DSM from the AP of that.  For that you will want to launch around 2.2 rather than 5.2 years before a regular Jool launch window, since you'll only be doing the one orbit before your gravity assist. For that you could actually use the third Jool transfer window in the game, which happens on Y3, d275, for a launch date around Y1, d190.  These numbers may not be 100% accurate, because my estimate of an extra 0.2 years is unfortunately just that, but there will be a decent amount of wiggle room on your second gravity assist if you find yourself coming in ahead of or behind Jool. Anyway, that was probably a fairly bewildering explanation, but if you look at my pix and then fiddle about some, I think you'll find its not as hard to work out as all that made it sound.  Just make sure to fly to LKO like 40 days ahead of the launch dates I suggested and quicksave as soon as you get there, in case you need to move your ejection burn forward or back.  You probably won;t need to, but if you do, whatever the phase angle is between your Jool orbit intersect and the target position marker from your best gravity assist, just move your ejection burn that many degrees forward/back in Kerbin's orbit on the next attempt and it should work fine. Good luck!

WOW, thank you for your detailed answer!!!

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On 7/2/2021 at 8:53 PM, The Aziz said:

The tool is almost too powerful for its own good, but then it uses a lot of cpu power toto calculate those things, and it can take a while. Though you need to enter the bodies manually, you need to know which planets you're going to use for assists beforehand.

I've resisted getting something like this in the past because I got a kick out of trying to figure these things out in a more seat-of-the-pants way, as if I were Jeb. It almost certainly never created the most efficient solution, but my sense of accomplishment about it was  greater.  It's the same reason I never really used MechJeb.  I wanted my designs and mission plans to be robust enough that I could fly them with my own hands.  But at this point, I feel like learning how to use a tool like the above-linked to plan long chains of assists could be a new frontier for me.

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2 hours ago, MAFman said:

WOW, thank you for your detailed answer!!!

You are very welcome! I'd love to see what your mission looks like when it's done. One thing I didn't mention before is that if you're going straight to the ~2:1 orbit like Juno, you won't need to do quite as big a retrograde DSM burn at AP as I did just to get to Jool. I deliberately sent myself to a higher-than-needed AP to maximize the amount of solar retrograde motion I could get out of my Jool encounter. You could probably get to Jool with only a ~150m/s retrograde burn there, but I have not personally tested that, at least not recently enough to remember the numbers.

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