The prehistory of endurance UAVs

The idea of designing an Unmanned Aerial Vehicle (UAV) that could remain in the air for a long time has been around for decades, but only became an operational reality in the twenty first century. Endurance UAVs for low-altitude and high-altitude operation, the latter sometimes referred to as "high-altitude long-endurance (HALE)" UAVs, are now in full service. This chapter describes the evolution of the endurance UAV concept.

Beamed power UAV experiments

The idea of using HALE UAVs as a cheaper alternative to satellites for atmospheric research, earth and weather observation, and particularly communications goes back at least to the late 1950s, with conceptual studies focused on UAVs with conventional propulsion, or new forms of propulsion using microwave beamed power or photovoltaic solar cells.

Raytheon suggested what would now be described as a HALE UAV helicopter operating using beamed power, flying at an altitude of 15 kilometers (9 miles), as far back as 1959, and actually performed a proof-of-concept demonstration in 1964, with a transmitting antenna powering a helicopter on a 20 meter (65 foot) tether. The helicopter carried a rectifying antenna or "rectenna" array incorporating thousands of diodes to convert the microwave beam into useful electrical power.

The 1964 demonstration received a good deal of publicity, but nothing came of it, since enthusiasm for Earth satellites was very high and the rectenna system was heavy and inefficient. However, in the 1970s, NASA became interested in beamed power for space applications, and, in 1982, published a design for a much lighter and cheaper rectenna system.

The NASA rectenna was made of a thin plastic film, with dipole antennas and receiving circuits embedded in its surface. In 1987, the Canadian Communications Research Center used such an improved rectenna to power a UAV with a wingspan of 5 meters (16 feet 5 inches) and a weight of 4.5 kilograms (9.9 pounds), as part of the "Stationary High Altitude Relay Platform (SHARP)" project. The SHARP UAV flew in a circle at 150 meters (490 feet) above a transmitting antenna. The UAV required 150 watts, and was able to obtain this level of power from the 6 to 12 kilowatt microwave beam.

Incidentally, the idea of flying a UAV using beamed power is still floating around. In 2002, a team of NASA researchers flew a small solar-powered RC airplane using a theater searchlight as a power source to drive its propeller, and the next year, 2003, they used a laser to keep the little airplane flying. The laser could track the flight of the aircraft to remain focused on its solar cells. The whole thing was strictly a proof-of-concept effort, since the flights were conducted indoors and the aircraft was too unsophisticated to be really called a "UAV", but it was an interesting exercise, and more may be made of the idea in the future.

olar-powered UAVs: HALSOL and SOLAR HAPP

In the 1980s, new attention was focused on aircraft propelled by solar power. Solar power cells, more technically known as "photovoltaic (PV)" cells, are not very efficient, and the amount of energy provided by the Sun over a unit area is relatively modest. This means that a solar powered aircraft must be lightly built to allow low-powered electric motors to get it off the ground.

Such lightly-built aircraft had been developed in the competition for the Kremer Prize for human-powered flight. The Kremer Prize had been set up in 1959 by Henry Kremer, a British industrialist, and offered 50,000 British pounds in prize money to the first group that could fly a human-powered aircraft over a figure-eight course covering a total of 1.6 kilometers (a mile). Early attempts to built human-powered aircraft had focused on wooden designs, which proved too heavy. In the early 1970s, Dr. Paul B. MacReady and his AeroVironment company took a fresh look at the challenge, and came up with an unorthodox aircraft, the "Gossamer Condor", that pilot Bryan Allen flew to win the Kremer Prize on 23 August 1977. The Gossamer Condor was basically a flying wing, modified with the addition of a gondola for the pilot underneath and a canard control surface extended in front, and was mostly built of lightweight plastics.

The next logical step was to build a solar-powered piloted aircraft. In 1980, Dupont Corporation backed AeroVironment in an attempt to build a solar-powered piloted aircraft that could fly from Paris, France to England. The first prototype, the "Gossamer Penguin", was fragile and not very airworthy, but led to a better aircraft, the "Solar Challenger". The Solar Challenger had a wingspan of 14.3 meters (47 feet) and a weight of 90 kilograms (200 pounds). Its wings were covered with 16,128 PV cells, with a total output power of 2,600 watts, about enough to drive a pair of hair driers. The Solar Challenger was capable of reaching an altitude of 3,660 meters (12,000 feet), and in July 1981 the aircraft accomplished the 262 kilometer (163 mile) flight from Paris to Manston in the UK.

