X-51A Scramjet Flight Good, Not Perfect (With Video)

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The Air Force's scramjet, the Boeing X-51A Waverider, set a new world record, Wednesday, when it ran for 200 seconds, burned JP-7, and reached a speed of about Mach 5 (roughly 1 mile per second). In 2004, NASA's X-43 flew much faster (Mach 9.7) but burned hydrogen and managed that for about 12 seconds before it melted. Early data show the unmanned X-51A suffered from some temperature problems of its own, though not as severe. It had been scheduled for a 300-second run topping out at Mach 6 near 70,000 feet. But temperatures at the rear of the engine bay grew higher than expected. After the vehicle reached roughly Mach 5, it began to slow down for reasons yet to be determined and the vehicle was destroyed by its operators.

In theory, the X-51A scramjet engine should be able to operate indefinitely -- so long as there is an indefinite supply of fuel. The engine has no moving parts but must be accelerated to very high speeds to work. In the case of the X-51A, it was launched from a B-52 mother ship at about 0.8 Mach and 50,000 feet. A booster then accelerated the vehicle to about Mach 5 where ethylene was used to ignite the scramjet before it transitioned to JP-7. A scramjet uses fuel to cool the engine which in turn heats the fuel that ignites when injected into the engine's supersonic flow. Ramjets operate similarly, but the airflow through the engine is subsonic. Aside from its longevity, the X-51A scramjet's flight still operated in the Mach 3 to Mach 6 regime shared by ramjet engines. Find more information in the military's release, here.