Basic orbit maneuvers
Understanding Kepler's laws
Kepler defined his laws before Newton found out how gravity causes these laws, so it is a nice set of rules of thumb for spaceflight.
- The first law defines what a orbit, in Keplers eyes, is: An ellipse (or circle) with the central body in the focal point of it.
- The second law describes the change in velocity with changing distance to a planet. The further you go away from the planet, the slower you get on the same orbit. It comes from the law of conservation of angular momentum.
- The third and last law defined the changes in Orbit period.
Raising the periapsis of your orbit is an important maneuver, and is used in a variety of ways, ranging from achieving a stable orbit to rendezvousing with another craft. Along with being quite important, it is also a relatively easy maneuver, so you should take the time to master it. To raise your periapsis, you must fire your main thrusters while facing prograde at your apoapsis. To do this, wait until the(Time to Apoapsis) readout on your Orbit MFD approaches about 90 seconds. At this point, it is necessary to engage the prograde autopilot ( ), and wait until your craft has turned completely prograde. If it has not already, wait until before engaging you main thrusters. Watch your periapsis altitude rise ( on your Orbit MFD), and kill the thrust when it has reached your desired altitude. As a general rule, anything above 200 km in LEO is safe.
Lowering the periapsis of your orbit is also a useful maneuver to master, as it plays an important role in deorbiting you craft. The procedure is similar to what is done to raise your periapsis, but inverse. Again, you must wait until you are nearing your apoapsis, but instead of turning prograde when, you will turn retrograde ( ). Once your craft has fully turned and is facing retrograde, engage your main thrusters, and monitor the rate at which your periapsis falls. Be careful to kill thrust before you periapsis has fallen below the surface.
Your apoapsis is raised and lowered in ways similar to the manipulation of your periapsis. These maneuvers are just as important as important as the maneuvers regarding your periapsis, and a similar amount of time should be dedicated to mastering and understanding them. Instead of burning at your apoapsis, as was done with periapsis manipulation, you will now burn at your periapsis. On your Orbit MFD, you will want to watch thereading, and wait for it to approach 90 seconds. Once it does, turn prograde ( ) and fire your main thrusters. You will notice the altitude of the apoapsis ( on the Orbit MFD) begin to increase. Kill thrust when it has reached a desired altitude.
Lowering, again, is the inverse of the procedure for raising. Once you reach your periapsis, turn retrograde (), and engage thrusters. The readout will begin to decrease. If you lower you apoapsis far enough, you may notice your apoapsis and periapsis flipping on the Orbit MFD. This is due to your apoapsis's altitude falling lower than your periapsis's altitude, and therefore it is no longer the apoapsis. Continue firing regardless of this, and your apoapsis (now the periapsis) will continue to lower.
Plane changes are performed to alter your orbital inclination. This is useful for matching orbits with a target vessel.
Deorbiting is an essential part of many missions. Although some craft do have retro engines, not all do. Therefor, the best option is to point retrograde and fire your main engines until your periapsis is beneath the surface of the body you are orbiting. Depending on the body, you may have to perform a reentry or use the hover engines.
A Hohmann transfer is an elliptical orbit which allows both the apoapsis and periapsis to be raised or lowered. It is done by firing the engines prograde at a desired point on the orbit. The apoapsis should coincide with the altitude of your next orbit. The engines are then fired prograde at the apoapsis of the new orbit to circularize it.
Due to the reversibility of orbits, a Hohmann transfer can lower an orbit. The engines are fired retrograde for the transfer orbit, and the circularization takes place with a retrograde burn at the periapsis of the transfer orbit.
Hohmann transfers are great for simple planet-to-planet or moon-to-moon transfers. They will need course corrections, and may be modified to encounter multiple objects (as in a slingshot).
A bi-elliptic transfer is a modified Hohmann transfer which uses a large orbit to raise a smaller orbit. It may seem like overkill, but certain cases may require less delta-v than regular Hohmann transfers.
As in a regular Hohmann, the engines are fired prograde at the desired point in the orbit, but the apoapsis of the transfer orbit overshoots the desired altitude. The engines are fired again at apoapsis so that the periapsis becomes the desired altitude. At the new periapsis, the engines are fired retrograde so that the apoapsis is lowered to the desired altitude.
If your orbit is too low, a backwards transfer with a bi-elliptical is not recommended, as your periapsis may be too low, and you will crash. Bi-elliptic transfers may be too lengthy for planet-to-planet transfers.