cycle, while not providing the absolute maximum cycle efficiency is very competitive on this “ideal” basis. Furthermore, the Brayton cycle operates continuously, whereas the Diesel and Otto cycles operate intermittently, so their net performance is not nearly as good as their ideal performance. As a result, the Brayton cycle proves to be superior to the Diesel and Otto cycles for many applications. In aviation, the Brayton cycle has historically been the cycle of choice when the effects of propulsion system weight, volume, and durability are factored into the entire aircraft and air transportation system.

The efficiency of the Brayton cycle is governed by the maximum compressor exit temperature, which is determined by high temperature material limits. The propulsion taxonomy in Appendix D describes a broad range of propulsion concepts and substantiates the conclusion that the conventional gas turbine engine and its variants based on the Brayton cycle will continue to be the primary aircraft propulsion system of choice at least through 2025. One of the variants entails a departure from the concept of isentropic compression and expansion. Here, two alternatives exist. The first is combustion in the turbine (i.e., interturbine burning). This alternative offers the possibility of reducing the temperature drop across the turbine while increasing the work done by the turbine, thus improving overall engine performance even if interturbine burning is active only during peak power (takeoff and climb). A second alternative is the introduction of volume cooling ahead of or in the compressor by introducing a mist of water or other coolant either ahead of or between compressor stages. This alternative has two benefits. The first is the increase in total pressure owing to the volume cooling, and the second is the increase in mass flow. The former has the effect of increasing the compressor efficiency in that the compressor outlet temperature (T₃) is reduced for a given pressure ratio. The latter increases the exit momentum flux, which could also be used to increase takeoff and climb performance. Either improvement could reduce the propulsion system weight fraction and improve aircraft efficiency. These modified Brayton cycles warrant research and could be incorporated into operational systems by 2025.

Propulsion system performance is directly related to the safety, capacity, mobility, noise, and emissions of individual aircraft and the air transportation system as a whole. Figure 4-4 shows the significant advances that have been made in

FIGURE 4-4 Predictions made in 1968 of subsonic thrust-specific fuel consumption, updated with data on operational systems developed since 1968. UEET, ultra-efficient engine technology; VAATE, versatile, affordable, advanced turbo engines; η_th, thermal efficiency; η_p, propulsive efficiency. Source: Jeffrey M. Stricker, Wright-Patterson Air Force Base, Aero Propulsion Laboratory, briefing to committee members S. Michael Hudson and Willard J. Dodds, January 13, 2003. Modification of data from L. Dawson. Propulsion. Aeronautical Journal of the Royal Aeronautical Society 72(September):209-229.