Pykrete

Pykrete is a composite material made of approximately 45 percent sawdust or some other form of wood pulp (such as paper) and 55 percent ice by weight. The properties of such a composite were apparently first noted by a couple of researchers at Polytechnic University of New York, and were investigated more thoroughly by Max Perutz. Its use was proposed during World War II by Geoffrey Pyke to the Royal Navy as a candidate material for making a huge, unsinkable aircraft carrier. Pykrete has some interesting properties, notably its relatively slow melting rate (due to low thermal conductivity), and its vastly improved strength and toughness over unmodified (crystalline) ice, actually closer to concrete. Pykrete is slightly more difficult to form than concrete, as it expands during the freezing process, but can be repaired and maintained from the sea's most abundant raw material: water.

History

Pyke managed to convince Lord Mountbatten of the worth of his project (actually prior to the invention of Pykrete) some time around 1942, and trials were made in two locations in Alberta in Canada.The idea for a ship made of ice impressed the United States and Canada enough that a convert|60|ft|m|sing=on-long, 1,000-ton ship was built in one month on Lake Patricia in the Canadian Rockies. It was however built of plain ice (from the lake), before Pykrete was considered. It took slightly more than an entire summer to melt.

Plain ice proved to be insufficiently strong. Pyke learned from a report by Herman Mark and his assistant Walter Hohenstein that ice made from water mixed with wood fibres formed a strong solid mass – very much stronger than pure water ice. Max Perutz later recalled:

cquote|Then, one day, Pyke handed me a report that he said he found hard to understand. It was by Herman Mark, my former professor of physical chemistry in Vienna, who had lost his post there when the Nazis overran Austria, and found a haven at the Polytechnic Institute of Brooklyn. As an expert on plastics, he knew that many of them were brittle when pure, but could be toughened by embedding fibres such as cellulose in them, just as concrete can be reinforced with steel wires. Mark and his assistant, Walter P. Hohenstein, stirred a little cotton wool or wood pulp - the raw material of newsprint - into water before they froze it, and found that these additions strengthened the ice dramatically.

When I had read their report, I advised my superiors to scrap our experiments with pure ice and set up a laboratory for the manufacture and testing of reinforced ice. Combined Operations requisitioned a large meat store five floors underground beneath Smithfield Market, which lies within sight of St. Paul's Cathedral, and ordered some electrically heated suits, of the type issued to airmen, to keep us warm at convert|0|°F|°C|abbr=on. They detailed some young commandos to work as my technicians, and I invited Kenneth Pascoe, who was then a physics student and later became a lecturer in engineering at Cambridge, to come and help me. We built a big wind tunnel to freeze the mush of wet wood pulp, and sawed the reinforced ice into blocks. Out tests soon confirmed Mark and Hohenstein's results. Blocks of ice containing as little as four percent wood pulp were weight for weight as strong as concrete; in honor of the originator of the project, we called this reinforced ice "pykrete". When we fired a rifle bullet into an upright block of pure ice two feet square and one foot thick, the block shattered; in pykrete the bullet made a little crater and was embedded without doing any damage. My stock rose, but no one would tell me what pykrete was needed for, except that it was for Project Habakkuk.Perutz, 2002, p83.]

Perutz would later learn that Project Habakkuk was the plan to build an enormous aircraft carrier, actually more of a floating island than a ship in the traditional sense. The experiments of Perutz and his collaborators in Smithfield Meat Market in the City of London took place in great secrecy behind a screen of animal carcasses. [cite journal
last = Gratzer
first = Walter
title = Max Perutz (1914–2002)
journal = Current Biology
volume = 12
issue = 5
pages = R152–R154
date = 2002-03-05
url = http://download.current-biology.com/pdfs/0960-9822/PIIS0960982202007273.pdf
doi = 10.1016/S0960-9822(02)00727-3
accessdate = 2008-01-12
format=PDF] [cite journal
last = Ramaseshan
first = S
title = Max Perutz (1914–2002)
journal = Current Science
volume = 82
pages = 586–590
date = 2002-03-10
publisher = Indian Academy of Sciences
issn = 0011-3891
url = http://hdl.handle.net/2289/728
accessdate = 2008-01-12] [cite journal
last = Collins
first = Paul
title = The Floating Island
journal = Cabinet Magazine
issue = 7
year = 2002
url = http://www.cabinetmagazine.org/issues/7/floatingisland.php
accessdate = 2008-01-12] The tests confirmed that pykrete is much stronger than pure ice and, unlike the latter, does not shatter, but also that it sags under its own weight at temperatures higher than -15 C. [cite journal
title = Max Perutz OM
journal = The Daily Telegraph
date = 2002-02-07
url = http://www.telegraph.co.uk/news/main.jhtml?xml=/news/2002/02/07/db0701.xml
accessdate = 2008-01-12]

Mountbatten’s reaction to the breakthrough is recorded by Pykes biographer David Lampe:

cquote|What happened next was explained several years after the war by Lord Mountbatten in a widely-quoted after-dinner speech. "I was sent to Chequers to see the Prime Minister and was told he was in his bath. I said, 'Good, that's exactly where I want him to be.' I nipped up the stairs and called out to him, 'I have a block of a new material which I would like to put in your bath.' After that he suggested that I should take it to the Quebec Conference."

