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. composite structures
. propulsion, and 
. microelectronics

Computational aerodynamics
When operating near its ceiling, a subsonic aircraft with a well-matched powerplant will fly in a narrow band between stall and Mach buffet (Mach divergence). In this situation small changes in Mach number and Reynolds numbers have a large effect on airfoil performance. Fortunately, the advent of computers with sufficient processing power and the development of elegant computational schemes has made possible the numerical simulation of practical aerodynamic configurations. The work of Drela and Giles al MIT [3] has produced ISES, the first code accurately modeling both Mach and Reynolds effects fast enough and inexpensively enough to be a practical design tool in the low Reynolds number regime. ISES was effectively validated through wind tunnel models and actual practice (including the wing and propeller airfoils that were the key to Daedalus), and it can confidently be used for design work at altitudes up to approximately 30km. In the region beyond 30km, additional corroborative data are needed from wind tunnel or flight experiments.

Composite structures
The strength and stiffness advantages that composite materials such as graphite-epoxy offer over conventional materials such as aluminum are well known. Equally significant for the aircraft proposed here is the fact that composite materials are generally amenable to small-scale manufacture. For example, while aerospace composites are generally cured at high temperature and pressure in large (and expensive) autoclaves, large reduction in temperature and pressure bring relatively small compromises in performance. Thus, both Voyager and Daedalus aircraft, each of which had a wingspan exceeding 30 meters and achieve performances two to three times better than the previous records, were manufactured by small teams using inexpensive facilities. Those same manufacturing approaches are directly applicable to high-altitude unmanned aircraft and make practical the development of custom air frames for the science community.

Propulsion
At 20km the air pressure is about 5% of its sea-level value. This drops to about 1% al 30km, and to one-third that value at 40km. The pressure ratios required to operate an internal combustion engine using ambient air thus become very large, posing formidable and costly development challenges which are further exacerbated by the relatively low mass flows and (correspondingly low Reynolds numbers) required by the lightweight unmanned aircraft. Studies to date suggest that =22km is a practical limit for a two-stage turbocharged engine, while 30km may achievable with a three-stage system. Achieving altitudes beyond 30km requires an alternative method. A variety of such systems have been investigated in [6], which concludes that an affordable near-term option is presented by an internal combustion engine operating on tanked liquid oxygen, while higher performance may soon be achieved with a fuel cell. The turbocharged engines benefit considerably from work done in the defense community during the early 1980s. while the fuel cells continue to undergo extensive development for both space and terrestrial use. The closed-cycle engine is presently under development through the government's small business innovative research program.

Microelectronics
The same rapid expansion in capability that has been brought to the area of personal computing during the past decade is opening new opportunities in the areas of lightweight, low-power, highly reliable control systems. Microprocessors place vast computing power aboard the platform. The Global Positioning Satellite (GPS) system currently provides two-dimensional positioning data anywhere in the world with accuracies of better than 10 m, with unit costs already below the $1,000 level. 

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