This success led in turn to AeroVironment concepts for a solar-powered UAV for HALE applications. A solar-powered UAV could in principle stay aloft indefinitely, as long as it had a power-storage system to keep it flying at night. The aerodynamics of such an aircraft were challenging, since to reach high altitudes it had to be much lighter per unit area of wing surface than the Solar Challenger, and finding an energy storage system with the necessary high capacity and light weight was troublesome as well.

In 1983, AeroVironment was able to obtain funding from an unspecified US government agency to secretly investigate the concept, which was designated "High Altitude Solar (HALSOL)". The HALSOL prototype first flew in June 1983. HALSOL was a simple flying wing, with a span of 30 meters (98 feet 5 inches) and a width of 2.44 meters (8 feet). The main wing spar was made of carbon fiber composite tubing, with ribs made of styrofoam and braced with spruce and Kevlar, and covered with thin Mylar plastic film. The wing was light but remarkably strong.

The wing was built in five segments of equal span. Two gondolas hung from the center segment, which carried payload, radio control and telemetry electronics, and other gear. The gondolas also provided the landing gear. Each gondola had dual baby-buggy wheels in front and a bicycle wheel in back for landing gear. HALSOL was propelled by eight small electric motors driving variable-pitch propellers. There were two motors on the center wing segment, two motors on each inner wing segment, and one motor on each outer wing segment. The aircraft's total weight was about 185 kilograms (410 pounds), with about a tenth of that being payload.

Nine HALSOL flights took place in the summer of 1983 at the isolated and secret Groom Lake base in Nevada. The flights were conducted using radio control and battery power, as the aircraft had not been fitted with solar cells. HALSOL's aerodynamics were validated, but the investigation led to the conclusion that neither PV cell nor energy storage technology were mature enough to make the idea practical for the time being. HALSOL was put into storage, and as it turned out, would be resurrected for greater glories later, as discussed later. For the moment, though, it remained a complete secret.

In the mid-1980s, not long after HALSOL went into mothballs, NASA awarded a contract to Lockheed to study a solar-powered HALE UAV named the "Solar High Altitude Powered Platform (Solar HAPP)" for missions such as crop monitoring, military reconnaissance, and communications relay. The Solar HAPP effort did not result in a prototype. Solar-powered HALE UAVs were a concept a bit ahead of their time, and early practical work on endurance UAVs focused on more conventional concepts.

Amber

Modern US government studies for endurance UAVs, powered by more or less conventional aircraft engines, began with a secret study begun by DARPA in the early 1980s, codenamed "Teal Rain".

In 1984, DARPA issued a $40 million US contract to Leading Systems Incorporated (LSI) of Irvine, California, to build an endurance UAV named "Amber". Amber was to be used for photographic reconnaissance, ELINT missions, or as a cruise missile. The US Army, Navy, and Marine Corps were interested, and DARPA eventually passed control over to the Navy.

Amber was designed by a team under Abraham Karem of Leading Systems. Amber was 4.6 meters (15 feet) long, had a wingspan of 8.54 meters (28 feet), weighed 335 kilograms (740 pounds), and was powered by a four-cylinder liquid-cooled piston engine providing 49 kW (65 hp), driving a pusher propeller in the tail. The wing was mounted on a short pylon above the fuselage. The cruise missile version of Amber would discard the wing when it made its final dive on a target.

Amber had an inverted-vee tail, which would prove a popular configuration for a pusher UAV, since it protected the propeller during takeoff and landing. The airframe was made of plastic and composite materials, mostly Kevlar, and the UAV had retractable stiltlike tricycle landing gear to ensure propeller clearance. Amber had a flight endurance of 38 hours or more.

The initial contract specified three "Basic Amber" A-45 cruise missile prototypes and three B-45 reconnaissance prototypes. Initial flights were in November 1986, with long-endurance flights the next year. Up to this time, Amber was a deep secret, but in 1987 details of the program were released.

Amber was only one of a number of different US UAV programs in planning at the time, and the US Congress became impatient with what was perceived as confusion and duplication of effort. Congress ordered a consolidation of UAV programs in 1987, freezing funding until June 1988, when the centralized Joint Program Office for UAV development, mentioned earlier, was established. Amber survived the consolidation of UAV efforts into JPO, resulting in the first "Amber I" reconnaissance UAV, which first flew in October 1989. Seven Amber Is were built, and were used in evaluations along with Basic Ambers through 1990. However, funding for reconnaissance assets was being cut, and in 1990 the Amber program was killed. LSI was faced with bankruptcy, and was bought out by General Atomics, who would later develop the Amber into an operational platform, the MQ-1 Predator [http://spyflight.co.uk/Predator.htm]

References

This article contains material that originally came from the web article [http://www.vectorsite.net/twuav.html "Unmanned Aerial Vehicles"] by Greg Goebel, which exists in the Public Domain.