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22/29 December 2012 | NewScientist | 65 Mackenzie was horrified. As an engineer, he understood that scaling up from 20 metres to 600 would bring immense complications. For a start, the model had only involved plain ice. “It was just to see if a refrigeration unit could sustain the ice structure,” says Langley. The full-size carrier would need a hull of much stronger stuff. Pleasingly mad As it turned out, even pykrete would have been inadequate. At temperatures above -15 °C, pykrete slowly deforms. Even with enough refrigeration plants to keep it cold, it still wouldn’t have been able to support a 600-metre ship. Steel reinforcement would be needed too. Probably a lot of it. And, of course, a vast quantity of wood pulp would be needed, the equivalent of a small forest for each carrier. Perutz later said he had come to realise that the construction and navigation of a bergship might prove “as difficult as a voyage to the moon”. In any case, this pleasingly mad idea was soon made redundant by somewhat more mundane developments, including longer-range bombers and new airbases in Iceland. In December 1943, project Habbakuk was dropped. Since the war, a few engineers have toyed with the idea of using pykrete for other purposes – such as protecting Arctic oil rigs from stormy seas, or making a quay in Oslo harbour. Although nothing has come of it so far, Langley is hopeful that pykrete will one day make more than an icy bath toy. “It still has amazing possibilities, but it has not found its niche yet,” she says. “We just have to get the right people interested.” In the spirit of Pyke’s original plan, perhaps some aspiring seasteaders will see the material’s potential, and build themselves a floating city of ice. n Stephen Battersby is a writer based in London Match this image to one on the grid on page 77, where you will find details of the competition K SPL > It can be painful and messy if your cup runneth over. How do you avoid a scalding, asks Bob Holmes Splash and burn N O ONE can accuse H. Norman Abramson of sitting in an ivory tower. “It is common everyday knowledge to each of us that any small container filled with liquid must be moved or carried very carefully to avoid spills,” the US engineer and fluid dynamicist wrote almost half a century ago. “Experience has taught us that the unrestrained free surface of the liquid has an alarming propensity to undergo rather large excursions, for even very small motions of the container.” Hardly rocket science, you might think. The irony is, it was. Abramson’s words come from the introduction to a 464-page report he wrote for NASA in 1966. At the time, the US space agency was aiming to catapult astronauts to the moon atop rockets filled with liquid propellants. Abramson’s magnum opus, entitled The Dynamic Behavior of Liquids in Moving Containers with Applications to Space Vehicle Technology, presented the sum of human knowledge on a phenomenon of life-or-death importance to the space race: sloshing. It is a problem even now. Fuel sloshing has probably caused the failure of several rocket launches, and in 1998 a slosh- induced tumble cut short a course adjustment and set back by a year the Near Earth Asteroid Rendezvous (NEAR) mission to the asteroid 433 Eros. In 2005, the European Space Agency launched an entire mission, dubbed Sloshsat, to study fluid dynamics in microgravity. NASA’s own investigations continue. Back on Earth, we are still baffled by the burning question Abramson touched on. Why does hot coffee leap from its cup so readily, and can we do anything about it? From space fuel to coffee, this much we know: sloshing results largely from the interplay of two things. One is a liquid’s inertia, which stops it following when its container changes speed suddenly. This causes waves. The other is resonance, in which these waves get bigger as a result of some external pulse, or forcing frequency, that roughly matches the liquid’s natural oscillation frequency. The same effect makes an unplucked guitar string vibrate if a sound with its natural pitch is played through a nearby speaker. The carried coffee problem is a tougher bean to grind, because the forcing comes not from a single frequency but from a complex biomechanical process with a simple name: walking. “It makes the story more interesting, but characterising the problem more difficult,” says Rouslan Krechetnikov of the University of California, Santa Barbara. Walking’s intricate rhythms, with accelerations in all three dimensions, mean sloshing is better understood when it happens in a spacecraft than in a common cup of coffee. It was the sight of mathematicians struggling with their mid-morning coffees at a conference in 2011 that convinced Krechetnikov and his
