EADS is hoping a newly approved joint project with Russian researchers will help pave the way to realising an air-breathing or rocket-propulsion technology that promises efficient operations from subsonic to hypersonic speeds up to Mach 5. But while detonation engines are tantalisingly simple, essentially comprising a tube closed at one end in which fuel and oxidiser are ignited to create a jet blast out of the open end, their success is to date mostly confined to laboratory testbeds.
A €1.7 million grant from Russia's Skolkovo Foundation, to be matched by EADS, is one of the first fruits of a collaboration deal sealed last year between EADS and a nascent science and technology city in the Moscow suburb of Skolkovo. The plan is to evaluate routes towards achieving a continuous detonation wave engine, and apply for further grants if early work is successful.
A detonation engine is a type of propulsion system that ignites the fuel and oxidiser mixture with supersonic detonation waves, which might be generated by the subsonic shock waves from an ignition process as used in a normal engine combustion chamber. As EADS chief technology officer Jean Botti describes them, detonation engines are believed to have a thermodynamic efficiency higher than other designs such as turbojets and turbofans because a detonation wave rapidly compresses the mixture and adds heat at constant volume.
However, while patents for detonation engines go back to at least 1960 and there has been considerable interest in them during the past decade, practical issues associated with initiating and sustaining detonation have kept them largely in the laboratory. Moreover, the continuous detonation engine (CDE) to be pursued by EADS and colleagues at Russia's Lavrentiev Institute of Hydrodynamics is a more daunting prospect.
EADS and Lavrentiev have done some preliminary theoretical and experimental work, though, and Botti says that, compared to a pulsed detonation engine (PDE), the continuous design allows an easier operation in a reduced-pressure environment and an increase in engine mass flow rate and thrust-to-weight ratio.
Botti is not alone in his optimism. The US military's Defence Advanced Research Projects Agency (DARPA), in its fiscal 2011 budget request, notes that "considerable progress has been made and the technology is believed mature enough to enable a dramatic new system capability". DARPA envisions a hybrid engine, combining either a pulsed or continuous detonation unit with a conventional gas turbine to achieve speeds in excess of Mach 4.
To date, however, it is pulsed engines that have made the running, and in early 2008 an aircraft powered by a US Air Force Research Laboratory-developed experimental PDE even got off the ground for "tens of seconds". The sortie by a modified Scaled Composites Long-EZ along the runway at Mojave, California, showed that the aircraft and its pilot could survive the noise and vibration of the PDE, which had four tubes and a peak thrust of about 200lb (0.9kN).
According to the aerodynamics research centre at the University of Texas at Arlington, PDEs offer numerous advantages over traditional jet engines.
PDEs do not need the heavy rotors and compressors of a jet engine, and in principle can be operated from standstill, whereas jets need to be spooled up to build enough pressure for combustion. Also attractive is the simple layout of a PDE: a tube and control valves for fluid entry and exhaust. A PDE's theoretical efficiency comes from the higher pressures created by detonation.
UTA sums it up: "PDEs bridge the gap between the subsonic regime and the hypersonic regime, when scram jets and rockets take over."
However, it continues, it is difficult to achieve consistent detonations, or even to achieve detonation at all. There are competing concepts in combustion chamber architecture and much work to be done to achieve a practical engine - and CDEs, barely beyond the concept stage, are even more difficult.