Juno style Kerbin fly-by to Jool

[Mad_Maelstrom]

What is Juno?

Juno is a spacecraft enroute to Jupiter. The unique trajectory Juno is taking to Jupiter requires less delta-v than a direct (Hohmann transfer) route. After a single orbit of ~2 years, Juno returned and flew by Earth to get a gravitational assist to Jupiter. Because the assist came from Earth, Juno could launch whenever the phase angle between Earth and Jupiter was right and didn't have to wait for other bodies (like Mars) to be in the right spot.

Who cares?

This method can be used to reduce the delta-v required to get to Jool by over 400 m/s! For some perspective, saving 400 m/s will reduce a chemical spacecraft's mass by more than 12% or a nuclear spacecraft's mass by 5.7%. The penalty for using this is one extra maneuver in space, one extra encounter with Kerbin, and ~2 extra Kerbin years of flight time.

How is it done?

Getting into an orbit with a period of approximately two Kerbin years will put a spacecraft on an encounter with Kerbin. The apoapsis of this initial orbit around the Sun (Kerbol) is important and sensitive to errors. The apoapsis determines where and how fast Kerbin will be encountered and how good the gravitational assist will be. A small retrograde burn is performed at apoapsis to put the spacecraft on an encounter with Kerbin. The approach speed will be greater than the speed the spacecraft originally departed Kerbin. Passing as close to Kerbin as possible (without impacting the atmosphere) will cause the greatest change in velocity and provide the greatest gravitational assist. Below is a graph providing the required delta-v (retrograde burn @ initial apoapsis) and expected final apoapsis after the Kerbin fly-by for a given initial apoapsis around the Sun.

Getting to Jool!

Before any interplanetary mission, the spacecraft cannot leave Kerbin until the planets are properly positioned. To get an encounter with Jool using this Kerbin fly-by maneuver Jool should be ~60 degrees ahead of Kerbin. A final apoapsis of ~70 billion meters should work well for an encountering with Jool. Based on the graph above, an initial apoapsis of 30.4 billion meters and a retrograde burn of 175 m/s will be enough to get to Jool. Click on the "spoiler" under the graph for guidance on how to use the graph. Getting into the right initial orbit will cost about 1340 m/s of delta-V if leaving Kerbin from a 75 km orbit. Perform the retrograde burn at apoapsis; it should only require ~175 m/s but perform the burn as necessary to encounter Kerbin. The encounter with Kerbin also provides a good opportunity to adjust the inclination of the trajectory to Jool. Below is a screenshot taken shortly after reaching interplanetary space, with a small maneuver node at the apoapsis of the initial orbit the spacecraft will fly-by Kerbin and reach Jool in under 4 Kerbin years.

Please share your experiences (good or bad) with trying this method to get to Jool.

Edited June 9, 2015 by Mad_Maelstrom
Correction to mass savings

[Goody1981]

On 24 May 2015 at 9:51 AM, Mad_Maelstrom said:

What is Juno?

Juno is a spacecraft enroute to Jupiter. The unique trajectory Juno is taking to Jupiter requires less delta-v than a direct (Hohmann transfer) route. After a single orbit of ~2 years, Juno returned and flew by Earth to get a gravitational assist to Jupiter. Because the assist came from Earth, Juno could launch whenever the phase angle between Earth and Jupiter was right and didn't have to wait for other bodies (like Mars) to be in the right spot.

This is very helpful - thanks so much for the guide!

I have been trying to do this for the last couple of days to get this happening (didn't know your guide was here until @PLAD linked it (thanks PLAD).

I made a mistake initially by getting a solar orbit of EXACTLY 2 years, and thinking I could get a boost off Kerbin when I encountered again. Though I timed it perfectly (departed 2 years before the launch window to Jool), I discovered I could only lower my Ap that way. On subsequent reading of a thread entitled "Kerbin-Kerbin slingshot" (of which PLAD was a large contributor as well) I found that I needed to make some burn to change my angle of approach during the Kerbin encounter.

I (perhaps stupidly) made a radial-out burn fairly close to my departure from Kerbin initially. This got me a similar encounter to the one you show in your pic, but it cost like 300 dV, so it looks like your 175 m/s is better. Plus mine only got me out to Dres (might go there instead haha).

Will use your method for the next time I try it!

Here's the discussion - very interesting and informative reading

[Tiberius K]

Agreed, this is excellent.  I will try this tonight.  It would be fun to send a small rocket up to resupply snacks as it passes Kerbin. 

 

For reference here is the actual Juno slingshot:

Juno actually encounters Jupiter after reaching a an Ap beyond the orbit.

[Eddy119]

Hi, I'm wondering if it's possible/if there's a difference between this and launching inwards closer to the sun for a 3:2 orbit (3 probe orbits every 2 Kerbin years) against Kerbin? I don't know how to calculate it...