Extending the Vision: The Evolution of Mini-Sniffer

The Mini-Sniffer program was initiated in 1975 to develop a small, unpi­loted, propeller-driven aircraft with which to conduct research on tur­bulence, natural particulates, and manmade pollutants in the upper atmosphere. Unencumbered and flying at speeds of around 45 mph, the craft was designed to reach a maximum altitude of 90,000 feet. The Mini-Sniffer was capable of carrying a 25-pound instrument package to 70,000 feet and cruising there for about 1 hour within a 200-mile range.

The Aircraft Propulsion Division of NASA’s Office of Aeronautics and Space Technology sponsored the project and a team at the Flight Research Center, led by R. Dale Reed, was charged with designing and testing the airplane. Researchers at Johnson Space Center devel­oped a hydrazine-fueled engine for use at high altitudes, where oxy­gen is scarce. To avoid delays while waiting for the revolutionary new engine, Reed’s team built two Mini-Sniffer aircraft powered by conven­tional gasoline engines. These were used for validating the airplane’s struc­ture, aerodynamics, handling qualities, guidance and control systems, and operational techniques.[899] As Reed worked on the airframe design, he built small, hand-launched balsa wood gliders for qualitative evalua­tion of different configurations. He decided from the outset that the Mini­Sniffer should have a pusher engine to leave the nose-mounted payload free to collect air samples without disruption or contamination from the engine. Climb performance was given priority over cruise performance.

Eventually, Reed’s team constructed three configurations. The first two—using the same airframe—were powered by a single two-stroke, gasoline-fueled go-cart engine driving a 22-inch-diameter propeller. The third was powered by a hydrazine-fueled engine developed by James W. Akkerman, a propulsion engineer at Johnson Space Center. Thirty-three flights were completed with the three airplanes, each of which provided experimental research results. Thanks to the use of a six-degree-of-freedom simulator, none of the Mini-Sniffer flights had to be devoted to training. Simulation also proved useful for designing the control system and, when compared with flight results, proved an accurate representation of the vehicle’s flight characteristics.

The Mini-Sniffer I featured an 18-foot-span, aft-mounted wing, and a nose-mounted canard. Initially, it was flown via a model airplane radio­control box. Dual-redundant batteries supplied power, and fail-safe units were provided to put the airplane into a gliding turn for landing descent in the event of a transmitter failure. After 12 test flights, Reed abandoned the flying-wing canard configuration for one with substantially greater stability.[900] The Mini-Sniffer II design had a 22-foot wingspan with twin tail booms supporting a horizontal stabilizer. This configuration was less susceptible to flat spin, encountered with the Mini-Sniffer I on its final flight when the ground pilot’s timing between right and left yaw pulses coupled the adverse yaw characteristics of the ailerons with the vehicle’s Dutch roll motions. The ensuing unrecoverable spin resulted in only minor damage to the airplane, as the landing gear absorbed most of the impact forces. It took 3 weeks to restore the airframe to flying condition and convert it to the Mini-Sniffer II configuration. Dihedral wingtips provided additional roll control.

The modified craft was flown 20 times, including 10 flights using wing-mounted ailerons to evaluate their effectiveness in controlling the aircraft. Simulations showed that summing a yaw-rate gyro and pilot inputs to the rudders gave automatic wings leveling at all altitudes and yaw damping at altitudes above 60,000 feet. Subsequently, the ailerons were locked and a turn-rate command system introduced in which the ground controller needed only to turn a knob to achieve desired turn­ing radius. Flight-testing indicated that the Mini-Sniffer II had a high static-stability margin, making the aircraft very easy to trim and min­imizing the effects of altering nose shapes and sizes or adding pods of various shapes and sizes under the fuselage to accommodate instrumen­tation. A highly damped short-period longitudinal oscillation resulted in rapid recovery from turbulence or upset. When an inadvertent hard – over rudder command rolled the airplane inverted, the ground pilot sim­ply turned the yaw damper on and the vehicle recovered automatically, losing just 200 feet of altitude.[901] The Mini-Sniffer III was a completely new airframe, similar in configuration to the Mini-Sniffer II but with a lengthened forward fuselage. An 18-inch nose extension provided better balance and greater payload capacity—up to 50 pounds plus telemetry equipment, radar transponder, radio-control gear, instrumentation, and sensors for stability and control investigations. Technicians at a sailplane repair company constructed the fuselage and wings from fiberglass and plastic foam, and they built tail surfaces from Kevlar and carbon fiber. Metal workers at Dryden fashioned an aluminum tail assembly, while a manufacturer of mini-RPVs designed and constructed an aluminum hydrazine tank to be integral with the fuselage. The Mini-Sniffer III was assembled at Dryden and integrated with Akkerman’s engine.

