The prehistory of endurance UAVs
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The idea of designing a 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.
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[edit] 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.
[edit] Compass dwell and the XQM-93
In the late 1960s, following the early microwave HALE vehicle studies, the US Air Force worked with LTV Electrosystems (later E-Systems) under the Compass Dwell program to build an endurance UAVs using much more conventional turboprop propulsion. At least part of the motivation or inspiration for this effort was derived from the Igloo White program, which was a multiservice attempt to cut the flow of supplies from North Vietnam to South Vietnam through the network of paths and roads running through Cambodia and Laos known as the "Ho Chi Minh Trail".
Igloo White involved seeding the region with thousands of seismic and acoustic sensors, most of them air-dropped, which would pick up indications of traffic along the trail and report them back to a central command center in Thailand, which would dispatch air strikes in response. The sensors were battery-operated and had limited range, so airborne radio relay aircraft orbited above the battle area to pick up the signals and pass them on to the command center.
Originally, the radio relays were EC-121 Warning Star aircraft, a military variant of the Lockheed Super Constellation four-piston aircraft, but these machines were expensive to operate. They were replaced by Beech Debonaire single-engine light aircraft, modified for the radio relay role and given the military designation of "QU-22A". They could be operated as drones, but apparently nobody trusted that as operational practice, and they were never flown unpiloted except as experiments.
LTV Electrosystems' development effort focused on an endurance aircraft that could be flown as a piloted aircraft or a UAV. A number of prototypes, including piloted and UAV versions, were built and flown. They were based on a Schweizer sailplane design with major modifications by Schweizer to accommodate a Pratt & Whitney Canada PT6A-34 turboprop engine, large fuel tanks, and operational payloads. The aircraft had fixed tricycle landing gear.
| LTV XQM-93 | ||
| Length | 9.14 meters | 30 feet |
| Wingspan | 17.4 meters | 57 feet |
| Empty weight | 1,090 kilograms | 2,400 pounds |
| Maximum loaded weight | 2,090 kilograms | 4,700 pounds |
| Cruising speed | 170 km/h | 105 mph / 91 knot |
| Service ceiling | 15,850 meters | 52,000 feet |
| Endurance | 30 hours maximum | |
| Range | 9,650 kilometers | 6,000 MI / 5,220 NMI |
| Unmanned aerial vehicle | ||
The first prototype, designated the "L-450F", was piloted. It first flew in February 1970, but was lost in an accident on its third flight in March 1970, the pilot bailing out safely. A second L-450F was built and used to complete the flight test program. The third prototype, the first UAV variant, was designated the "XQM-93" and flew in early 1972. It had no cockpit or other provisions for piloted flight. It could carry a payload of 320 kilograms (700 pounds). The Air Force ordered four XQM-93s but it is unclear that all were actually delivered, since Compass Dwell was cancelled that year.
These specs are actually for the piloted L-450F, but the general configuration of the XQM-93 appears to have been very close to of the L-450F.
Martin Marietta also built two prototypes of the "Model 845A" for the Compass Dwell program, these machines also being based on a Schweizer sailplane and similar in configuration to the XQM-93. Test flights were performed in 1971, with one of the prototypes staying aloft for almost 28 hours, but the Model 845A was cancelled along with the XQM-93.
The Ryan and Boeing Compass Cope UAVs, discussed earlier, were other early implementations of endurance UAVs. The XQM-93 and the Compass Copes looked forward to aircraft that would come into service in two decades, but nothing much came of it at the time.
[edit] Solar-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 "Gossammer 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.
[edit] Condor and AMBER
| Boeing Condor | ||
| Length | 20.7 meters | 68 feet |
| Wingspan | 61 meters | 200 feet |
| Empty weight | 3.630 kilograms | 8,000 pounds |
| Loaded weight | 9,070 kilograms | 20,000 pounds |
| Cruising speed | 370 km/h | 230 mph / 200 knot |
| Service ceiling | 20,000 meters | 65,000 feet |
| Endurance | 2.5 days | |
| Unmanned aerial vehicle | ||
In the mid-1980s, Boeing developed a large and capable HALE UAV named the "Condor" that was a significant milestone in the development of endurance UAVs. The Condor featured lightweight composite and honeycomb structures, autonomous controls, high altitude aerodynamics, and a fuel-economical propulsion system.
The Condor was rolled out in March 1986, with first flight on 9 October 1988. It set an altitude record for piston-powered aircraft of 20,420 meters (66,980 feet) during its 141-hour flight test program, and stayed aloft for two and a half days during one of its test flights. Boeing consulted with Dick Rutan, pilot of the Earth-circling "Voyager" aircraft, on the design of the Condor.
