Propulsion System for Very High Altitude Subsonic Unmanned Aircraft

This paper explains why a spark ignited gasolineengine, intake pressurized with three cascaded stagesof turbocharging, was selected to power NASA'scontemplated next generation of high altitudeatmospheric science aircraft. Beginning with the mosturgent science needs (the atmospheric samplingmission) and tracing through the mission requirementswhich dictate the unique flight regime in which thisaircraft has to operate (subsonic flight @ >80 kft) webriefly explore the physical problems and constraints,the available technology options and the cost driversassociated with developing a viable propulsion systemfor this highly specialized aircraft. The paper presentsthe two available options (the turbojet and theturbocharged spark ignited engine) which arediscussed and compared in the context of the flightregime. We then show how the unique nature of thesampling mission, coupled with the economicconsiderations pursuant to aero engine development,point to the spark ignited engine as the only costeffective solution available. Surprisingly, this solutioncompares favorably with the turbojet in the flightregime of interest. Finally, some remarks are madeabout NASA's present state of development, andfuture plans to flight demonstrate the three stageturbocharged powerplant.INTRODUCTIONBecause of the increasing influence of man-madepollutants and their potential impact on Earth'satmosphere, the science community is expendingconsiderable effort to gain a better understanding ofits detailed chemistry and dynamics. Much of the workinvolves the development of more sophisticatedcomputer models of the atmosphere. These arevalidated through correlations with observed data,which includes both remote sensing and in situmeasurements. At present, the highest prioritymeasurements are in situ measurements at altitudesabove 73 kft to over 80 kft, especially within 12 ° ofthe Equator. The in situ measurements are hardest toobtain since they involve physical samples taken byairborne instruments. Aircraft are the preferredinstrument platform because of the length anddirectedness of the flightpath, which allows largenumbers of samples to be obtained at specifiedlocations in the atmosphere, at the specific timesdictated by science opportunity.The most urgent need is for an aircraft that can flylong distances at altitudes significantly above 80 kft.The aircraft presently used for sampling, even thehigh altitude ER-2, are not capable of flying muchhigher than 73 kft. While balloon borne instrumentscan reach altitudes as high as 130 kft, the undirectednature of balloon flight limits the geographic coverageand spatial resolution that is needed (the coverageachieved by a single airplane flight is the equivalent of10--100 simultaneous balloon flights). There are alimited number of supersonic aircraft capable of flyingover 80 kft, but these aircraft achieve high altitude byflying supersonically. The aerodynamic heating andshock associated with supersonic flight causechanges to the air sample which negate themeasurements being made. The airplane whichperforms the sampling mission must be subsonic.This presents a contradiction--the most straight-forward way to achieve high altitudes is to fly fast, butthis airplane must fly high and slow--a very difficultthing to achieve. Because of the exponential lapse ofair density with altitude, a subsonic aircraft flying at80 kft altitudes cannot generate much lift (see Fig. 1).Even at reasonably high speeds (M = 0.5) thedynamic pressure available limits wing Ioadings to only7 to 12 psf; more like a sailplane than a poweredaircraft.THE PROPULSION CHALLENGEIt is widely acknowledged that the propulsionsystem is the most difficult technical challenge.Whether it is manned or unmanned, an aircraftdesigned to fly subsonically >80 kft for >4 hrs willNASA/TM--1998-206636 1