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This Paper reports a recently completed Phase I SBIR program which showed that Loop Heat Pipe (LHP) condenser tubing can be embedded into the composite skin of a UAV, essentially turning the aircraft skin into a radiator. Test panels were fabricated by Teledyne Ryan Aeronautical which incorporated processed LHP condenser tubing into the composite skin material of the Global Hawk. The thermal characteristics of the composite structure as heat rejection fin were experimentally determined. The overall system was modeled and calculated that the upper wing of the Global Hawk would be capable of rejecting more than 50 kW under MIL-STD-210B, 1% Hot Day conditions. Phase II is underway; it will demonstrate the system operation and could lead to all-composite LHP radiator panels.

A passive anti icing system for the engine cowl of the Global Hawk UAV was designed and evaluated under a NASA Phase I SBIR Program. The system utilizes five Loop Heat Pipes to remove 3.8 kW of waste heat from the hydraulic system and deliver it to the engine inlet during icing conditions. During non-icing conditions the heat is bypassed to relief valves on the fuel ullage tank to maintain their temperature above freezing. The system masses 18.5 kg and replaces some 15 lbs of existing equipment. A Phase II program to integrate the system aboard the aircraft is underway with the goal of starting flight tests in early 1999.

Loop Heat Pipes combine the advantages of both heat pipes and Capillary Pumped Loops, while overcoming the limitations of each. Loop Heat Pipes provide very high thermal transport capacities; they can transport heat over long distances, through small cross-sectional tubes and have the capillary pumping capacity to overcome high gravitational heads. Most of these features are available from capillary pumped loops (CPLs), but unlike CPLs, Loop Heat Pipes are inherently self priming and totally passive in operation. This paper describes the operating principles of Loop Heat Pipes, provides performance data from hardware tests, describes some areas of ongoing development, and discusses applications, terrestial as well as space, where Loop Heat Pipes could confer major benefits.