A CRPU was essentially a steel tube, 27 cm in diameter and 16 meters long, joined from a few shorter tubes. The CRPU was divided into three sections by aluminium bulkheads, with additional stiffening rings between bulkheads. Forward, the majority of the tube contained a mixture of nitric acid and nitrogen tetroxide oxidisers. Next was a section of kerosene fuel. This was commercial-grade kerosene, not the more expensive RP-1. Last was the engine section. A fuel line carried nitric acid around the kerosene, into the engine.