Difference between revisions of "Tutorial: The Berry Maneuver"

Revision as of 18:02, 7 August 2015

Virtually all tutorials and explanations of how to get to another planet or moon say first get into a similar, slightly smaller or larger orbit, then WAIT for your target to get close. This waiting can take years if you are going to another planet. The waiting is NOT NECESSARY. With the Berry Maneuver, it is very easy to align your Hohmann transfer orbit directly with your target the first time. The Berry Maneuver also makes gravity assists much easier. Any orbiting object encounter can be achieved simply.

This tutorial assumes familiarity with many important concepts in orbital mechanics:

• Hohmann Transfer Orbit
• Orbit Apoapsis and Periapsis
• Prograde and Retrograde burns
• Ascending Node and Descending Node
• Normal and Anti-Normal burns
• Radial-In and Radial-Out burns
• The Navball!
• Adding Maneuvers to your flight

All these concepts are covered in some fashion elsewhere in this wiki; they will be described briefly here.

The Berry Maneuver

The first step is to get into circular orbit. If you have not achieved orbit around Kerbal yet, another tutorial explains how. To circularize your orbit, raise your periapsis until it is about the same altitude as the apoapsis. To do so, burn prograde at the apoapsis. The orbit does not need to be perfectly circular. The purpose of circularizing the orbit is so you can start your Hohmann transfer orbit at any point. This step isn't strictly necessary but makes the next steps easier to see and easier to control.

Click on your target body to set it as the target. This will add two new icons to the map, the ascending node and descending node of your orbit relative to the target. Before you can encounter the target, you need to adjust your orbit to the same plane as the target. Do this by setting a normal or anti-normal burn near one of the nodes. When the angle of inclination is 0.0, you are on the same plane.

Now you're ready to set a Hohmann transfer orbit to meet your target. Add a maneuver point to your orbit, and add a prograde burn (retrograde if the target has a smaller orbit) until the apoapsis of the new elliptical orbit just touches the target orbit (periapsis if the target is smaller). On the orbital map, you can tell the orbits are touching when two new icons appear on the target orbit, one white arrow pointing up and one white arrow pointing down. These show where you will be when you hit the target's orbit, and where the target will be. It shows more information, like how far you will be from the target, but we don't need that. All that matters is that if the two points are separated you are not yet in a trajectory that will meet the target.

The next step is to position the maneuver point on your orbit so that you encounter the target. To drag the maneuver point, you need to click and hold on the center circle. When your pointer is hovering over the circle it will light up, then click and drag. Drag the maneuver point around your orbit and watch the two white encounter arrows on the target orbit. As you move the maneuver point they will get closer together or move apart. Slowly drag the maneuver point all the way around the orbit until the arrows are on top of each other. If the arrows are very far apart, you probably will not be able to encounter the target during this orbit. If you cannot get the white arrows close to each other, cancel this maneuver and make one orbit and try again. The arrows will get closer together next time around.

The point here is that it is incorrect to match orbits with the target then wait for the target to encounter you. Rather, before you leave your original orbit, drag the maneuver point until the apoapsis of your transfer orbit encounters the target first try. This is WAY faster, and puts you in complete control of going where you want to go.

Once the two white arrows are almost on top of each other, very slowly drag the maneuver point back and forth. You should be able to get the arrows right on top of each other. When they are on top of each other, they will disappear, and your orbit trajectory will change to an encounter trajectory. Voila.

Now perform the burn you set. You will go right to the target body first time.

As you approach the target body you will need to adjust your orbit using prograde and retrograde, and radial in and out, so that you approach the target on the side you want (behind it for gravity assist). It's also important to make sure your encounter orbit has a periapsis! If you don't see one, it means you're going to crash into the target, and that's no good.

That's it. If your goal is to orbit the target, then do a retrograde burn at the encounter periapsis. If your goal is gravity assist, then get the periapsis as close as possible to the target body, and/or prograde burn at periapsis to help throw your ship faster.

With this technique you can gain complete control of your encounters, and go where you want to go rather than waiting for the target to come to you.