Tutorial: Gravity Assist

Over-simplified example of gravitational slingshot: the spacecraft's velocity changes by up to twice the planet's velocity

A Gravity Assist a.k.a. Swing-By or Slingshot is a very fuel-conserving method to increase your speed and direction. With the exception of Kerbol, every celestial body orbits another larger body. Whichever celestial body that you're currently in the sphere of influence of, that body's own revolution velocity is also influencing your craft. By entering the sphere of influence of a new body, you can use that body's revolution velocity along with its gravity to your own advantage to increase your craft's speed and change its heading.

• Length: 30 minutes
• Difficulty: Medium
• For version: 0.18.4

How to do it

In this example we will leave from Kerbin to a trajectory to Duna by performing a gravity assist around the Mün. Usually you need to raise your apoapsis to 85 million km to escape from Kerbin. With this method we will just have to get to the Mün orbit (12 million km). This will save you fuel for several hundred m/s of delta-v.

1. Get your ship into a circular Low Kerbin Orbit
2. Zoom out so you can see the orbit of Kerbin around the sun.
3. Arrange for a fly-by with the Mün just like you learned in the "Going to the Mün" ingame tutorial, but arrange for it to happen while the mün is heading into the direction you want to go. In this case we want to get further way from the sun, so do it when the Mün is going in that direction - the moment it intersects the orbit of Kerbin.
4. Accelerate time until you reach the sphere of influence of the Mün
5. Now plan another course correction maneuver. The maneuver is more effective when you are close, so try to pass the Mün as close as possible without colliding. Your Mün periapsis needs to be behind Mün on Müns trajectory. When you pass in front of Mün, you will lose speed instead of gaining it. Also watch the declination of your trajectory - when it gets too high it will be problematic.
6. Accelerate time until you left the SoI of the Mün.
7. When you check the map you will notice that you are on a much wider orbit than you were before the Mün fly-by (likely with an apoapsis beyond Minmus orbit), but it's not a Kerbin escape trajectory yet. The Mün hasn't yet given you all the speed it can.
8. So perform another correction maneuver which leads you to another close Mün fly-by and again try to pass it as close as possible.

Congratulations, you should now be on an orbit leading you in the general direction of Duna with plenty of fuel left for the return trip. When you want to go back, perform the same trick with Ike.

Reverse Gravity assist

Pass in front of a celestial body to brake and behind a celestial body to accelerate

Using a reverse gravity assist around Tylo to get into a Jool orbit. The necessary course correction required less fuel than getting close enough to Jool for an aerobrake maneuver. It is also much safer than aerobraking.

Should you enter the Duna-Ike system in a suitable trajectory, you can perform a reverse-gravity-assist to lose speed instead of gaining it. To do so just do an Ike-fly-by. In order to lose speed instead of gaining it, your Ike periapsis needs to be in front of Ike's trajectory instead of behind it.

Powered Gravity assist

To get even more out of a gravity assist, you can perform a powered gravity assist. As you know, accelerating at your periapsis has the greatest effect on your apoapsis. This still applies to your periapsis during a gravity assist. When you accelerate during your fly-by, this fuel is used very efficiently to increase the apoapsis of your final trajectory. This is, however, a very difficult technique, because it is hard to control your final trajectory, even when using a maneuver node.

See also

• Using Gravity Assists To Save Fuel by Scott Manley on YouTube
• How to perform a gravity assist (Narrated)
• Gravity assist on Wikipedia