v1.4.1 / chapter 17 of 19 / 01 mar 08 / greg goebel / public domain
* Another growth field in UAVs are miniature UAVs, ranging from "micro-aerial vehicles (MAVs)" that can be carried by an infantryman to man-portable UAVs that can be carried and launched like an infantry anti-aircraft missile. This chapter outlines developments in miniature UAVs.
* The notion that small, even very small, UAVs might have practical uses arose in the early 1990s. In 1992, DARPA conducted a workshop titled "Future Technology-Driven Revolutions In Military Operations". One of the topics in the workshop was "mobile microrobots". The idea of using very small "microdrones" was discussed, and after initial skepticism the idea started to gain momentum.
The RAND Corporation released a paper on the microdrone concept in 1994 that was widely circulated. DARPA conducted a series of "paper studies" and workshops on the concept in 1995 and 1996, leading to early engineering studies by the Lincoln Laboratories at the Massachusetts Institute of Technology (MIT), and the Naval Research Laboratory (NRL) in Washington DC.
The studies demonstrated that the concept was feasible. In 1997, DARPA then began a multi-year, $35 million USD development program to develop "micro aerial vehicles (MAVs)". The MAV project's goals was to develop a microdrone whose largest dimension was no more than 15 centimeters (6 inches); would carry a day-night imager; have an endurance of about two hours; and be very low cost. It would operate with a high degree of autonomy and was to be used in the squad-level combat environment. MAVs capable of hovering and vertical flight would be used to scout out buildings for urban combat and counter terrorist operations. A MAV could be included in a pilot's survival kit. A downed pilot could use it to keep a lookout for enemy search parties, or relay communications to search and rescue units.
* This phase-one DARPA study ended in 2001, and was followed by a phase-two study that focused on particular vendors with an intent to develop MAVs closer to operational specification. A number of different MAVs were developed as part of these DARPA efforts:
The CalTech / Aerovironment MicroBat ornithopter was test-flown for short distances under battery power. Researchers performing flight tests with the MicroBat said it tended to attract small birds when it ran low on power and fell to the ground. The birds clustered near the floundering ornithopter in what seemed to be a desire to help.
Other research groups also worked on ornithopters. A Georgia Tech group built a rubber-band powered ornithopter and also did research on a chemically-powered "artificial muscle" propulsion system. Interestingly, there are stories that the CIA tinkered with tiny ornithopters decades earlier in an unsuccessful attempt to build artificial birds to carry microphones to the windows of target buildings.
While the first Black Widow prototype was a flat disk with a single vertical stabilizer and a propeller in the front, it was followed by an improved Black Widow that looked a little like a thin portable CD player with tapered edges and cut-off corners; a propeller in front; and three fins on the back. It did not have autonomous navigation capabilities, and was controlled essentially like a hobbyist's RC airplane.
Along with the flight prototypes, the DARPA effort considered subsystems design. A useful operational MAV needed a lightweight, highly efficient engine with a power source with high energy density. Electric motors were becoming available that met the requirement, but power sources were more troublesome. Lithium batteries were marginal; new compact fuel cells were in development but weren't expected to be available for several years.
One particularly intriguing option for both propulsion and power was a button-sized silicon microturbine ("jet") engine developed at MIT during the 1990s. Silicon was actually a good structural material at such scales, though increasing operating temperature would have dictated use of silicon carbide.
A production device was envisioned as a centrifugal-flow turbojet engine about two centimeters across burning natural gas, with a single turbine disk for compression and a single disk for exhaust rotation. The design didn't look much like a conventional turbojet, resembling more a tiny flat cylindrical box with an inlet hole on one side and an exhaust hole on the other. It was expected to have a thrust-to-weight ratio of about 100 -- incredible compared to any "macroscale" engine, but a logical consequence of scaling the technology down in size -- and run at about 1.2 million RPM, making bearings a tricky issue. Since it could "spool up" in about a millisecond, it was envisioned as operating in a pulsed mode to conserve fuel and also provide a throttling scheme.
Other tricky issues were control systems, since a MAV couldn't be flown like a model airplane and would have to be able to tolerate turbulence and wind gusts, and miniaturizing navigation, communications, and sensor systems, as well as ensuring that they didn't interfere with each other. DARPA specified that the payload would be no more than 15 grams.
* As extreme as MAV specifications were, a team under Ilan Kroo at Stanford University worked on an even more extreme design in the form of a centimeter-wide four-rotor "mesicopter" using microcircuit fabrication techniques. The work was funded by NASA.
