The X-51A WaveRider maintained its speed for more than 3 minutes. The X-43A reached the speed of Mach 9.8 in 2004. However, that top speed lasted for only 10 seconds.
“The X-51 launched at about 10 a.m. from here, carried under the left wing of an Air Force Flight Test Center B-52 Stratofortress. Then, flying at 50,000 feet over the Point Mugu Naval Air Warfare Center Sea Range, it was released. Four seconds later an Army Tactical Missile solid rocket booster accelerated the X-51 to about Mach 4.8 mach before it and a connecting interstage were jettisoned. The launch and separation were normal”, Mr. Brink said. (www.af.mil courtesy)