THE CHALLENGE APU's (auxiliary power units) for aircraft impose a challenging combination of demands for small-scale engines: compact & light weight, dependable, smooth, quiet, reliable operation across large differential temperatures and altitudes, simple operation, safety, and longevity. To date, designers and manufacturers have had to focus on large power APU's for aircraft given the  intrinsic limitations of decades-old, small gas turbine engine technologies.  Such gas turbines have a practical power floor that has prevented their utilization in small-scale aircraft APU applications.  Conventional internal combustion and rotary engines in this power domain are too heavy and large, have excessive vibration and noise, lack aviation-fuel capability, and possess unacceptable longevity. Mallen's solution is as technically sound as it is revolutionary. Robust materials and a multi-fuel, clean combustion system are combined within a lightweight proprietary spool geometry with turbine smoothness and extreme compactness. The advanced configuration of the Mallen design represents a significant breakthrough for small-scale power systems.