In the 1960s, many rotary piston engine developments were launched based on the principles established by Felix Wankel and his colleagues at NSU. While most of these engines were petrol-fuelled, Rolls-Royce (Crewe) was uniquely commissioned in 1964 by the Military Vehicles and Engineering Establishment (MVEE) to develop a diesel variant for use as a high power-to-weight ratio engine in fighting vehicles. This presentation explores the engines design concept, technical challenges, extensive development programme, and the successes and setbacks that shaped the project. Although the programme ended in 1974, the engines had already demonstrated successful operation on testbeds and in a vehicle. The talk will also provide an update on the recommissioning of one of the surviving engines. The presenter briefly worked on the project in the 1970s and is now involved in restoring the engine at the Anson Engine Museum.