Hypersonic jet Waverider eyes Mach 6
The US military has conducted a test flight of its hypersonic jet Waverider, aiming to reach Mach 6 (3,600mph; 5795km/h) above the Pacific Ocean.
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 in 2003.
The military used a B-52 bomber 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 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 essentially a repeat of last summer's attempt, when the hypersonic aircraft reached Mach 5, but the engine failed to attain full power.
European aerospace and defence giant EADS believes that hypersonic passenger flights are likely to appear in the near future. In 2011, EADS presented its own concept of a commercial high-speed aircraft designed to fly at Mach 4.
"The business community who wanted to be in New York in three hours made Concorde highly viable, and now there's interest on both sides of the Atlantic to jump a generation and go from supersonic flight to hypersonic flight," EADS' vice-president of business development, Peter Robbie, told the BBC.
"Such an aircraft will be very expensive, of course, because of the enormous amounts of energy that is required to get to such speeds.
"But the idea of going from Tokyo to Paris in two-and-a-half hours is very attractive for the business and political community - and I think that by about 2050, there may be a viable commercial aircraft."
In August 2011, US military scientists attempted to get another unmanned hypersonic experimental aircraft to reach Mach 20 - 20 times the speed of sound.
However, they lost contact with the Falcon Hypersonic Test Vehicle 2 (HTV-2) after it had separated from its rocket.