About 60% of the Condor's loaded weight was fuel, all of which was carried in wing tanks. The UAV had a boxy fuselage that was 1.32 meters high and 0.86 meters wide (52 by 34 inches) that was designed for mounting antennas and sensors. The UAV's large size gave it a large payload capacity, and during test flights it carried about 815 kilograms (1,800 pounds) of instruments. The Condor could be broken down to allow it to be flown to remote sites in a large transport aircraft.
The Condor was powered by two six-cylinder liquid-cooled Teledyne Continental piston engines, each with 131 kW (175 hp). The engines featured a two-stage turbocharging for high altitude operation, and drove three-blade composite tractor propellers 4.9 meters (16 feet) in diameter. The engines drove the propellers through a two-speed gearbox that shifted the propellers to higher RPM at high altitude.
The Condor used duplicate, redundant flight control computers. It was capable of operating autonomously from takeoff to landing, using a flight control program consisting of 60,000 lines of FORTRAN code, with some assembly language optimization. Communications links allowed a mission to be modified in flight. The Condor took off on a dolly with outriggers that drop off on the wingtips, and lands with a skid and a nosewheel. The scheme reduced the weight penalty of full landing gear. Boeing invested over $100 million USD on the Condor, with some assistance from DARPA, but found no buyers for the aircraft and ultimately mothballed it.
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.
[edit] General Atomics GNAT-750
| GNAT 750 | ||
| Length | 5 meters | 16 feet 5 inches |
| Wingspan | 10.75 meters | 35 feet 4 inches |
| Height | 0.5 meters | 1 foot 8 inches |
| Empty weight | 254 kilograms | 560 pounds |
| Maximum loaded weight | 517 kilograms | 1,140 pounds |
| Maximum speed | 212 km/h | 132 mph / 115 knot |
| Service ceiling | 7,620 meters | 25,000 feet |
| Endurance | 48 hours | |
| Unmanned aerial vehicle | ||
Amber died, to be quickly resurrected. In 1988, LSI had begun development of a simplified version of the Amber named the "Gnat 750", intended for foreign sales. The Gnat 750 made its first flight in 1989.
The Gnat 750's configuration was similar to that of the Amber, except that the Gnat 750's wing was mounted low on the fuselage, instead of being mounted on a pylon on top. The Gnat 750 was somewhat larger than the Amber, but weighed less and could carry a heavier payload.
The Gnat 750 is powered by a Rotax 912 piston flat-four four-cycle engine with 64 kW (85 hp). The UAV can fly to an operational area from 2,000 kilometers (1,240 miles) away and loiter there for 12 hours before returning home.
Eight Gnat 750s were in development when General Atomics bought out LSI. General Atomics continued the program, which led to a contract from the Turkish government for a number of the UAVs in 1993.
By this time, the breakup of the old Communist states of Eastern Europe was in full swing, and the US government wanted to obtain an intelligence asset to help it deal with trouble spots in the region, specifically the former Yugoslavia. A contract was issued to General Atomics for Gnat 750s with minor modifications. The Gnat 750s were to be operated by the CIA.
The program encountered a number of difficulties, much of them due to bureaucratic factionalism and squabbling. One Gnat 750 crashed during tests when it was hit by a gust of wind, causing it to indicate zero airspeed. The UAV's software decided that meant it had landed and shut down the engine, causing the Gnat to fall to earth.
The Gnat 750 effort squeaked through, and in early 1994 the CIA sent a team equipped with a Gnat 750 to Albania to monitor events in the former Yugoslavia. The operation was not a success. The Gnat 750 suffered from a number of bugs and was limited by bad weather, and the team was finally withdrawn. However, the Gnat 750 continued to be built, leading to an "Improved Gnat" or "I-Gnat" variant, with a turbocharged engine and general overall refinements to increase reliability, reduce maintenance, and enhance capability. The Gnat 750 also led to a next-generation derivative, the "Gnat 750-45", much better known as the "Predator", discussed later.
General Atomics also used the Gnat 750 as the basis for a tactical UAV, known as the "Prowler". It looks much like a Gnat 750, but is cut down in size, with a span of 7.31 meters (24 feet) and a length of 4.24 meters (13.9 feet). It has an endurance of over 16 hours, and some commonality with Gnat 750 subsystems.
[edit] References
This article contains material that originally came from the web article Unmanned Aerial Vehicles by Greg Goebel, which exists in the Public Domain.
