Re: hydrogen powered planes

From:         ehahn@bass.mitre.org (Ed Hahn)
Organization: The MITRE Corporation, McLean, Va.
Date:         21 Nov 95 01:19:23 
References:   1
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In article <airliners.1995.1810@ohare.Chicago.COM> gaia1@aol.com (Gaia  1) writes:

   When is the aviation industry going to be serious about the looming
   shortage of fossil fuels and their disastrous effects on the atmosphere--
   CO2 contributes over 59% of the greenhouse gases to global warming?

   When is the new airplane going to be reinvented? So much is happening with
   the reinvention of the automobile.

<snip>

   How long have particulalry those communities that suffer most from large
   airports with their concomitant noise, land, water and airpollution wait
   for a fuel cell powered airplane????
----
Despite the bias (which I will not address one way or another) that
appears in this article, there are several technical problems to be
overcome regarding aviation use of H2 which need to be addressed:

1) storage capacity:  Gaseous H2 would take up too much volume to be
useful given existing aircraft.  Totally new aircraft would need to be
designed, which would be quite large.  Liquid H2 would require
cryogenic storage capabilities, which again would require entirely new
aircraft designs to be efficient.

(Note - the X30 (National Aerospace Plane) concept had a proposal to
use Liquid H2 for several reasons - not only would it produce more
thrust, but it could also be used as a heat sink to keep the skin
temperature down.)

2) safe storage:  Unlike jet fuel, which has a very narrow fuel-air
ratio in which it will burn, H2 is quite flammable over a wide range
of fuel-air ratios.

3) NOx emissions:  While CO2 emissions would be reduced by H2 as fuel,
nitrous oxide emissions would still be a concern.

These are the technical issues w.r.t. a H2 BURNING aircraft.  The
original poster spoke of "fuel cell powered airplanes", which of
course is another can of worms:  a fuel cell by definition produces
ELECTRICITY (and water).

Looking at a fully loaded B747 (3.9e+5 kg), estimate 160,000 lbs
(7.12e+5 N) of thrust (40,000 lbs per engine), on a back-of-the-
envelope takeoff roll of 6000ft (1830m), you get a total energy
requirement of 1,300,000,000 Joules, or an average power output of 30
Megawatts over the 45 second takeoff roll.  (Yes, that number is
order-of-magnitude comparable to a fully operating nuclear power
plant.)

Even if you ignore the fuel cell weight that would be required to
generate this massive amount of electrical energy for an aircraft to
take off, and ignore the efficiency factors, the electric motors
required to turn propellors would be necessarily huge and heavy.

Furthermore, any large propellor powered aircraft wouldn't decrease
the noise pollution mentioned in the article.

Bottom line:  electrically powered aircraft have many more problems
than H2 burning aircraft, which in turn have several problems.
"You canna change the laws of physics."

ed

--------   Ed Hahn | ehahn@mitre.org | (703) 883-5988   --------
The above comment reflects the opinions of the author, and does not
constitute endorsement or implied warranty by the MITRE Corporation.
Really, I wouldn't kid you about a thing like this.