* While the US was flying Lightning Bugs over Vietnam, the Americans were also working on long-range UAVs, primarily for missions deep into China. These long-range UAV programs did not result in operational systems, but after the Vietnam War there was increasing interest in development of "high-altitude long-endurance (HALE)" UAVs, not merely to obtain long range but also persistence over an operational area. This chapter describes the origins of the endurance UAV concept.
* Although the Model 147s Lightning Bugs had the range to cover any target in North Vietnam, they did not have the range to fly deep into China and back out again. In particular, the Chinese nuclear development facility at Lop Nor was far out of reach of the Lightning Bugs, and was barely within reach of the Lockheed U-2 spyplane, which had become far too vulnerable to SAMs. US intelligence clearly needed a long-range drone with a high degree of survivability. Such requirements spelled out a completely new design, not a modification of a target drone.
While Ryan worked on the Lightning Bugs, the company pursued advanced drone concepts on a part-time basis. After discussions with the CIA that went nowhere, in early 1966 Ryan pitched their advanced reconnaissance drone concepts to the NRO. The NRO was interested, and opened up a design competition, originally designated LONE EAGLE, but later changed to COMPASS ARROW. Ryan was pitted against North American Aviation, with Ryan winning the competition in June 1966. The new Ryan design was designated the "Model 154 / AQM-91A Firefly". The basic design concept resembled that of the Model 136 Red Wagon drone that Ryan had proposed earlier in the decade, but which had been turned down in favor of modified Firebees. The name "Firefly" was resurrected for the new drone, though it was also referred to as COMPASS ARROW after the program name.
Like the Model 136 Red Wagon, the Model 154 had an engine on its back to reduce its radar and infrared signature as seen from below, as well as twin inward-canted tailfins to conceal the exhaust stream. The Model 154 was a substantially more sophisticated design, however, leveraging off what Ryan had learned since the early 1960s.
The Model 154 had sloped flat sides to deflect radar signals, and was built using a high percentage of plastic composites, which had lower radar reflectivity than metal. The aircraft was powered by a General Electric YJ97-GE-3 turbojet providing 17.8 kN (1,815 kgp / 4,000 lbf) thrust, with the engine exhaust mixed with cool air to reduce infrared signature. The YJ97 was something of a unique engine, not used in any operational aircraft, derived from a General Electric demonstrator engine designated the "GE1". The Model 154 was to carry electronic countermeasures to further improve its survivability.
RYAN MODEL 154: _____________________ _________________ _______________________ spec metric english _____________________ _________________ _______________________ wingspan 14.5 meters 47 feet 8 inches length 10.4 meters 34 feet 2 inches empty weight 1,725 kilograms 3,800 pounds max loaded weight 2,450 kilograms 5,400 pounds maximum speed 815 KPH 505 MPH / 440 KT service ceiling > 23,800 meters > 78,000 feet range 7,030 kilometers 4,370 MI / 3,800 NMI _____________________ _________________ _______________________
The Model 154 was to be launched by a DC-130 Hercules director aircraft, and recovered in midair by helicopter. It had a precision-navigation autopilot system, a reconnaissance payload, and a self-destruct system to ensure that none of its sensitive gear fell into enemy hands. The primary reconnaissance payload was an Itek panoramic camera, but in principle it could also carry infrared cameras or a SIGINT payload. The guidance system was designed to provide navigation accuracies with an error of no more than half a percent, meaning that if it flew a thousand kilometers, it would be no more than five kilometers off. The guidance system proved very tricky, and first powered flight of a Model 154 did not take place until September 1968.
The test flights were conducted over the US Southwest. The project was a deep black secret, but on 4 August 1969 one of the prototypes decided to stop working and parachuted itself to ground inside the Los Alamos nuclear research complex during lunch hour. Unfortunately, it didn't land in a restricted area, and local newspeople were able to take pictures of the aircraft. The pictures were published in local papers. The Air Force released a statement that the aircraft was a "high altitude target", but though such a statement might have been believed in 1959, it wasn't in 1969.
Test flights were halted for a few weeks while procedures were reviewed. The flights then resumed, culminating in long-range evaluations in late 1971. The Model 154 passed with flying colors, exceeding its altitude requirements by a good margin, and proving almost invisible to radar. However, by that time the need for the Model 154 had evaporated. In July 1971, President Nixon began a diplomatic effort to build ties with China, and reconnaissance overflights were canceled. Satellite reconnaissance capabilities had improved through the 1960s, leading to the first launch of the advanced "Big Bird" satellite on 15 June 1971, which could provide strategic intelligence without provoking the Chinese. In addition, adversary air defenses had improved and the Model 154 was no longer seen as very survivable.
