Turbine engines have been used as high reliable, safe engines in airline transportation. Safety is the most important factor in the social use of aerial robots. We started research on Aerial Cargo Robots (ACR) in 2004. The first flight of an ACR prototype was successfully achieved on November 22, 2005. The ACR prototype consists of a flexible airfoil, twin micro-turbo-jet engines and a gravity center control unit. The ACR meets the following requirements for safety: touchable, i.e., without propellers or rotors; a low sink rate the same as a parachute, i.e., below 1.0 m/sec; a low stall speed, i.e., less than 30 km/h; and a redundancy arrangement control system. The most important safety specification is the use of a silent turbojet engine for the ACR thruster. This paper reports the results of turbojet engine development for aerial robots.