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outlined in figures 2, 3 and 4 emerged as the solution. Its salient features are as follows: 1. unmanned operation, for safety and small size; 2. a 50 kW engine that burns on-board fuel (gasoline or liquid methane) and oxygen (diluted by recirculated exhaust gas) and can therefore run at a very high altitude; 3. provision for disassembly and operation from remote locations; and 4. sizing for payloads comparable with Harvard's O/CIO/BrO instrument, i.e. about 50 kg. While this design was evolving, NASA, NOAA, and NSF also began calling for new high-altitude aircraft [5][8] for studies of not only ozone chemistry, but also atmospheric radiation and stratosphere/troposphere mixing (both vital to understanding the "greenhouse effect") and emissions from proposed supersonic airliners. NASA has recognized Perseus as the first aircraft able to meet the new requirements, and is therefore funding its development. Meanwhile, several investigators have begun designing lightweight instruments in anticipation of in situ programs beginning in 1994. Perseus B Several months after the project began. Perseus attracted the attention of Kerry Emanuel at MIT. Professor Emanuel maintains a program of hurricane researches [4][9], and saw that the economics of unmanned aircraft would allow much expanded measurements for study of not only hurricanes but also various problems in climate, meteorology, and chemistry. However, these problems call for modification of the original Perseus design. A very high ceiling is no longer necessary, but longer range and endurance become important as does higher payload weight. Fortunately, giving up altitude in favor of range and payload is a natural trade, since at lower altitude the aircraft can fly at higher weights, and moreover can use turbocharged engines with much lower propellant consumption that the gasoline/oxygen power plant on Perseus A. Figure 5 shows what Perseus B would look like, and figure 6 shows its estimated range and endurance. These would be sufficient for a single aircraft to circle above a hurricane for most of its life cycle, or to do a global-scale chemistry survey over nearly half the planet's circumference. II-7