The Model 154 program lingered on for a few more years, but the drones were finally put into mothballs in 1973, and scrapped a few years after that. 28 had been built, including 20 production machines. They were twice as expensive as projected, largely because Ryan had recklessly underbid the contract to get the job. The high unit price helped put defense officials off drones, since it suggested they weren't as cheap a solution as their advocates claimed. Incidentally, in 1974 NRO abandoned its aircraft reconnaissance assets, handing them over to the Air Force. The NRO had become focused on space reconnaissance, with aerial assets being not much more than distraction, all the more so because of interservice squabbling over them.
* Ryan updated the Model 154 design for a more advanced derivative, the "Model 235", for the USAF COMPASS COPE drone program. The Air Force had initiated COMPASS COPE in 1971, specifying a robot aircraft that could take off and land from a runway like a manned aircraft, eliminating the need for a launch aircraft, and operate at high altitudes for up to 24 hours to perform surveillance, communications relay, or atmospheric sampling.
Boeing was originally selected as a sole source, with the USAF awarding the company a contract for two "YQM-94A" (later "YGQM-94A") demonstrators. However, Ryan then pitched the Model 235 as an alternative, and the next year, 1972, the Air Force agreeably awarded Ryan a contract for two "YQM-98A" (later "YGQM-98A") demonstrators as well.
The Boeing YQM-94A was variously known as the "COMPASS COPE B", "COPE B", or "B-Gull". It was basically a jet sailplane, with long straight wings, a twin fin tail, retractable tricycle landing gear, and a turbojet perched in a pod on its back. The engine was a GE J97-GE-100 providing 23.4 kN (2,390 kgp / 5,270 lbf) thrust.
BOEING YQM-94A COMPASS COPE B: _____________________ _________________ _______________________ spec metric english _____________________ _________________ _______________________ wingspan 27.4 meters 90 feet length 12.2 meters 40 feet height 3.86 meters 12 feet 8 inches loaded weight 6,520 kilograms 14,400 pounds max speed at altitude 805 KPH 500 MPH / 435 KT service ceiling > 16,700 meters > 55,000 feet endurance > 17 hours _____________________ _________________ _______________________ Speed statistic is approximate.
The COMPASS COPE B was strictly a demonstrator, and so it was radio controlled, with no autonomous guidance capability. It had a TV camera in the nose to allow it to be flown from a ground station. Initial flight of the first demonstrator was in July 1973, but the machine crashed on its second flight, a few days later. The second demonstrator performed its first flight in November 1974, and went on to complete the evaluation program.
The Ryan YQM-98A was variously known as the "COMPASS COPE R", "COPE R", or "R-Tern". Its general configuration was similar to that of the Boeing COMPASS COPE B, resembling a jet sailplane with a twin-fin tail, retractable tricycle landing gear, and an engine in a pod on its back. The engine was a Garrett YF104-GA-100 turbofan, with 18.0 kN (1,835 kgp / 4,050 lbf) thrust. The COMPASS COPE R had a clear resemblance to the Model 154, though its wings were straight instead of swept.
RYAN YQM-98A COMPASS COPE R: _____________________ _________________ _______________________ spec metric english _____________________ _________________ _______________________ wingspan 24.75 meters 81 feet 2 inches length 11.4 meters 37 feet 4 inches height 2.4 meters 8 feet loaded weight 6,480 kilograms 14,310 pounds max speed at altitude 805 KPH 500 MPH / 435 KT service ceiling 21,300 meters 70,000 feet endurance > 28 hours _____________________ _________________ _______________________
Initial flight of the first COMPASS COPE R demonstrator was in August 1974. However, the Boeing COMPASS COPE B won the competition in August 1976 on the basis of lower cost, with the company awarded a contract to build preproduction prototypes of the "YQM-94B" operational UAV. The YQM-94B was to be bigger, with a length of 15.2 meters (50 feet) and a loaded weight of 7,800 kilograms (17,220 pounds). It was to be powered by a GE TF34-GE-100 turbofan with 26.7 kN (2,720 kgp / 6000 lbf) thrust, similar to the engine used on the Lockheed S-3A Viking carrier-based antisubmarine aircraft. The YQM-94B was to have an autonomous navigation system.
