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ISRU SSTO and returning to Kerbin from the Jool system...


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I find myself in need of a pointer or two getting back from Laythe, and I do know this is one of the more challenging things to accomplish.

PS4 so this is all stock - I’m at that stage you get to where your tech tree is maxed out and have moved into to SSTO infatuation.

Using ISRU and have been wheels down and back from everywhere in the Duna and Kerbin systems, and have now worked my way out to Laythe via refueling at Minmus-> Dres -> [forced to learn about gravity assists here] -> Jool -> Laythe.

To hopefully make it easier to get back I refueled on Pol (probably moot given the numbers) so I’m in LPO with 4069 fuel remaining, and as creative as I’ve managed to get is to try to use gravity braking using Kerbin -> Kerbin -> Kerbin -> [out of fuel] which doesn’t seem to be slowing me down much as far as I can tell (I know to pass in front of the body to slow down, behind it to speed up).

So before I go all Blunderbirds and mount a rescue mission I first wanted to ask about other options.  I’ve been considering stepping down with refueling stops at Dres -> Duna -> then to Kerbin, or experimenting with gravity braking at other bodies like Eve.

I know I can get home slowly, but would like to learn a better way...

 

 

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23 minutes ago, Gregbert said:

I know I can get home slowly, but would like to learn a better way...

Unfortunately it is fast or cheap. Pick your poison.

36 minutes ago, Gregbert said:

experimenting with gravity braking at other bodies like Eve

Eve is quite convenient for gravity assist, both for traveling to and from Kerbin. If you are feeling brave enough you may skim its atmosphere for further deltaV savings (granted that it will be tricky to manage both heat and drag for a useful trajectory afterwards, to say the last. However, capturing and refuel at Gilly may also be an option)

 

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i had some success going for duna and doing some aerobraking there during the gravity assist. multiple kerbin assists are feasible.

 

then again, with 4000 m/s, you can easily get back to kerbin from pol without any problem. 2000 are more than enough.

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4,000 m/s dV should be more than enough to return from Jool to Kerbin, including capturing propulsively into Kerbin orbit. A thought occurs based on your description (but a picture would be even more helpful) - where is your transfer trajectory intercepting Kerbin?

You want the PE of your transfer to be a close as possible to the PE of Kerbin. This way your craft's velocity and Kerbin velocity are mostly aligned, so you only need to cancel out the hyperbolic excess. If you're intercepting Kerbin away from the PE of the transfer orbit, then you'll have a significant radial component. Hopefully this sophisticated diagram help show what I mean:

n6rR5UU.png

Another suggestion: Tylo and Laythe are wonderful for gravity assists when arriving/leaving Jool, due to their large mass and high orbital velocity.
With a little practice you can also eject directly from Laythe/Tylo orbit into a Kerbin intercept.

Edited by ManEatingApe
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11 hours ago, Gregbert said:

I’m in LPO (low Pol orbit) with 4069 fuel remaining, and as creative as I’ve managed to get is to try to use gravity braking using Kerbin -> Kerbin -> Kerbin -> [out of fuel] which doesn’t seem to be slowing me down much as far as I can tell

4096 in what units? If it is m/s of delta-V you can use one of the planning-tool webpages (link) to find the right time to make the most efficient transfer.  It is no fun to copy the 'transfer details' onto console (nor is it on PC) so you aim to make the transfer orbit look like a Hohmann transfer that reaches Kerbin parallel to Kerbin's orbit, as the @ManEatingApedrew above.  You need to leave when Pol is about 16° before  it is between Joon and the Sun (the link above says 90°+16°=106° to the retrograde direction of Jool's orbit) on whichever orbit of Pol is closest to the your chosen best transfer-date from the planning-tool.  Then aiming so you leave in the forward direction of Pol's orbit gives you most advantage from Pol's speed, and should get you close to the desired Hohmann transfer.   You will surely need a correcting maneuver while still far from the Sun, to reach Kerbin parallel to Kerbin's orbit and low enough to aerobrake.

The tool says it takes 1260m/s of delta-V to leave an orbit around Jool near Pol's orbital altitude.  You will probably need more to correct for Pol's inclination, and various other reasons, but that might leave you 2800m/s to slow down at Kerbin

You can use a gravity assist from Tylo to leave Jool (at the same time and in the same direction as you would have left from Pol).  Then you have more fuel for slowing down enough to aerobrake.  And, Tylo gravity assists are fun.

The Kerbin-Kerbin-Kerbin . . .  pattern of gravity assists can slow you down relative to the Sun, but as you noticed you keep returning to Kerbin at the same speed relative to Kerbin, just at different angles each time, which doesn't help. 

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One of the long-time YouTubers  posted a video dong something very similar, using gravity-assists instead of fuel.

https://youtu.be/8eGottzfcyY?t=1259

The return is from Bop, rather than Pol in the OP.  Matt uses gravity assists from Tylo and Laythe.  He does not worry about the timing of leaving Jool, no worries about transfer windows, because he can adjust the years-long path from Jool to take long enough that it meets Kerbin parallel to Kerbin's orbit. 

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