So, I've been reading a bit about thermal rocketry, from NTRs to beamed propulsion ideas. Now the basic concept is to squeeze more Isp out of traditional propellants such as liquid hydrogen. Specifically, if you heat H2 to around 2700K (depending on pressure), it breaks up into quantities of H1, which gives a big boost in exhaust velocity and thus Isp.
I'm sure I'm not the first person to think of this, but instead of elaborate schemes to beam energy to a moving craft, why not just use an energy source while on the ground to create heat, perhaps a dense plasma like in fusion experiments, and then use that to heat the propellant that will lift the craft into orbit? Basically plug the rocket in. You only need around ~10 minutes to get into orbit, so this system would only have to be viable for a short time. You could use reflective barriers to minimize radiative heat loses and channel them to the heat exchanger.
I guess this is more of a thought experiment than a serious proposal. What are the drawbacks of this sort of design?