The next new aircraft to roll out of the U.S. Air Force hangar may be a powered sensor. Scientists at the U.S. Air Force Research Laboratory at Wright-Patterson Air Force Base, Ohio, have developed radar arrays that can serve as aircraft skin and as structural components. Their research is opening up entirely new radar capabilities as well as materials advances.
 
This work is emerging from the SensorCraft program, which aims to produce a high-altitude unmanned aerial vehicle (UAV) that could loiter over an area of interest for 40 to 50 hours (SIGNAL Magazine, February 2001, page 16). While this program is geared toward producing a vehicle in the next decade, some of the advances could be implemented in existing aircraft well before delivery of the specialized surveillance and reconnaissance vehicle.
 
The key to the SensorCraft lies in its skin. Instead of being an aircraft equipped with advanced sensors, the vehicle would be an aircraft built out of advanced sensors. John Perdzock, SensorCraft lead in the Air Vehicles Directorate of the U.S. Air Force Research Laboratory, explains that the program’s radar research largely has focused on breakthroughs in two areas: low-band and high-band antennas. An original aircraft design featured a joined-wing configuration that resembled two letter “Vs” attached at the ends. This approach was favored for placing radar antennas on the outside of the aircraft so that the radar could scan completely around the aircraft’s exterior.
 
But scientists at the laboratory now are exploring a concept known as Endfire for a low-band radar element to perform 360-degree scanning. Perdzock explains that a traditional flat-panel radar antenna emits a signal perpendicular to its surface. Endfire technology would emit a radar signal from any of the four sides of the flat panel, parallel to the plane of the surface.
 
This new capability allows engineers to look at a totally different airframe design. Instead of the joined wing, the aircraft could have a more conventional swept wing configuration similar to that of the B-2.
 
Engineers have built several different configurations of this technology, including a square array five elements by five elements. Another array under construction would be up to 20 feet long x 10 feet wide. This rectangular array would be more consistent with the type of antenna that would form a wing, Perdzock observes.
 
Similar work characterizes the high-band array, which operates in the X band. Perdzock recounts that engineers have built arrays in a solid form that constitutes a load-bearing structure. The transmit/receiver part of the radar is bonded to that load-bearing antenna using conventional composite bonding technology. The result is a thin, compact X-band array that can go into the skin of an aircraft.
 
The large size of these antennas provides a higher gain, which in turn allows maximizing the power performance of all the aircraft’s radar antennas. Perdzock relates that scientists have developed a square-foot construct that addresses all of the array’s requirements. This one square foot of X-band radar features more than 200 elements, and a high percentage of these must work to ensure that the antenna functions properly. One of the program’s challenges has been to get a high yield out of that array, he observes.
 
Another effort underway has produced a test article that is 20 feet long x 2 feet wide. This unit lacks the necessary electronics, but it features all of the antenna elements along with the bonded feed network that would be attached to the transmit/receive chips. They were omitted to spare expense, Perdzock allows. Absent active radar components, this construct was tested last year successfully for withstanding primary aircraft structural loads. The tests demonstrated that this X-band antenna concept can be used in many aircraft areas, Perdzock says. “We’re quite excited about the capability structurally.” He adds that the next step is to verify the construct electrically.
 
The arrays are the key part of the aircraft, but the vehicle’s structure also is undergoing substantial engineering. Perdzock says that a “significant amount of activity” aims at maturing the basic airframe concepts and the related air vehicle technologies.
 
Beginning with the original concept of building an aircraft around a sensor package, engineers worked to determine just what would compose that package. Their work defining the low- and high-band radar constructs led to the two vehicle variants—the flying wing and the joined wing. The design of either aircraft would mandate a fairly large vehicle with a wingspan of from 150 feet to 200 feet, Perdzock points out.
 
With that large conceptual design on paper, scientists now are identifying which technologies will be needed to realize the goals of the SensorCraft. But the program’s testing regimen has not been without trial and error.
 
Perdzock relates how the first attempt at a five-element array was a failure. Engineers went back to the drawing board to undertake a much more rigorous buildup to design and testing. The steps have taken longer than envisioned, but progress has been steady.
 
The biggest hurdle has been understanding the nature of “this fundamentally new way of doing business” with radars, Perdzock allows. “We’ve never attempted something as ambitious as a structural X-band antenna,” he states.
 
These tests, which are designed to validate the maturity of both the arrays and the structural aspects of the vehicle, are generating results that are not limited just to the SensorCraft, Perdzock notes. Virtually any radome, bay door, fuselage door, or generic panel or wing could serve as a low- or high-band radar antenna. The laboratory already is looking at opportunities to deploy these technologies before the SensorCraft is delivered.

The full version of this article is published in the March 2007 issue of SIGNAL Magazine, in the mail to AFCEA members and subscribers March 1, 2007. For information about purchasing this issue, joining AFCEA or subscribing to SIGNAL, contact AFCEA Member Services.