The U.S. Air Force has released new details of the record-breaking hypersonic test flight conducted by the Boeing-built X-51A Waverider demonstrator on May 1. The diminutive scramjet-powered vehicle achieved a blistering Mach 5.1, covering 230 naut. miles in just over six minutes over the Point Mugu Naval Air Warfare Center Sea Range in the Pacific.
The success of the flight, which included 240-seconds of powered flight in scramjet mode – the longest ever achieved by an air-breather, is particularly important to the Air Force as it seeks to develop a hypersonic capability. Interest in high-speed, long range strike weapons which can fly at hypersonic speeds grew in the 2000s with the requirement to prosecute fleeting targets in counter-terrorism operations, and is assuming fresh importance with the strategic ‘pivot’ to Asia. Although the Air Force says there is no immediate successor to the X-51A program, it adds work on the technology demonstrator “will pay dividends to the High Speed Strike Weapon program currently in its early formation phase with AFRL.”
The May 1 flight was also the fourth, and final, chance to achieve the full mission targets of the $300 million X-51A effort which began in 2004 with the aim of proving the viability of air-breathing, high-speed scramjet propulsion. The first mission, two years ago, achieved partial success when it reached Mach 4.88 under scramjet power in May 2010, but both this mission and the second and third flights in March 2011 and August 2012, ended prematurely after various malfunctions.
The last of the four vehicles that was tested on May 1 incorporated improvements and lessons learned from the three former flights. These included better sealing between interfaces in the engine flow-path that are thought to have suffered “burn-through” on the first flight, allowing hot gases to penetrate the vehicle’s interior and prematurely ending the flight. Additionally, it incorporated hardware and software changes to counter issues thought to have brought the second flight to a premature end after only 9.5 sec. of powered flight at around Mach 5. On that flight the vehicle experienced an inlet un-start during the switch to hydrocarbon fuel, effectively blocking flow through the engine and shutting it down. Finally, the mission also included changes to the hypersonic cruiser’s control fins, one of which failed on the third mission, causing it to go out of control only 16 seconds onto the test while still under boost.
This week’s mission began when the X-51A was released at 10.55 am on May 1 from a U.S. Air Force B-52H flying at 50,000 feet. Following release, a solid rocket booster accelerated the vehicle to about Mach 4.8 before the booster and a connecting interstage were jettisoned after 26 seconds. The vehicle accelerated to Mach 5.1 powered by its Pratt & Whitney Rocketdyne SJX61 dual-mode ramjet/scramjet, which burned all its JP-7 jet fuel over 240 seconds. The X-51A then made a controlled dive into the Pacific at the conclusion of its mission.
Tucked under the B-52H's port wing, the X-51A is pictured prior to launch on May 1