Difference between revisions of "Maneuver node"

Revision as of 04:43, 6 May 2013

Maneuver node in orbital view

Maneuver marker on navball

A maneuver node is a planned velocity change along an orbit. Multiple maneuver nodes can be added which will affect the following maneuver nodes. After adding a node, it shows the velocity change needed to reach the next new orbit. If multiple maneuver nodes have been planned, only the first one will be displayed on the navball - once it has been completed, it should be deleted so that the next one becomes visible.

Adding a Maneuver Node

Two maneuver nodes with one in edit mode

Click on the vessel's orbital line and a popup will give the option to add a new maneuver node. Once created, selecting and right-clicking the node gives the option to delete it. If the maneuver node has been passed, selecting it immediately puts it in "delete" mode.

Hovering over a node displays the time of arrival and the Delta-V needed to perform the maneuver.

Directions

Because there are three dimensions of movement in space (or air), there are three basic axes on which to burn, with two directions per axis; thus, all maneuvers can be described as a combination of movement in three separate directions.

Prograde and retrograde

These vectors directly change the speed of the craft. Burning prograde will increase velocity, raising the altitude of the orbit on the other side, while burning retrograde will decrease velocity and reduce the orbit altitude. Both of these vectors are directly visible on the navball.

This is the most efficient way to change the orbital shape, so whenever possible these vectors should be used.

Normal and anti-normal

The normal vectors are orthogonal to the orbital plane. Burning normal or anti-normal will change the orbital inclination. On the navball, the normal and anti-normal vectors are located on the equator line directly between the prograde and retrograde markers.

These vectors are generally used to match the orbital inclination of another celestial body or craft, and the optimal time to do this is when the current craft's orbit intersects the orbital plane of the target. To match the orbital inclination, either perform a normal burn at the ascending node or an anti-normal burn at the descending node - when the correct magnitude is reached, the relative inclination at the selected node will reach 0° and the nodes will start to move rapidly toward each other's positions.

Radial in and radial out

The radial-in vector points directly toward the orbited body (center of the brown hemisphere on the navball), while the radial-out vector points directly away from it (center of the blue hemisphere). Performing a radial burn will rotate the orbit around the craft. Radial burns are the least efficient way of adjusting one's path - it is much more effective to use prograde and retrograde burns.

During ascent from the launch pad, your craft is effectively burning radially outward.

Using

Upon selecting a maneuver node, six symbols will appear, each corresponding to the vectors above. Each symbol can be clicked and dragged to adjust the amount of velocity change desired on each axis. Pulling a symbol away from the center of the node increases the speed change along the selected axis, and pushing a symbol toward the center of the node is equivalent to pulling the symbol on the opposite side. The further the symbol is pushed or pulled, the more rapidly the adjustment will be made.

After adding a maneuver node, three new values will appear next to the navball:

1. Velocity change required, in meters per second
2. Estimated burn time
3. Time to maneuver node (or time since the node was passed)

Because a maneuver node assumes an instantaneous velocity change, a perfect burn is impossible; to get the best results, it is recommended to burn half of the time before the node and the other half directly afterwards.

Maneuver nodes can be used to reach a specific orbit without needing to continuously monitor the orbital map. When performing an aerobraking maneuver, the current maneuver node will automatically update the needed velocity change as the craft's velocity decreases; this can be used to perform manual corrections if the aerobraking turns out to be insufficient or excessive.

Prograde

This is the direction of your velocity (velocity=speed+direction) and is symbolized by a yellow circle with three little wings at 90 degree angles from each other, exactly the same as it appears on the navball.

Burning in this direction will increase your apoapsis or periapsis, depending on where you are in your orbit. Think of it as increasing what your height above the ground will be on the opposite side of your orbit. This has other applications but this should help you get the gist of it.

Increasing the apoapsis.

Retrograde

This is the opposite direction of your velocity and is symbolized by a yellow circle with a cross in it as well as three wings at a 120 degree angle from each other, exactly the same as it appears on the navball.

