The US Air Force is to conduct a key test of an experimental aircraft designed to fly at six times the speed of sound - about 3,600mph.
The unmanned X-51 WaveRider is expected to reach Mach 6 after being dropped by a B-52 bomber and taking flight off the Southern California coast near Point Mugu.
Engineers hope the X-51 will sustain its top speed for five minutes, twice as long as it had gone before.
Last year, in its most recent test, the X-51 fell for about four seconds before its booster rocket ignited, but the aircraft failed to separate from the rocket and plunged into the ocean.
Designed by Boeing, the aircraft is intended to allow the Pentagon to deliver strikes around the world within minutes.