Since the evaluation of the COMPASS COPE prototypes had shown the YQM-98A to be superior to the YQM-94A in some respects, Ryan challenged the award. It did Ryan no good, since the entire COMPASS COPE program was axed in July 1977, apparently because of difficulties in developing the sensor payloads.
BACK_TO_TOP* The US pursued another path towards advanced reconnaissance drones in parallel with the Model 154. In the early 1960s, Lockheed had developed the Mach 3 "A-12" spyplane, which quickly evolved into the famous "SR-71 Blackbird" strategic reconnaissance aircraft. After the destruction of Powers' U-2 over the USSR in 1960, concepts for an A-12 drone were proposed. Kelly Johnson, in charge of Lockheed's secret "Skunk Works" that had built the A-12, thought the A-12 itself would be too big and complicated to make a useful drone, but felt that the design and technology could be leveraged into a smaller ramjet drone that could perform the same mission. The drone could be launched by the A-12, it being one of the few aircraft fast enough to allow the drone's ramjet engine to light up properly. The ideas congealed into a formal study for a high-speed, high-altitude drone begun in October 1962. The NRO backed the idea, with funding scraped up for initial development. The drone itself was given the preliminary designation of "Q-12", and was a very deep secret.
Kelly Johnson wanted to power the Q-12 with a ramjet built by Marquardt for the Boeing BOMARC long-range SAM. Marquardt's plant was close to Lockheed's, helping ensure security, and the two companies had collaborated on several programs in the past. Conversations with Marquardt engineers indicated the BOMARC ramjet could be used, though the engine, ultimately designated the RJ43-MA-11, needed some work, since it wasn't designed to burn for much longer than it took a BOMARC to hit a target a few hundred kilometers away. The Q-12's engine had to operate for at least an hour and a half, much longer than any ramjet built to that time.
In order to reduce weight and cost, the Q-12 was not designed to be recoverable. Instead, it would eject its nose section, containing the camera payload and the expensive guidance system. The nose section would descend by parachute for recovery.
A mockup of the Q-12 was ready by 7 December 1962. Radar tests indicated that it had an extremely low radar cross section. Wind tunnel tests also indicated the design was on the right track. The CIA was not enthusiastic about the Q-12 at first, mostly because the agency was overextended at the time with U-2 missions, getting the A-12 up to speed, and covert operations in Southeast Asia. In contrast, the Air Force was interested in the Q-12 as both a reconnaissance platform and a cruise missile, and the CIA finally decided to work with the USAF to develop the new drone. Lockheed was awarded a contract in March 1963 for full-scale development of the Q-12.
The major initial problem confronted by the design engineers was launch of the Q-12 from the A-12 mother ship. The Q-12 was to be carried on the back of the A-12, with an uncomfortably small amount of clearance between the A-12's fins, and the potential for disaster during separation of the drone from the mother ship was obvious.
* The design was finalized in October 1963, and the designations were changed. The drone was now known as the "D-21", while the A-12 launch aircraft was known as the "M-21". "M" stood for "Mother" and "D" stood for "Daughter". The project now had the codename "Tagboard".
The production UAV, the "D-21A", looked like a stovepipe with a cone in its inlet, with a tailfin and wings running the length of the stovepipe that gave the drone something of the look of a sweptback manta ray. It was mostly made of titanium, with some elements made from radar-absorbing plastic composites.
LOCKHEED D-21A: _____________________ _________________ _______________________ spec metric english _____________________ _________________ _______________________ wingspan 5.8 meters 19 feet length 13 meters 42 feet 10 inches launch weight 5,000 kilograms 11,000 pounds maximum speed 4,300 KPH 2,700 MPH / 2,300 KT service ceiling 29,000 meters 95,000 feet range > 5,550 KM > 3,450 MI / 3,000 NMI _____________________ _________________ _______________________
The reconnaissance payload and guidance systems were carried in a "Q-bay" about 1.9 meters (six feet) long. These systems were built into a module that plugged neatly into the bay and was known as a "hatch". As per the original design concept, the hatch would be ejected at the end of the mission, and the aircraft would then blow itself up with a self-destruct charge. The hatch would be snagged out of the air by a C-130 Hercules, a technique that had been refined by the Air Force to recover film canisters from reconnaissance satellites.
