The design originally featured a seven-stage compressor based on their
Adder engine — the Viper is in effect a large-scale Adder. Like the similar
J85 built in United States, the Viper was originally developed as an
expendable engine for production versions of the
Jindivik target drone. Like the J85, the limited-life components and total-loss oil systems were replaced with standard systems for use in crewed aircraft. Because it was initially developed as an expendable engine, the Viper was subject to many recurring maintenance issues. This led to the development of the first
Power by the Hour program in which operators would pay a fixed hourly rate to Bristol Siddeley for the continual maintenance of the engines. In the 1970s, Turbomecanica
Bucharest and Orao
Sarajevo acquired the license for the Viper engine, which propelled various Romanian and Yugoslav built aircraft. ==Variants==