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MaverickSawyer

Kerbosynchronous Transfer Orbit planning...

Question

I've never been able to understand how to time the transfer burn of a satellite so that it will wind up in a stationary orbit above a particular point on the globe. I can do the actual burn, the tuning of the orbit for full stationary, all that... I just can't figure out when to start the burn so that I can park a satellite above, say, the KSC.

 

Does anyone have any good tips or rules of thumb?

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It's Kepler's law.

For Keostationary orbit the altitude is 2863333m, or 3463333m form Kerbin's center. For a Hohmann transfer form a 75000m orbit, the Pe is 675000m from the center and the Ap is 3463333m, so the semi-major axis is

(675000+3463333)/2 = 2069167m.

By Kepler's law the time it takes for the transfer is

(2069617  / 3463333 )^( 3 /2 ) / 2  = 0.23 Kerbin day

In other words your Ap should be a little less than 1/4 of a circle (83 degrees to be precise) ahead of your target, by the time you burn.

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My method is to put myself in a medium high circular orbit. Then the important point is to estimate by how many degrees my ship exceeds the planet's orbital speed in one orbit. Let's say it's 30 degrees per orbit. The transfer will generally take exactly one quarter of that, because my orbit is decreasing linearly from 30 degrees per orbit to zero degrees per orbit, and it's half an orbit. So the average during the transfer is 7.5 degrees. So I eyeball it, waiting until I'm about 7.5 degrees behind my target point on the surface and then do the burn.

 

Edited by bewing

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Set the planned burn up on your map screen, then click on it so the 6 symbols appear as if you were going to edit it. Instead of clicking on a symbol, click and hold the central circle, and you can drag the burn location back & forth along the orbit line until the result is in the right place.

 

Works best if you're in a relatively close orbit to where you intend to end up, obviously.

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A method that I use, if I'm not in a mood to do much of previous planning, is like this:

 

1) Get the satellite to LKO

2) Raise the Ap to keostationary height (2 863 330 m)

3) When at Ap, raise the Pe so that your orbital period is a bit less than a Kerbin day, let's say 5,5 hours

4) Wait for the KSC to be roughly underneath when you are at Ap

5) Burn at Ap to circularize and correct the orbital period to 1 day

 

It's not 100% precise, but doesn't require any maths or much planning and is more than sufficient for most purposes.

Hope that helps,

Michal.don

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