Editing OTRAG (rocket) (section)

==Design==
[[File:OTRAG CRPU shape.jpg|thumb|left|upright=1.75|OTRAG CRPU]]

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 [[Staging (rocketry)|stage]] by dropping outer CRPUs, leaving the interior ones to continue with the payload.

A CRPU was essentially a steel tube, 27&nbsp;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 [[Dinitrogen tetroxide|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-fed cycle (rocket)|pressure alone]].{{citation needed|date=November 2013}}  This eliminated the need for [[turbopump]]s.  The engine was [[Ablation|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 (rocketry)|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 [[Hypergolic fuel|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 hard limit on the engine diameter, preventing use of an efficient, high-expansion nozzle for the upper stages.  However, ganging CRPUs into three stages was sufficient to reach orbit.  Meanwhile, the low cost of each CRPU, after the [[Economy of scale|economies of scale]] gained by producing hundreds or possibly thousands of them per year, would have still left the vehicle cheaper than its contemporaries.

The company's baseline launcher design claimed to lift one metric ton to orbit.  It would have consisted of a third stage core of four CRPUs, surrounded by a second stage of twelve CRPUs, in turn surrounded by the first stage's 48 CRPUs.{{citation needed|date=November 2013}}  Larger vehicles and capacities would be achieved with greater numbers of CRPUs, possibly including several hundred per flight for a heavy launcher.

The company forecast that CRPUs would eventually be so cheap, recovering and refurbishing a launcher would be no better than simply building more units.{{citation needed|date=November 2013}}  The use of storable propellants and few moving parts meant that launch-site operations would also be very simple.  These advantages were expected to overcome the disadvantages of small [[specific impulse]] and [[payload fraction]].