Then opportunity came knocking, in the form of a Florida-based company called Astrotech Space Operations. The company had no interest in scramjets per se; it merely wanted to expand its sounding rocket business (selling cargo space for microgravity science experiments) into the Asia-Pacific region. What better way to make a public relations splash than by carrying “some sexy payload,” in Paull’s words, on the two demonstration flights the company had planned? An intermediary made the introductions, and in 1998 the two parties signed an agreement. Astrotech would provide the Terrier-Orion rockets for two launches; Paull would equip them with a cutting-edge scramjet experiment. The HyShot program was born.
Now the pressure was on: “We had to figure out how to make the engine fly and not fall apart” in a test time window hundreds of times longer than the one available in wind tunnels, Paull says.
Space programs from around the globe have tried to tackle the same perplexing dilemma for decades. The United States and Russia, in particular, have invested millions. What’s the carrot motivating their research? First and foremost, they hope one day to use scramjets as a cost-effective rocket replacement in space launch vehicles. Military planners want to add hypersonic missiles to their arsenals. On the commercial end of things, a scramjet-powered passenger airplane could, in theory, reduce travel time, allowing you to fly from, say, London to Sydney in two hours.
Indeed, eyeing such payoffs, the U.S. government began funding scramjet research in the 1960s; it now sponsors some half a dozen scramjet programs in the Department of Defense and NASA (see “A Matter of Seconds,” p. 76). Since 1994, NASA has worked with the much-lauded Russian program, which has launched some of the most successful tests to date. Today, half a dozen other countries have substantive programs as well.
Yet for half a century the scramjet has remained (excuse the pun) a pipe dream. Putting the simple theory into practice is fraught with engineering challenges. To begin with, there is the difficulty of igniting fuel with air that is traveling at supersonic speed. “It is like lighting a match in the middle of a blowing hurricane,” says Robert Mercier, head of HyTech, the U.S. Air Force’s scramjet program, located at Wright-Patterson Air Force Base in Ohio.
Then there is the heat issue: A vehicle traveling that fast can reach a temperature of 3,600 degrees Fahrenheit—similar to what the Apollo capsule experienced on reentry, and hot enough to warp, if not melt, most materials. (Some researchers are experimenting with heat-absorbing fuels that, prior to combustion, would circulate through channels in the engine walls to cool them down.) Also, to work effectively, scramjets must be integrated with the airframe; how best to do this remains a question.
The most advanced wind tunnels can accelerate a scramjet model to the required speeds and temperatures for only a few milliseconds. But actual flight tests are expensive, logistically challenging, and considerably vulnerable to things going wrong.
Just ask NASA. In June 2001, the agency hoped to record the first scramjet-powered hypersonic flight in a much-ballyhooed trial (the first of three in its Hyper-X program) off the California coast. But the rocket booster malfunctioned and, before the scramjet could be released to allow the real experiment to begin, propelled itself and its payload straight into the Pacific Ocean.
Because different scramjet teams have focused on different pieces of the research puzzle and pursued slightly different near-term goals, the question of who can rightfully claim some nominal “first” usually hinges on semantics and nuances; “It’s a bit fuzzy,” Paull says. What is beyond dispute is that until July 30 no one had demonstrated purely supersonic combustion in a scramjet hurtling unaided through the atmosphere.
The scramjet rode as the payload on a Terrier-Orion sounding rocket, which flew into space in a parabolic arc. The scramjet separated from the booster on the upward trajectory, rotated at its apogee toward Earth, and eventually operated for five seconds (at a speed of almost Mach 8) before it hit the ground. Attached instruments measured various parameters and transmitted a stream of data that researchers can now use to calibrate their design, analysis, and test tools to real flight conditions.