This article discusses that today’s squad leader must still risk troops to scout out what lies over the next hill, beyond the next tree line, or inside the next building. The Department of Defense is trying to help ground troops at the platoon, company, or brigade level with this crucial task by giving them tiny spy planes, called micro aerial vehicles (MAVs), to search the local terrain. Planners at the Defense Advanced Research Projects Agency (DARPA) envision equipping small combat units with their own “organic” intelligence assets that can locate and monitor possible threats. DARPA planners define a MAV as semiautonomous airborne vehicles, measuring less than 6 inches in any dimension and weighing about 4 ounces that can accomplish a useful military mission at an affordable cost. The most likely parameters to sense for MAV stabilization are inertial angular rate, differential and absolute pressure, acceleration, and the Earth’s magnetic and electrostatic fields; optical sensing could be used for angular position and rate stabilization. Developing useful micro aerial vehicles is going to be a severe design engineering challenge.
Keeping aware of situations amid the chaos of combat is one of the most critical but troublesome tasks battlefield commanders must face. Since the airplane was developed, the upper echelons of the armed forces have benefited from ever-greater access to aerial reconnaissance data with which to plan their battles. In recent years, portable satellite data links have started to bring theater-level surveillance information to the lower levels of the military hierarchy nearly in real time. Large-area intelligence assets like spy planes, unmanned drones, and satellites are not always able to provide detailed small-area information to frontline commanders in a timely manner, however. Today's squad leader must still risk troops to scout out what lies over the next hill, beyond the next tree line, or inside the next building.
The Department of Defense is trying to help ground troops at the platoon, company, or brigade level with this crucial task by giving them tiny spy planes, called micro aerial vehicles (MAVs), to search the local terrain. Planners at the Defense Advanced Research Projects Agency (DARPA) envision equipping small combat units with their own "organic" intelligence assets that can locate and monitor possible threats.
Technical evaluations conducted at the Massachusetts Institute of Technology's Lincoln Laboratories in Lexington and the Naval Research Laboratory in Washington, D.C., have concluded that the concept is workable. DARPA is currently launching a three-year, $35 million program to develop MAVs. Negotiations are now being conducted that will lead to Small Business Innovation Research Grants and other types of research and development awards to a range of organizations, including university laboratories, aerospace firms, and small businesses. The agency also plans to select a number of efforts for MAV system development and demonstration.
Several prototype MAV technologies have already shown some promise. Engineers at AeroVironment Inc. in Simi Valley, Calif., have flown a palm-size disk-wing airplane for 16 minutes on lithium battery power. The small Black Widow MAV prototype, which looks like a discus with a propeller, tail, and flaps, awaits completion of its miniaturized computer flight-control, navigation, and communications systems.
Progress is also being made in addressing the need for substantially longer-lasting power sources. IGR Enterprises Inc., a small technology company in Beachwood, Ohio, is developing very lightweight, one-time-use solid-oxide fuel cells that have several times the energy density found in lithium batteries. M-DOT, a technology firm in Phoenix, is working on a diminutive gasturbine engine that will produce approximately 1.4 pounds of thrust.
Another company currently receiving government support is Aerodyne Corp. in Billerica, Mass. Engineers there are working on a radical hover-vehicle design, a "fin-stabilized oblate spheroid" that flies as a lifting body. The football-like aircraft will use eight ventrally located microturbofans such as the miniature-scale turbine engine M-DOT is developing.
At the same time, even more unconventional flight technologies are being pursued. Engineers at several locations-including the Georgia Tech Research Institute (GTRI) in Atlanta; SRI International in Menlo Park, Calif.; Vanderbilt University in Nashville, Tenn.; and the California Institute of Technology in Pasadena-are investigating the wing-flapping technology that would make bird- , bat-, or insect-like "ornithopters" possible.
DARPA planners define a MAV as a semiautonomous airborne vehicles, measuring less than 6 inches in any dimension and weighing about 4 ounces, that can accomplish a useful military mission at an affordable cost (less than $1,000 if it is to be a throwaway system). Nominal performance goals include real-time imaging, navigation, and communications capabilities, a range of up to 6 miles, and a top speed of up to 30 miles per hour, during missions lasting 20 minutes to 2 hours.
"These systems are at least 10 times smaller than any current flying system," said James McMichael, DARPA's MAV program manager. "They will be uniquely suited to the challenges of small unit operations and operations in urban terrain. For the first time, they will give individual soldiers and Marines an asset they own and control that can provide real-time situational awareness and reconnaissance information."
