High Heat Sink Fuels for Improved Aircraft Thermal Management

Aircraft subsystem and engine heat loads are increasing at a rapid rate.¹ Fuel is used in integrated aircraft thermal management systems to cool aircraft subsystems and the engine lubricating oil. All current U. S. fighter aircraft circulate fuel on the airframe to match heat loads with available heat sink. Future aircraft will be required to circulate fuel in excess of that required for propulsive energy through the airframe and engine to assure component life and integrity. These thermal stresses push current fuels JP-4 and JP-8 to bulk fuel temperatures as high as 163°C at the inlet to the mainburner fuel nozzles and to wetted wall temperatures of 205°C inside the fuel nozzle passages.² At these conditions, engine fuel nozzles, afterburner spray assemblies and manifolds are plugging, causing increased maintenance and cost. In some instances fuel degradation changes the spray pattern in the combustor or afterburner leading to damage to engine components. Improved engines and high capability subsystems being developed will increase the frequency and magnitude of thermal stability problems in the future. To reduce fuel fouling in current systems and to provide additional heat sink and thermal stability for future systems the U. S. Air Force is developing an improved JP-8 fuel (JP-8+100) that offers a 100°F (56°C) improvement in thermal stability and the potential of a 50% increase in fuel heat sink capability. The approach chosen by the Air Force is to develop an additive package which would cost less than $0.001 per gallon. The development of JP-8+100 allows the beginning of an era to develop improved aircraft thermal management schemes that can exploit the 50% increase in fuel heat sink, the higher potential operating temperatures and potentially reduce the amount of fuel recirculated through the aircraft. One concept that offers potential is the “Hot Tank” concept.¹ This paper will explore thermal management limitations of JP-8, a summary of the effort to develop JP-8+100, and the potential exploitation of the improved heat sink offered by JP-8+100.