By Summer 1943, the Jumo 004A engine had passed several 100-hour tests, with a time between overhauls of 50 hours being achieved. However, the Jumo 004A engine proved unsuitable for full-scale production because of its considerable weight and its high utilization of strategic material (Ni, Co, Mo), which were in short supply. Consequently, the 004B engine was designed to use a minimum amount of strategic materials. All high heat-resistant metal parts, including the combustion chamber, were changed to mild steel (SAE 1010) and were protected only against oxidation by aluminum coating. The total engine represented a design compromise to minimize the use of strategic materials and to simplify manufacture. With the lower-quality steels used in the 004B, the engine required overhaul after just 25 hours for a metallurgical test on the turbine. If it passed the test, the engine was refitted for a further 10 hours of usage, but 35 hours marked the absolute limit for the turbine wheel. While BMW's and Junkers' axial compressor turbojet engines were characterised by a sophisticated design that could offer considerable advantage – also used in a generalized form for the contemporary American Westinghouse J30 turbojet – the lack of rare materials for the Jumo 004 design put it at a disadvantage compared to the "partly axial-flow" Power Jets W.2/700 turbojet engine which, despite its own largely centrifugal compressor-influenced design, provided (between an operating overhaul interval of 60–65 hours) an operational life span of 125 hours. Frank Whittle concludes in his final assessment over the two engines: "it was in the quality of high temperature materials that the difference between German and British engines was most marked"