the inlet provides most of the compression for ramjets, the high speeds at which scramjets operate allow them to take advantage of the compression generated by shock waves, primarily oblique shocks. Very few scramjet engines have ever been built and flown. In May 2010 the Boeing X-51 set the endurance record for the longest scramjet burn at over 200 seconds. P&W J58 Mach 3+ afterburning turbojet Turbojet operation over the complete flight envelope from zero to Mach 3+ requires features to allow the compressor to function properly at the high inlet temperatures beyond Mach 2.5 as well as at low flight speeds. The J58 compressor solution was to bleed airflow from the 4th compressor stage at speeds above about Mach 2. The bleed flow, 20% at Mach 3, was returned to the engine via 6 external tubes to cool the afterburner liner and primary nozzle as well as to provide extra air for combustion. The J58 engine was the only operational turbojet engine, being designed to operate continuously even at maximum afterburning, for Mach 3.2 cruise. An alternative solution is seen in a contemporary installation, which did not reach operational status, the Mach 3 GE YJ93/XB-70. It used a variable stator compressor. Yet another solution was specified in a proposal for a Mach 3 reconnaissance Phantom. This was pre-compressor cooling, albeit available for relatively short duration. Hydrogen-fuelled air-breathing jet engines Jet engines can be run on almost any fuel. Hydrogen is a highly desirable fuel, as, although the energy per mole is not unusually high, the molecule is very much lighter than other molecules. The energy per kg of hydrogen is twice that of more common fuels and this gives twice the specific impulse. In addition, jet engines running on hydrogen are quite easy to build—the first ever turbojet was run on hydrogen. Also, although not duct engines, hydrogen-fueled rocket engines have seen extensive use. However, in almost every other way, hydrogen is problematic. The downside of hydrogen is its density; in gaseous form the tanks are impractical for flight, but even in the form of liquid hydrogen it has a density one fourteenth that of water. It is also deeply cryogenic and requires very significant insulation that precludes it being stored in wings. The overall vehicle would end up being very large, and difficult for most airports to accommodate. Finally, pure hydrogen is not found in nature, and must