v1.4.1 / chapter 18 of 19 / 01 mar 08 / greg goebel / public domain
* This chapter covers a variety of topics that didn't fit neatly into the rest of the document.
* One specialized application of UAVs is for aeronautic research. It is in principle much cheaper to test unusual aircraft configurations by implementing them as small UAVs, instead of large piloted aircraft. NASA has proven enthusiastic about the use of UAVs for such purposes, and has conducted a number of aeronautic test programs using UAVs.
NASA had long had a tradition of flying scale models of aircraft, sometimes with rocket boosters, for aerodynamic tests, but in general these vehicles were basically instrumented flying wind-tunnel test models and were not really UAVs. However, in the early 1970s, NASA built three 3/8ths-scale unpowered drone versions of the new McDonnell Douglas F-15 fighter to confirm that it was as agile as the designers hoped it would be. These three machines were referred to as "remotely piloted research vehicles (RPRVs)" and were taken aloft over Edwards Air Force Base (AFB) in California by the NASA B-52 carrier aircraft, to be released at high altitude. They would glide back to earth and land with retractable skids on the dry lakebed at Edwards. The RPRVs could also be fitted with a parachute to be snagged by a helicopter in flight for recovery, just like the old Lightning Bugs.
Each drone weighed 1,099 kilograms (2,425 pounds) and had a length of 7.01 meters (23 feet). They were made of aluminum, wood, and fiberglass and cost only $250,000 USD each. They provided extremely valuable flight data, and the exercise was regarded as highly successful. At least one remained in NASA service after the F-15 development program was completed, being used to perform spin testing and the like.
* The "Drones for Aerodynamic & Structural Testing (DAST)" program was conducted from 1977 to 1983 at the NASA Langley and Dryden Flight Centers. It involved flights of modified Ryan Firebee II supersonic target drones to test new wing designs and wing control systems. The Firebee II was selected because it had supersonic performance, its wings could be easily replaced, it used only tail-mounted control surfaces, and because it was available at low cost from the US Air Force.
After initial test flights with a Firebee II in its normal configuration but with added instrumentation, NASA fitted a Firebee II with an aeroelastic, supercritical research wing suitable for a Mach 0.98 cruise airliner. A total of ten flights were made, with initial launches from the NASA Boeing B-52 bomber and later from a DC-130 Hercules drone controller aircraft, and a NASA Lockheed F-104 Starfighter performing chase duties. The DAST drones were radio-controlled and recovered by parachute with helicopter snatch. The program encountered difficulties, with two crashes, one in 1980 and one in 1983, and was abandoned after the second crash.
* In the wake of the successful F-15 RPRV program, two "Highly Maneuverable Aircraft Technology (HIMAT)" UAVs were developed by NASA and Rockwell International in the 1970s, and used in 26 flight tests from 1979 to early 1983, with the initial flight on 27 July 1979. The HIMAT UAVs were about half the size of an F-16 fighter.
HIMAT was a joint effort of NASA's Dryden Flight Research Center at Edwards AFB, the NASA Ames Research Center in California, and the Air Force Flight Dynamics Laboratory at Wright-Patterson Air Force Base in Ohio. HIMAT was intended to demonstrate new composite materials, digital flight and engine control systems, and high-maneuverability aerodynamic concepts such as winglets and canards.
Each HIMAT vehicle was a swept-wing canard aircraft with a fighter-like configuration, featuring wings in the rear and horizontal control surfaces in the nose. HIMAT was 7.16 meters (23 feet 6 inches) long, had a wingspan of nearly 4.9 meters (16 feet), and a loaded weight of 1,830 kilograms (4,030 pounds). HIMAT was powered by a GE J85 afterburning turbojet engine with 22.3 kN (2,270 kgp / 5,000 lbf) thrust, the same engine used in the Northrop F-5E fighter. HIMAT's top speed was Mach 1.4, and the UAV could easily out-turn an F-16. About 30% of the airframe was made of composite materials, mostly fiberglass and graphite-epoxy. Cost of the two UAVs was $17.3 million USD.
The HIMATs were launched at high altitude from NASA's B-52 carrier aircraft. They were remotely piloted from the ground, using a TV imager to give the "pilot" a view of what was going on. A Lockheed F-104 chase plane, fitted with auxiliary remote controls, was used as an emergency backup. At the end of a test flight, the ground-based pilot would land the HIMAT on the dry lakebed at Edwards. The HIMAT was designed to be easily modified to a number of alternate aerodynamic configurations, such as forward-swept wings, but it appears that these alternate configurations were never implemented. The two HIMAT UAVs are now on display, one at the NASA Ames Research Center, and the other at the Smithsonian Air & Space Museum in Washington DC.
