At that speed it could travel from London to New York in about an hour. Results from the test flight are expected later on Wednesday. 

 

The Pentagon and Nasa hope to use the project to develop faster missiles.

 

During a test in June 2011, Waverider travelled at Mach 5, but failed to reach the target speed.

 

It is one of several projects currently under way to create a hypersonic aircraft.

 

The research could also be used to build a commercial plane, able to reach much higher speeds than today's jets, after Concorde was decommissioned（退役的） in 2003.

 

The military used a B-52 bomber¹ to lift the wingless unmanned jet from US Edwards Air Force Base in California to 50,000 ft (15,250m) between 1700 and 1800GMT.

 

After being dropped from the B-52, the engine of the X-51A Waverider was supposed to ignite（点燃） after a free fall of about four seconds.

 

The 8m long craft was then expected to climb to 70,000ft (21,300m) and eventually reach Mach 6. 

 

The results of the test flight would be available later on Wednesday, a military spokesman said in a statement.

 

The Mach number is the ratio of the speed of an object to the speed of sound. Mach 1 is the speed of sound - approximately 768mph, depending on various factors including temperature and altitude. 

 

So Mach 6 is six times the speed of sound. Concorde's cruising speed was Mach 2 - it flew from London to New York in just over three hours.

 

The Waverider test flight was expected to last for about five minutes before the craft fell into the Pacific.

 

The test is essentially² a repeat of last summer's attempt, when the hypersonic aircraft reached Mach 5, but the engine failed to attain³ full power.

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