The OTRAG rocket was a modular satellite-delivery rocket developed by the OTRAG company in the 1970s and 80s. The OTRAG rocket was to become a rocket built up from several mass-produced units, intended to carry satellites with a weight of 1-10 tons or more into orbit. Mass production meant that the vehicle was projected to have been 10x cheaper than conventional vehicles of similar capability. Design Various OTRAG rockets could be built up from the company's CRPUs (Common Rocket Propulsion Unit). A sounding rocket would bundle four or more CRPUs in parallel, topped with the payload. An orbital launcher would use dozens to hundreds of CRPUs, depending on payload mass. The launcher would then stage by dropping outer CRPUs, leaving the interior ones to continue with payload. 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. The design of the CRPU was extremely simple. The tubing was strong enough that the propellants were fed to the engine by pressure alone. This eliminated the need for turbopumps. The engine was ablatively cooled, eliminating the need for fine fuel passages and heat-resistant kerosene. The engine did not gimbal; instead, the vehicle was steered by throttling one side's CRPUs versus the opposite side. Thus, the engine was simply built into the tube walls, with the only mechanisms being the throttling valves. No separate pressurising system was included; the tanks were simply left with an ullage space, which was then filled with gas to a few hundred psi. Because of the narrow tubing, the bulkheads between sections could be simple plates, instead of domes like virtually all other rocket stages. There was no ignition system; instead, a slug of furfuryl alcohol was injected before the kerosene. The furfuryl alcohol ignited spontaneously upon contact with the nitric acid. The use of ablative cooling, high-pressure steel construction, and large "empty" spaces meant that a CRPU was heavy, with relatively low performance. The diameter of the tubing also put a