Design of such a small aircraft was constrained by the fact that at it such scales, the air becomes a highly viscous medium, or in aerodynamic terms a mesicopter had a low "Reynolds Number". Basic aerodynamics of the mesicopter were defined by a cycle of computer simulation, followed by tests of model components. The research led to mesicopter rotor designs where the rotor looked much more like the blades of an ordinary room fan than the rotor of a conventional helicopter. Fabrication of components was tricky and it is unclear if the Stanford group ever flew a mesicopter of the size envisioned.
* The DARPA MAV effort ended in 2000 and the results of the effort were somewhat negative, demonstrating that a 15 centimeter UAV was simply too small to be useful or even workable, at least at the time. However, though the size was unrealistic, the basic concept seemed valid even if it was recognized that a larger machine was needed over the short run. DARPA did begin a follow-on effort in the spring of 2002, working with the US Army on a larger ducted fan vehicle as a follow-on to SLADF under the "Organic Air Vehicle (OAV)" program. Allied Aerospace, which had bought out Micro Craft, demonstrated a scaled-up SLADF, while Honeywell performed tests with their own ducted-fan vehicle, named "iSTAR". However, neither vehicle seemed particularly promising and the program was cut short.
As an example of how defense research programs tend to disappear and reappear, it was revived as "OAV-2" in 2004, with DARPA specifying a diesel-powered ducted-fan vertical-takeoff UAV with a weight of 51 kilograms (112 pounds), including a payload of 10 kilograms; a range of 10 kilometers (6.2 miles); a top speed of 92 KPH (50 KT); the ability to hover in a 37 KPH (20 KT) wind; an endurance of two hours; and a ceiling of 3,350 meters (11,000 feet).
The OAV was be carried, launched, and recovered on a Humvee truck, using a crew of two soldiers, who would be able to get it flying in five minutes. Its sensor systems were to provide targeting data to within 10 meters (33 feet) to support non-line-of-sight weapons. The UAV would have autonomous flight capabilities with the ability to maneuver in cluttered terrain using an all-weather obstacle-avoidance system, and DARPA wanted it to have the ability to land and conduct observations from its landing site. Other possibilities were use of the UAV for communications relay, SIGINT, countermeasures, or even armed attack.
Honeywell submitted a improved version of the iSTAR, while BAE Systems entered its ducted fan designs, and Aurora Flight Sciences entered the Goldeneye. The Goldeneye won the competition in the summer of 2005. Since DARPA only does demonstration programs, it was unclear if there was any commitment to go to production with the program.
* Aerovironment has also worked on follow-ons to its Black Widow, named the "Wasp" and the "Hornet". The Wasp is a flying wing, with the wing in the form of a rectangle with a slightly swept leading edge. It is propeller driven, with the propeller in front. The Wasp's main improvement over the Black Widow is that the lithium-ion battery and wing structures are one and the same, allowing maximum battery capacity relative to MAV size. The Wasp has a wingspan of 33 centimeters (13 inches) and a weight of grams 210 grams (6 ounces). Like the Black Widow, the Wasp is radio controlled.
In the spring of 2003, Aerovironment performed the first flight of the Hornet, which was similar to the Wasp but had a straight rectangular wing with a slightly greater span of 38 centimeters (15 inches) and, more significantly, was powered by fuel cells. The fuel cells were built into the top of the wing, where they combined oxygen in the ambient air with hydrogen produced internally by the MAV through reaction of a hydride material with water.
The fuel cell system was expected to provide three times the endurance of batteries of comparable weight, though early flights were limited by the tendency of the fuel cells to dry out. DARPA was actually more interested in the battery powered Wasp, but other interested parties in the US defense establishment, particularly the NRL, were very intrigued by fuel cells, and so DARPA hedged its bets. Ultimately, Aerovironment engineers want to fit their MAVs with an autopilot and a color video camera.
The French, who seem to be at least as infatuated with high-tech as the Americans, have done work along similar lines, with the French Ministry of Defense (DGA in its French acronym) sponsoring a flight demonstrator, the "Mirador". It was a fixed-wing, propeller-driven aircraft 25 centimeters (10 inches) long and was powered by miniature fuel cells that gave it an endurance of about 20 minutes. It was built by the French defense aerospace research agency ONERA, working with the Royal Military Academy of Brussels, and was primarily intended to be a testbed for miniature sensor technologies.