Burning in this direction will decrease your apoapsis or periapsis, depending on where you are in your orbit. Think of it as decreasing what your height above the ground will be on the opposite side of your orbit.

Decreasing the apoapsis.

Normal

This direction is both perpendicular (i.e. forming a 90 degree angle) to the orbital plane (the plane formed by one complete orbit along a particular orbital path or line) and perpendicular to the orbited body's radial direction or parallel to the surface (i.e. in the same direction as). It is symbolized by a pink triangle with a dot in the center.

This direction has two major uses. Primarily, this maneuver is used to change the orbital plane by rotating it in the counter clock-wise direction (viewed from the side) around the point at which the burn takes place. The secondary use is to rendezvous with an orbiting object (ship, moon or planet); this is done by burning in this direction at the ascending node (i.e. the point at which your orbit intersects with the orbital plane of your target and begins passing beneath it)

Tilting the orbital plane counter-clockwise.

Anti-normal

This direction is both perpendicular to the orbital plane and perpendicular to the orbited body's radial direction or parallel to the surface. It is also 180 degrees from Orbit Normal or in the opposite direction. It is symbolized by a pink triangle with little wings on each side.

This direction has two major uses. Primarily, this is used to make changes to the orbital plane by rotating in the clock-wise direction around the point at which the burn takes place. The secondary use is to rendezvous with an orbiting object (ship, moon, or planet); this is done by burring in this direction at the descending node (i.e. the point at which your orbit intersects with the orbital plane of your target and begins passing above it).

Tilting the orbital plane clockwise.

Radial in

This direction points in the direction of the orbited body, i.e. it is perpendicular to the surface of the orbited body. It is also perpendicular to the direction of travel and Orbit Normal. It is symbolized by a blue circle with what looks like a four leaf clover in it (actually 4 inward wings).

This direction is used to rotate the orbit about the point of burn without changing the orbital plane (think spinning a hula hoop with a stick). This direction will rotate the orbit counter-clockwise with a maximum change in angle of less than 90 degrees; beyond this point, the orbit would pass through the center of mass of the orbited body, and the ship would traverse a slow spiral in towards the center of mass of the orbited body.

This is primarily used in rendezvous to line up two irregular orbits.

Rotating the orbital plane counter-clockwise.

Radial out

This direction points away from of the orbited body; also, it is perpendicular to the surface of the orbited body. It is also perpendicular to the direction of travel and Orbit Normal. It is symbolized by a blue circle with a dot in the center and four wings at 90 degrees from each other.

This direction is used to rotate the orbit about the point of burn without changing the orbital plane. This direction will rotate the orbit clockwise with a maximum change in angle of less than 90 degrees; beyond this point, all horizontal momentum would be cancelled (this not possible in real life, but it is possible in Kerbal Space Program; normally, the ship would traverse a slow spiral away from the center of mass of the orbited body).

This is primarily used in rendezvous to line up two irregular orbits.

Rotating the orbital plane clockwise.

Burning

The nav ball at the bottom shows the blue marker the vessel has to be oriented to perform the desired maneuver during the burn.

To perform the desired maneuver, you have to initiate a burn at the time indicated. For long burns, you will want to perform approximately one half of your burn done before you actually hit the node (and the remainder afterwards).

If you are using SAS, you can line up your navball indicator with the blue burn direction indicator as soon as you are done setting up the maneuver node (see screenshot) - once the maneuver has been defined, the burn direction will not change, regardless of your craft's position.

As you complete your burn, the marker may begin to move away from your current heading; this is merely because you were not perfectly lined up. This can generally be ignored, since it is usually a small error rather than a large one.

Once the desired velocity change approaches zero, reduce your throttle so as to avoid overshooting; once it reaches zero (or reaches a sufficiently small value), stop burning (press "x"). Once the maneuver is completed, the blue marker becomes meaningless, so you should delete the node. This is especially important if you have set up multiple maneuver nodes, since the next node will not show up until you delete the current one.