Third X-51A Hypersonic Test Targeted For Mid-August http://www.boeing.com/defense-space/military/waverider/docs/X-51A_overview.pdf http://en.wikipedia.org/wiki/Boeing_X-51 The Boeing X-51 ( X-51 WaveRider ) is an unmanned scramjet demonstration aircraft for hypersonic ( Mach 6, approximately 4,000 miles per hour (6,400 km/h) at altitude) flight testing . It successfully completed its first free-flight on 26 May 2010 and also achieved