The DGA envisioned an operational MAV as about 40 centimeters (16 inches) long, with a weight of less than 1.5 kilograms (3.3 pounds), an endurance of 15 minutes or more, a ceiling of 100 meters (330 feet) and an operating radius of a kilometer (0.6 mile). French industry has come back with MAV concepts of their own, with Sagem developing the "Odin", a little ducted fan UAV with a weight of 4 kilograms (8.8 pounds) and an endurance of 30 minutes.
* The notion of bird-sized or even insect-sized MAVs hasn't disappeared. In late 2005, DARPA opened up a competition for "nano-UAVs", specified to weigh no more than 10 grams, including a 2 gram payload, and with dimensions no greater than 5 centimeters (2 inches) in any direction. Range was a maximum of a kilometer, with the machine to be able to hover for a minute or so at maximum range. The goal was to build a vehicle that could be used to check on adversary forces in buildings and tunnels.
Some of the MAV effort may have gone underground. Unverified rumors were also floating around at the time about feathery little MAVs and ornithopters that were powered by concentrated radio beams, a scheme which would eliminate the difficulty of providing a MAV with a fuel supply but which would generally limit the vehicles to line-of-sight operation.
The DARPA effort also did get a lot of people thinking about tiny aircraft. MAVs have attracted a hobbyist and amateur community, somewhat along the lines of the "robot war" competitions that make it onto TV, and yearly competitive events have been conducted. These home-built MAVs are of course relatively unsophisticated, but have demonstrated a great deal of ingenuity. Possibly one of these days somebody is going to come up with an idea that will catch on.
* The US Army has been interested in developing MAVs that could be deployed as munitions, fired from artillery or unguided rocket launcher pods. A research team at the Massachusetts Institute of Technology (MIT) developed a prototype artillery-launched UAV. The UAV, named the "Wide Area Surveillance Projectile (WASP)", no relation to the Aerovironment Wasp, was fired out of a 127 millimeter (5 inch) naval gun.
The MIT group modified a standard illumination flare round to serve as the external case. After firing, the shell popped out six fins to keep it from tumbling. Once the shell was 20 kilometers (12.4 miles) downrange, a parachute popped out the tail to extract the drone. The parachute slowed the drone, which then unfolded into flight configuration. The WASP had a folding vee tail, a folding two-blade propeller up front, and two straight folding wings. The wings were folded into six sections and unfolded into a total span of 94.5 centimeters (3.1 feet). Once unfolded, the right wing was higher on the fuselage than the left, a result of the packaging scheme.
The WASP drone had a flight endurance of fifteen minutes, including ten minutes of powered flight and five minutes of glide. It had a tiny camera in its lower fuselage, and relayed both imagery and its own current GPS coordinates back to the warship or artillery battery that fired it. At least two WASP prototypes were built and tested.
The WASP appears to have been a one-shot demonstration program, but in 2006 ILC Dover, a company that manufactures a range of interesting and unusual items such as space suits, gas masks, protective suits, and various inflatable technologies, announced an inflatable wing system that could be used for gun-launched or air-dropped projectiles. Although the company has flown demonstrators, it is unclear if any production UAV using the technology is in the works.
* The Army has also worked on a UAV that can be launched out of a 70 millimeter (2.75 inch) unguided rocket pod mounted on a helicopter and could also be carried by larger UAVs. Aerovironment has developed such a "wing-store UAV" named the "Switchblade" that features a forward-mounted prop driven by an electric motor, twin pop-out tailfins, and a set of pop-out wings fore and aft. It may also be airdropped by various kinds of aircraft.
Raytheon has worked on an airdrop UAV named "SilentEyes". SilentEyes looks like a simple metal cylinder with a rounded nose, straight folding wings mounted in the middle of the UAV and with a noticeable dihedral, and a folding inverted-vee tail. The UAV is 46 centimeters (18 inches) long and less than 7 centimeters (2.75 inches) in diameter. It can carry an infrared or color TV camera, as well as a jammer payload or a warhead.
* Galileo Avionica of Italy is currently working on their own "parasite" UAV, called simply the "Miniature Air Launched Payload (MALP)", to be carried on a Falco or similar UAV. The MALP has large cruciform tailfins, small cruciform nosefins, and "switchblade" wings stowed back along the fuselage that pop out straight when the UAV is released. It is intended to carry imaging or other sensors to probe dangerous targets.
