A spark-ignited reciprocating gasoline engine, intake-pressurized with three cascaded stages of turbocharging, was selected by NASA's Environmental Research Aircraft and Sensor Technology (ERAST) Program, managed at the Dryden Flight Research Center, to propel the next generation of remotely piloted atmospheric-science airplanes. Scientific needs for sampling the atmosphere, and providing unique images of the Earth, dictate the unique required flight regime; namely, subsonic speeds at altitudes >80 kft (>24 km) and for periods exceeding 24 hours.
The unique nature of the sampling mission, coupled with the economics of engine development, point to the turbocharged, spark-ignited reciprocating engine (see figure) as the only cost-effective option. The table shows the subtle physical advantages of the reciprocating engine over the turbojet for high-altitude, low-speed flight, arising from its near-stoichiometric combustion. Its low specific air consumption reduces the amount and weight of turbomachinery required to generate power at >80 kft, which apparently results in lower weight and lower thrust specific fuel consumption than a turbojet in this flight regime, despite the large heat exchangers.
The turbocharged engine is low cost because of the existing technology base of mass-produced automotive and general aviation hardware that can be adapted to build such an engine. In addition, the turbocharged engine is technically competitive with the turbine engine at high altitudes. Although the power density of a turbine engine is higher than the reciprocating engine at normal altitudes, the reciprocating engine begins to compare favorably with turbine engines at altitudes above 80 kft, especially at the lower speeds where inlet precompression is not available.
This work was done by James L. Harp, Jr., of ThermoMechanical Systems, Inc., with contributions from NASA Lewis Research Center for Dryden Flight Research Center. To obtain a copy of the paper, "Propulsion System for Very High Altitude Subsonic Unmanned Aircraft," access the Technical Support Package (TSP) free on-line at www.nasatech.com em> under the Machinery/Automation category.
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ThermoMechanical Systems, Inc.
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