先前最长的冲压式喷气发动机下极超音速飞行测试，是在2004年由NASA的X-43进行的，速度更快，但只持续了10s。
The longest previous hypersonic scramjet flight test, performed by a NASA X-43 in 2004, was faster, but lasted only about 10 seconds and used less logistically supportable hydrogen fuel.
5月25日的尝试将会是本财政年内唯一的一次极超音速飞行测试，按照原计划，2010年将有三次测试，但计划有所改变。
The May 25 attempt will be the only hypersonic flight attempt this fiscal year, a change from the original test plan, which was to fly once in December 2009 and three more times in 2010.
的极超音速X - 43A条飞行速度更快，但X - 51A的飞行时间。
The hypersonic X-43A flew faster, but the X-51A flew longer.
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