Help with manoeuvre to Jool using Mun gravity assist and Oberth effect

[THX1138]

Hi. I want to send a vessel from Kerbin to Jool using the Mun for a gravity assist and thrusting at the best time to benefit from the Oberth effect. 

I made an image to demonstrate when I think I should be trying to encounter the Mun on the basis of angles given by http://ksp.olex.biz/ and when I should burn for the Oberth effect assuming that I only do one pass around the Mun (I'll actually do two).

 

In order to determine what direction to burn, I made a manoeuvre node at a location after Kerbin escape and pulled the prograde thing on that node. Then I set the SAS to hold the attitude at the blue target marker and started the burn five days early. Is that right?

Edited June 22, 2016 by THX1138

[ineon]

2 hours ago, THX1138 said:

I want to send a vessel from Kerbin to Jool using the Mun for a gravity assist

This is really hard to do. If you have huge amounts of time and the ability to run and re-run bunch of launches then you might be able to do it. It is likely that you will need larger course corrections when you leave Kerbin SOI because of the inaccuracy of doing this maneuver and end up using more dV this way. As you be travelling fast passed the Mun you will only have a small window to benefit from the Oberth effect

However, if you want to try it or you want to do it because it is cool (a good reason) then here is what I would do:

• Get mechjeb if you don't already have it
• Work out the escape angle that you want to achieve. You have linked to  http://ksp.olex.biz/ so I will assume that you are familiar with the escape angle.
• Wait until the Mun is approximately at 90 degrees to the escape angle
• Burn from LKO to Mun SOI so that you are passing behind the Mun relative to the direction you want to escape
  • Note this will mean going retrograde around the Mun if it is between Kerbin and Kerbol
• Now here is the tricky part: getting the right Munar periapsis 
  • Too high and you won't get much Oberth effect
  • Too low and when you burn you will burn into the Mun
• Set up a maneuver node at or near Munar periapsis to complete your escape of Kerbin SOI
  • Escape at the same angle that you would from LKO

Good luck - you will need it. And if you do manage it post some pics here (or in the tutorials) for other to follow  

[Tatonf]

This is a very bad idea. The Oberth effect on Mun is very small compared to Kerbin's one, and the benefit of the gravity assist is quite small too. You'll end up using more delta-V this way than doing it in LKO.

The only way a Mun gravity assist could be profitable is to do almost the entire burn in LKO, and end up with a trajectory that will pass in Mun's SOI so you can grab a bit of velocity. But as ineon stated, this is very difficult, and the faster you go, the less profitable it is (I would be surprised if you gain more than 20 m/s).

Finally, the true only way to make Mun gravity assist interesting for going to Jool is to use it for going to Eve, then gravity assist there, then make two gravity assist at Kerbin. But this is only for expert players.

 

[ajburges]

The only report I've seen of successful Munar sling shots is carefully crafted multi-encounter sling shots.

In summary: you eject from the Kerbin system in such a way that you will reencounter it (via resonant orbits). Then you proceed to slingshot through the system ensuring you keep resonant orbits with Kerbin until you gain enough velocity for an encounter with the desired body.

This approach does give incredible dV savings, but a the cost of being tricky and taking in game years for a transfer.