Micro aerial vehicles may be regarded as "six-degree of-freedom" sensor platforms that will enable a broad spectrum of small-unit and special operations. Missions might include video and multispectral (infrared) reconnaissance and surveillance, battle-damage assessment, targeting of weapons on key installations, placement of autonomous sensors, a communications relay, or the detection of hazardous substances or land mines. Other uses are also under consideration, such as monitoring hostage situations or weapons-ban treaties, patrolling national borders, and searching for disaster survivors.
Work on the MAV concept began in the early 1990s, when a government-funded Rand Corp. study stated that extremely small reconnaissance vehicles with tiny sensors should be feasible. By 1994, researchers at Lincoln Labs had begun considering the issue, said William R . Davis, leader of the labs' Optical Systems Engineering Group, which he said was involved early on for its expertise in advanced sensors, communications, and aerodynamics.
After consulting with DARP A and potential field users, the Lincoln Labs team came to some basic conclusions regarding the vehicles' core mission. To be truly useful, MAVs need to carry a short-range day/ night area imaging system with enough resolution for operators to discern important details in the transmitted scene. The system must feature an accurate geolocation capability so users will know where the images come from. Sufficient vehicle range and real-time communications are also key. Moreover, MAVs have to be lightweight and robust enough to be carried in a backpack. And, if possible, the systems should be sufficiently inexpensive to be expendable.
Another crucial requirement is for the craft to be "covert-difficult to see, hear, and otherwise detect, so it doesn't give its presence away nor compromise the operator's location," Davis said. "We asked ourselves: Looking at it from a systems point of view, what's the smallest vehicle we can get by with?" By and large, the answer was that an optimal MAV should be as close as possible to a flying sensor chip.
An airplane, the saying goes, is nothing more than a series of compromises flying in close formation, so imagine the severe compromises that were needed to design a tiny unmanned plane like a MAV. According to the team at Lincoln Labs, the MAVs will require high degrees of system integration with unprecedented levels of multifunctionality, component integration, payload integration, and minimization of interfaces among functional elements. One key engineering issue will arise from close-coupled, dynamic electromagnetic and thermal interactions that are brought about by close proximity.
Among the specific significant engineering challenges to successful MAV deployment are ultra-compact, lightweight, high-power- and high-energy-density propulsion and power sources; novel concepts for lift generation; flight stabilization and control for aerodynamic environments with very low Reynolds numbers; lightweight, secure, low-power onboard electronic processing and communications with sufficient bandwidth for real-time imaging; microgyro scopes and very small onboard guidance, navigation, and geolocation systems; a high degree of functional/physical design synergy achieved through highly integrated electromechanical multifunctional modules (for example, combined flight-control, collision-avoidance, navigation, and communications systems); advanced lightweight, strong structures; high g-hardening and special packaging for projectile-release systems; and last but not least, the development or modification of a variety of advanced MAV -tailored sensors.
Propulsion Defines Aerodynamics
Davis said that "the most challenging near-term technical development item for MAVs is the propulsion system and the related aerodynamic issues. [However,] if you have a good propulsion system, you can overcome most problems with aerodynamics."
"Propulsion is definitely the long pole in the tent," said Richard Foch, head of the vehicle research section in the off-board countermeasures branch of the Naval Research Laboratory's Tactical Electronic Warfare Division. "These systems require a method to generate enough aerodynamic thrust in an extremely efficient manner. Given a good power source and propulsion ' system, the aerodynamics for MAVs don't look too bad," he said. "Of course, developing an airplane without a power plant is a fairly risky business. But the tiny machines we're considering have a lift-to-drag ratio between 3 and 10, so you can calculate how much energy is needed to make it fly."
According to the Lincoln Labs engineers, a 6-inch propeller-driven vehicle with a lift-to-drag ratio of 5 will require about 2.5 watts of shaft power for cruising and double that for climbing, turning, or hovering. This low power regime means standard model-airplane engines are four or five times too big, according to Davis. Of the three general classes of available power systems-mechanical-energy storage, electric drives, and thermal-cycle machines-only a few seem suitable.
Internal combustion engines have the most near-term promise, Davis said. Mechanical-energy storage systems using springs, compressed gas, or flywheels are not deemed practical.
Electric propulsion is also promising. Electric motors of the required size are available using electrochemical batteries, fuel cells, microturbine generators, thermal photovoltaic generators, solar cells, systems. The first three sources are considered the most practical because calculations indicate that a power density of about 300 milliwatts per gram and an energy density of about 700 joules per gram are required for a robust electric system.
Foch noted that new small motor designs such as the brushless neodymium-iron-boron magnet type are now running at 90-percent efficiencies. A lightweight power system comprising a high-efficiency electric motor and the best lithium batteries would run 20 to 30 minutes, he said. Although current lithium battery performance is marginal for this application, its performance should improve in the near future.
