The Nimbus was based on the Turbomeca Artouste engine, which was originally used as an APU on airliners. From Janes Aero Engines:
Derived from a French design, this small helicopter engine is no longer in military/naval service. In 1958 the Nimbus free-turbine turboshaft was derived from Turbomeca designs by Blackburn Engines, originally with the designation A.129. In 1960 Blackburn was one of several aerospace companies purchased by the Hawker Siddeley Group. It later became a unit of Bristol Siddeley Engines, and with the purchase of that company in 1966 the Nimbus became a Rolls-Royce engine. When it passed out of British military service in 2001 all rights to it were transferred to H+S Aviation of Portsmouth, Hampshire. Nimbus-powered helicopters are still flying. The Nimbus was based on the Turbomeca Artouste, for which Blackburn obtained a manufacturing licence. To the fury of the French company, Blackburn added two axial compressor stages ahead of the centrifugal compressor, a second turbine stage to the gas generator, and a free power turbine to drive the output shaft. From the free turbine, the output is taken via a two-stage gearbox. The Nimbus, then known only as the A.129, was first run as a turbojet in July 1958, and as a turboshaft with a mainshaft drive gearbox in the following month. When ready for production in 1960 the rated power was 722 kW (968 shp). Production engines were de-rated to suit the applications.