fuel has too low energy density to enable a step forward in spaceflight
problem with spaceflight is that fuel has too low energy density, if an electric ramjet could be built, then lasers based on earth or lasers based on space based solar platforms could use lasers to beam energy to spacecraft that uses that energy in an electric ramjet, then that spacecraft would not have to use fuel while inside atmosphere, but when spacecraft leaves atmosphere, then spacecraft would have to use onboard fuel for it's rocket engines