management is often integrated into the power system because much of the thermal load is often generated by electrical devices. The operation of hydraulic system components and other actuators is also closely related to electrical system design requirements for peak versus average power.
Much of the technology used in the design of conventional aircraft is directly applicable to UAVs. In fact, two Air Force programs, More Electric Aircraft and More Electric Engine, have advanced the state of the art for onboard power systems. This section reviews present technologies and identifies technology needs that could be addressed as part of a comprehensive UAV research program.
The choice of an electric power system is dictated in large part by the mission requirements, specifically the amount of power required and the time over which the power is to be delivered (i.e., the total energy required). Although systems are usually described in terms of average power, the peak power can also determine the size of the overall system. (Peak power can often be accommodated through a power conditioning system.)
Figure 6-1 provides an overview of several options for a prime power source for a range of average power levels and flight durations. For modest loads and short flight times (minutes), batteries can provide hundreds of watts of power or more. Batteries are attractive because of their relatively low cost and modularity, especially at small sizes, but they have low power and energy densities compared to other alternatives. Fuel cells can provide power from hundreds of watts to hundreds of kilowatts. Because fuel cells have excellent efficiency, they may be an option for very long-endurance missions. However, fuel cells have not yet been developed for use in aircraft, and current fuel cell systems are relatively complex and require inconvenient fuels (e.g., hydrogen). At higher power levels, kilowatts to megawatts, conventional dynamic conversion systems, such as turbines or diesel generators, come into play. For extremely long operating times and modest loads, solar-battery systems might be applicable. Each of these alternatives is discussed below.
The prime power source is the first of several subsystems necessary to provide electrical power. The overall power system shown in Figure 6-2 reflects the many choices that are available. The selection of a prime power source will be determined by mission requirements and platform constraints. After the prime power source has been selected, the subsystems related to power conversion, power storage, and power management must be defined. The conversion process may be as simple as a battery or as complex as a gas turbine generator. Storage subsystems may be necessary for start-up, peak power, and transients. The power