airplanes and space flight

In article ,
Mikko wrote:

But 30,000ft and, say, 600mph isn't much of a boost. It's a long, long,
long way from orbit. The difficulty of getting to orbit is better
described by "17500 miles per hour" than any figure of altitude.

Why is that speed needed?

Because that's orbital velocity in low-Earth orbit. Any slower than
that, and you're falling to the ground.

Only thing I can think is, that since earths gravity effects the craft
whole time, the longer it takes, the more gravity will "drag back" the
craft?

Hmm, I suppose that's one way of looking at it. Here's another way:
gravity is pulling you toward the ground at (roughly) 9.8 m/sec. But
the Earth is also round; if you are moving forward fast enough, then by
the time you fall to the Earth, it's no longer there -- it's curved away
and is now behind you a bit. At 17500 mph or so, you're moving so fast,
that the Earth is *always* behind you; you're going around it as fast as
you are falling towards it.

Hmm, I think that was a lousy explanation. Surf the web a bit and I bet
you'll find better ones.

In pithy form: orbiting is the trick of falling towards the ground and
missing.

But also gravity gets smaller when you get more away from the earth?

Yes, but not for a LONG way. You can pretty safely ignore that effect.

How high does one have to go to have only half of gravity?

Well, gravity (like pretty much anything else, due to basic geometry)
falls off with the square of the distance. So you can write g2/g1 =
(r1/r2)^2. The radius of the earth is 6400 km; call that r1, and you
want the r2 where g2/g1 is 0.5. 0.5 = (6400/r2)^2, do the algebra, r2
comes to about 9100 km, or about 2700 km altitude.

So this decrease of gravity with altitude isn't much help in reaching
orbit.

What if someone built a 30,000 ft high tube, similar to magnetic trains
- electrical magnets around it. Then you could just put metallic
cargo inside - without any engine or fuel, and shoot it up. The tube
would have to be high enough that there is no air where the cargo comes
out, and maybe part of the tube would have to be a vacuum.

Yes, this has been explored before (it's generally called a mass driver
-- try a google search). It would be a massive engineering project,
probably more so than you realize, if you want to be able to launch
anything other than bulk materials like water. You'd need a barrel
length of over 50 km to keep the acceleration tolerable for humans (say,
2 Gs or so).

Propably not something to do today, but still lot shorter than the
"space-lift", and it would give near 100 % payload.

By "space-lift" I assume you mean a space elevator. Yes, it's
dramatically shorter than that, but has a number of additional
operational complications, like keeping the thing up in the sky and
getting your payloads to and from it. It also doesn't help much with
getting stuff back down, which is itself a rather hard problem.

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