How to get the speed at apoapsis from an Orbit?

[martincmartin]

I'm trying to get the speed at apoapsis from an Orbit.  I see getOrbitialVelocityAtUT() and timeToAp, but what is Universal Time?  Does it start now?  At the start of the epoch?  How do I get the current time since the epoch?

[sarbian]

The current time is Planetarium.GetUniversalTime ()

Here is how to get your next Ap & Pe UT :

https://github.com/MuMech/MechJeb2/blob/master/MechJeb2/OrbitExtensions.cs#L190-L219

Current time + timeToAP would work too.

[Padishar]

Using an intermediate time makes this quite a bit less efficient than it can be.  All you need is vis-viva:

https://github.com/CYBUTEK/KerbalEngineer/blob/master/KerbalEngineer/Flight/Readouts/Orbital/SpeedAtApoapsis.cs#L53-L62

 

[Padishar]

Also, if you don't want to write it all out then you could just use:

double speed = orbit.getOrbitalSpeedAtDistance(orbit.ApR);

...which also just uses the vis-viva equation...

Edit: though you should check how that behaves when there is no Ap and a general check for a NaN return value would be a good idea too...

Edited March 10, 2016 by Padishar