The 15-horsepower, hydrazine-fueled piston engine drove a 38-inch-diameter, 4-bladed propeller. Plans called for eventually using

a 6-foot-diameter, 2-bladed propeller for high-altitude flights. A slightly pressurized tank fed liquid hydrazine into a fuel pump, where it became pressurized to 850 pounds per square inch (psi). A fuel valve then routed some of the pressurized hydrazine to a gas generator, where liquid fuel was converted to hot gas at 1,700 degrees Fahrenheit (°F). Expansion of the hot gas drove the piston.[902] Since hydrazine doesn’t need to be mixed with oxygen for combustion, it is highly suited to use in the thin upper atmosphere. This led to a proposal to send a hydrazine-powered aircraft, based on the Mini-Sniffer concept, to Mars, where it would be flown in the thin Martian atmosphere while collecting data and transmitting it back to scientists on Earth. Regrettably, such a vehicle has yet to be built.

During a 1-hour shakedown flight on November 23, 1976, the Mini­Sniffer III reached an altitude of 20,000 feet. Power fluctuations pre­vented the airplane from attaining the planned altitude of 40,000 feet, but otherwise, the engine performed well. About 34 minutes into the flight, fuel tank pressure was near zero, so the ground pilot closed the throttle and initiated a gliding descent. Some 30 minutes later, the Mini­Sniffer III touched down on the dry lakebed. The retrieval crew, wearing
protective garments to prevent contact with toxic and highly flamma­ble fuels, found that there had been a hydrazine leak. This in itself did not account for the power reduction, however. Investigators suggested a possible fuel line blockage or valve malfunction might have been to blame.[903] Although the mission successfully demonstrated the opera­tional characteristics of a hydrazine-fueled, non-air-breathing aircraft, the Mini-Sniffer III never flew again. Funding for tests with a variable – pitch propeller needed for flights at higher altitudes was not forthcoming, although interest in a Mars exploration airplane resurfaced from time to time over the next few decades.[904] The Mini-Sniffer project yielded a great deal of useful information for application to future RPRV efforts. One area of interest concerned procedures for controlling the vehicle. On the first flights of Mini-Sniffer I, ordinary model radio-control gear was used. This was later replaced with a custom-made, multichannel radio-control system for greater range and equipped with built-in fail­safe circuits to retain control when more than one transmitter was used. The onboard receiver was designed to respond only to the strongest sig­nal. To demonstrate this feature, one of the vehicles was flown over two operating transmitter units located 50 feet apart on the ground. As the Mini-Sniffer passed overhead, the controller of the transmitter nearest the airplane took command from the other controller, with both trans­mitters broadcasting on the same frequency. With typical model radio­control gear, interference from two simultaneously operating transmitters usually results in loss of control regardless of relative signal strength.[905] A chase truck was used during developmental flights to collect early data on control issues. A controller, called the visual pilot, operated the air­plane from the truck bed while observing its response to commands. Speed and trim curves were plotted based on the truck’s speed and a recording of the pilot’s inputs. During later flights, a remote pilot con­trolled the Mini-Sniffer from a chase helicopter. Technicians installed a telemetering system and radar transponder in the airplane so that it could be controlled at altitude from the NASA Mission Control Room at Dryden. Plot boards at the control station displayed position and alti­tude, airspeed, turn rate, elevator trim, and engine data. A miniature

television camera provided a visual reference for the pilot. In most cases, a visual pilot took control for landing while directly observing the air­plane from a vantage point adjacent to the landing area. Reed, how­ever, also demonstrated a solo flight, which he controlled unassisted from takeoff to landing.

"I got a bigger thrill from doing this than from my first flight in a light plane as a teenager,” he said, "probably because I felt more was at stake.”[906]

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