Fuel cells, meanwhile, are not yet sufficiently small for the MAV application, but the technology, which should be ready in three or four years, is considered to be a good bet, according to Foch. IGR has demonstrated the technical feasibility of small nonregenerative solid-oxide fuel cells that could provide more than two to four times the energy density (in weight and volume) of the best nonrechargeable lithium batteries, said Arnold Z. Gordon, IGR's president. Roughly the size of a 1-centimeter-tall playing card and weighing a mere 25 grams, the fuel cell "should provide all the power a MAV should need," he said.
Gordon said that his firm's proprietary solid-state power unit spontaneously generates electric power with the addition of fuel and air. Almost the entire power unit, he said, is made out of steel; the sole exception is its solid ceramic electrolyte, which also serves as the permeable membrane. Gordon noted that the ceramic electrolyte "is formulated as a composite, which provides it with useful mechanical viability. Previous solid electrolytes were very brittle, while the new design can flex a bit."
The system's oxidant is ambient air, so " all that's needed are two holes for air coming in and going out." (Gordon did not reveal the type of fuel to be used.) He added that the power unit, which runs hot, fits in a heat-exchanger insulation unit that protects surrounding apparatus and preheats the incoming air. Operation would be controlled by special-purpose, low-frequency, low-power integrated circuits.
Unlike most refuelable fuel cells, the IGR device would run to completion once the reaction is started (approximately 1 or 2 hours). In addition to clean, quiet operation with instant start-up and no cold-weather problems, the device is nontoxic and has an essentially infinite shelf life with no maintenance, Gordon said.
A promising but technically difficult power source is the microturbine-a microelectromechanical-systems( ME MS-) based gas-turbine-engine/electric-generator set the size of a shirt button that weighs a mere 1 gram. The micro turbine is now under development in an ambitious project at MIT led by Alan Epstein (see "Turbines on a Dime," October 1997). This technology seems at least three or four years away at best.
Thermal-cycle machines such as rockets, pulse jets, steam-cycle engines, microturbine fan jets, and Sterling and internal-combustion engines are possible MAV power sources. Internal-combustion engines seem to hold a great deal of promise. While the thermal efficiencies of internal-combustion engines at this small scale are likely to be only about 5 percent, power densities are typically about 1 watt per gram, and the engines use high-energy fuels. So far, however, truly suitable internal-combustion engines have not yet been built. Noise and reliability issues must also be overcome.
The small fan jet-the M-DOT unit and a variant of the MIT microturbine-is similarly attractive. Jon Sherbeck, M-DOT's director of engineering, is leading the effort to develop a scaled-down version of a conventional jet engine that produces 1.4 pounds of thrust. Using off-the-shelf parts such as dental-drill bearings, the M-DOT group is running a 3-inch-long, 1 %-inch-diameter turbine that weighs only 85 grams.
"You can't just shrink a 747 proportionally down to 6 inches and expect it to fly."
The Problems of Being Small
"You can't just shrink a 747 proportionally down to 6 inches and expect it to fly," said Samuel Blankenship, principal researcher at GTRI and coordinator of Georgia Tech's Focused Research Program for Microflyers. Because of their small size and low airspeed, MAVs will fly at Reynolds numbers lower than for conventional aircraft. The first challenge is to create an efficient wing design that can provide enough lift and sufficiently low drag for a vehicle in that size range, where aerodynamic behavior is different from that of larger, faster aircraft.
Viscous forces are more significant when you get down to this size and airspeed range. The MAVs have proportionally larger drag compared with a larger vehicle, so they are operating at a low Reynolds number. "Before MAVs," said Foch, "it used to be that a low-Reynolds-number regime was 100,000 to 1 million; now low is 5,000 to 80,000, which is pretty much outside the current database."
In addition, boundary-layer characteristics are different. Boundary layers tend to be laminar rather than turbulent in this flight regime, he said. There are also different separation effects: The airflow tends to detach easily, causing a lot of separation in the boundary layer. These aerodynamic conditions are expected to drive designers to new wing sections and wing-body configurations to obtain optimum performance.
Model-airplane experience will undoubtedly help the design effort. Foch cautioned that it is relatively difficult to do wind-tunnel tests on the thin airfoils required at this small size. The forces being monitored are so slight that "acoustic noise and vibration tend to pollute the data," he said. "They can also trip the boundary layer."
Georgia Tech, MIT, the Naval Research Laboratory, and the University of Not re Dame in Notre Dame, Ind., are said to be working on the sensitive balances needed to do this work.
The small sizes of MAVs pose another design complication in modeling airflow, according to Foch: "In conventional airplanes, we normally treat wing design as a two-dimensional problem. But because we're living with so much separated flow, and we're trying to take advantage of the vortices that form, we can no longer treat it as a 2-D problem. You have to consider three-dimensional effects in the span wise direction. For example, the transient sideways momentum has a big effect on the stability of the vortices that are creating the extra lift you need."
