Once in space, I guess rail guns can already achieve 1.5 km/sec easily. And I guess it is possible to push it up to 3 km/sec with tech improvements. If you are able to reach 5km/sec, you effectively have a LOX/RP1 engine right there. And you wouldn't have the heat and all those issues associated with ion thrusters. It seems this can work for orbit transfers between planets better than currently proposed tech. Since no one is trying it, I guess there is something wrong with this, so what is it?
[Physics] Why can’t we use railguns to propel rockets once in space
rocket-sciencespace
Related Solutions
Rory Alsop explained why the idea is wrong, but it may originated from the following reasoning.
When a space rocket takes of, it does so vertically. At that time it is fully loaded with fuel and hence its acceleration is slow. When you watch a video of a space rocket take-off, it seems to crawl along the launch tower.
However, in order to achieve orbit, the rocket has to travel 7 km/sec horizontally. To achieve that, after a while the rocket's path starts to curve towards the horizontal. At that point the first stage may already have dropped off and a large amount of fuel has been burned, so the rocket is a lot lighter. Because the acceleration is inversely proportional to the mass the rocket will be accelerating significantly faster at that point. At the same time, because the rocket is now fairly high up, the air pressure has dropped significantly, and the reduced drag also increases acceleration.
Hence, the rocket accelerates faster when it is going horizontally. Somebody could then take that as meaning "faster than upwards as well as downwards"
Edit
Another issue is the is the "dynamic pressure" which is created by the speed and air-drag. Because of this, the engines may not be run at full power until past the "max-q" point. In the case of the Shuttle, the main engines ran at 65% for the first minute or so of the flight. Only then was it throttled up to 100%, increasing acceleration. See http://www.aerospaceweb.org/question/aerodynamics/q0025.shtml
There are several issues with using ramjets on rockets:
The startup problem. Ramjets rely on the supersonic velocity of the vehicle to create the compression needed for the combustion chamber to operate. So you'll need something (traditional motor stage, air breathing engine, etc) to get the craft moving to the point the ramjets can actually function.
Ramjets have relatively low thrusts. They don't weigh much because they have no moving parts, but they don't produce a whole lot of thrust. So a rocket would need a large number of the engines to function.
Ramjets will eventually stop working as the speed gets higher. At some point, the duct upstream of the combustion chamber will not slow down the incoming air enough. This will make the air in the combustion chamber supersonic instead of subsonic. This is what a SCRAMJET engine is used for, but that's a slightly different configuration. So either the ramjet would need to have moving parts to change its geometry to handle subsonic and supersonic flow in the combustion chamber, or you would need to shut down the ramjets and open up some scramjets (which means the rocket would now have 3 different types of engines).
Ramjets, and scramjets, are air-breathing engines. For anything that gets very high in to atmosphere, there isn't much air left. So the engines won't work very well anymore. Traditional rockets carry both their fuel and oxidizer so they work anywhere.
All of that said, there is considerable research going on in the area. The USAF is actively funding research into them, and the Russian military has a pretty cool design for some self-guiding bombs that drop off of an ICBM and fly to their targets using scramjet engines.
Best Answer
A major problem with your idea is the following: If you're looking to eliminate the need for a supply of propellant with the railgun, it won't happen. The impulse imparted to the rocket is, by Newton III, imparted o on the railgun and so each launch will de-orbit the latter. It will need a constant supply of fuel to keep its orbit stable. A given quantity of fuel imparts a given impulse, so whether impulse is imparted directly to the rocket through chemical engines or whether to the launcher, you won't save on the need for propellant.
The other engineering issues aside (like rail wear cited by Dmckee):
another major reason that if you planned to boost humans in this way, or delicate instruments, the rails would need to be very long. Not impossibly so, but clearly a project hugely beyond the ISS, for example. Suppose we are in LEO and wish to boost to Earth escape speed: we need a delta-V of about $3.5{\rm km\,s^{-1}}$ ($11.2{\rm km\,s^{-1}}$ from LEO orbital speed of $7.8{\rm km\,s^{-1}}$). If you did this at constant acceleration $a$, then (calculating in a frame comoving with the LEO orbiting railgun):
$$s=\frac{(\Delta V)^2}{2\,a}$$
and limiting $a$ to, say, $5\,g$, I get a rail length of about $123{\rm km}$.