“Pykrete” is an intriguing material—and while it was ultimately deemed unsuitable for military purposes in World War II, it is easy to make at home and experiment with.

World War II was the largest land war in human history, but it also included the most far-flung naval campaigns ever fought. Taken independently, World War II in Europe and the Pacific War would each still rank as history’s two largest and most complex naval wars by a wide margin.

Projecting power beyond one’s immediate sphere of influence has always been among the most resource-intensive endeavors that nations face. That job has historically fallen to navies, though long-range airpower has eased some of that burden. Even so, strategic aircraft based abroad still rely on naval power for their logistics. This problem’s persistent difficulty is sometimes illustrated by the means by which planners consider actually achieving it. And in World War II, a variety of factors led the Allied powers to consider a fascinating, yet ultimately ludicrous, new way to project airpower in the Atlantic: building new aircraft carriers out of a special mixture of ultra-strong, heat-resistance ice.

The “Atlantic Air Gap”: Where Allied Planes Couldn’t Reach U-Boats

Aircraft carriers allowed the United States and Japan to each project air power across the Pacific Ocean in the early stages of World War II. The pivotal battle of the Pacific War, the Battle of Midway, was fought entirely between aircraft carriers, with no American or Japanese surface vessels ever making contact with each other. US aircraft carriers continued to lead the US across the Pacific, until suitable island bases could be acquired from which to stage the B-29 heavy bomber against the Japanese Home Islands. Even with those bases, naval tactical air was a major factor until war’s end, and remains so today.

The Atlantic Campaign, however, was different. It was primarily a commerce war, with Germany employing U-boats (and the very occasional surface raider) to disrupt the logistical lines from North America to the British isles. Convoy tactics and escort ships like destroyers and frigates played a major role in protecting the merchant ships necessary for the war effort. But airpower was also crucial. Aircraft could cover more area faster, and could attack U-boats far more quickly than even the fastest destroyer. And unlike surface combatants, the U-boats had no way to fight back against aircraft; their only defense was to dive and wait until the aircraft went away. Land-based air was a critical piece of the convoy system, and by the middle of the war—after early problems with coastal defense were ironed out—ships became much safer while in range of coastal air cover.

The most dangerous part of the trans-Atlantic convoy route, by far, was the “Atlantic Air Gap,” the area in between the coasts where land-based air simply could not reach, no matter where they were based. Even stretching the range of existing aircraft with extra fuel tanks meant that the planes couldn’t carry weapons, and their loiter time over the convoy was almost non-existent. There was no point to it.

The easy answer was to include aircraft carriers in the convoy escort. But the US Navy had the Allies’ only viable carrier fleet, and by the time the US entered the war, its carriers were badly needed in the Pacific. As Midway demonstrated, carriers were the new kings of naval combat; they simply had more important things to do than escort duty. Even the relatively antiquated USS Ranger (CV-4), which was left in the Atlantic after it was deemed too slow to compete with Japanese warships in the Pacific, was detailed to higher-priority fleet support duties.

As the war went on and US naval strength grew, small escort carriers with decreasing usefulness in the Pacific were eventually assigned to convoy duty. However, in the meantime, the Allies looked for new and novel ways to cover the air gap.

“Project Habakkuk”: An Iceberg Aircraft Carrier?

One of the strangest, yet most intriguing, ideas came from a British inventor named Geoffrey Pyke.

Pyke was attached to the British War Ministry and began to wonder whether the large naturally occurring North Atlantic icebergs could be employed as makeshift airstrips for anti-submarine aircraft. The advantages to this were clear: unlike an existing boat, an iceberg was made entirely of inherently buoyant ice, and could not be torpedoed by an enemy vessel and sunk. However, Pyke quickly learned that floating natural icebergs were inherently unstable and had a propensity to randomly capsize, making them impractical to station military assets on.

Undaunted, Pyke came across a report written some years earlier about how a compound of 14 percent wood pulp (sawdust) or similar material, mixed with 86 percent water, created a far stronger and more stable form of ice when it was frozen. In essence, the cellulose from the wood pulp reinforced the existing ice matrix, making it tougher and more resistant to cracking. Made and maintained properly, the mixture would rival reinforced concrete for strength and durability. It also decreased heat conductivity relative to regular ice, slowing its melting rate.