The Russian Tula organization, which makes the "Smerch (Tornado)" multiple-launch rocket system, has introduce the "R-90" tactical UAV, which is launched as a payload on a Smerch 30 centimeter (1 foot) diameter rocket. The rocket can deliver the UAV to a target area up to 90 kilometers (55 miles) away, with the UAV operating for up to 30 minutes to relay TV images along with target coordinates back to a control station. The R-90 has a weight of 42 kilograms (92 pounds), and can also be launched from a pod on a helicopter or fixed-wing aircraft.
* There is actually a great deal of activity in the small UAV field, with a systems now being acquired in quantity and some being used in combat. In 1999, the US Army bought four Aerovironment Pointer small UAVs for testing in the service's "Military Operations In Urban Terrain" and was enthusiastic about the usefulness of the Pointer. The Pointer system is too large to be conveniently carried by soldiers and is normally hauled around in a Hummer vehicle or the like, and so the Army asked Aerovironment if the company could come up with a more portable solution. Aerovironment agreeably developed a half-sized control system and a cut-down version of the Pointer called the "Raven" (no relationship to the Flight Refueling Raven).
The Raven has the same configuration and central pod of the Pointer, but a shorter tail and a wing reduced to a 52% span of 1.34 meters (4 feet 5 inches). The Raven has an endurance of 80 minutes. The entire system can be carried by two soldiers. Following the Afghanistan campaign in 2001:2002, the US SOCOM ordered 80 Ravens from Aerovironment, which was more than the total number of Pointers that had been sold to that time. The US Army also placed orders for up to 105 Ravens in the late summer of 2003 after the US occupation of Iraq led to persistent insurgent attacks on US forces. The UAV was given the designation of "RQ-11A" and proved an outstanding success.
An upgraded version of the Raven, the "RQ-11B", went into production in 2006; improvements included more endurance, a lighter ground-control system, better sensors, and a laser target designator. By early 2008, the Army had obtained over a thousand Ravens, and the Marines are also interested in buying the Raven.
* US forces are also using other mini-UAVs in Iraq. The Lockheed Martin "Desert Hawk" weighs 3.2 kilograms (7 pounds), has a wingspan of 1.32 meters (52 inches) and a length of 86.4 centimeters (34 inches). It is made mostly of plastic foam, suggesting something like a Nerf toy, and uses an electric motor driving a pusher propeller as a powerplant, making it very quiet. It is launched with a bungee cord, carries three small CCD cameras, and has an endurance of about an hour. It flies mostly under autonomous control, with the "pilot" keeping track of what's going on with a laptop computer.
The Desert Hawk was designed by Lockheed Martin for the Air Force on a quick-reaction contract issued late in the winter of 2002, with the first system delivered in the early summer. It was designed quickly because it leveraged heavily off of technology and design studies developed for the MicroSTAR MAVs; the DARPA effort paid off even in the short run.
In addition, the Honeywell work on ducted-fan UAVs led to the introduction of a ducted-fan MAV with a loaded weight of 8.4 kilograms (18.5 pounds) that was introduced to the Iraq theater in 2007, primarily to help troops find "improvised explosive devices (IEDs)". The machine is known as the "RQ-16A G-MAV", the "G" indicating that it is gasoline-powered, using a high-end RC aircraft piston engine manufactured by 3W of Germany. Honeywell developed a prototype in 2005, coming up with a new design based on the company's previous work with DARPA, with the US Army performing evaluations in the summer of that year. After modifications as indicated by the trials, an initial batch of G-MAVs were sent to Iraq to see how they fared in a war theater.
Honeywell is working on further improvements, including a sensor turret to replace the current fixed visual / IR cameras and improved flight control software. The company is also working with RCV Engines of the UK to develop a heavy-fuel / diesel version, the "D-MAV", with twice the endurance of the G-MAV. The RQ-16 is the basis for the "Class I UAV" required for the Army's "Future Combat System (FCS)", with the MAV in development to feature a full autonomous flight control system capable of being programmed with a hundred waypoints. The Class I UAV is expected to be available for evaluation no earlier than 2009.
The US military has become extremely enthusiastic in using mini-UAVs for operations in Iraq. One of the frontline officers to make initial use of the Raven in the theater spotted a team of insurgents planting a bomb the first time he used the UAV operationally, and the good word spread like wildfire. So many little UAVs are flying there that they are beginning to become a hazard in air operations, with a few near misses with helicopters and at least one collision, with no serious damage to the helicopter. Operational procedures are being tightened up.