The M-21 was a two-seat version of the A-12, with a pylon on the fuselage centerline between the tailfins to carry the drone in a nose-up attitude. A periscope allowed the back-seater, or "Launch Control Officer (LCO)", to keep an eye on the D-21. Two M-21s were built, along with an initial batch of seven D-21s for test flights.
The first flight of the M-21 and D-21 combination was on 22 December 1964. The D-21 remained attached to the M-21 throughout the flight, since it was simply to study aerodynamics and other systems issues. Refining the scheme until an actual release seemed possible proved troublesome, and the first launch was not until 5 March 1966. The release was successful, though the drone hovered above the back of the M-21 for a few seconds, which seemed to one of the flight crew like "two hours". Kelly Johnson called it "the most dangerous maneuver we have ever been involved in, in any airplane I have ever worked on." The D-21 itself crashed after a flight of a few hundred kilometers.
This was still not too bad for the first flight of such an advanced machine, but the CIA and the Air Force remained unenthusiastic about the program. Kelly Johnson conferred with Air Force officials to see what he could do to tune the project more closely to the service's needs. Among other things, Johnson suggested launching the D-21 from a B-52 bomber and using a solid rocket booster to get the drone up to speed.
A second successful launch took place on 27 April 1966, with the D-21 reaching its operational altitude of 27,400 meters (90,000 feet) and speed of Mach 3.3, though it was lost due to a system failure after a flight of over 2,200 kilometers (2,200 NMI). This was regarded as very satisfactory progress. The successful tests sharpened the interest of the program's government backers, and by the end of the month a contract for 15 more D-21s had been placed.
A third successful flight took place on 16 June 1966, with the D-21 flying through its complete mission, though the hatch wasn't released due to an electronics failure. However, a launch attempt on 30 July ended in disaster. The D-21 collided with the M-21 on release, destroying both aircraft. The two crewmen ejected and landed at sea. The pilot, Bill Park, survived, but the LCO, Ray Torick, drowned when his pressure suit leaked.
* All the fears about launching the D-21 from an A-12 had been proven justified, and Kelly Johnson immediately canceled any more launches from the M-21. However, he felt that the B-52 launch scheme was still practical, and the D-21 program remained alive and well, being re-codenamed "SENIOR BOWL".
Adapting the D-21 for launch from a B-52 was not trivial. The drones had to be broken down, modified, and reassembled to allow fitting the attachment points on top to link the drone to the B-52's pylon and the points on the bottom to link the drone to its solid-rocket booster. The modified drone was designated the "D-21B". The booster was no little RATO pack: it was a solid-fuel rocket with a length of 13.5 meters (44 feet 4 inches) and a weight of 6.025 tonnes (13,290 pounds), making it longer and heavier than the drone itself. The booster had a single small tailfin on the bottom to ensure that it flew straight. The tailfin folded to ensure ground clearance. The booster had a burn time of about a minute and a half, and a thrust of 121.4 kN (12,380 kgp / 27,300 lbf).
Two B-52Hs were modified to launch the D-21Bs. They were given two very large underwing pylons to carry the drones, replacing the smaller pylons used for the B-52's Hound Dog cruise missiles. Two independent LCO stations were added at the rear of the bomber's flight deck, along with command and telemetry systems; a stellar navigation system to ensure that the drones were launched from well-defined coordinates to reduce flight guidance error; and a temperature control system to keep the drones at a stable temperature before launch.
First attempted launch of a D-21B was on 28 September 1967, but the drone accidentally fell off the B-52's pylon; its booster lit, but the D-21B went straight into the ground. Kelly Johnson called the incident "very embarrassing." Three more launches were performed from November 1967 through January 1968. None were completely successful, so Johnson ordered his team to conduct a thorough review before renewing launch attempts. The next launch was on 30 April 1968, and was also a failure. The Lockheed engineers went back to the drawing board once more, and on 16 June 1968 they were rewarded with a completely successful flight. The D-21B flew a test mission at the specified altitude and course over its full range, with the hatch recovered successfully, though it didn't have a camera payload.
The troubles were not over yet, however. The next two launches were failures, followed by another successful flight in December. A launch near Hawaii in February 1969 to simulate an actual operational flight was a failure as well, but the next two flights, in May and July, were both successes.
Tagboard now appeared ready for operational flights. The first operational mission, part of a program designated "Senior Bowl", was on 9 November 1969, with a D-21B sent to observe Lop Nor. The Chinese never spotted the stealthy drone, but it disappeared and was not recovered. Once again, the Lockheed engineers went back to the drawing board. Another test flight was conducted on 20 February 1970, and was successful. However, the next operational mission was not until 16 December 1970. The D-21B made it all the way to Lop Nor and back to the recovery point, but though the hatch was dropped as planned, it did not deploy its parachute and was destroyed on impact.
