NASA Mini-Sniffer

The initial "Mini-Sniffer I" had a wingspan of 5.5 meters (18 feet), tailfins on the wingtips, and canards on the nose. It used a gasoline-powered piston engine and performed a dozen low-altitude flights to validate the design.

Mini-Sniffer II.

The Mini-Sniffer I was then modified into the "Mini-Sniffer II" by removing the canards and the wingtip tailfins, then adding tail booms and extending the wings, giving it a wingspan of 6.7 meters (22 feet). It was still powered by a gasoline engine, and made 21 flights to a maximum altitude of 6,100 meters (20,000 feet).

To get to much higher altitudes, Reed planned to use an unusual engine that burned hydrazine. A normal internal combustion engine burns gasoline with air to generate power, but at 21 kilometers the air is too thin to keep it running. Instead of gasoline, Reed planned to use hydrazine, or N2H4, which breaks down spontaneously when run across a catalyst, generating heat to produce steam to drive the engine. Hydrazine is a corrosive, toxic, and unstable propellant, but its ability to "burn" without oxygen makes it useful for spacecraft thrusters and for such high-altitude engine applications.

"Mini-Sniffer III" was a new build aircraft, similar to Mini-Sniffer II but with a longer fuselage and the hydrazine engine. It was designed to carry an 11.3 kilogram (25 pound) payload to 70,000 feet (21,000 m) or higher. However, the Mini-Sniffer III only made a single flight to 6,100 meters (20,000 feet), and was not flown again because fuel leaks made it hazardous to handle. NASA lost interest in the idea, and the concept of a high-altitude UAV was abandoned until later when the NASA ERAST Program resurrected the idea.