The US Air Force has set 27 October as the first flight date for the Boeing X-51A Waverider, but there is already talk of expanding the rigidly controlled hypersonic test programme.
Boeing confirms it has submitted pricing and technical data requested by the Air Force Research Laboratory to increase the number of test flights from four to six.
While more flight tests would add a safety margin in case of failures, the AFRL is also eyeing additional test points, such as using the X-51A's global positioning system antenna to demonstrate waypoint guidance during the unpowered, terminal phase.
However, Charlie Brink, the X-51A programme manager, is reluctant to expand the requirements for the experiment, which senior USAF leaders have described as the military's most promising and ambitious hypersonic effort.
"We've got our requirements down very well," says Brink. He is especially wary of "requirements creep" adding to the complexity, cost and risk of the experiment.
Efforts are being made to build support to fund a follow-on programme with an X-51B demonstrator. Its case could be bolstered by showing the X-51A's ability to more closely mimic a real weapon, such as following GPS-guided waypoints to a target.
The AFRL has carefully crafted the four test vehicles to emphasise safety tolerances. The 680kg (1,500lb) hypersonic cruiser is designed with a safety factor of two, Brink says.
Boeing has relied on readily available materials for the vehicle, such as an aluminium frame and tungsten nose, instead of more exotic structures. The Lockheed Martin F-35 programme donated a surplus full authority digital engine control system for the Pratt & Whitney F119 engine, rather than assume the risk of integrating an all-new control system for a brief flight-test phase.
"I didn't want to have a scramjet experiment, and spend all the money fixing a structural problem," Brink says.
The stack test vehicle is scheduled to enter calibration loads testing in mid-April, says Joseph Vogel, Boeing's X-51A programme manager. The first flight test vehicle is scheduled to start electro-magnetic interference testing aboard the Boeing B-52H launch platform on the ground at Edwards AFB in California on 1 June, Vogel says. Captive carry tests over the Edwards flight-test range should begin in August, followed by a dress rehearsal, including a picket line of Lockheed P-3C Orions relaying telemetry data in September.
After the initial flight test, the AFRL plans to conduct the second flight test on 15 January, followed by the third in mid-February and the fourth in mid-March.