Background The concept of using beamed power for aircraft propulsion was invented almost single-handedly by
William C. Brown. After joining
Raytheon in the 1940s, Brown started work on improving their
magnetron products. This led to the development of the
crossed-field amplifier, a simple, reliable and highly efficient
microwave amplifier. He later worked with colleagues to develop the
rectenna, which receives microwaves (the (an)"tenna") and converts them directly to
DC power (the "rect"ifier). Brown now had a system that could convert input power to microwaves with up to 70% efficiency and convert it back to electric power with 70% efficiency, resulting in an overall efficiency of about 50%. Brown looked for applications of the technology, working on both
solar power satellites (SPS) and the
High Altitude Powered Platform (HAPP) concept. This research went as far as flying a model
helicopter using beamed power in 1965.
NASA also invested in the rectenna concept as part of their SPS work. This was tested in a ground-to-ground experiment in 1975, and as part of this they developed lightweight versions of the rectenna. In 1982, Brown and James Trimer (of NASA) announced a new version of the rectenna using
printed circuit techniques that reduced the weight by ten times. The economics of the system were attractive as a replacement for conventional satellites even in some large-area deployments. The CRC estimated the aircraft would cost about $100,000 each, and operate for $2 to $3 million a year. In contrast, just launching a satellite cost about $150 million. Additionally, whereas a satellite of the era might have a lifetime of about 10 years, the aircraft could be periodically returned to the ground for servicing and upgrades, allowing it to operate indefinitely. They felt this would be attractive to
third world markets. Compared to SHARP, their MILAX (Microwave Lifted Airplane Experiment) vehicle had two new design features. The rectenna on the aircraft was embedded in the wings and tail surfaces, eliminating the need for the separate antenna body. The broadcast antenna was based on an active
phased array, allowing it to steer without physical movement. The system was tested by mounting the broadcast antenna on the back of a light truck, and driving it around while MILAX followed. Another beamed-power experiment in Japan was the ETHER project, which beamed 5.8 kW of power to a helium-inflated airship. In the years since the system was first proposed, advances in solar cells and battery technology have upset the initial calculations. The
NASA Pathfinder demonstrated long-duration solar powered flight in a role essentially identical to SHARP. In the 2000s,
Titan Aerospace began development of such a vehicle specifically for the communications role, in this case as an
internet relay. ==References==