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The  Onera micro turbine

French aerospace agency ONERA has tested a tiny combustion chamber for use in a prototype micro gas turbine that could be built by 2010 for use in a miniature Unmanned Aircraft.

ONERA develops micro-UAV gas turbine engine

By Rob Coppinger

DATE: 8 February 2008

Tested at ONERA's Laerte laboratory in Palaiseau, near Paris, the combustion chamber has a volume smaller than 10mm³ and is intended for a micro-turbine that is 20mm in diameter by 20mm high. This will power a micro-UAV with up to a 20 cm wingspan, says ONERA.

Air and fuel, either hydrogen or propane, are injected separately into the micro-turbine, which produces mechanical energy that is then converted into electricity. "The power required is between 50 and 100W [for] an expected UAV endurance of 30min," says ONERA.

The agency has concluded that fuel cells of sufficent power density will not exist for some time and that batteries will always be too heavy.

Hydrogen is used as a fuel because its chemical reaction occurs in 50µs, a tenth of the time for normal hydrocarbons, ensuring that ignition occurs before the gas exits the tiny combustion chamber. Another major challenge is the thermal losses caused by the chamber's relatively large surface area, which can extinquish combustion.

The next step is to study the rotation of a centripetal silicium turbine wheel 8 mm in diameter with paddles 400 µm in height. These would be manufactured using either silicon etching or micro-machining. The work will investigate the ability of the micro-turbine to attain rotation speeds of several hundred thousand revolutions per minute. The target speed is 1 million rpm. With such high rotation speeds, tiny hydrodynamic bearings will be used so their gases can keep the rotating surfaces from catastrophically touching.

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