* HIMAT seemed like a good enough idea to be revived in a new form in the following decade. In March 1996, McDonnell Douglas, working with NASA, rolled out the "X-36", a quarter-scale flying model of a stealthy manned jet fighter. The X-36 was an arrowhead-like aircraft with chiseled lines and no tailfin, performing turns with wingtip "drag rudders". It was designed to study technologies for building agile, stealthy fighters.
The X-36 was a little over 5.5 meters (18 feet) long, including its test boom, with a wingspan of 3.17 meters (10.4 feet), a weight of almost 590 kilograms (1,300 pounds), an aluminum frame with graphic-epoxy skin, and a small Williams Research turbofan engine with 3.1 kN (320 kgp / 700 lbf) thrust, giving it a top speed of Mach 0.6. It had a vectored exhaust for maneuverability, was remotely piloted from the ground using a TV camera in the cockpit to keep the operator informed, and had a parachute for emergency recovery.
Total program costs were kept low by avoiding redundancy in flight systems and by designing the X-36 to take off and land on its own, eliminating the need to use NASA's B-52 as a launch platform. Two X-36s were built, with 31 test flights beginning in May 1997 and ending in November of that year. The two aircraft were then put in storage at NASA Dryden flight center for other possible users, though use of them has been limited by the fact that some of the data obtained by the X-36 program remains classified.
* In 1996, NASA introduced a small drone named LoFLYTE that featured a wedge-shaped "waverider" configuration designed for hypersonic flight. In reality, LoFLYTE had a top speed of about 450 KPH (280 MPH), having been built to study flight control systems for high-speed aircraft based on "neural net" logic systems. LoFLYTE was 2.5 meters (8 feet 4 inches) long, had retractable landing gear, and was powered by a turbofan engine with 225 newtons (23 kgp / 50 lbf) thrust. It was built by Accurate Automation with funding from NASA, the USAF, and the US Navy.
In 2001, NASA followed up the LoFLYTE program with a more focused UAV effort to support the agency's X-43 experimental hypersonic propulsion vehicle program. NASA researchers wanted to know how a vehicle designed for hypersonic flight would handle at the low speeds experienced after ground takeoff and before landing, and so decided to build two low-cost UAVs to find out.
The "X-43ALS", where the "LS" stands for "low speed", was built by Accurate Automation. It resembled the X-43 hypersonic vehicle aerodynamically, had a length of 3.7 meters (12 feet), weighed about 82 kilograms (180 pounds), and was powered by a small jet engine that provided about 540 newtons (55 kgp / 120 lbf) thrust. Top speed was about 550 KPH (345 MPH). First flights were in early 2002.
The "X-43BLS HYSID (Hypersonic Systems Integrated Demonstrator)" was built by SWB Turbines. It was a larger machine, with a different configuration that features canard control surfaces. The X-43BLS had a length of 4.6 meters (15 feet), a wingspan of 2.75 meters (9 feet), a weight of 32 kilograms (180 pounds), and was powered by three SWB-100 turbojets, with 475 newtons (48.5 kgp / 107 lbf) thrust each.
* In 2007, NASA test-flew another research UAV, the "X-48B", which was a demonstrator for a "blended wing body (BWB)" transport, built in cooperation with the Air Force Research Laboratory and with Boeing as the prime contractor. It was more or less a flying wing, with a stingray-like cranked delta configuration, winglets, three small turbojet engines mounted on the rear, and fixed tricycle landing gear. The X-48B had a span of 6.4 meters (21 feet), a wing area of 9.3 square meters (100 square feet), and was powered by three JetCat P200 RC modeler turbojets with 245 N (25 kgp / 55 lbf) thrust each. Maximum speed was a modest 220 KPH (135 MPH).
The X-48B was built of composite materials, with the airframe construction by Cranfield Aerospace in the UK as a Boeing subcontractor. Two were built, with the second machine acting as a backup in case the first came to ruin.
* One of the really interesting applications of UAVs is their potential use for Mars exploration. The idea of flying through the thin air of Mars precedes manned spaceflight. Back in the 1950s, COLLIER'S magazine published a popular series of articles by space pioneers promoting the exploration of space, with spectacular illustrations by painter Chesley Bonestell that are still impressive today. One of the most prominent pioneers, Werner von Braun, envisioned spacecraft landing on Mars using huge long wings.
This turned out to be overoptimistic, since space probes sent to fly past Mars in the 1960s showed that the Martian atmosphere was about ten times thinner than had been believed, with less than one percent of the atmospheric density of the Earth at sea level. Building something to fly through such a thin atmosphere was no simple task.
