The US Air Force plans to add up to six more test flights of Boeing's expendable X-51A Waverider hypersonic scramjet vehicle, in preparation for an operational missile programme.

The rapid identification and prosecution of targets in denied areas, or Riptide, programme aims to deliver a capability to intercept intercontinental missiles in their terminal flight phase.

Boeing is currently under contract to perform four X-51A flights, but the USAF will fund between another two and six tests under an "X-51A Plus" programme to start in 2011. The additional vehicles will have propulsion improvements, waypoint navigation and an ability to hit a precise location.

Boeing X-51A Waverider
 © Mike Cassidy/USAF
A USAF Boeing B-52H bomber carries Boeing's X-51A Waverider hypersonic scramjet

The first air launch of an X-51A from a Boeing B-52H bomber is expected "in the next few weeks", says Boeing, with Mach 6.0 "a target". The original four tests have been planned to demonstrate acceleration from M4.0 to M6.0 during 300s of flight.

The first flight event has slipped from an original target of August 2009, with a delayed captive carry test having been conducted with a B-52 only last December.

A first flight during May would make the $250 million programme two years behind schedule. Boeing advanced space exploration director Steve Johnston says some suppliers had "manufacturability" issues with some parts for the X-51A, and that those delays combined with limited access to the carrier aircraft.

Tests could follow at four- to six-week intervals if the first flight is successful, with vehicles three and four also to receive enhancements including waypoint navigation.

Source: Flight International