|Apoapsis AMSL:||Velocity:||Units:||Ignore Mouse|
The blue circle is Kerbin itself, the light blue circle around it is the top of the atmosphere. You can click+drag on the left of Kerbin to set periapsis, or on the right of Kerbin for apoapsis. You can also use the text boxes to enter altitudes and velocities numerically.
You need to specify two values in all: either altitudes of periapsis and apoapsis, velocities at periapsis and apoapsis, or both altitude and velocity at either periapsis or apoapsis. You select the values you want to enter with the Parameters menu, the remainder of the information will be computed from the values you put in. If you enter altitude and a velocity above escape velocity, it'll give you excess velocity at infinity. The apoapsis and periapsis textboxes are altitudes above mean sea level (AMSL), the text report below has both altitudes AMSL and distances from the center of Kerbin. Note that if you specify values that lead to an apoapsis lower than periapsis, the plotted orbit and contents of the text fields will be swapped automatically.
If the input mode is Apsides/Velocities, dragging the mouse sets the apsis altitude and calculates new velocities for both apsides. Values from the textboxes are taken from the column being modified: editing altitude computes new velocities, editing a velocity computes new altitudes.
If the input mode is Apsis-Velocity, dragging the mouse sets the apsis while holding the velocity for that apsis constant, computing a new altitude and velocity for the opposite apsis. Values in the text boxes are taken from the row being modified, setting velocity and altitude for the apsis being modified and computing new ones for the opposite apsis.
The Apsides/Apsis-Vel input mode combines these. Editing the apoapsis and periapsis fields or dragging the mouse acts like Apsides/Velocities mode, editing the velocity fields acts like Apsis-Velocity mode. Thus you can either adjust both altitude and velocity for one end of the orbit or altitude of each end of the orbit without changing mode. (This was the original input mode, but was lost in version 0.2 when the different input modes were added)
You can easily create a circular orbit from an elliptical orbit: simply attempt to drag periapsis to a higher altitude than apoapsis, or apoapsis to a lower altitude than periapsis. Each acts as a higher/lower limit for the other, allowing you to easily create a circular orbit from either altitude of an elliptical one. This is intended to make Hohmann transfer maneuvers easier to compute...just set up the initial orbit, adjust periapsis or apoapsis to the desired altitude and burn to match the computed velocity, adjust again to make a circular orbit and do another burn when you reach your destination orbit's altitude.
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Version 0.3.5. See this KSP forum thread for updates.