Beyond supersonic speeds lies the realm of the hypersonic--faster than Mach 5.0, or in somewhat more down-to-earth terms, in excess of just under 4,000 miles per hour. On Tuesday, the U.S. Air Force plans to venture into that realm with the test flight of a chisel-shaped aircraft called the X-51A Waverider.
The test won't last long. The sleek, 14-foot-long X-51A will zip through the air for only about five minutes, but in that time it could hit a speed of Mach 6, or six times the speed of sound. It'll be pushed to that blink-of-an-eye tempo by what's known as a scramjet engine, "scramjet" being the compression of the phrase "supersonic combustion ramjet." This kind of engine is designed to burn oxygen from the atmosphere, unlike conventional rocket engines whose functioning requires a combination of oxidizers and big fuel tanks.
Update, Wednesday 7:42 a.m. PDT: Tuesday's planned flight of the X-51A was postponed by a day because of freighter traffic in the Pacific Ocean test area.
Update, Thursday 12:26 p.m. PDT: The X-51A flew later Wednesday morning. To find out how well it performed, see "X-51A races to hypersonic record."
Tuesday's test, weather permitting, will be the first free flight for the X-51A. The scramjet aircraft has been aloft twice before, both times attached to the wing of a B-52. And that's how Tuesday's flight will begin, too. The B-52 will take off from Edwards Air Force Base in California, then head out to a Pacific ocean test range where the Waverider will be released at approximately 50,000 feet. A solid rocket booster on the X-51A will accelerate the test vehicle to about Mach 4.5, at which point the scramjet engine will kick in to push the vehicle toward 70,000 feet and Mach 6, according to the Air Force. At the end of its five-minute autonomous flight, the X-51A will head for a planned splashdown and crackup--the Air Force does not plan to recover the vehicle.
"In those 300 seconds, we hope to learn more about hypersonic flight with a practical scramjet engine than all previous flight tests combined," said Charlie Brink, X-51A program manager with the Air Force Research Laboratory's Propulsion Directorate, in a statement last week.
The Air Force says this will be the only hypersonic flight this year, largely for budgetary reasons but also to allow for maintenance to the Air Force Flight Test Center's B-52.
This image from December shows the X-51A in the air attached to the B-52, marking the first of its two captive flights. That trip was to check out the communications and telemetry systems, as well as the B-52's handling with the Waverider aboard.
In its autonomous flight Tuesday, the X-51A is expected to transmit "vast amounts of data" to ground stations. A U.S. Navy P-3 Orion aircraft also will relay telemetry data to people on the ground.
Expectations are that the X-51A program could lead both to hypersonic weapons--think of a cruise missile, but faster--and to cheaper space planes as eventual follow-ons to the soon-to-be-retired space shuttles.
The Rocketdyne SJY61 scramjet engine has virtually no moving parts, while the exterior has minimal wings. "Hypersonic combustion generates intense heat," goes part of the Air Force's explanation of the technology, "so routing of the engine's own JP-7 fuel will serve to both cool the engine and heat the fuel to optimum operating temperature for combustion."
Air enters the engine through a scoop at the front of the X-51A. "It is critical for air entering the inlet to be turbulent at hypersonic speeds, or the engine could 'unstart,' causing it to crash," according to a 2008 news release from the Purdue University School of Aeronautics and Astronautics, which has assisted in research for the X-51A program. "For this reason, air has to be converted to a turbulent flow before entering the inlet. This conversion is accomplished using a raised strip of metal placed near the inlet to 'trip' the air from smooth to turbulent."
Meanwhile, airflow over the top of the Waverider has to be smooth to minimize damaging friction and heat.