Rocket Science.

[Bear Cub]

I know this will be a stupid question to all those who know the answer...but I don't, and I'm curious.

The Space Shuttle...Orbiter...call it what you will.

Without wishing to "put it down" with my choice of words - it launches, it orbits the Earth, it (sometimes) meets up with stuff and docks with it, then it comes back.

During its trip, it has the ability to change it's "altitude", its speed, its orbit (?)...pretty much fully manoeuvrable (sp?).

So.....why does it have a launch window?

Last launch had a window of six minutes. What would happen if it went at seven minutes past...or eight?

If it can move around once its up there...and its only orbiting the Earth (and they know where that is at all times)...why can't it go whenever the weather is good enough on both sides of the Atlantic?



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[Genghis the Engineer]

You'll find that most astronautical engineers will quietly put down the shuttle (which is really a re-usable space station, not a spacecraft in the "star-wars" sense) as a very expensive way of achieving something that was done much more efficiently with a lot of 1960s / 1970s Saturn V / Soyuz / Molniya technology. If it was otherwise, the US wouldn't still be launching most of it's satellites on old fashioned rockets.

The capability to manoeuvre in orbit is extremely small and limited to very small orbit corrections for matching, it's simply not possible to lug up enough fuel to make major orbital corrections. Also orbital corrections are basically limited to (a) forward thrust = a high higher and slower, or (b) reverse thrust = a bit lower and faster, (c) lateral thrust = a very slight alteration to the orbital plane.

The Launch window is defined in order to hit the right orbit, at the right stage, get that badly wrong and you end up several hundreds of miles out - which is not a great problem if you just wanted to be in orbit, but to match orbits with an existing satellite presents a few difficulties.

The other point of-course is that because it isn't an aircraft that can just take-off and land(whatever NASA try to claim) there are hundreds of support staff and organisations that need to all get organised for the same time, which restricts them a bit to slots planned well in advance.

G

[Bear Cub]

Understand everything you say, G.

But if everybody is there, at their desks, for a six minute window...why not stay to minute seven..or eight... and still launch.

Agree, too, with the "catch the previously launched star" comment...but if just the change of altitude (BTW...what do they use as a vertical reference?) changes the speed - and I've heard that before, then surely once the orbiter is pushed in the right direction with a short blip of 'burn' it should not really pose a big problem.

I KNOW I'm looking at this very simplsticly, but I'm still curious.

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[Dr. Red]

I think predicted weather patterns has something to do with it.

Also, the precise position of the launch site in relation to sattelites (which are not always geostationary) means only certain routes and time windows are possible for fuel economy, and to make sure the shuttle can make it back to base.

Remember, when doing big astrodynamic (my new word) calculations, little errors quickly multiply, and you wouldn't want to launch late and end up on the wrong side of the earth for re-entry because of a little delay!

And there's the American fixation with accuracy, and also because it's so cool to go 5... 4... 3... 2... 1...

BLAST OFF! WHEEE!

[Genghis the Engineer]

Satellites dealt with by the Shuttle are never in a Geostationary (Clarke) orbit (GEO) which is far further than it can reach. The shuttle is operating in LEO (Low Earth Orbit) where the time around the world is around 90 minutes give or take.

Getting into the right orbit (something around 500km) is easy, but that orbit only passes the same spot on the earth's surface every few days. So, the Shuttle has to launch such that it is meeting the orbit at the same point as it's target (give or take a few hundred miles), if it doesn't it will forever be following something the other side of the world doing almost exactly the same speed.

To use an aviation analogy, imagine you want to fly formation circuits with another aeroplane of the same type that is flying overhead the circuit patternn. If you take off a little before it passes directly overhead, it will slowly overtake you as you climb then you should be able to formate somewhere downwind. But if you take-off at the same time, you will be chasing it but never quite getting there - you can catch up a bit by tighter circuits but not too tight because of the aircraft's manoeuvre limits (or in the shuttle's case the atmosphere), or allow it to slowly catch you up by flying wider circuits - but in the Shuttle's case it hasn't got the fuel to go to a vastly higher orbit.

So, just as you would have had to take-off at a pretty precise point to formate on this chap, so does the shuttle. The difference for the Shuttle is that the "circuit" as opposed to the orbit - the time for something to pass over the same spot again, is days or weeks long.

G

[Dupre]

The only thing I can think of is this:

Say the orbit speed is 30 000 km/h (don't know - just did a rough calculation). Say you can't be more than 1500km out (3 minutes cruise time) as you won't have enough time/fuel to get to your docking point if you're out by more.
If the perfect time to leave is 17:30, you can leave between 17:27 and 17:33 and still be within 1500km. Any more and you're not going to make it.

Just my 2 cents worth.

[Quidnunc]

Bear Cub -
I think you're getting at the idea of changing speed once you've moved orbit. This isn't possible as, for a specific mass and radius of orbit there is only one possible speed. Too fast and it's off to the stars, too slow and it's "welcome home, boys. SPLOSH!"

[Bear Cub]

Dupre...seems reasonable...but some of the launches have more glass (much larger windows).

Dr Red....also make sense and is often the second thing people say when I ask this question (the first being fuel load) - but I doubt that even NASA meteorologists can predict weather forecasts down to six minutes for some eight sites around the globe (is it eight - or six?, something like that).

Quid...I'll go with that too...but you say yourself "for a given radius of orbit there is only one speed"....so change the radius with a burn and change the speed to catch the satellite...no?

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[EchoTango]

To add to the complexity of the problem, remember that the orbit of a low earth orbit satellite is seldom circular. It is elliptical. As is the orbit of the vehicle trying to meet up with it. So the two axes of each orbit must match up in some way.

[rightstuffer]

Here's my 6 penn'orth. Bear Cub, you said
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"for a given radius of orbit there is only one speed....so change the radius with a burn and change the speed to catch the satellite...no?"
For a given orbit, if you acellerate (in the direction of the orbit) you will automatically move outwards into a larger orbit. Paradoxically, if you are trying to catch up with another satellite at the same orbital radius, you need first to slow down! That then drops you into a lower orbit, allowing you to cut the corner and 'undertake' your target before speeding up again and climbing to meet it!

Ever heard of 'Shepherd Moons'? (I know its a CD by Enya, but the real 'shepherd moons' are two satellites of Saturn (I think it's Saturn) which share very close synchronous orbits; one slightly higher than the other. The inner one gradually gets ahead of the outer one then, years later, slowly catches it up from behind. At that point they swap orbits and speeds.


"Coming nurse..."

[Bear Cub]

Rightstuffer....

The only Shepherd Moons I've heard of is, in fact, the Enya CD (but mine won't play - faulty track).

But I do have a cat called Saturn.

Don't 'spose any of that helps really, does it?

<<edit point: I also have a cat called, Orbit, another called Sputnik and another called Cosmic" end edit>>
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[This message has been edited by Bear Cub (edited 22 December 2000).]