Letting Jool reverse a trajectory?

[Delay]

Hi,

In my KSP 1.3 career, I have accepted a contract to put a satellite into a Kerbol orbit. I already talked about it in the difficult contracts thread; basically I need a highly eccentric orbit around Kerbol... that goes retrograde. It has 179° inclination.

Because the orbit goes beyond Jool, the idea to use its gravitational influence to do the work for me seems to be the best option to save fuel and time.

The problem is that in over 3 (real) days of trying this kind of maneuver, I never consistantly got it to actually reverse my trajectory. I either get really close to that or Jool kicks me back to Kerbol (pushing up the Ap, but not reversing).

Is this maneuver actually possible? If yes, how do I perform it (consistantly)?

Edited November 24, 2017 by Delay

[Human Person]

It is possible! Others and me did it in this challenge. My 2nd post shows exactly what kind of transfer to jool I chose.

Basically, you want to arrive at jool with some extra energy. Such a high speed encounter can be achieved e.g. by choosing a fast (some people might say inefficient) transfer, so make sure you’re a little late in your transfer window. The extra delta v needed for your exit burn is well spent!

The requirement is simple: when doing a gravity assist, you leave the SOI with the same velocity you entered it with, so you want to make sure to enter Jools SOI with a higher velocity than Jool’s orbital velocity itself and leave it in a retrograde direction.

Another approach some people in the challenge took is doing a bielliptical transfer. This requires an extremely high solar apoapsis (way bejond Jool), where your orbital velocity is so small, you can flip your orbit easily. The is method takes a long time (approaches in the challenge range from multiple decades to millennia ofr mission time). 

Brute forcing yourself into a retro solar orbit is possible and has been demonstrated by a few people as well.

Edited November 24, 2017 by Physics Student

[bewing]

Or you can use Tylo to change your trajectory a bit before you get to Jool. It's a lot harder to do it with just a single slingshot.

 

[Kryxal]

I accidentally did this once back in 0.23.5, went on a close Laythe flyby for science and ended up retrograde ... but bi-elliptic is simpler if you have the time.

[linuxgurugamer]

On 11/24/2017 at 4:06 PM, Physics Student said:

It is possible! Others and me did it in this challenge. My 2nd post shows exactly what kind of transfer to jool I chose.

Basically, you want to arrive at jool with some extra energy. Such a high speed encounter can be achieved e.g. by choosing a fast (some people might say inefficient) transfer, so make sure you’re a little late in your transfer window. The extra delta v needed for your exit burn is well spent!

The requirement is simple: when doing a gravity assist, you leave the SOI with the same velocity you entered it with, so you want to make sure to enter Jools SOI with a higher velocity than Jool’s orbital velocity itself and leave it in a retrograde direction.

Another approach some people in the challenge took is doing a bielliptical transfer. This requires an extremely high solar apoapsis (way bejond Jool), where your orbital velocity is so small, you can flip your orbit easily. The is method takes a long time (approaches in the challenge range from multiple decades to millennia ofr mission time). 

Brute forcing yourself into a retro solar orbit is possible and has been demonstrated by a few people as well.

For the brute force, you need something on the order of 20,000 dV, and another 20,000 if you want to return

[Cpt Kerbalkrunch]

10 hours ago, linuxgurugamer said:

For the brute force, you need something on the order of 20,000 dV, and another 20,000 if you want to return

Yeah, that's an awful lot; and definitely not worth it for a satellite contract. At least he doesn't need to return, though. So unless he wants to spend a century or so doing multiple gravity assists, Jool is definitely the way to go. Without even using Tylo or Laythe, it costs about 3,500m/s from LKO, depending how high you need your Pe to be, with just a single maneuver. Then whatever you need to circularise and adjust orbit to match up. When you combine time and cost, I don't think there's a better way.

Of course, I'm always thinking in terms of chemical rockets. I know you can pack some serious d/v into an Ion vessel, I just couldn't handle the time it would take. I would consider 3 or 4 minutes to be a long burn. Nukes or Ions would drive me nuts.

Edited November 26, 2017 by Cpt Kerbalkrunch

[Delay]

56 minutes ago, Cpt Kerbalkrunch said:

I would consider 3 or 4 minutes to be a long burn.

I've decided on doing a bi-elliptic transfer with Ion engines (because of the dV). It's too close to wait for Jool to be in position, unfortunately.

It will approximately take 16 minutes at full physics time warp for a 2900m/s burn. And I have a 3011m/s burn as well.

[Cpt Kerbalkrunch]

1 hour ago, Delay said:

I've decided on doing a bi-elliptic transfer with Ion engines (because of the dV). It's too close to wait for Jool to be in position, unfortunately.

It will approximately take 16 minutes at full physics time warp for a 2900m/s burn. And I have a 3011m/s burn as well.

That's actually not so bad, if you can handle the burn. Have you considered a Spark or 2? They can't match a Dawn, or even a Nerv for ISP, but combined with Oscar B's and the smaller FL tanks, they have great capabilities. And can really cut down that burn time. You'll add mass though, so I think you'd have to experiment a bit to see if you dig it. Sounds like a solid mission plan either way, though. Hope all goes well.

And truth be told, I dig the flip at Jool for the "cool factor" as much as the d/v savings. Watching the planet flip your orbit through sheer speed is pretty awesome. Makes me feel like I'm on the Enterprise, swingin' around the sun to go back in time. 

Edited November 26, 2017 by Cpt Kerbalkrunch

[Starman4308]

If it's for a satellite contract, there may not be time to go for a bi-elliptic.

What I might try is adding a small burn deep inside Jool's gravity well, to take advantage of the Oberth effect and exit Jool's SOI with more relative velocity than you started. You'd set up the slingshot mostly as normal, with the exception that you probably want to aim your return vector a bit towards the sun*. Then, just set up a maneuver node at peri-Jool and add a bit of prograde until you get a solar periapsis that isn't a sundive.

In short, at the periapsis of a Jool slingshot, you're deep in a gravity well; use the Oberth.

*The most efficient direction to be exiting Jool's SOI is directly retrograde of it. Since you'll be adding velocity at peri-Jool and effectively straightening out the U-turn a bit, you want to set up the slingshot deliberately curved inwards.