Tutorial: Advanced Orbiting
- → See also: Hohmann transfer orbit on Wikipedia
The Hohmann transfer is the most frequently used method of changing orbital altitudes while keeping the same inclination. The ending orbit may be around the same celestial body as it began or for travelling to another body, such as between Kerbin and the Mun.
It involves first entering an eccentric orbit, then circularizing once reaching the desired orbital altitude. Thus, there are two burns to be made, ideally using engines with high thrust-to-weight ratios; low TtW can require up to 40% greater Δv from having to start earlier at less efficient points than apoapsis or periapsis are for changing orbits.
To transfer from a lower orbit to higher:
- Burn prograde at periapsis until the apoapsis reaches the desired altitude.
- Upon reaching the raised apoapsis, burn prograde until periapsis rises to the desired altitude.
To transfer from higher to lower:
- Burn retrograde at apoapsis until the periapsis reaches the desired altitude.
- Upon reaching the lowered periasis, burn retrograde until apoapsis falls to the desired altitude.
- → See also: Bi-elliptic transfer on Wikipedia
The bi-elliptic transfer can be more efficient (but slower) than the Hohmann transfer orbit in some cases (when going from a very tight orbit to a very large one: the ratio must be higher than ~12:1). This is because changes of speed are more efficient at low speeds (and therefore at high altitudes).
- Start by burning prograde (most efficiently at periapsis) until orbit becomes highly elliptical with the apoapsis higher than starting and desired orbits.
- At apoapsis, burn prograde until the periapsis reaches the altitude of your desired orbit.
- Upon reaching periapsis, burn retrograde until your orbit is circularized.
Opposite burns at these same points will lower your orbit. This can be used to enable aerobraking to lower your orbit height.
Orbital plane alignment
An important part of intercepting another orbiting body is to align your orbital plane with the target's orbital plane.
Start by select the destination body as a target. This will show several new points on your orbit, in particular ascending node (AN) and descending node (DN). These are the points where your orbit crosses the plane of the other body's orbit. ("Ascending" is from the point of view of a prograde (eastward) orbit. If you're orbiting retrograde, your orbital plane "descends" below the other one at the "ascending" node.)
Set up a maneuver node at the next one of these nodes. The maneuver you want is pure normal, in the opposite direction from the type of node it is. For the descending node, burn normal ("up", a pink triangle with a dot in the centre); for the ascending node, burn antinormal ("down", an upside-down pink triangle with radial lines and a dot in the centre). See Maneuver node to see what these symbols look like.
- Don't attempt to match planes anywhere other than an ascending or descending node. It won't work and you'll waste fuel trying.
- It is easier to match orbital planes if your orbit is roughly the same shape (especially regarding eccentricity) as the target, mostly because you can line up the orbits in the map view much more easily.
- Changing inclination is most efficient when you're moving slowly, i.e. high in the orbit. If you're aiming for a polar orbit, arrange that while you're still far away from the body rather than doing it after you've arrived. If you need precision afterwards, start with a high altitude parking orbit.
- If you're merely changing orbits, for example to fulfil a "put a satellite in a particular orbit" contract, try to combine the two maneuvers by making the Hohmann transfer from an ascending or descending node and adding a normal component to the burn. Matching planes this way is highly efficient.
- On the other hand, if you need to rendezvous with a body, it's often necessary to make the plane change as a mid-course correction.
The Exley maneuver
(As of version 0.17.1)
Named by its author, wiki user Sir Exley, it is an approach to getting an encounter with a target planet without having a precise launch window planned by entering an eccentric orbit whose apoapsis meets the orbital path of your target and a periapsis whose altitude has a lower altitude around the sun that the target.
In order to easily meet with a target planet's sphere of influence, you will need to perform a few burns while at either the periapsis or apoapsis of your transfer orbit.
Planets outside your original orbit
If you are meeting with a planet whose orbit is outside of your starting orbit, create a transfer orbit such that your apoapsis is as close as possible to your target planet's orbit.
Next, make a few orbits until the target planet is slightly in front of you when you reach your apoapsis. Begin a prograde burn until you see your orbit cross the target planet's near your apoapsis for a fraction of a second. If you overshoot, simply turn around and burn retrograde until the cross orbit is visible again. If your transfer orbit exceeds the planet's orbit, then you have gone too far, and have either missed the cross orbit, or do not have an apoapsis close enough to the target orbit to be affected by the planet's sphere of influence.
Planets inside your original orbit
If you are meeting with a planet whose orbit is inside of your starting orbit, create a transfer orbit such that your periapsis is as close as possible to your target planet's orbit.
Next, make a few orbits until the target planet is slightly behind you when you reach your periapsis. Begin a retrograde burn until you see your orbit cross the target planet's near your periapsis for a fraction of a second. If you overshoot, simply turn around and burn prograde until the cross orbit is visible again. If your transfer orbit goes within the planet's orbit, then you have gone too far, and have either missed the cross orbit, or do not have a periapsis close enough to the target orbit to be affected by the planet's sphere of influence.
Once you are in the cross orbit, burn retrograde until the orbit goes around your target planet.