Slingshot Mun 01

Note: this is not a tutorial. It is an experiment to try to prove some points. I have eyeballed some datas in a rough manner and made a huge mistake by subtracting phase angle instead of adding them. Exp 002 will try to be more pertinent.

This goal of this experiment is to intercept Minmus with an unplanned slingshot with the Mun. No math, just fiddling with nodes.

For this experiment i will use a small probe in a sandbox game. The probe will be put in orbit with hyperedit or debug menu.


we do a Hoffman transfer to Mun in a way we pass the moon as near as we can in a prograde direction. We are thrown away beyond of the Minmus orbit. Our Mun periapsis is 8km.


If we do the same with a higher periapsis (let’s try 100km) we are thrown less far. Sounds logical; the nearer a body, the stronger the gravity, hence the gravity assist.


And if we try the same periapsis of 9km in a retrograde direction, we are thrown in the other direction.  (periapsis: 6,7million, apoapsis:30,5million.


What we can see is with a minimal dv difference (about 2m/s) we have HUGE differences.

So now, let’s try to do the burn to Mun at phase angle.

phase angle from Mun to Minmus is 90° according to “Interactive illustrated interplanetary guide and calculator for KSP”

Screen Shot 2017-01-16 at 22.59.25.png

Let’s warp until we are in this situation.I have eyeballed. It won’t be really precise.

Screen Shot 2017-01-16 at 23.08.50.png

Let’s try our Hoffman transfer again.


And … it’s a fail. Of course. The time we encounter the Mun, the phase angle between Mun and Minmus will have changed. A lot. As we can see, it has changed of about 70°. We want to arrive at Mun periapis WHEN phase angle between Mun and Minmus is 90°.

So let’s try at Mun-Minmus phase angle – 70° = 20°. BIG MISTAKE HERE, i should had add the phase angle, not subtract them. should be around 150, 160°

Hey Hey Hey what we have here? an encounter with minmus with a gravity assist from the Mun for the same price (in m/s)


Because Minmus is inclined i will arrive at Minmus in a polar situation. I need a minimal correction mid course to achieve an equatorial orbit.

In this situation, without numbers, it is difficult to say if it is worthily; i gained so much speed than the dv i spared could be less than the dv i could need to brake and circularise around Minmus in comparison of a direct Hoffman transfer.

Another point is: i encountered Minmus on a return trajectory. I expected to encounter it on the forward trajectory.

On next experiment, i’ll try to be more precise. I’ll use alarm clock which can give me the phase angle between mun and minmus in any situation. I won’t eyeball datas.


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