The Mini-Sniffer high-altitude drone of the 1970s, discussed earlier. suggested a workable solution to the problem. Its hydrazine engine could operate in the thin carbon-dioxide atmosphere of Mars. Although the atmosphere on Mars at ground level is thinner than the Earth's at a height of 21 kilometers (70,000 feet), Martian gravity is only a third that of the Earth's, and so the Mini-Sniffer could be regarded as a prototype of a Mars airplane.
In the second half of the 1970s, many researchers in the NASA Mars community were very interested in development of a Mars airplane, since the Viking landings in 1976 had done much to generate enthusiasm for Mars exploration. In 1978, members of the Mars community got in touch with Dale Reed, the father of the Mini-Sniffer, and discussed the possibilities for a Mars airplane.
This was in the days when NASA thought in grand terms, and some of the concepts for Mars airplanes were impressive. The biggest had a wingspan of 21 meters (70 feet) and a mass of 545 kilograms (1,200 pounds). It had an inverted-vee tail, a configuration that would be incorporated into Earth-based drones in the coming decades. The Mars airplane had to be folded to fit into an "aeroshell", shaped like a shallow cone and covered with a heat-resistant material, in order to survive falling into the Martian atmosphere at high speed. Once the aeroshell completed entry into the atmosphere, a parachute would extract the Mars airplane, which would unfold, start its engine, and fly away.
Initially, the Mars airplane was to simply crash at the end of its flight, but the planetary scientists involved in the discussions suggested that its sensors might be useful even when the Mars airplane was grounded after it ran out of fuel. If the airplane could land and take off again, so much the better. The airplane could fly around and explore for a time, land to take samples or simply wait for updated mission flight plans, and then take off again to fly to other sites on the planet.
To explore landing technologies, Reed took a sailplane that had a high tee tail, and connected a cable to pivot the tailplane up from the rear. Pivoting up the tailplane pitched the sailplane's nose up, causing it to float down in a reasonably controlled fashion. A Mars airplane could use hydrazine thrusters to make a soft landing, and use the thrusters to launch itself again.
* The investigations resulted in a scheme to send twelve airplanes to Mars. Three spacecraft would be launched, each with an aeroshell derived from the Viking Mars landers, and with each aeroshell carrying four airplanes. The aircraft would have a maximum range of up to 5,600 kilometers (3,500 miles), and the fleet would be able to explore Mars in great detail over wide areas. The plan was too ambitious and never went beyond the "paper plane" stage. NASA gave up on Mars airplanes in the 1980s, and in fact NASA planetary science missions fell on hard times in that decade. However, research on aircraft technologies for human powered and high altitude flight that were applicable to Mars airplane designs continued.
In 1992, NASA introduced the "Discovery" program to develop low-cost space probes, reviving the agency's sluggish planetary exploration efforts. One early Discovery proposal was for a "Mars Airplane" mission. The proposal was made by John Langford, who ran Aurora Flight Sciences, builder of the first ERAST drones for high altitude research.
One of the veterans of the NASA Mars Airplane effort in the 1970s, Larry Lemke of NASA Ames, came up with another Mars Airplane proposal. Lemke's proposal was based on a scaled-down 180 kilogram (400 pound) version of one of the big NASA Mars airplanes designed in the 1970s, and envisioned it hopping from site to site, taking pictures and obtaining surface samples. There were other Mars Airplane proposals. NASA Ames also suggested a flying wing design named MAGE, and a team formed by NASA Jet Propulsion Laboratory (JPL), AeroVironment Corporation, and others suggested a mission that would drop six gliders over the length of the huge Vallis Marineris canyon on Mars.
* NASA didn't bite on the Mars Airplane proposals, at least not immediately. A Princeton University physicist named Edgar Choneri was the one who got the ball rolling again. Choneri noticed the fact the 2003 Mars window coincided with the centennial of the Wright Brothers flight in 1903, and mentioned it to Norman Augustine, a former chief executive of Lockheed Martin who was living in Princeton. Augustine had NASA Administrator Daniel S. Goldin's ear and sold him on the concept. In early 1999, Goldin announced that the agency planned to send a small robot aircraft to Mars, to arrive on 17 December 2003 to commemorate the first flight of the Wright Brothers' aircraft, exactly a century before.
The Mars Airplane effort was one of the low-cost experimental "Mars Micromissions" that NASA was planning in cooperation with CNES, the French space agency, and was partly intended to serve as a project that would bridge the gap between NASA's aeronautics and space research centers. Bridging the gap proved a little troublesome, but after some wrangling on who should do what, NASA's Langley Research Center in Hampton, Virginia, was assigned to build the Mars Airplane for launch in November 2002. The only catch was that the Mars Airplane project had to be done on a shoestring budget. These missions were to be launched as microspacecraft payloads on an French Ariane V booster, hitching a ride into space as the Ariane put communications satellites or other payloads into Earth orbit. A microspacecraft would be tossed off into space on a highly elliptical orbit, and then sent on Moon flybys that would eventually "slingshot" it towards Mars.