* The Naval Research Laboratory and US Marines Warfighting Laboratory have developed a man-portable UAV of roughly the same size as the Aerovironment Raven, the "RQ-14 Dragon Eye" -- not the same machine as the BAI Aerosystems Dragon. The Dragon Eye is a tailless design with a rectangular wing and twin props. It is designed to fit into a backpack, with a weight of 2.25 kilograms (5 pounds) and a span of 1.14 meters (3 feet 9 inches). It can be launched by hand or bungee slingshot and has a GPS-INS-based waypoint navigation system.
One of the interesting features is that the operator monitors Dragon Eye operation through "video goggles" connected to a laptop computer. The control system weighs about 5.4 kilograms (12 pounds). The Dragon Eye's endurance is an hour. It was quickly put into service with the US Marine Corps, and the Marines were so enthusiastic about it that they ordered over a thousand of them, with Aerovironment performing production. An "RQ-14B" was introduced that used the same control system as fitted to Aerovironment mini-UAVs.
Work is underway to improve on the Dragon Eye. Marines like to launch the UAV off of rooftops in Iraq, and they want a longer span to make it easier to land on the same rooftop it was launched from; the longer span will also improve flight endurance. A better imager is being considered, as well as a payload to allow a Dragon Eye to act as a communications relay for another, permitting a wider radius of action. Yet another improvement is a control system with eight instead of four channels, doubling the number of Dragon Eyes that could be flown in the same area; and a wearable control system. This control system will not only be compatible with the Dragon Eye, but also the "Dragon Runner" ground robot, said to be a four-wheel vehicle about the size of a breadbox, and the "Small Unit Remote Sensing System", which is based on a static module with EO/IR imagers and an acoustic sensor.
The proliferation of small UAVs is becoming something of a nuisance, and not surprisingly the US armed services are considering a common "Joint Small Unmanned Aircraft System (J-SUAS) that will be used by different services. The effort is now in its preliminary stages.
* Of course, man-portable UAVs are being developed or are being sold by other countries. French work in this field was mentioned earlier, and European EADS organization has designed a small UAV named the "DRAC Tracker", which features a wide-span wing, twin booms for payload and so on, and a central pod with tractor and pusher propellers. It has a weight of 7.5 kilograms (16.5 pounds), a span of 1.4 meters (4 feet 7 inches), and an endurance of an hour.
In the spring of 2003 Elbit of Israel introduced two electrically-powered man-portable UAVs, the "Skylark" and the "Seagull". Both of these UAVs have a launch weight of about 5.5 kilograms (12 pounds), a speed of from 35 to 70 KPH (20 to 40 knots), and can carry either a color daylight imager or an infrared imager. The Skylark is of conventional configuration, resembling nothing so much as a large kid's rubber-band airplane with a pod under the fuselage. It has an endurance of two hours. The Skylark has been obtained by the Swedish Army.
The Seagull is much less conventional, in the form of a boomerang-shaped flying wing with wingtip fins and a pusher propeller. Size, performance, and payload details of the Seagull are similar to those of the Skylark, but the endurance is stretched to six hours.
* Rafael of Israel has built a man-portable UAV also named the "SkyLite", which is fired out of a tube like an antitank missile, and has an endurance of about an hour. It can be launched from a vehicle mount or shoulder-launched by a soldier.
The SkyLite has a certain general resemblance to the Raytheon SilentEyes, being a tube a 110 centimeters (3 feet 7 inches) long with a glass sensor nose; a pusher propeller powered by an electric motor; pop-out straight wings with a span of 150 centimeters (4 feet 11 inches); and a cruciform pop-out tail. It has a launch weight of 6 kilograms (13.2 pounds). It was originally named "Skylark", but Rafael decided to change the name to avoid confusion with the Elbit Skylark. It has been adopted by the Israeli Defense Forces.
A "SkyLite B" was introduced in 2007, featuring endurance stretched to three hours; span increased to 240 centimeters (7 feet 11 inches); a vee tail replacing the original three-fin tail; weight increased to 8 kilograms 17.6 pounds); and a digital encrypted communications system.
IAI Malat has also introduced their own small UAV line, designated "BirdEye", which includes the 5 kilogram (11 pound) "BirdEye 500" and the 500 gram (1.1 pound) "BirdEye 100". Sources also mention a Malat micro-UAV, the "Mosquito", though this may be the same as the BirdEye 100. Malat has been promoting the BirdEye 500 for both military and civilian uses, with civilian uses including urban security, crime-fighting, and traffic observation.
