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Navigating in the Jool System


Hegemon
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How much Delta-V is needed in Keribn orbit (100km) in order to arrive at the Jool system with a reasonable amount of delta-v for navigation? Every time I send a craft to Jool, I get there and either shoot right out of the system, crash into Jool or a moon, or end up stranded in a ridiculously useless orbit.

At the moment I have some frogs stranded on/around Pol and it seems like getting there was a fluke. Appreciate any assistance!

(PS- It doesn't help that once you enter the Jool system, Jool itself becomes untargetable)

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These are my go-to delta-V maps for more complex missions. They'll cover things like Hohmann transfers between Joolian moons, or a direct departure from Laythe to Kerbin. They are pre-1.0 so the atmospheric values aren't valid, but everything else is the same.

https://www.reddit.com/r/KerbalAcademy/comments/1qu5jv/deltav_charts/

I drew this to show how your flyby of Jool should relate to the orbit of your target moon to make a direct powered capture or aerocapture into orbit of that moon.

https://flic.kr/p/vKzszJ

And the final "piece of the puzzle" is that by making a mid-course correction and trading prograde/retrograde and radial/antiradial components of the burn, you can keep a certain trajectory past Jool, for example tangent to Laythe's orbit, and adjust the time of that trajectory, thereby enabling you to encounter Laythe (or any other moon round any other planet) in the place and time you desire.

Edited by cantab
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Jool is a magical land of free dV. All you need is patience and experience, and a willingness to fail a lot before you get it right. And after you get it right, because Jool loves to keep throwing you (out of orbit) with (literal) curveballs in the way of moon encounters.

It's really hard to explain, at least for me, but what you should do is get a ship in equatorial Jool orbit and just play with encounters and maneuver nodes. See what you can do with a Tylo encounter, or a Laythe encounter.

Spoiler Alert: You can do a LOT. You can get into a nice orbit of Jool for FREE with a Tylo encounter. You can also get back to Kerbin for almost the same amount of FREE though that's a lot harder.

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Standard way of getting low into the Jool system using Tylo (but this is actually too close, it'll probably make adjustments for later encounters quite expensive):

Spoiler

H8NQ92r.png

A little less close to Tylo on the flyby giving a direct transfer to Pol's orbit, followed by a second Tylo flyby for a less eccentric orbit ready to visit the inner moons (all the nodes are 0 m/s, just placed to get the next segment of the trajectory to show):

Spoiler

FZR7bXg.png

 

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6 hours ago, Hegemon said:

How much Delta-V is needed in Keribn orbit (100km) in order to arrive at the Jool system with a reasonable amount of delta-v for navigation? Every time I send a craft to Jool, I get there and either shoot right out of the system, crash into Jool or a moon, or end up stranded in a ridiculously useless orbit.

At the moment I have some frogs stranded on/around Pol and it seems like getting there was a fluke. Appreciate any assistance!

(PS- It doesn't help that once you enter the Jool system, Jool itself becomes untargetable)

Well, as for the dV to get to Jool (or anywhere else) and play around with the moons, consult the dV map.  There's one on the KSP Wiki that's probably still good (was for 1.0.5).  In general, all about 2500 just to get a good Jool encounter.  After that, it's a question of how much you want to do.  Note that in 1.1 we got this big inflatable heatshield so aerocapture at Jool is an option again, so figure maybe 3000 to get into an orbit in the vicinity of the large moons, fix your inclination, etc.  Less if you can arrange a gravity brake off Tylo.

Getting around between the moons is highly variable.  You can, as @5thHorseman says, play Joolian Roulette, which means getting into an elliptical orbit that crosses the orbits of the various moons, then just wait.  Pretty soon you'll get an encounter with one of them, probably not the one you want, but by tweaking the encounter you can get a gravity assist to put you into a more desirable orbit that gets you where you want to go.  It just might take months and many times going around Jool, but it hardly costs any fuel.  But that's typical of KSP.  In many cases, getting somewhere is a choice between burning more fuel or taking more time to get there.

If you prefer to have more control over where you're going and don't want to spend a lot of time doing it, then you can transfer from moon to moon fairly cheaply.  This is the same as transferring from planet to planet, just on a smaller scale, so you need to wait for transfer windows.  Still, you can move between the big inner moons for only a few hundred 1-way, and somewhere about 1200 to get from Laythe to Pol.  The only real trick is is that when going between the big, inner moons, you have to do it slightly inefficiently to avoid the weirdness associated with such massive bodies in close proximity.  You need to leave one of these moons on a path that diverges from its orbit rather quicker than you would going from say Kerbin to Duna, or the moon you just left will catch you from behind and pull you back.  Anyway, figure on another 2000-3000m/s to see all the sights from low orbit (not counting any landings and takeoffs).  So it's useful to have the ISRU and make some fuel along the way at Vall or maybe Pol (I prefer Vall).

And then you still need some fuel to get home.

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Using jools moons to give you a 'gravity brake' is probly the best thing since the oberth effect.  I've even had a tylo encounter that brought my PE down so far that I would of gone inside of jool if I had not changed my tylo PE.  Since it takes close to 100 days to get to jool PE once you've entered it's SOI, you can even adjust your orbit to try different moons to slow you down for a relatively cheep price

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