Conventional power-trains for aviation are expensive on initial cost, operation and maintenance. Power-trains for vertical takeoff encounter even more challenges because it is difficult to satisfy propelling power efficiency for both low speed vertical takeoff (thrust due to air mass) and high speed cruise (thrust due to jet speed). Turbofan is a successful solution but expensive with high maintenance, it is not practical for average users.

Piston engines usually are not considered for aviation power-train due to poor power ratio. A clean, two-stroke hybrid Fuel-Air Injection (FAI) engine is proposed here to serve two purposes for vertical takeoff aviation, (1) Duct fan drive for low speed vertical takeoff (2) Compressed air generation for rocket engine during high speed horizontal cruise.