Twenty years ago, long before ‘biofuel’ became a marketing hook, the Soviet Union quietly flew for the first time a modified version of the Tupolev Tu-154B to demonstrate how aircraft could be powered with an alternative energy source.
Designated the Tu-155 the aircraft took to the air on 15 April 1988, its thrust partly generated using liquid hydrogen.
The tri-jet, registered SSSR-85035, was fitted with a thermally-insulated 17.5m3 (618ft3) fuel tank aft of the passenger cabin which contained liquid hydrogen at a temperature of minus 253°C. This tank provided fuel to a new engine, the Kuznetsov NK-88, mounted in the number three position on the starboard side.
Some 30 other systems were installed in the aircraft including pressurisation equipment, cryogenic pumps and injection mechanisms, and safety and monitoring systems to protect against leakage and possible explosion. Specialised rigs were built to refuel the aircraft on the ground.
Although liquid hydrogen was used for the first few flights, the focus quickly shifted to liquid natural gas, in line with the Soviet energy strategy at the time.
Liquid natural gas was stored in the Tu-155’s cryogenic tank at minus 162°C, and the aircraft performed its first flight with the fuel on 18 January 1989.
“During this time the deficit of traditional fossil fuels worsened,” says the Tupolev design bureau, adding that this spurred the research into cryogenic alternatives. “Use of liquid natural gas in aircraft significantly reduced adverse environmental impact.”
Tupolev subsequently proposed a further modification, the Tu-156, which was fitted with three cryogenic-fuel Kuznetsov NK-89 engines. Diagrams showed the aircraft fitted with around 70 seats in a layout heavily constrained by the presence of the rear fuel tank.
While the design bureau has since claimed to be pursuing several related cryogenic-fuel aircraft programmes – such as the Tu-204K, Tu-334K and twin-turboprop Tu-136 – nothing of note has emerged. The Tu-155, which performed around 100 flights, was eventually retired to the Zhukovsky airfield outside Moscow.