[mrfusion]

But why can't we use the railgun to replace just the first stage of a rocket? Due to the tyranny of the rocket equation that would mean outsized savings in fuel and payload.

It would be exiting the barrel at far less than mach 27 so this wouldn't be an issue.

[benihana]

Even without a first stage, wouldn't that mean that the railgun would need to account for dealing the energy needed to accelerate a couple hundred thousand tons of rocket fuel to any reasonable speed? As per Newtons 3rd law?

[Crito]

This is correct. And the power needed for larger rockets is well beyond what we currently generate with traditional power plants on earth.

For instance, the space shuttle with SRBs and SSME's firing would require more than five Hoover Dams to match (11.7 gigawatts, vs 2.074 gigawatts). The Three Gorges Dam puts out 18.3 gigawatts, but the Saturn V first stage did somewhere around 190 gigawatts.

You could get away with less of course, since with rockets the first stage must lift the immense mass of itself, but with a railgun you will have significant efficiency losses with all of the energy conversions involved. I would expect a heavy-lift railgun to consume at least a significant percentage of a large nation's entire power supply. So we'd be looking at ludicrously sized capacitor banks, or some very large power plants that sit idle without anything to do while we aren't launching rockets.