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Aviation History
1989
1989 - 0410.PDF
Soviet designer Tupolev describes hydrogen fuel safety measures Hydrogen-fuelled aircraft flies The Tupolev Tu-155, a modified Tu-154 fuelled 1> i ige-:, fi*r* flew on April 15. These photovrr \i, :,-» .„.•- • • - Above From Flight, May 7, 1988. Below The modified NK 88 turbofan Fundamentally new design and operating procedures have had to be adopted to enable the experimental Tupolev Tu-155 airliner to use volatile, highly explosive liquid hydrogen fuel, which has to be carried under pressure at extremely low (cryo genic) temperatures, says the Soviet design bureau. General designer Alexei Tupolev tells Flight that exten sive ground-rig tests preceded last year's flight trials of the highly modified Tu-154 trijet. The aircraft has a hydrogen- fuelled Kuznetsov NK88 turbofan in its starboard nacelle. Leak peril Tupolev describes the research programme as the "practical beginning of cryogenic avia tion". He says that, in contrast to conventional kerosene fuel systems, any leak or depres- surisation results in almost instantaneous vaporisation of the liquid hydrogen, which rapidly fills the entire compart ment and ignites from the minutest spark, even at extremely low vapour concen trations. This radically changes the concept of inflammability and fire safety, says Tupolev, because all wiring and electrical devices aboard an aircraft become potential ignition sources. Tupolev has therefore "contained" the hydrogen fuel tank, the fuel-feed control system, and the engine feed pipes between bulkheads in the Tu-155 fuselage. The compart ment formed between the bulk heads is then pressurised to a higher level than the vapour pressure within the fuel tank. "Spark arresting equipment" is fitted in the buffer compart ment, while information systems provide the crew with "timely warning" of any hydro gen vapour escaping into it, "and thus prevent fatal accidents", according to Tupolev. The starboard nacelle is also "contained", and has spark arresting equipment installed. The engine fuel- control systems and feed pipes are mounted within a stainless- steel "hydrogen gadget container" installed in the upper section of the nacelle. Flight and emergency vent pipes exit above the tailplane, while the hydrogen jettison pipe leads out through the aircraft skin and down to the centre engine nozzle, where it ignites like gas from the flare stack of an oil rig. As a fire precaution, all of the aircraft's hydraulic pipes, kerosene fuel lines, and elec trical wire bundles are laid beneath covers on the exterior skin of the fuselage, and are ventilated by the airflow during flight. They are covered with sealant where they pass through the skin. Alexei Tupolev asserts that liquid hydrogen has a calorific value three times that of kerosene, and that it can soon be produced in sufficient quan tities for aviation purposes at acceptable cost. He says that the difficulties of using hydrogen as aircraft fuel made it "highly advisable" to conduct a "practical verification" with the Tu-155. The flight trials enabled systems failures to be simulated, and intensive manoeuvres to be undertaken at a wide range of aircraft speeds and altitudes. Besides the fire problem, the biggest difficulty is the necessity to work at cryogenic tempera tures. Kerosene requires no thermal insulation of tanks or pipes, and neither congeals nor boils under normal circum stances. Because liquid hydro gen boils at -253°C, it needs special insulation. Despite this, the fluid is permanently boiling aboard the aircraft, so it is also necessary to maintain a high pressure in the tank to minimise vapour losses through the vents at high altitude. Fuel feed The Tupolev design team has not attemped to feed hydro gen to the engine in liquid form because vaporisation in the fuel pipes makes it extremely difficult to ensure stable flow and regulate com bustion. Instead, the fuel is deliberately vaporised by raising its temperature to -170°C in a heat exchanger before it is fed to the injectors. Because liquid hydrogen is only a quarter as dense as kerosene, it is practically impos sible to accommodate it in the wings: "hence the need to develop light but strong fuse lage tanks", according to Tupolev. He says that the rear- fuselage installation used in the Tu-155 proved "simple enough" because the experi menters chose an aircraft with a large cabin, which was required to carry sufficient fuel only for sorties of up to lj hr. Before the Tu-155 flew, the hydrogen tank, the fuel-feed system, and the engine were tested on a rig mounted inside a giant cryostat. A second "sophisticated" rig checks out the fuel-tank pressurisation. 16 FLIGHT INTERNATIONAL, 18 February 1989
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