Researchers are developing a new hypersonic engine based on detonations, which are explosions that send waves at rates faster than the speed of sound, with the ultimate aim of producing a propulsion system for the interplanetary planes and rockets of the future that harnesses the same type of energy emitted by supernovas.
With assistance from the U.S. Naval Research Laboratory, University of Central Florida engineers have constructed and tested a small-scale version of a detonation propulsion system powered by the continuous detonation of ultrahigh-purity hydrogen and air, as described in a paper published Monday in the Proceedings of the National Academy of Sciences. The project has received funding from the Air Force Office of Scientific Research, the National Science Foundation and NASA.
Because of the high-velocity shockwaves they produce, detonations are known to be unpredictable and chaotic. They are usually the byproduct of other phenomena, such as a fighter jet breaking the speed of sound or a dynamite explosion delivering a powerful shock wave to its surroundings. But researchers recognized that harnessing detonation energy in a controlled manner could provide a more powerful and efficient propulsion mechanism for planes and rockets than what is currently possible.
"The energy released out of this could actually propel an aircraft anywhere from Mach 6 to about Mach 17" — from six to 17 times the speed of sound — Kareem A. Ahmed, an associate professor at the University of Central Florida and the paper's corresponding author, told The Academic Times. "It has been shown numerically before, but here, we actually show it experimentally."
The team's design directs detonation waves across an angled surface, allowing for a more stable system. The process involves working against a detonation's natural tendency to violently spread away from the source of the explosion. "There's an explosion, and it wants to propagate upwards," Ahmed said. "But we want to feed it in with the right mixture and the right flow conditions to keep it held steady."
Previous experimental versions of detonation-powered engines have used controlled detonation pulses as power, but the University of Central Florida and NRL researchers are exploring an alternative method that enables continuous detonation for a more sustained power supply. The method is currently being studied on a small scale: the apparatus that feeds and detonates the hydrogen and air is only around two by two inches in size, according to Ahmed.
The group was inspired by type Ia supernovas, which are believed to explode at rates above the speed of sound, therefore qualifying as detonations. Most jets take air from their surroundings and heat it internally by burning fuel, with the ensuing combustion propelling the jet through the sky. But that combustion spreads outward at speeds below the speed of sound. Detonation, meanwhile, creates a wave of energy that exceeds the speed of sound, meaning it could potentially power faster airplanes and rockets. Other experimental propulsion technologies include the use of a nuclear reactor to thrust a spacecraft forward.
The fastest official speed for a manned air-breathing jet was reached in 1976, when a Lockheed SR-71 Blackbird reached 2,193 mph, or 3.3 times the speed of sound, at its high-flying altitude. Rocket-powered jets have reached hypersonic speeds — over five times the speed of sound — with one manned experimental aircraft reaching a blistering 4,520 mph over the Mojave Desert in 1967. Unmanned aircraft and rockets have gone much faster; an experimental NASA X-43A aircraft reached Mach 9.6 in 2005.
Humans are capable of enduring even higher speeds. In 1968, for example, the Apollo 10 spacecraft reached 24,816 mph during reentry into the atmosphere. But that kind of velocity cannot yet be sustained in a safe, controlled and comfortable environment.
The University of Central Florida engineers think that their system could one day provide a highly precise flow of energy that would make hypersonic travel more feasible. The researchers hope to eventually test their detonation system on a larger scale, first on the ground and then in a test flight, with the aim of eventual commercial use in terrestrial or space trips. But first, they say they will continue to adjust different flight parameters in an experimental setting, simulating how their system would react when a plane is climbing, descending or landing on a runway.
"This was just a discovery," Ahmed said. "Now that we were able to hold it, we want to understand a little bit more. And by understanding it, then you could actually start to tailor it further, for different conditions."
The study "Standardized detonation for hypersonic propulsion," published May 10 in the Proceedings of the National Academy of Sciences, was authored by Daniel A. Rosato, Mason Thornton and Kareem A. Ahmed, University of Central Florida; and Jonathan Sosa, Christian Bachman and Gabriel B. Goodwin, U.S. Naval Research Laboratory.