Another challenge arises from limited propeller efficiencies. "At 6 inches," Foch said, "propellers are still big enough to operate reasonably efficiently," adding that 3 inches is the lower limit. "Below that size, you might have to flap wings, but that is second-generation technology that still needs a lot of basic research." Fifty-percent efficiency has been demonstrated with 5-centimeter-diameter propellers rotating at 25,000 rpm. Larger. propellers could be more efficient, but increased torque and extra mass for gear reduction are needed.
These constraints provide opportunities to develop new airframe configurations including variations on wing-tail and flying-wing configurations as well as hoverers, with emphasis on trading aerodynamic performance for propulsion and payload integration requirements.
The diminutive vehicle also needs a flight-control system that can maintain its course in the face of turbulence or sudden gusts of wind. Operations out of the line of sight mean "a soldier can't fly the vehicle like a model airplane," Davis said. His team has determined that the prototype could rely on tiny sensors that measure airspeed, acceleration, and atmospheric pressure as well as on electrical actuators for flight surfaces to execute maneuvers.
A flight-control system is required to stabilize the MAV, or at least augment its natural stability, and to execute maneuver commands. It may also have to stabilize the line of sight if the vehicle has an imaging mission. Flight-control components include actuators for aerodynamic controls, motion sensors, and processing.
Aerodynamic control could be achieved using conventional control surfaces with discrete actuators, distributed microflaps, or warped lifting surfaces, depending on the airframe configuration. Very small electric motors could serve as actuators for 6-inchclass vehicles. Additional candidates include piezoelectric actuators (both bulk and thin-film devices) and large number of MEMS devices, which could be electromagnetic, electrostatic, piezoelectric "inchworm," or ultrasonic-wave devices.
AeroVironment's disk-wing MAV, for example, uses a 2-gram flight- control system with a flight computer, a command receiver, and three Smoovy micromotors used as control flap actuators. The micromotors, from RMB in Switzerland, are said to be the smallest electric motor in production; each 3-millimeter-diameter devices weighs Y3 gram.
The most likely parameters to sense for MAV stabilization are inertial angular rate, differential and absolute pressure, acceleration, and the Earth's magnetic and electrostatic fields; optical sensing could be used for angular position and rate stabilization. Small pressure sensors and accelerometers, which could measure altitude and angle of attack, are available now, but inertial angular-rate sensors would produce the most robust control systems. MEMS Coriolis-force angular-rate sensors would be adequate for stability augmentation (not inertial navigation), but further work is needed to miniaturize the associated electronics. A processor will be required for the flight-control functions, and commercial microcontrollers will probably have enough capability for the first-generation MAVs. Advanced abilities, such as autonomous control, will need custom chips.
Once in the air, the MAV will need to maintain communications with its human controllers. Such links could take several forms. The simplest is a direct line-of-sight system, while a vehicle flying beyond or below the line of sight would require an overhead communications relay-another flying vehicle or satellite.
Antennas tend to b e a big problem for MAVs, as the small dimensions limit radio frequencies and range. On top of that, engineers must isolate the system from electromagnetic and radio-frequency interference. The communications electronics will need to be extremely mass- and power-efficient, with capabilities stripped down to the bare minimum.
Proposed means of making MAVs autonomous include using a geographic information system to provide a map of the terrain, or a Global Positioning System (GPS) satellite, which determines location by triangulating from satellite signals. GPS capability would greatly enhance a MAV's capabilities, but current mall units need at least 0.5 watts of power and have antennas weighing 20 to 40 grams. In addition, GPS systems need a substantial amount of data-processing power to work. "We'd really like a GPS, but right now the electronics are too power-hungry and the antennas are too big," Davis said. "It all has to be downsized."
Furthermore, for the machines to be useful, MAVs will have to carry payloads ranging from television cameras to infrared and chemical/ biological sensors in a package weighing just 15 grams. These advanced sensor systems will be the basic cost driver for the MAV systems.
Now under development are 1-gram charge-coupled-device (CCD) video cameras. To provide enough resolution to classify vehicles and detect personnel at about 100 meters high, for example, these video devices will require focal planes with about 1,000 by 1,000 pixels. The best infrared sensor candidates (in the 3- to 5-micron band) are platinum silicide CCD or complementary-metal-oxide-semiconductor arrays. Biological and chemical-agent detectors will require substantial development, according to experts. Airborne chemical sensors now weigh about 5 kilograms , for example, while biological sensors are at an even earlier development stage.
Developing useful micro aerial vehicles is going to be a severe design engineering challenge, Davis said. Retaining the needed performance while meeting the Pentagon's low-cost goals will be particularly difficult: "After all, we don't want a Swiss watch but a Swatch."