The Mars Airplane microspacecraft was constrained to fit into an aeroshell no more than about 76 centimeters (30 inches) in diameter. This meant a very small airplane that had to fold up into a compact package, implying a complicated folding scheme. To make matters worse, the thin Martian atmosphere would not provide enough resistance to help unfold the airplane, and so it had to be unfolded with springs, increasing weight and complexity.
NASA Langley issued a request to potential contractors for a Mars Airplane in September 1999. The request described general specifications for what the Mars Airplane had to do, but did not specify how it was to be implemented. However, Langley had investigated some possibilities on their own to focus their own thinking and provide some hints to contractors. Langley came up with a number of designs, focusing primarily on aircraft powered by a hydrazine thruster rocket, which the design engineers felt would be simpler and more reliable than a propeller-driven design using a hydrazine engine. Some of the designs used straight wings, others had swept wings.
The designs all had to confront the difficulties imposed by the thin Martian atmosphere, which reduced lift to a bare minimum. For example, once deployed, the Mars Airplane would fall like a brick for several kilometers until it finally obtained enough lift to fly straight and level. Even when it was flying it would have very limited maneuverability, with, for example, a turning radius of several kilometers.
The Mars Airplane would have to use a preprogrammed autopilot, since the time lags in communications between Mars and Earth were far too long to allow it to be remotely controlled. It would carry a two kilogram (4.4 pound) science payload, with cameras for observing terrain, a magnetometer to measure magnetic fields, and a miniaturized spectrometer to perform analyses of materials. Data from the Mars Airplane would be sent to a orbiting spacecraft for storage and then relay it back to Earth. Dependence on an orbiter meant the Mars Airplane wouldn't be able to usefully fly for longer than about 20 minutes, the total amount of time an orbiter was in line of sight.
* Unfortunately, after the loss of two NASA Mars probes in 1999, the agency was forced to reconsider Mars efforts, and Mars Airplane work was put on hold. Some of the companies involved with Mars Airplane studies have continued research at a low level. NASA is now conducting a series of low-cost planetary "Scout" missions, and one of the early proposals was for a Mars airplane, designated the "Aerial Regional-scale Environmental Survey (ARES)". ARES was designed by NASA Langley and Aurora Flight Sciences, and of course it was just a continuation of Langley's earlier work. Langley researchers had already tested a 50% scale prototype, named the "Mars Eagle", in the fall of 2002. There was no commitment to ARES, but the idea hasn't gone away.
* One of the less obvious issues concerning UAVs is integration of their operations into open "national airspace", where they may fly alongside private and commercial aircraft, in contrast to "special use" airspace, which includes "Restricted", "Warning", and "Military Operations" areas where airliners and private pilots are not allowed to fly. Currently, UAV operations in national airspace are considered on a case-by-case basis by the US Federal Aviation Administration (FAA), and lead times run to months.
There are projections that the US military could be operating thousands of UAVs in a decade, with some of these aircraft carrying munitions, and the number of US commercial UAVs could be even larger. Figuring out how these UAVs can operate safely alongside commercial traffic is a nasty bureaucratic issue. The US Department of Defense, the FAA, and NASA have been considering how UAV traffic over the US should be regulated, and what implications UAVs have for international air traffic agreements. International regulations for commercial UAVs are regarded as particularly important, to allow American businesses to operate and sell UAV technology in other countries. Initial work suggested that UAVs should be divided into three regulatory classes:
Measures under consideration are to allow a medium-altitude, long-range UAV into the US national airspace if it were escorted by a piloted chase plane, or if it has adequate sensors to "see and avoid" commercial traffic and is monitored at all times by a ground operator. The ground operator will likely have to be a qualified pilot who knows the rules of the airways, and would be in two-way communications with the air-traffic control network.
UAVs would likely not be allowed to operate on a normal basis in high-volume "Class B" airspace around major airline hubs such as Chicago, New York, and Los Angeles. The FAA would also require certification that the communications link to a UAV be reliable and resistant to interference.
There are a large number of confusing issues to consider, since the possibilities for UAVs are open-ended. Small, long-range UAVs like the Aerosonde that could be used for weather or traffic observation are one complication, as well as UAVs like the Scaled Composites Proteus used as relays for high-bandwidth communications. NASA has worked with industry on a program to sort out the issues and hand the results over to the FAA, while European UAV vendors have formed their own consoritium to provide data to the European aviation regulatory body, the European Aviation Safety Agency (EASA). At last notice, progress was slow and frustrating.
