N.V. was a freshly minted aeronautical engineer when a famous author, explorer and balloonist, who had made numerous voyages across Africa in helium balloons, asked for his help designing a hot-air blimp:
Your correspondent was soon to learn that it wasn’t a matter of starting with a blank piece of paper. The hot-air blimp’s colourful envelope of polyurethane-coated Terylene had already been sewed up—so pictures could be taken and articles written to help raise money for the planned expedition. The blimp’s long, thin cigar shape would have been fine for an original Zeppelin with its rigid internal skeleton. But it was far from ideal for a non-rigid blimp that derived its shape solely from the slightly higher pressure of the warmer air within the fabric envelope.
Nevertheless, a scale model was duly carved from polystyrene foam, its centre of pressure estimated, and the model set up in a wind-tunnel at Imperial College. A series of low-speed stability tests to measure pitch and yaw quickly determined the size of the control surfaces needed to keep the craft straight and level and pointing in the desired direction.
The results were not encouraging. With no inner structure to brace the enormous cruciform tail-fins and rudder required to do the job, all your correspondent could suggest was to use pressurised hoops made from thin rubber tubing (like the inner tubes of bicycle tyres) attached at various points towards the rear of the envelope. Inflated to high pressure, these would form a reasonably stiff frame for holding the fabric-covered control surfaces in place.
Unfortunately, with no going back to the drawing-board allowed, the design proved much too unwieldy—and the world’s first thermal airship failed to get off the ground. A decade later, Cameron Balloons of Bristol, England, licked most of the problems and is now the most successful maker of hot-air craft in the world, with separate operations in Ann Arbor, Michigan, as well as Bristol.
There are trade-offs between hydrogen, helium, and hot air:
Hydrogen is the lightest of all gases, but has a propensity to catch fire. The Hindenburg disaster in 1937, caught on film and seen by millions around the world, put paid once and for all to hydrogen’s use in commercial balloons and airships. The only reason it was used in the first place was because of the ease with which it could be made (by electrolysis of water).
The next best lifting agent is helium. Though twice as heavy as diatomic hydrogen, helium provides only 8% less buoyancy. Better still, it is inert and a fire extinguisher to boot.
The problem with helium is that there are only 16 plants worldwide for extracting it from natural gas. Meanwhile, supplies are dwindling. Unlike other fossil reserves such as oil and natural gas, which can always be made synthetically if necessary, helium is an irreplaceable, non-renewable resource accumulated over billions of years from the slow radioactive decay of uranium and thorium. The biggest user is NASA, followed by hospitals for their magnetic-resonance imaging machines and flat-panel display makers. The price, currently around $5 a litre, is rising steadily.
Hence hot air. It may have only a third of the lifting capacity of helium, but it costs just a twentieth as much after taking into account the price of the propane burner and fuel for producing the hot air, and the greater overall simplicity of a thermal airship. The downside is that thermal airships tend to be rather large for the modest payloads they can carry. The world’s largest, the 300,000 cubic-foot AS-300 built by Lindstrand Technologies of Oswestry, England, was designed to deposit a pair of botanists onto a rainforest canopy. Fill an envelope that size with helium instead of hot air and it would cost over $40m for the gas alone.