Rail-Gun Space Launcher

[Zkribbler]

The rail gun (a.k.a. mass driver) uses electromagnetic impulses to accelerate an object. The navy is currently developing weapons systems using this devise.

IIRC, it is not practical for space launches from Earth, altho it would work on the Moon. ---But for the life of me, I can't remember why.

My best guess is that, in reaching Earth escape velocity, an unexceptable amount of air resistence (& with it: friction heat) will be encountered. Right??

Someone clue me in, pleeeeease!

[KrazyHorse]

In addition to the fact that the atmosphere would make anything you tried to launch go "boom", the moon also has a velocity-to-orbit ~6 times lower than the Earth's. Now, ke = mv^2/2, so the energy needed would be 36 times higher. Which means that given equal lengths of track the force needed would be 36 times higher.

So there are a lot of problems with railguns on earth...

[Kuciwalker]

The problem with the force being higher is that the accelerations tend to do bad things to the payload. Lunar railguns would presumably be sending raw materials and such.

[Zkribbler]

I don't see the problem as being accelateration, so much. The rail-gun could be really long, so the vehicle could ease up to the necessary speed.

After I made the opening post, I remembered that Earth Escape Velocity is 25,000 miles per hour. In the thin upper reaches of the atmosphere, that might be doable, but down here on terra firma, there'd be a lot of air friction and a lot of heat! As KH said: Launch vehicle go BOOM!

[Kuciwalker]

The rail-gun could be really long

Assuming you use a circular accelerator, the acceleration is the square of the velocity divided by the radius. Let's say the accelerator is 10 miles in diameter; that leaves an acceleration of ~800 g, well outside the tolerances for most things. To cut that to 10 g we'd have to make the accelerator 800 miles in diameter.

[Zkribbler]

Well....so much for my brilliant idea on how to leave all that heavy fuel behind.

[KrazyHorse]

From what I figure, given a fairly aerodynamic vessel of radius ~10 meters, the energy released in its passage through the atmosphere would be about 1 Hiroshima in scale.

[KrazyHorse]

The atmosphere's so bad that it actually manages to suck away most of the energy of the vehicle before it reaches orbit. Which means you're already on the losing end of the energy equation.

It's the rocket dilemma all over again.

[KrazyHorse]

Not to mention the fact that the instant you're released your deceleration due to air resistance = 5000 g or so...

[Kuciwalker]

So you could build a really tall tower - maybe even have it support itself with a counterweight past geosync - and launch the thing from a railgun above the atmosphere!

[notyoueither]

Originally posted by Kuciwalker

The rail-gun could be really long

Assuming you use a circular accelerator, the acceleration is the square of the velocity divided by the radius. Let's say the accelerator is 10 miles in diameter; that leaves an acceleration of ~800 g, well outside the tolerances for most things. To cut that to 10 g we'd have to make the accelerator 800 miles in diameter.

Use the Eiffel Tower as the centre.

Takes care of most of the French and the Anglos at the same time, alnog with a fair few Krauts.

Et voila, old Europe is no longer a concern.

[Pekka]

Can we object to this, or does it only apply to non-US places developing new tech and military industry?

[notyoueither]

You would object to making the French nothing more than bell-hops for space exploration?

[Pekka]

No?

[notyoueither]

Good answer, even if lacking a little in the conviction department.