Traveling at speeds exceeding 3,800 mph, the X-51a Waverider, a joint project between the US Air Force and Boeing, could go from from New York to London in under an hour – if it doesn’t crash that is. A recent test of the scramjet-powered vehicle this summer proved unsuccessful, but new research may open the door to safe flight at hypersonic speeds, more than five times the speed of sound.
Scramjets achieve such high speeds because of their unique air-breathing design. As opposed to traditional jets, scramjets use air from the atmosphere to ignite their fuel, providing thrust. Because scramjets capitalize on rapidly-moving air to produce thrust, they don’t need heavy fuel tanks to fly.
While scramjets have had a few successful tests in the upper atmosphere, the most recent test failed due to a phenomenon known as unstart. If the airflow in an engine changes suddenly, a shockwave will propagate through the engine and cause it to overheat. Preventing unstart has become increasingly important to researchers as test flights have ramped up.
“There’s a very narrow band between having too little thrust and unstart, and you have to be in there,” said Johan Larsson, a mechanical engineer at Stanford University’s Center for Turbulence Research.
In late November, Larsson’s colleagues presented their ongoing research on unstart conditions at the American Physical Society’s Division of Fluid Dynamics Annual Meeting. The researchers focused their efforts on modeling the delicate balance between providing enough thrust to exceed Mach 5 while preventing catastrophic engine failure. The scientists found a way to model scramjets’ extreme sensitivity to small changes in qualities such as heat and air speed, which are significant at such high speeds.
When a scramjet is traveling more slowly, it can revover from small amounts of airflow change or turbulence. When a jet approaches hypersonic speed, however, small changes can cause the scramjet to become unstable and unstart. If this happens, the jet’s fate is typically sealed, and there is little hope for restarting the engine.
“With this model, we can develop a reduced order model to optimize the scramjet geometry and minimize the likelihood of unstart,” said Joseph Nichols, a scientist based at Stanford’s Center for Turbulence Research and co-presenter of the research.
Scientists don’t solely rely on computer models before test flights, however. Instead, computer models are often compared to wind tunnel tests, and these two areas of research tend to build off of each other. But a full understanding of unstart conditions is still being developed.
Although scramjets have been tested on the ground and more recently in the air, computer models of hypersonic flight are still in their infancy. Because there are so many physical properties involved, modeling scramjets can require enormous computing power. For example, Larsson runs about 1000 processors for several days just to complete one of his simulations.
“The real challenge in predicting unstart is having an accurate model,” said Chris Goyne, an aerospace engineer who specializes in scramjets, from the University of Virginia in Charlottesville.
For now, scientists are focusing on making their models as accurate as possible while limiting the amount of computer power needed. As models become more sophisticated, scientists hope that they can use the insights they develop to build scramjets that avoid unstart during flight. Before performing additional test flights, scientists want to take as much wind tunnel and modeling data as possible.
“Flight testing is expensive and high risk,” said Goyne. “It involves taking the lessons learned from the ground and applying them to the air.”
Another test flight of the X-51a Waverider was tentatively scheduled for this fall, but that flight may have been postponed.