The development of such a cutting-edge technology was a protracted process, which was aggravated by shortages of exotic metals required for the manufacture of some parts operating under extremely high temperatures and strong mechanical forces. German metallurgists managed to reduce their use to a minimum by developing special alloys, in which only small proportions of the rare metals were used. This technical compromise solved some problems but caused other technical problems, because these alloys were not as strong as they were supposed to be. Early jet engines thus suffered from low reliability. In autumn 1944 the average running life of the Jumo 004 engine was only 10 hours and pilots complained repeatedly about their poor reliability. The equivalent BMW 003 engine was even more problematic: in early November 1944 one of Heinkel’s test pilots reported an average of 7.4 failures per running hour with this engine after a visit to Arado’s ﬂight test base. Most of the failures were due to ﬁres and breaking turbine blades. It is interesting to note that besides offering dramatic increases in aircraft performance, early jet engines did not require special fuels and could operate on almost any type of fuel. It was reported that successful experiments were even carried out in operating the Jumo 004 jet engine with Rumanian crude oil.The main type of fuel used for their propulsion was J2, a diesel-type fuel. It was a low-octane fuel that required less reﬁning than the fuel used in normal piston engines. Therefore, jet propulsion also offered simpliﬁed fuel logistics.