The third operational flight, on 4 March 1971, was even more frustrating. Once again, the D-21B made it all the way to Lop Nor and back again, and properly discarded the hatch. The hatch actually deployed its parachute, but the midair recovery failed, and a destroyer that tried to pick the hatch out of the sea simply ran it down. The hatch sank and was lost.
The fourth, and as it turned out last, flight of the D-21B was on 20 March 1971. It was lost over China on the outbound leg, apparently having been shot down. In July, the D-21B program was canceled Although the program had suffered from more than its fair share of bugs, it appears the main reasons were the same as those that led to the cancellation of the Model 154 Firefly: Nixon's rapprochement with China and operational introduction of the Big Bird reconnaissance satellite.
* When Ben Rich, Kelly Johnson's successor at the Skunk Works, visited Russia in the 1990s after the fall of the USSR, a contact gave him a package that contained parts of the D-21 that had disappeared on the first operational flight. It had crashed in Siberia. The Soviets had apparently been puzzled as to what it was, but it appears that they also obtained the wreckage of the D-21 lost on the fourth operational flight. The Tupolev design bureau reverse-engineered the wreck and came up with plans for a Soviet copy, named the "Voron (Raven)", but it was never built.
38 D-21s were built, with 21 expended. The other 17 were put in mothballs at the Davis-Montham Air Force Base "boneyard" near Tucson, Arizona. Since the base is open to the public, the exotic D-21s were eventually spotted and photographed, leading to wild speculations as to their nature that were inflamed by misinformation generated by the Air Force. For example, they were described as test machines used in development of the A-12 / SR-71. The full details didn't come out until 1993, when an author named Jay Miller published a book titled LOCKHEED'S SKUNK WORKS: THE FIRST 50 YEARS that gave a reasonably complete account of the D-21 project. The mothballed drones were passed off to the US National Aeronautics & Space Administration (NASA), which took four, and to a number of air museums.
In the late 1990s, NASA considered using their D-21s to test a hybrid "rocket-based combined cycle (RBCC)" engine, which operates as a ramjet or rocket, depending on its flight regime. However, this idea was abandoned, with NASA preferring to use a derivative of the agency's X-43A hypersonic test vehicle for the experiments.
The Seattle Museum of Flight was one of the museums that received a D-21. The museum also had an A-12, and museum volunteers built a pylon to allow mounting a D-21 on its back. The combination is the central exhibit in the main display area and is one of the most spectacular sights available at any air museum.
BACK_TO_TOP* 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 the agency 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.
* 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 engined airliner, 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 on an experimental basis.
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.
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 canceled that year.
LTV XQM-93: _____________________ _________________ _______________________ spec metric english _____________________ _________________ _______________________ wingspan 17.4 meters 57 feet length 9.14 meters 30 feet empty weight 1,090 kilograms 2,400 pounds max loaded weight 2,090 kilograms 4,700 pounds cruising speed 170 KPH 105 MPH / 91 KT service ceiling 15,850 meters 52,000 feet range 9,650 kilometers 6,000 MI / 5,220 NMI endurance 30 hours maximum _____________________ _________________ _______________________ 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 canceled along with the XQM-93. The XQM-93 and the COMPASS COPEs looked forward to aircraft that would come into service in two decades, but nothing much came of the effort at the time.
BACK_TO_TOP* 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-power 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). In the early 1970s, Dr. Paul B. MacReady and his AeroVironment company 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 very strong.
The wing was built in five segments of equal span. Two gondolas hung from the center segment, with the gondolas carrying 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, since the UAV 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 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.
* 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" piloted 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, 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 providing 131 kW (175 HP). The engines featured a two-stage turbocharging system 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.
BOEING CONDOR: _____________________ _________________ _______________________ spec metric english _____________________ _________________ _______________________ wingspan 61 meters 200 feet length 20.7 meters 68 feet empty weight 3.630 kilograms 8,000 pounds loaded weight 9,070 kilograms 20,000 pounds cruise speed 370 KPH 230 MPH / 200 KT service ceiling 20,000 meters 65,000 feet endurance 2.5 days _____________________ _________________ _______________________
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 it ended up in the Hiller Museum in San Carlos, California.
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