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Aviation History
2003
2003 - 0247.PDF
TECHNOLOGY PROPULSION GUY NORRIS / LOS ANGELES Mixed results for pulse detonation US Air Force technology promises to improve efficiency for high-speed aircraft, but big hurdles remain As part of ongoing research efforts to develop a practical pulse detona tion engine (PDE), the US Air Force Research Laboratory at Wright- Patterson AFB, Ohio, has revealed mixed results from tests of a deto nation-driven turbine. The study, performed by the labo ratory's propulsion directorate, is one of several under way to prove the viability of the PDE concept for future propulsion needs ranging from missiles and boosters to high speed aircraft and reusable launch vehicles. Although the basic PDE cycle promises simplicity and effi ciency, it faces several hurdles, some of the most major being the ability to aspirate the PDE at subsonic speeds without significantly decreas ing performance, and to extract auxiliary power for accessories. PDEs are air-breathing jet engines with no moving parts and the potential to operate from sta tionary to Mach 4. Combustion takes place in an open-ended tube in which fuel is mixed with air and detonated. As the detonation wave travels down the tube at supersonic speed, it draws in fresh fuel and air and the cycle is repeated. For aspiration and power extrac tion the USAF has been testing the ability of a compressor and turbine to stand up to the harsh pulsing flow of a PDE. The latest tests were a follow-on to earlier experiments in which a coupled PDE-turbocharger demonstrated shaft power extrac tion and self-aspiration. For this series a Garrett T3 auto motive turbocharger was attached to a 0.9m (3ft)-long detonation tube. The turbine was spun to over 130,000RPM and evaluated at "vir tually all" significant operating conditions, says the study. Results of several test runs were "surprisingly different" to theoret ical predictions, says the USAF. A University of Cambridge project aimed at enabling mechanical sys tems to "learn" to adapt to damage in real time could improve air safety by helping to prevent acci dents such as the 1992 loss of an El Al Boeing 747-200 Freighter in Amsterdam, say UK researchers. Model Predictive Control, a fault-tolerant control system being developed by the university's engi neering department, aims to restore as far as possible a system's normal behaviour by using the remaining functioning controls, by comparing actual behaviour to that of a reference model. Speaking to the UK Institute of Measurement and Control, Jan Maciejowski, head of the control group at Cambridge, said the controller was tested on models representing El Al flight 1862, which crashed after the number three and four engines detached from the aircraft. The crew managed to keep the 747 flying for about 8min, but lost control at low speed after deploying high-lift devices, and crashed into an apartment block. "Delft University had very detailed models of the aircraft's "Even accounting for the 60-75% efficiency of the turbine/compressor in the total work, the overall effi ciency of the detonation blowdown drops by a factor of around four when driving a turbine as opposed to making pure thrust." In addition, significant back-pressurisation of the detonation tube was seen. The results were not all negative, however, and the USAF says the tur bine stood up well and was still functioning after more than 50,000 detonations. Saying the result shows "cause for optimism", it adds that no visible pitting or discolouration was seen on the unit. behaviour before and after the engine separation," says Macie jowski. "Using simulated pilot con trol inputs, and by comparing the reference aircraft model's expected behaviour to that observed in the damaged model, the controller attempted to track that behaviour, to some extent hiding the effects of the damage from the pilot," says Maciejowski. The key to adapting the system - which is already used in the petro chemical industry - to aircraft applications is to speed up the con troller's learning phase. "An adaptation like that required for the flight 1862 sce nario would need to be made within a couple of minutes," Maciejowski says, "but this should be possible with today's computing power." While primary uses for the tech nique are still in other industries, the group is hopeful that the avia tion safety community will show interest, leading to real applica tions. Maciejowski says the tech nology may be of particular inter est in autonomous unmanned air vehicles, where controllers often struggle to cope with failures. The SR-71 's Mach 3 turbojet will provide a reference PROPULSION R-R and US university to look at cool turbine technology Rolls-Royce is to work with Purdue University in Indiana, USA, to research propulsion technologies for future aircraft capable of speeds of up to Mach 7. The R-R University Technology Centre (UTC) will study the use of fuel to cool engine turbomachinery and the effects of fatigue on engine life. Stephen Heister, professor at Purdue's School of Aeronautics and Astronautics, says that using fuel as a heat sink requires detailed knowl edge of the high-temperature properties of jet fuel. "We hope to use fuel to cool the air going into the turbine, as well as for turbine blade cooling," says Heister. "But we need to understand the fuel's physical and chemical properties in high temperature and pressure environments," he adds. Heister says R-R has already demonstrated fuel properties at tempera tures up to 200°C (400T), but Purdue now wants to investigate properties up to 430°C. "Potential problems include chemical changes in the fuel affecting its behaviour in the combustor, and fuel coking causing carbon deposits on fuel line walls, which could impede heat transfer and lead to burning of components," says Heister. Researchers will also study suit able fuel injectors for the combustion of high-pressure supercritical fuel. Heister says the Pratt & Whitney J58 turbojet, developed for the Lock heed SR-71 Blackbird, is being used as reference technology for Purdue's high-Mach engines, for aircraft with speeds of up to 4,300kt (8,000km/h). SAFETY MICHAEL PHELAN / LONDON Fault-tolerant system could prevent crashes 32 4-10 FEBRUARY 2003 FLIGHT INTERNATIONAL www.flightinternational.com
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