air, thereby enhancing its heat sink, decreasing its mass flow, and improving engine SFC. At the same time, employing cooled cooling air (CCA) might enhance the turbine blade thermal gradient and accelerate thermal fatigue, which means the turbine materials would need better low-cycle fatigue capability. Future development of low-fuel-consumption engines with CCA or engine power generation for directed-energy weapons will dramatically increase the heat load rejected to the fuel. The fuel heat sink requirement is shown in Figure 6-2.
The thermal management challenge is exacerbated at higher Mach numbers, where fuel temperatures can exceed 900°F at Mach 4 and 1300°F at Mach 7 (hypersonic). The problem is not the high fuel temperatures per se—it is preventing any resulting fuel system deposit (coking) from crippling the operation of the engine and limiting its life. The quality of a fuel is measured by a rule-of-thumb bulk and wall temperature limit corresponding to 2,000+ hours of fuel system life. Thus, the development and use of high-heat-sink fuel for thermal management is a paramount challenge. Research into fuel additives and deoxygenation systems is required to increase fuel system life, which is typically a few hundred hours, by an order of magnitude at temperatures above 500°F.
The Air Force Research Lab (AFRL) high-heat-sink fuel program seeks to increase the thermal stability of JP-8 from its current operating temperature of 325°F to 550°F (120 percent higher heat sink) and eventually to prefect the fuel designated as JP-900 for the hypersonic flight regime. Ideal gas turbine fuel attributes are envisioned in Figure 6-3.
Jet fuels are complex, and technology transition often involves multiple partners such as engine companies, airframers, fuel system suppliers, additive manufacturers, university researchers, the Air Force Petroleum Office, the Defense Energy Support Center, and the Air Force system program offices. Also, coordination with commercial aviation and industry through the International Air Transport Association, the American Society for Testing and Materials, the Federal Aviation Administration, and the North Atlantic Treaty Organization is required. Interchangeability of military and commercial fuels is a key logistical benefit.
AFRL has been conducting research in the following areas to enhance fuel thermal stability using fuel deoxygenation, advanced additives, and surface coatings:
Evolving technologies such as nanoparticles could allow developing the JP-900 type fuel. Nano fuel technology shows promise in enabling advanced smart additives. Advanced fuel system