The successes of the P-47N and P-51 gave the impression that the escort fighter was a concept worth continuing after the end of the war. The high fuel use of early jet engines made such aircraft difficult to design, and a number of experimental designs were tried that used mixed power, typically a turboprop and jet, but these failed to meet performance requirements. A new concept, the XF-85 Goblin microfighter, planned to act as a parasite fighter for the Convair B-36, was tested with a B-29 Superfortress and found to be utterly impossible to use operationally. Later the FICON project attempted a similar solution, docking jet fighters with heavy bombers via a trapeze mechanism or their wingtips.