Boeing X-51 Waverider

Lore & Background

The X-51 program originated from the Air Force Research Laboratory's HyTECH program in the 1990s, which led to the development of the SJX61 hydrocarbon-fueled scramjet engine by Pratt & Whitney. This engine was originally intended for the canceled NASA X-43C, then applied to the AFRL's Scramjet Engine Demonstrator program in late 2003. The flight test vehicle was designated X-51 on 27 September 2005. The program was a cooperative effort by the United States Air Force, DARPA, NASA, Boeing, and Pratt & Whitney Rocketdyne, managed by the Aerospace Systems Directorate within the AFRL.

In flight demonstrations, the X-51 was carried by a B-52 to about 50,000 ft and released over the Pacific Ocean. It was initially propelled by an MGM-140 ATACMS solid rocket booster to approximately Mach 4.5, after which the booster was jettisoned and the vehicle's SJY61 scramjet accelerated it to near Mach 6. The X-51 used JP-7 fuel, carrying 270 lb onboard. The term 'waverider' refers to aircraft that take advantage of compression lift produced by their own shock waves.

Ground tests began in late 2005, with wind tunnel tests at NASA Langley Research Center in July 2006 and a simulated Mach 5 flight in April 2007. The first captive flight on a B-52 occurred on 9 December 2009. The first powered flight on 26 May 2010 reached Mach 5 and 70,000 ft, flying for over 200 seconds, though not the planned 300 seconds. The second test flight on 13 June 2011 ended early due to an inlet unstart event. The third test flight on 14 August 2012 ended when the craft lost control after separating from its booster, due to an unlocked aerodynamic fin. The fourth and final test flight on 1 May 2013 was fully successful, with the X-51 accelerating to Mach 5.1, flying for 210 seconds under scramjet power, and totaling over six minutes of flight time.

Reader's Guide

The X-51 Waverider program demonstrated the longest air-breathing hypersonic flight to date, with its final flight on 1 May 2013 achieving over six minutes of flight and 210 seconds at speeds above Mach 5. This success marked the completion of the program and provided critical data for practical hypersonic applications. The Air Force Research Laboratory viewed the flight as research for future missiles, reconnaissance, transport, and air-breathing first stages for space systems.

DARPA once considered the X-51 a stepping stone to the Blackswift hypersonic demonstrator, which was canceled in October 2008. In May 2013, the U.S. Air Force planned to apply X-51 technology to the High Speed Strike Weapon (HSSW), a missile similar in size to the X-51, expected to fly in 2020 and enter service in the mid-2020s. The HSSW was to have a range of 500–600 nautical miles, fly at Mach 5–6, and fit on an F-35 or in the internal bay of a B-2 bomber.

The program's significance lies in its validation of scramjet propulsion for sustained hypersonic flight, overcoming earlier limitations such as the X-43's 12-second burn time. The X-51's longest hypersonic flight time of 140 seconds under scramjet power (on its first flight) and 210 seconds on its final flight set benchmarks for air-breathing hypersonic technology.

Did You Know?

• The X-51 completed its first powered hypersonic flight on 26 May 2010, reaching Mach 5 and 70,000 ft.
• The longest flight on 1 May 2013 lasted over six minutes, with 210 seconds at speeds above Mach 5.