directed, controlled, steered or ‘navigated’, thereby distinguishing it from a
balloon), speeds of about 80 mph could be achieved—very fast for the early
1900s. Between 1910 and 1914, *35,000 passengers passed between cities
in Germany and other European countries on dirigibles such as the Graf
Zeppelin, Although the Hindenburg disaster was many years in the future,
the ‘Achilles heel’ of the early airship was always its ‘lifting gas’, hydrogen, a
choice dictated because of the American embargo on helium at the time.
Hydrogen of course is the lightest element and even then, it was relatively
cheap to produce. But the lower flammability limit of hydrogen in air at
atmospheric pressure is just 4%, the upper limit is 75%. The explosive limits
at atmospheric pressure are 18.3–59%.
9 Robert’s innovation therefore was
really quite obvious—especially considering his association with Ramsay.
Replace hydrogen with a more benign lifting gas viz., helium. Helium is
neither flammable, nor a supporter of combustion, but it is expensive,
non-renewable, (released helium escapes the earth’s atmosphere) and its
buoyancy is about 8% less than that of hydrogen—although Robert calculated just over 6% less in his patent. Airships filled with helium therefore are
inherently safer but less practical, for they lift smaller loads and travel shorter
distances because they can carry less fuel. Helium airships for example would
not have had the range to cross the Atlantic with obvious economic consequences. To help overcome these difficulties but at the same time preserving
safety and minimizing costs, Robert’s patent proposed substituting a proportion of the expensive helium with cheaper hydrogen, but within
non-explosive limits.
10 These he suggested were between fifteen and
twenty-five percent hydrogen—‘perhaps even more’. The total buoyancy he
claimed would increase ‘from one to two percent’ which meant an increase in
‘anything from two to ten percent in carrying capacity’.
Now discussions regarding the use of helium/hydrogen mixtures persist to
this day, but detailed studies show that no mixture would be safe for use in an
airship unless it contained <8.7% hydrogen in admixture with helium
9
—a
much lower limit than Robert proposed. Even so, Robert’s idea never ‘took
off’. The superior economics of hydrogen eventually led to more than
30 years of passenger travel on German commercial Zeppelins during which
time tens of thousands of passengers flew over a million miles on more than
2,000 flights without a single injury. But the inevitable conclusion of the
contest, ‘economics versus safety’, finally came at Lakehurst, New Jersey on
06 May 1937 with the catastrophic explosion of the hydrogen filled
‘Hindenburg’ bringing an abrupt end to the age of the airship. Nowadays,
an *8% substitution of helium by hydrogen produces little economic
advantage, and even if theoretical calculations and experiments show that we
322
D. Sheppard
balloon), speeds of about 80 mph could be achieved—very fast for the early
1900s. Between 1910 and 1914, *35,000 passengers passed between cities
in Germany and other European countries on dirigibles such as the Graf
Zeppelin, Although the Hindenburg disaster was many years in the future,
the ‘Achilles heel’ of the early airship was always its ‘lifting gas’, hydrogen, a
choice dictated because of the American embargo on helium at the time.
Hydrogen of course is the lightest element and even then, it was relatively
cheap to produce. But the lower flammability limit of hydrogen in air at
atmospheric pressure is just 4%, the upper limit is 75%. The explosive limits
at atmospheric pressure are 18.3–59%.
9 Robert’s innovation therefore was
really quite obvious—especially considering his association with Ramsay.
Replace hydrogen with a more benign lifting gas viz., helium. Helium is
neither flammable, nor a supporter of combustion, but it is expensive,
non-renewable, (released helium escapes the earth’s atmosphere) and its
buoyancy is about 8% less than that of hydrogen—although Robert calculated just over 6% less in his patent. Airships filled with helium therefore are
inherently safer but less practical, for they lift smaller loads and travel shorter
distances because they can carry less fuel. Helium airships for example would
not have had the range to cross the Atlantic with obvious economic consequences. To help overcome these difficulties but at the same time preserving
safety and minimizing costs, Robert’s patent proposed substituting a proportion of the expensive helium with cheaper hydrogen, but within
non-explosive limits.
10 These he suggested were between fifteen and
twenty-five percent hydrogen—‘perhaps even more’. The total buoyancy he
claimed would increase ‘from one to two percent’ which meant an increase in
‘anything from two to ten percent in carrying capacity’.
Now discussions regarding the use of helium/hydrogen mixtures persist to
this day, but detailed studies show that no mixture would be safe for use in an
airship unless it contained <8.7% hydrogen in admixture with helium
9
—a
much lower limit than Robert proposed. Even so, Robert’s idea never ‘took
off’. The superior economics of hydrogen eventually led to more than
30 years of passenger travel on German commercial Zeppelins during which
time tens of thousands of passengers flew over a million miles on more than
2,000 flights without a single injury. But the inevitable conclusion of the
contest, ‘economics versus safety’, finally came at Lakehurst, New Jersey on
06 May 1937 with the catastrophic explosion of the hydrogen filled
‘Hindenburg’ bringing an abrupt end to the age of the airship. Nowadays,
an *8% substitution of helium by hydrogen produces little economic
advantage, and even if theoretical calculations and experiments show that we
322
D. Sheppard