US Edwards Air Force Base in California - The American hypersonic jet WaveRider is to undergo another test flight above the Pacific Ocean aimed at reaching Mach 6.
At a speed of more than 4,300mph (6,900km/h), it could travel from London to New York in about an hour.
WaveRider is one of several projects currently under way to create an aeroplane able to reach much higher speeds than today's jets, after Concorde was decommissioned in 2003.
During a test in June 2011, WaveRider failed to reach the target speed.
A B-52 bomber will lift the wingless unmanned jet from US Edwards Air Force Base in California to 50,000 feet (15,250m).
The craft will then be dropped, and after a free fall of about four seconds, its engine is supposed to ignite.
X-51A WaveRider should then climb to 70,000ft (21,300m) and eventually reach Mach 6.
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 is expected to last for about five minutes. At the end of it, the aircraft will break into pieces and fall into the Pacific.
In August 2011, US military scientists attempted to get another unmanned hypersonic experimental aircraft to reach Mach 20 that is 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.