Still unsure of how practical the idea was, Pyke presented the idea of making floating aircraft platforms from this material to his superior, Admiral Lord Louis Mountbatten, who was then the Chief of Combined Operations. Mountbatten was enthusiastic, as was Prime Minister Winston Churchill. From that idea sprang “Project Habakkuk,” a British project to develop and create huge mid-ocean platforms from which aircraft could be staged.

Max Perutz, a scientist who had worked with Pyke on the earlier iceberg project and found that they were unsuitable, began work in a secret facility in London located in the Smithfield Meat Market. The animal carcasses reportedly screened the work. Perutz dubbed the material “pykrete” in Pyke’s honor.

“Pykrete” Was Better Than Regular Ice—but Had Its Own Problems

Project Habakkuk was not the first to propose a ship made from ice. The US and Canada sponsored a joint test on Alberta’s Patricia Lake, which saw the construction of a 60-foot-long ice ship, displacing 1,000 tons. But that ship was built from ordinary lake ice, with no sawdust leavening. The ship lasted the entire summer before melting, but it demonstrated that ordinary ice was too weak to support aircraft. 

Pykrete was better positioned to bridge this limitation. The story goes that Mountbatten brought a block of pykrete to the 1943 Allied Quebec Conference, along with a block of standard ice. Placing both on the floor, he drew his service pistol and shot the normal ice, shattering it. He then shot the block of pykrete. The bullet reportedly ricocheted, creasing the trouser leg of US Navy Admiral Ernest King before hitting a wall. That story’s veracity is dubious. Different versions of it still persist—meaning something probably happened along those lines, but it may not have involved Mountbatten or the infamously irascible King.

Tests were conducted, and pykrete proved its remarkable durability. But the tests revealed weaknesses, too. Pykrete had to be maintained at a temperature of -15 C or it began losing stability and collapsing under its own weight. That became a problem when British requirements called for a 2,000-foot-long flight deck, displacing 2.2 million tons. (By comparison, the standard World War II American Essex-class fleet carrier was 873 feet long and displaced 27,100 tons.) Reports vary over whether any actual pykrete test vessel was ever built, but the dates do not line up, so this appears not to have happened.

Efforts to solve the heat problem created their own cascade of problems. The pykrete ship would have needed to include machinery to maintain its necessary temperature, while also having engines and all the machinery to propel and maintain it at sea—not to mention all the support material for the aircraft and the problem of housing the crew and pilots. By mid-1943, as the war turned for the better in both theaters, it became clear that more conventional means would solve the problems at hand more efficiently, and the Habakkuk project was shelved.

Pykrete Is Still Around, with Uncertain Uses

Today, some 80 years later, pykrete’s properties are well-proven, but no practical large-scale use for it has ever been found.

This is not from lack of trying. In 1985, the government of Norway reportedly studied a proposal to construct a pykrete quay in Norway’s Oslo Harbor, where the cold temperature might keep it solid indefinitely. Despite the potential economic advantages relative to concrete, pykrete’s unreliability was a safety concern, prompting the use of traditional building materials instead.

The Vienna University of Technology and the Eindhoven University of Technology each constructed pykrete domes on their campuses in 2011 and 2014, respectively. Both projects used the technique of spraying the composite onto a large, inflated balloon, then deflating the balloons once the freezing was complete. The pykrete was found to have interesting properties when being shaped, similar to those of glaciers. But those structures were not permanent—both eventually melted—and no one has ever successfully used pykrete for a real-world project.

Experiments on pykrete, however, have never truly stopped, and easy recipes abound for making the compound at home. Perhaps some young future genius will yet find a use for this intriguing, but so far experimental, material.

About the Author: William Lawson

William Lawson is a military historian focusing on World War II and 20^th century conflicts and the American Civil War. His specialty is operational level warfare, especially American amphibious doctrine. He writes on history, politics, and firearms for multiple publications and historical journals. He serves on the editorial advisory board for the Saber & Scroll Journal and Military History Chronicles and is a member of the Society for Military History and the American Historical Association. Lawson is based in Virginia.

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