[Lars Larsen (OP)]

What do you think, would  it be possible to use rail guns to launch satellites in an orbit?

I think the answer is No, because the satellite had to be accelerated to escape velocity, that means it had to cross the thick lower layers of the atmosphere at a speed of at least 40.000 km/h - I think it would melt down within no time.

Another issue is, that you would need to calculate the vector and kinetic energy of the object with a practically impossible accuracy in order to launch it to the proper orbit (you would need to consider wind, atmospheric turbulences and so on in every layer of the atmosphere, e.g.).

Furthermore I think, even if it would be possible, it would  always be cheaper to just build a rocket anyway.

What do you think?

[daveshorts]

You can't launch an object into orbit using just a gun of any type, as if an object is in a bound orbit it will come back to the point it started. If the gun is on the surface the shell will always come back to the surface. That is not to say that it wouldn't be useful. You can use a gun to fire a small rocket which will give your payload enough kick to get it into orbit.

Gerald Bull got most of the way there with the HARP project that was canceled in the 60's for mostly political reasons
http://en.wikipedia.org/wiki/Project_HARP

A gun has lots of advantages, the main one being that you don't have to lift the fuel, which make the expensive moving part far lighter. However the accelerations are high so you could only launch very tough payloads such as fuel or food.

[Lars Larsen (OP)]

Thank you very much for your answer.

I don't understand your statement, that ANY object fired by a gun would come back to earth. Wouldn't an object that is accelerated above escape velocity be able to leave the earth for good?

[daveshorts]

Any object in a bound orbit - one that isn't above escape velocity and orbiting in an ellipse will come back.

[LeeE]

Daveshorts: I think you may have overlooked the fact that the projectile won't be moving through a uniform medium at launch.  I think it may be possible to launch a projectile with an appropriate trajectory and velocity such that the degree of slowing as it ascends through the atmosphere leaves it in a stable elliptical orbit, albeit one that decays relatively rapidly as it would have to cut through the extreme upper atmosphere once on each orbit.

Basically, you'd be using atmospheric slowing to translate the return point of the orbit above the surface of the Earth, but as a consequence, the return point would still have to be within the atmosphere (haven't done the maths to prove it though - too much hard work - so I'm quite happy to be proved wrong []).

Oh - incidentally, electromagnetic catapult launchers are being developed to replace the old steam catapults used on aircraft carriers.

[daveshorts]

Ok I was simplifying a bit, I was assuming there were no non-gravitational forces on the object after launch. You could use some sort of hypersonic lift from the upper atmosphere to push you into a more circular orbit, but you have a similar problem, if there is enough atmosphere to get lift from there is enough to get drag from too.

 That is not to say it might not be useful in some circumstances, but you are still going to need the rocket to get into a stable orbit.

[syhprum]

Although gun type launchers are not practical to launch satellites from the Earth they would be quite possible on the Moon or even Mars.
Who knows they may one day be used to harvest the Helium 3 that we will be using in our fusion type power stations in a few years time.