The demonstration in Churchill's streaming bath had been most dramatic. After the outer film of ice on the small pykrete cube had melted, the freshly exposed wood pulp kept the remainder of the block from thawing. [Lampe, 1959, p136.]

As an after dinner speech, Mountbatten’s story may have been elaborated somewhat to entertain his audience — the mental image of Churchill in his bath perhaps smoking one of his trademark cigars is rather amusing — and the incident may have been entirely imaginary. The story does, however, convey the sense of excitement generated by the discovery.

Despite these tests, the main Project Habakkuk was never put into action. The funds simply were not available due to other WWII projects, as well as the belief that the tides of the war were beginning to turn in favour of the Allies using more conventional methods.] .

According to the memoirs of British General Ismay:

Since WWII pykrete has remained a scientific curiosity, unexploited by research or construction of any significance. In 1985, pykrete was considered for a quay in Oslo harbour. [A New Chip Off an Old Block, Paul Breeze. The Guardian, 1 August 1985 p13 column F.]

New concepts for pykrete however crop up occasionally among architects, engineers and futurists, usually regarding its potential for mammoth offshore construction or its improvement by applying super-strong materials such as synthetic composites or Kevlar.

Durability

The durability of Pykrete is up for debate. It is often asserted that Pykrete has a crush resistance of greater than 3,000 pounds per square inch (21 MPa) so a very short 1 inch (25 mm) column could support the weight of a typical car. However, no published figures support this. Perutz himself gives a value of around 1100 lbs/sq. in..

A September 1943 proposal for making smaller pykrete vessels included the following table of characteristics. Strengths are in units of pounds per square inch:The National Archives, ADM 1/15677 - Proposals and inventions of Mr Geoffrey Pyke; gravity propelled ball bomb, pykrete and power driven rivers.]

A British service rifle bullet (a .303 caliber) will penetrate only 16.5 cm (6.5 inches) of pykrete. Of course ice always slowly deforms under pressure anyway, and this isn't really affected by the presence of the pulp.

Here is a modern demonstration, for example:

Manufacture

Pykrete can be easily formed using water and some form of wood pulp, even newspaper or paper towels. (Other fibrous materials might also be suitable, but there seems to have been little investigation of alternatives.) The mixture can be molded into any shape and frozen, and it will be extremely tough and durable, as long as it is kept at or below freezing.

Trivia

Tradition asserts that Mountbatten managed to convince Winston Churchill of its usefulness by bursting in on Churchill when he was having a bath, and plunging a block of pykrete into the bathwater. This may or may not have happened. Another tale is that at the Quebec Conference of 1943 Mountbatten brought a block of Pykrete along to demonstrate its potential to the entourage of admirals and generals who had come along with Winston Churchill and Franklin D. Roosevelt. Mountbatten entered the project meeting with two blocks and placed them on the ground. One was a normal ice block and the other was Pykrete. He then drew his service pistol and shot at the first block. It shattered and splintered. Next, he fired at the Pykrete to give an idea of the resistance of that kind of ice to projectiles. The bullet ricocheted off the block, grazing the trouser leg of Admiral Ernest King and ending up in the wall. The Admiral may or may not have been impressed by Mountbatten's unorthodox demonstration. According to Perutz's own account [cite book
last = Perutz
first = Max
authorlink = Max Perutz
title = Essay 'Enemy Alien' in I Wish I Made You Angry Earlier
publisher = Oxford University Press
location =
series =
year = 2002 - paperback
isbn = 0-19-859027-X ] , however, the incident of a ricochetting bullet hitting an Admiral actually happened much earlier in London and the gun was fired by someone on the project — not Mountbatten.

References

Notes

General references

*cite book
last = Ismay
first = General Lord
authorlink = Hastings Ismay, 1st Baron Ismay
title = The Memoirs of Lord Ismay
publisher = Heinemann
series =
year = 1960
isbn =
*cite book
last = Lampe
first = David
title = Pyke, the Unknown Genius
publisher = Evans Brothers
location = London
series =
year = 1959
isbn =
*cite book
last = Perutz
first = Max
authorlink = Max Perutz
editor = Gerald Seligman
title = The Journal of Glaciology
series = Volume I (January 1947 - October 1951)
chapter = A Description of The Iceberg Aircraft Carrier [1948]
publisher = The British Glaciological Society
location =
year =
isbn =
*cite book
last = Perutz
first = Max
authorlink = Max Perutz
title = I Wish I Made You Angry Earlier
publisher = Oxford University Press
location =
series =
year = 2002 - paperback
isbn = 0-19-859027-X

External links

* [http://www.combinedops.com/Pykrete.htm Pykrete - Ice Ships in the Rockies]
* [http://www.royalnavalmuseum.org/info_sheets_Habbakkuk.htm WWII Aircraft Carrier]
* [http://www.goodeveca.net/CFGoodeve/pykrete.html Pykrete ... or, The Myth that Wouldn't Die...]