# Re: question about 24 turnaround of VAB launch

Ruben Velasco (heston@arrakis.es)
Fri, 17 Dec 1999 21:34:39 +0100

```Hi!

Aaron Brown <asb@CSUA.Berkeley.EDU> said:

>I noticed that when the current VAB launch has been delayed, it has been
>delayed for exactly 24 hours.  My question is why?  It would seem that if
>they have a specific orbit to get the satellite into, then the launch
>window would shift, as it has when they have delayed the current shuttle
>launch.  What is it that dictates that the VAB launch window does not
>shift?

I'm not 100% sure of this. You have been warned :-)

If the orbit's plane remained fixed in space (with respect to distant
quasars or whatever you consider 'fixed' enough), a 24h delay would
really mean a 23h 56min 4s delay (the lenght of the sidereal day, or more
exactly, the time the Earth takes to complete one revolution, which is not
the same!).

But Terra is to be launched on a sun synchronous orbit. That means that
its orbit's plane precesses eastward about one degree each day due to the
Earth's oblateness (the same angle as the Sun moves in our sky during that
period, that's where the name comes from) How much will it take until the
Earth rotates that extra degree? 3 min 56 s.  So we have
23h56min4s + 3min56s = 24h exactly
(surprise!) because the sun-sychronous orbit has that property BY DEFINITION.

So they don't need to "accomodate" the next launch in the launch window as
someone has said before. The launch will take place at the same local time
if everything goes OK.

However, in the STS-Hubble case, each "24 h" delay has a monster shift
because 1) Hubble of course goes form West to East, so precession goes
West, and you need an earlier launch each day. 2) Hubble orbits closer
to the Earth than typical heliosynchronic satellites so effects are greater
and 3) Its inclination is lower, and precession is maximum with inclination
zero and null in polar orbits.

Just my 2 cents, as you say there in the US. If there's any specialist out
there, correct me if I am wrong. I'm not a rocket scientist although you may
know it by now anyway.

-
Ruben Velasco <heston@arrakis.es> 37.3877N 6.0008W +39m WGS84
PGP KeyID 37219E45  Fingerprint = 96575B8713370081 1BC5D43D324B3F4D

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