It seems that for most forms of modern transport there is a
technology in one form or another which currently sits as an
alternative to fossile fuels, and most particularly oil. Electric
motors generally sit at the heart of these designs, powered by some
energy source ranging from batteries to nuclear reactors. As far as
I'm aware though, there is currently no technology remotely to step
in for the jet engine in air transport. Using, as these engines do,
great quantities of fuel in order to ferry people and freight
around the globe, there seems a risk that the liberation of
international travel to the masses which their technology
facilitated could soon be under threat. Airlines are apparantly
already struggling to maintain a business model largely targeting
customers of average wealth, while rising fuel costs push up
operating costs. Presuming then that there is no equivalent
technological solution ready before oil is simply to scarce to be
thrusting millions of travelers through the skies for their yearly
holiday (not that I can afford that as it is), perhaps aviation
will have to take a look back at a technology mostly lost to the
tattered black and white film reels of the early 20th century.
HISTORY
The earliest successful ariships were actually products of the late
19th century, and so pre-dating the aeroplaine. During the First
World War German airships, by then the most advanced in production,
were used for bombing raids on London, in turn encouraging work on
a British airship fleet. Following the end of the war, this set the
stage for a game of one-upmanship between nations in developing
ever larger airships through the 20s and 30s. However by pushing
the technology so as to contruct what are still the largest
aircraft that have ever been built, failures too frequently lead to
disaster. The British R101, having been extended beyond the
original design length, crashed on its maiden voyage and ended
British airship development in 1930. The German programme of course
met the firery fate which now defines the end of the airship era,
with the Hindenburg disaster of 1937. By the Second World War
airships were only used by the United States and reduced to minor
roles. They were most effective in patrolling for submarines, which
they could follow slowly above in order to deploy depth charges. In
the last years of the war, both the Germans and the British had
developed jet engine powered fighter aeroplanes, and so the
technology which would go on within a couple of decades to
revolutionise international travel.
Airships have continued to find minor civillian uses for
advertising, aerial camera platforms, and sight-seeing tours.
Surprisingly the Zeppelin company, synonymous with the great German
airships of the early 20th century, continues to supply these
applications with their Zeppelin NT airships. Lifted now of course
by Helium rather than the flammible Hydrogen which burnt in the
Hindenburg.
LEARINING FROM THE PAST
Obviously the airship can never compete in speed with the jetliner,
but as an alternative, requiring a power sourse for propoltion
which might be practically replaced by electric motors, it may
offer an alternative after the jet engine has become too expensive
for all of the tasks currently relying on it. So what new
technologies and ideas could be applied to this old detour in
aviation history? How could the failings that sealed the airship's
fate before the jet era, now be overcome as that era appears to
draw towards an end?
LIFT
Let's begin with the main characteristic of an airship, that it is
lighter than air. Historically this question has come down to using
either Helium or Hydrogen. The unavailability of Helium to Germany
during the 30s sealed the Hindenburg's fate, and would seem to
answer the question clearly in favour of Helium even though the
non-flamibile gas offers slightly less lift. However Helium, which
for commercial use is extracted from natural gas deposits, is
recently increasing in cost as a result of deposits being
depleated. It seems doubtful that the volumes required for a
large-scale adoption of airships could be economically obtainable.
Hydrogen may be more reliable for long-term supply, as it can be
easily separated from water, and if Hydrogen fuel cells prove
popular for motor vehicles then the supply chains established to
serve that industry should help bring down the costs of using this
gas in airships as well.
Then there's also the mysterious third option: no gas at all. While
gasses with less density float in air, ideal lift is acheived with
no density, no gas, at all. The vacuum airship has been a
theoretical enigma dating back to the 17th century, with its
proponents stuck with the problem of having to resist the whole of
the Earth's asmosphere trying to push its way into the gassless
void, while not making a surrounding structure that is too heavy.
In fact it has been shown mathematically that a sphere in which to
keep a vacuum sufficient to lift itself would have to be made of a
material _stronger_ than diamond.
Nevertheless, an interesting patent was submitted in 2005:
It claims a mathematical proof that a sandwidch of a honeycomb
structure made of thin aluminium, between a specialised
high-strength carbon-based material, would be able to withstand the
atmospheric pressure while being light enough to overcome it's own
weight and more. As attractive as it is to imagine an airship
lifted by just turning on a vacuum pump though, I'm not aware of
any attempt to physically build a vacuum baloon to this design.
