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Uninhabited Air Vehicles: Enabling Science for Military Systems 8 Research on Vehicle Subsystems Part II of this report has focused on research opportunities for major vehicle subsystems, including aerodynamics (and vehicle configuration), airframes (with a focus on materials and structures), propulsion, power and related technologies, and controls. The committee analyzed subsystem needs based on three notional vehicle types indicative of the range of technologies required to support general advances in the USAF’s capability of designing, producing, and fielding the generation-after-next UAVs. The three vehicle types were: HALE (high-altitude, long-endurance) vehicles HSM (high-speed, maneuverable) vehicles very low-cost vehicles The committee identified crosscutting research opportunities, that is, research that would benefit all of the vehicle types, as well as research opportunities especially important to specific vehicle types. CROSSCUTTING TECHNOLOGIES The committee identified crosscutting research opportunities for vehicle subsystems in four areas: (1) computational modeling and simulation; (2) propulsion technology for small engines; (3) integrated sensing, actuation and control devices; and (4) controls and mission management.
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Uninhabited Air Vehicles: Enabling Science for Military Systems Recommendation. The U.S. Air Force long-term UAV research program should focus on crosscutting subsystem technologies. Computational Modeling and Simulation The need for affordability and short design cycles that underlies much of the interest in UAVs will require changes in design practices, resulting in increased reliance on computational modeling, simulation, verification, testing, and training. Although these technologies could greatly reduce cost and cycle time, they also raise serious concerns about the fidelity of models and about their inherent uncertainties. Unfortunately, many sources of real error, from the intrinsic variability of the real world being modeled to the multitude of assumptions and approximations introduced in the modeling and simulation steps, cannot presently be accounted for formally and explicitly. Research opportunities for the development and validation of computational modeling and simulation tools are listed below: development, validation, and application of computational tools for major subsystem design, including unsteady, nonlinear, three-dimensional aerodynamics models; structural analysis and aeroelasticity models; aerodynamic modeling concepts for designing vehicle control systems; propulsion system models; and simulation models for assessing new control laws validation of manufacturing process models for UAV components clarification of the role of uncertainty in computational analysis integration of models and simulations to provide “virtual mockups” for testing and evaluation of the total system Propulsion Technologies for Small Engines In the past, development costs have been a major factor in the development of UAV propulsion technology. The development of an all-new gas turbine for a tactical military aircraft can cost more than $1 billion, an inconceivable expense for a low-cost UAV development program. To meet program budget constraints, the practice has been to adapt existing devices, usually at the expense of both performance and reliability. The cost of new technology, especially of new concepts, will be as high for UAV development programs as it has been for conventional aircraft unless new ways of developing propulsion systems can be perfected. To address this concern, the committee recommends that research be focused on technologies to enable development of small, low-cost turbine engines. The following topics should be considered:
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Uninhabited Air Vehicles: Enabling Science for Military Systems low-cost, high-temperature materials and coatings cooling schemes to reduce the need for costly air-cooled parts technology and approaches to reducing leakage through clearances between stationary and rotating parts bearing and lubrication systems that will be more reliable after long-term storage small, low-cost propulsion system accessories (e.g., fuel pumps, engine controls, and electrical generators) Integrated Sensing, Actuation, and Control Devices Sensors and actuators are essential for aircraft operation. Minimizing the weight and volume of sensors, actuators, and other subsystems will be critical for UAVs, which will have stringent size and payload limitations. Emerging MEMS technology can provide transducers as small as tens of microns. By integrating microtransducers with CMOS electronic circuitry, a cost-effective, integrated system capable of sensing, analyzing, and actuating becomes feasible. Potential MEMS-based sensors include inertial sensors, aerodynamic sensors, structural sensors, and surveillance sensors. MEMS-based transducers may have many innovative uses, including the following: structures that are responsive to load variations aerodynamic flow control situational awareness (e.g., collision avoidance and detection of biological and chemical agents) Controls and Mission Management Technologies The single fundamental feature that distinguishes UAVs from other aerial vehicles is control. UAVs rely more on autonomous internal machine and remote links to humans than any other systems. The utility and effectiveness of UAVs will require exploiting the capabilities, and recognizing the limitations, of controls and mission management technologies. The committee envisions that UAVs will operate in integrated scenarios with the following features: several vehicles with specified missions; communication links among vehicles and between vehicles and remote human-operated control sites; and the capability to use sensors and information processing systems located onboard each vehicle, on other vehicles, and at ground sites. Important areas for research in controls for UAVs include the following: rapid (automated) design and implementation of high-performance control laws
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Uninhabited Air Vehicles: Enabling Science for Military Systems robust vehicle management functions (e.g., to carry out mission sequences) mission management technologies, including real-time path planning and control of dynamic networks RESEARCH ON SPECIFIC VEHICLE TYPES In addition to the crosscutting vehicle subsystem technologies just described, the committee identified research opportunities that would support the development of each notional vehicle type. Recommendation. As the long-range plans and priorities for UAVs emerge, the U.S. Air Force should include the applicable research opportunities in the long-range research program. High-Altitude, Long-Endurance UAVs HALE vehicles were analyzed as a focal point for technical advances for reconnaissance and surveillance aircraft a generation beyond current UAVs. The key attributes of HALE vehicles will be operation at very high altitudes (> 65,000 feet) and long endurance (from days to “indefinite” duration). Key subsystem technologies that will enable the development of HALE UAVs are listed below: vortex drag reduction (e.g., lifting systems and tip turbines) laminar-to-turbulent transition for low Reynolds numbers aeroelastic controls high-compression operation of gas turbines or piston engines alternative propulsion systems (e.g., fuel cells, solar cells, and energy storage systems) materials and designs for aeroelastic tailoring low-rate manufacturing technologies for ultra-lightweight airframe structures High-Speed, Maneuverable UAVs HSM UAVs were analyzed as a focal point for potential second-generation UCAVs. The goal of HSM vehicles will be to carry out high-risk combat operations at a significantly lower cost than for inhabited vehicles. The key consideration for HSM vehicles will be survivability, which will require trade-offs of stealth and maneuverability against speed, maximum altitude, and damage tolerance. The following key subsystem technologies will enable the development of HSM UAVs:
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Uninhabited Air Vehicles: Enabling Science for Military Systems nonlinear, unsteady aerodynamics simulation of flow fields for complex configurations modeling tools for propulsion-airframe integration stiff, lightweight structures for highly-loaded propulsion systems fluid seals high-load, long-life bearings probabilistic structural design methods for a high-speed, high-g environment automated manufacturing processes for high-performance structural materials high-temperature composite materials1 Very Low-Cost UAVs Very low-cost UAVs were considered as a focal point for trade-offs of cost against performance in vehicle design. The following key subsystem technologies will enable the development of very low-cost UAVs: very low Reynolds number aerodynamics bearings for long-term storage low-cost accessories for propulsion systems (e.g., fuel pumps, engine controls, and electrical generators) structural design criteria for expendable, low-use systems expanded suite of structural materials (including low-cost, commoditygrade materials) modular designs for low-cost manufacture 1 Some important research and development programs in composite materials and structures, such as NASA’s High Speed Research Program, have recently been discontinued.
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