In 1992, the late Jordin Kare proposed a simpler, nearer term concept which has a rocket containing liquid hydrogen.Kare, J. T. (1992). Development of Laser-Driven Heat Exchanger Rocket for Ground to-Orbit Launch. Washington, DC International Astronautical Federation Congress. The propellant is heated in a heat exchanger that the laser beam shines on before leaving the vehicle via a conventional nozzle. This concept can use continuous beam lasers, and the semiconductor lasers are now cost effective for this application.
