The Difference Engine: Not all hot air

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THE conversation started over lunch in a pub across the street from London’s Waterloo Station in the 1960s. The proposal, sketched out on the proverbial paper napkin, was to design a hot-air blimp—not another propane-fired hot-air balloon (several of which had recently taken to the skies) but an actual thermal airship.

The difference between balloons and blimps could not be more fundamental. While both rely on lighter-than-air buoyancy to stay aloft, balloons go where the wind tells them—like a leaf in a stream. Since the Montgolfier brothers’ experiments in the late 1700s, balloons have been steered mainly by changing altitude, with the drifting occupants seeking a wind at a different level that will blow them, hopefully, in the desired direction. By contrast, blimps—and airships in general—are powered and steerable craft that go more or less where they please.

That was the whole point of building a blimp rather than a balloon that rainy lunchtime in London. At the time, your correspondent was a freshly minted aeronautical engineer. Across the table from him was a famous author, explorer and balloonist, who had made numerous voyages across Africa in helium balloons. As a platform drifting quietly across the savannah, a balloon was a wonderfully non-invasive way of filming wildlife. Inevitably, however, the spectacular herd of wildebeest or whatever was way off on the horizon—and there was no way to get close. With a steerable blimp, by contrast, endless footage could be shot for television to pay for the expedition.

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.

Why hot air rather than hydrogen or helium anyway? 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.

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