Laser propulsion using a molecular absorber

Unlike the conventional approach of using a laser sustained plasma to heat a propellant, molecular absorption of laser energy makes it possible to avoid the frozen flow losses associated with the high temperature and complex chemistry of a plasma. The molecular absorption concept is developed by exploring several thermodynamic pathways using a 1-D fluid theory for energy addition in the supersonic regime and different pathways are shown in H-K coordinates. The absorption physics of a promising molecular absorber, SF6, is described at arbitrary laser beam intensities using a two-temperature non-equilibrium model, which is then applied to calculate the nozzle length required to achieve a specific impulse of 250 sec through a 300 K isothermal expansion in the supersonic section. The results of this conversative example case for energy addition illustrate that over a length of less than 1 m laser power on the order of 20 kW can be absorbed in the supersonic region of a 10 g/sec H₂ flow without creating a plasma.