[TriLUG] OT -- NASA Shuttle Launch Today

Joseph Mack NA3T jmack at wm7d.net
Fri Jul 8 14:09:01 EDT 2011


On Fri, 8 Jul 2011, Brian Daniels wrote:

> On 07/08/2011 01:15 PM, Joseph Mack NA3T wrote:
>> A shuttle launch to the ISS is at an inclination of 51.6deg,
>
> You're right.  They're burning more fuel to make that angle,

yes understand.

> but the orbital plane is being set during the launch, not 
> afterward as I was thinking.
>
> The two days appears to be taken up by the maneuvering 
> changes that are still needed to bring the Orbiter and ISS 
> into alignment:
>
> http://www.airliners.net/aviation-forums/military/read.main/84859#4

Great link. The author knows a lot about what's going on.

this explains why they launch NE to the ISS and not SE. This 
was another mystery to me.

It also gives a margin of +/-5 mins for launch, which is 
wider than I thought

> "The Space Station itself is not flying directly over 
> Launch Pad 39A at that moment. Waiting for such a 
> coincidence would limit launch opportunities to only a few 
> times a year. Instead, the Shuttle goes into the same 
> orbit as the Station and then speeds up or slows down to 
> catch up with the Station. That usually takes two days 
> (the Russians do it the same way with Soyuz and Progress 
> spacecraft.)"

I had thought that they launch the shuttle as the ISS is 
coming up from behind (like a relay race with the lead 
runner accelarating as the runner with the baton comes up 
from behind). For this to work, the two have to be in the 
same inclination (which we have) and in the same plane and 
in the same phase (not sure if plane and phase are right 
terms here). I'd assumed then that the shuttle would arrive 
at apogee meeting the ISS and would burn the OMS to 
circularise the shuttle's orbit and there you'd have it half 
an orbit later. From the link you provide, this coincidence 
only occurs a couple of times a year.

Let's see if I've got this. The orbits of the shuttle and 
the ISS have the same inclincation. However the ISS can pass 
over Cape Canaveral to the west or the east and earlier or 
later. Presumably as long as the shuttle stays at an 
inclination of 51.6deg, the E/W offset and the 
early/late offset are equivalent and can be made up by the 
shuttle flying a little bit faster or a little bit slower 
than the ISS. You don't want the difference in velocity to 
be too much, or you'd need to burn more fuel for the shuttle 
to dock with the ISS. So the approach is slow (about 2days).

As Neil describes, there's plenty to do in those two days, 
so it's not time wasted.

Thanks Joe

-- 
Joseph Mack NA3T EME(B,D), FM05lw North Carolina
jmack (at) wm7d (dot) net - azimuthal equidistant map
generator at http://www.wm7d.net/azproj.shtml
Homepage http://www.austintek.com/ It's GNU/Linux!



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