Laser propulsion to orbit

From ground to orbit. If perfects, it could really drop costs to orbit. [Link]

A lightcraft, then, flies on a beam of laser light, turning its energy into thrust. Earlier designs examined the concept from various directions, including one that used a heat-exchanger aboard the rocket and transferred the beamed energy in such a way as to heat a working fluid like hydrogen or ammonia that would be carried onboard. That produces thrust through expansion through a nozzle, much like a chemical rocket.
Another possibility is to carry an onboard solid propellant. But the latest incarnation of the lightcraft operates in dual mode, using air as described above (turned into a plasma by the laser) and then switching to laser thermal rocket mode at higher altitudes (above about thirty kilometers).
The latter concept, of course, demands a small onboard fuel supply, but nothing like the massive fuel/payload ratios we see in today’s rockets. We’re talking about a spin-stabilized, single-stage transportation system to orbit. In its ‘airbreathing’ mode, the engine pulses at a variable rate to achieve what the authors call a ‘quasi-steady thrust,’ one that depends upon the Mach number and altitude along the craft’s flight trajectory.