The Challenges of High Altitude Gas Turbine Engine Cycles

Abstract

High altitude flight, approaching 65,000 ft and above, is becoming increasingly important for various types of aircraft missions. Unmanned Aerial Vehicles (UAV), either for military reconnaissance or commercial/scientific study, are leading the way. For the military commander, high altitude, long endurance (HALE) flight for UAVs provides a good field of view for a long period of time and provides reasonable safety of flight. The upper atmosphere also provides the possibility for uncrowded, direct commercial flights. In addition, supersonic business jets are looking for a flight regime that also could be in the upper atmosphere. Typically, commercial, off-the-shelf engines are adapted for use in high altitude aircraft. While this has provided some success, the engine performance is marginal and the cycles are not optimized for this high altitude environment. Aircraft such as the Predator C and the Global Hawk are already operating in the high altitude environment with turbofan engines. More study is needed to determine what engine cycle is best suited to high altitudes. The smaller engines currently used in HALE UAVs carry a unique set of challenges which constrain the problem of high altitude propulsion hardware choices. Turbine engines have the most promise, especially turbofans, because of the higher speeds that are possible for the aircraft. Preliminary turbofan cycle analysis indicates that higher bypass-ratios and high compressor pressure ratios will be needed requiring more power output from the turbine however, high altitude limits how large these values can and should be. High altitude flight drives the cycle to be designed and sized at the high cruise altitude resulting in considerable impact on the off-design engine performance. Small gas turbine engine technology predictions show that fan pressure ratios of 1.76, compressor pressure ratios of 16.6, bypass ratios of 4.54, and a thrust specific fuel consumption of 0.393 /hr are possible in the near future (sea level reference). The cycle studied found a fan pressure ratio of 1.57, compressor pressure ratio of 16.7, bypass ratio of 5.45, and a thrust specific fuel consumption of 0.436 /hr (sea level reference) to be typical for a small gas turbine engine designed to fly at 65,000 ft. High altitude flight also brings other issues. Environmental impact must be considered in any high altitude application. High altitude reconnaissance aircraft often carry an increased sensor array adding more electric power requirements to the cycle. Long endurance means the engine cycle must be extremely fuel efficient. If stealth considerations are to be incorporated in the aircraft design, then the engine must be embedded in the fuselage limiting engine cross-section. Last, engine operational control will be a key technology for high altitude, low Reynolds number conditions.