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 economies of scale gained by producing hundreds or possibly thousands of them per year, would have still left the vehicle cheaper than its contemporaries.