Kerbisynchronous over KSC: I'm missing something

[ikehaiku]

Beware: some orbitals mechanics inside.

I'm rusty...haven't really played for some 2 years or more.

So, I want to launch a sat. in a Kerbisynchronous equatorial orbit right over the Space Center.

As you know, KSC is located at 74°W.

I am in a LKO - eccentricity of 0 , inclination of 0°, Apo/Peri of 100km. You know, basic.

I want to reach an apoapsis of 2868.7km - but I need to time my Hoffman transfer burn so that I'm right over KSC when reaching the desired apoapsis.

I still got a calculator, that I've made some times ago:

https://docs.google.com/spreadsheet/ccc?key=0AuhR8b-OEB0OdDVORnh5ZHVtQmp1NTdBUE85OGtZV2c#gid=0

It gives me an Hoffman Transfer time from 100km to 2868km of 1h23m50s - give or take, 85 minutes.

Given that Kerbin rotates at 1° per minutes, to me, I need to burn right when I'm over the 159°W point on Kerbin - that is, 74° (KSC Location) + 85° (Kerbin rotation during the Hoffman Transfer).

But....I'm ending over the 64/65°W point. I know there are some approximations, but any way I try, either manual or with Mechjeb, I'm always in this ballpark, 10° away from my intended destination.

So...I'm missing something. There is a parameter somewhere that I'm forgetting.

Any of you have an idea?

Thanks!

[Starwhip]

Unless you burn instantly, the burn itself takes time. In my experience with orbital rendezvous, this time is not insignificant.

Try accounting for it.

[ikehaiku]

Yes - but I tried taking that into account to no avail.

I first try with a probe that had a low TWR (around 1.4) - I needed around a 45s burn. So I tried to burn 50/50 around the "burn time". I gave it a few tries. I tried burning more "before". I tried burning more "after"

I even relaunched another probe, this one with a 3.5 TWR - cutting the burn time to a few seconds. Again, did some tests.

And I'm always short. It seems I need to burn over the 170°W point , not the 160°W point (again, all things considered equal)

And I don't understand why, and I won't be able to sleep.

Yes, there is always the possibilities that approximations and burn time are playing a role - but I'm 90%, if not 99%, sure it doesn't equal to a 10° difference.

[Redhaze]

be aware - kerbin's surface gravity may have changed since then, throwing off your calculations exponentionally.

[GoSlash27]

ikehaiku,

Funny, I just did this very thing yesterday for a challenge here

Because there's a challenge on, I can't just come out and tell you...

I think what you're missing is that you don't need to start your transfer at KSC+transfer time, but rather the opposite.

Your periapsis needs to be at where KSC's antipode will be 85 minutes after the burn.

Apologies,

-Slashy

[ikehaiku]

I think you're right - I'm just messing up things in my head...so, back to the drawing board - and I may enter your challenge then