Objectives:
- • To assess the viability and performance of microwave-/solar-powered aircraft for reconnaissance missions. The flight performance and mission potential of a power-harvesting aircraft are compared to a standard aircraft.
- • To optimize the vehicle design and trajectory to meet the needs of the mission while minimizing predetermined costs, such as power consumption, vehicle size, weight, etc. The vehicle and its trajectory must be designed together in order to meet these needs.
- • To build and characterize aircraft structures with embedded power harvesting capabilities. These structures will be tested mechanically and their energy harvesting capabilities will be determined in controlled environments.
Background
Unmanned Aerial Vehicles, or UAVs, are used in many applications to gather intelligence without risking human lives. These aircraft, however, have limited flight time because of their reconnaissance payload requirements coupled with their limited scale. A microwave-powered flight vehicle, on the other hand, would be able to perform a reconnaissance mission continuously. Using beamed microwave energy from a remote source on the ground, the airplane gathers energy using on-board antennas. A rectifying antenna, or rectenna, harvests power from the incident radiation and rectifies it into a form usable for charging a storage battery/capacitor, powering on-board electronics, and driving DC motors for thrust and control surface actuation. Thus, the power needed to maintain flight can be remotely transmitted, either continuously or in bursts.
The addition of microwave power harvesting to a battery-powered aircraft adds unprecedented surveillance capabilities to a typical Intelligence, Surveillance, and Reconnaissance (ISR) platform. Figure 1 depicts the regions of operation of such an aircraft. Region I is bounded by the equilibrium power radius req; it represents the area in which the aircraft can loiter indefinitely. In this area the power harvested from the ground transmitter is greater than the power required for flight. Region II is fully, radially observable: in this area the plane can circle the transmitter at least once. The stored energy is continually decreasing in this region, and the plane eventually requires a return to Region I to recharge. Region III represents the area where the plane can only fly a partial arc before returning to Region I. Here, the aircraft flies directly out to a specified radius, sweeps out an arc over the area of interest, and then returns to Region I. The length of this arc decreases from 360 degrees to 0 at the no-return radius rnr, since at this radius, the aircraft only has enough energy to fly out and directly back again. Region IV represents an area that cannot flown through from an arbitrary exit point from Region I. Finally, Region V covers the area outside the no-return radius. If the aircraft flies into this region, it will run out of stored energy (and, hence, thrust) before it makes it back to Region I. It should be noted, however, that this plot is generated under the assumption of constant altitude flight; any altitude the aircraft may sacrifice can extend these radii and enlarge the entire plot.
Figure 1: Surveillance envelope, indicating regions of coverage
Microwave power transmission was pioneered by William Brown, of Raytheon Corporation. Brown invented the first rectifying antenna, which simultaneously converts microwave radiation to electrical energy and rectifies the energy into a direct current. Brown used his rectenna to create a helicopter run by microwave energy transmitted from the ground. To prove his invention, the helicopter hovered stationary at a height of 60 feet for over ten hours.
The next significant contribution to fuel-less flight was made by the Stationary High Altitude Relay Platform, or SHARP, program. The team was lead by Ron Barrington; however, the aircraft was developed and tested by Professor James DeLaurier. An airplane with a wingspan of 4.5 meters was first flown in 1987. The SHARP program used a cluster of microwave dish antennas on the ground to allow the aircraft to circle above the transmitters and used a rectenna array mounted to the underside of the aircraft to convert the radiation into usable power. Despite these achievements, no other significant work has been performed on microwave powered aircraft since.
Figure 2: Microwave-powered helicopter platform (left), SHARP prototype (right)
Although both of these projects were very successful, they utilized large external rectenna arrays that were not integrated into the aircraft structure and, therefore, did not present the most aerodynamically efficient designs. Indeed, the relationship between geometry and received energy plays a significant role in the ability for a rectenna to receive and convert microwaves into electrical energy without sacrificing aircraft performance. Micro-scale and flexible rectennas, however, provide the possibility for a more efficient solution. Using conformal antenna shapes, large gains in aerodynamic performance may be achieved with only a small reduction in power conversion efficiency. Integrating the rectenna into the structural members also may yield a reduction in take-off weight. Two candidate wing designs – one traditional and the other specifically designed for beamed power reception – are depicted in Figure 3, indicating the received power density. These plots show that careful consideration of aerodynamics and power harvesting capabilities can yield an overall more efficient design.
Figure 3: Incident power density for elliptical (left) and bulbous (right) wings
Publications
- Wickenheiser, A. and Garcia, E. "Conceptual Design Considerations for Microwave- and Solar-Powered Fuel-less Aircraft", Journal of Aircraft (in review).
- Wickenheiser, A. M. and Garcia, E. "Mission Performance of a Solar- and Microwave-Powered Aircraft", Smart Structures and Materials 2008: Active and Passive Smart Structures and Integrated Systems II, March 10-13, San Diego, CA. published in: Proc. SPIE Vol. 6928, 2008. (PDF)