A Matter of Seconds
Before HyShot’s July 30 flight added another five seconds to the tally, the few minutes of scramjet flight data that had been gathered had all come from Moscow’s Central Institute of Aviation Motors. The Russian bureau conducted experiments in the 1990s, giving three scramjets rides on the noses of rocket-powered SA-5 anti-aircraft missiles. On the last launch, supported by NASA in 1998, the scramjet operated for 77 seconds with internal flows that were subsonic during parts of the flight.
Aerodynamicists have used supercomputers to simulate the airflow within scramjets, but combustion chemistry in turbulent flow has proved too complex, even for the most powerful computers. Flight tests produce far more reliable data—when they work. Had NASA’s Hyper-X program succeeded last June with the launch of the X-43A, it would have added 12 seconds to the total. (The X-43A also hitched a ride on a rocket, an Orbital Sciences Pegasus, launched from the wing of a B-52 from NASA’s Dryden Research Center in California.) Unlike the Australian and Russian engines, the X-43’s scramjet is integrated with the vehicle’s airframe. The 12-foot-long, 2,200-pound aircraft will separate from its rocket booster and fly alone through the atmosphere. The scramjet will ignite for the 12-second experment after the booster has accelerated the vehicle to Mach 7. The next launch attempt, according to program manager Lawrence Huebner, will take place in summer 2003.
In an independent effort, the U.S. Air Force is broadening the range of scramjet fuels, with an eye toward using the engines in operational missiles. Scramjets built to date, including those for the X-43A, have used hydrogen. It mixes and burns readily in an engine, but it is difficult to store and handle. The Air Force program, called HyTech, is relying on JP-7, a hydrocarbon fuel in use at most military bases.
Scramjet fuels must mix and burn far more rapidly than JP-7 can in ordinary circumstances. But the HyTech effort “cracks” the fuel’s molecules, breaking them into fragments that mix and burn more easily. In the laboratory, the cracking is accomplished by heating the fuel in a separate installation, but engineers believe that in flight the heat from the engine can break down the JP-7 molecules. The fuel, circulating as a coolant through the walls of an operating scramjet, will absorb heat and crack before it is injected into the combustor.
In the spring of 2000, and again in November of that year, a scramjet that had been built by Pratt & Whitney and that used cracked JP-7 as fuel produced a significant, though classified, amount of thrust in a wind tunnel. That version, built of heavy copper, weighed about 2,000 pounds and had no cooling. But a follow-on test engine using flight-weight components and weighing less than 200 pounds is now being tested with JP-7 as both the fuel and the coolant.
Program manager Robert Mercier expects to begin testing a third engine, this one a complete flight-ready system incorporating fuel pumps and valves, in 2003. In the meantime, NASA, a partner in the HyTech project, plans to develop an advanced X-43 to test the HyTech engine in flight in 2006 or 2007. The mission goal will be to accelerate under scramjet thrust from Mach 5 to Mach 7.
But first the next X-43A, which already is being prepared at the Dryden center, must collect its 12 seconds of scramjet flight data. Lawrence Huebner points out that although 12 seconds seems very short, it was the duration of the Wright brothers’ first powered flight in 1903.