HSSW meaning in Governmental ?

The Boeing X-51 Waverider is an unmanned research scramjet experimental aircraft for hypersonic flight at Mach 5 (3,300 mph; 5,300 km/h) and an altitude of 70,000 feet (21,000 m). The aircraft was designated X-51 in 2005. It completed its first powered hypersonic flight on 26 May 2010. After two unsuccessful test flights, the X-51 completed a flight of over six minutes and reached speeds of over Mach 5 for 210 seconds on 1 May 2013 for the longest duration powered hypersonic flight.

Waverider refers in general to aircraft that take advantage of compression lift produced by their own shock waves. The X-51 program was a cooperative effort by the United States Air Force, DARPA, NASA, Boeing, and Pratt & Whitney Rocketdyne. The program was managed by the Aerospace Systems Directorate within the U.S. Air Force Research Laboratory (AFRL).