http://www.grc.nasa.gov/WWW/RT/2004/...S-johnson.html

"Future advanced aircraft fueled by hydrogen are being developed to
use electric drive systems instead of gas turbine engines for
propulsion. Current conventional electric motor power densities cannot
match those of today’s gas turbine aircraft engines. However, if
significant technological advances could be made in high-power-density
motor development, the benefits of an electric propulsion system, such
as the reduction of harmful emissions, could be realized."

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Mark