20 July 2009

Hydrogen bomb

The invention: Popularly known as the “H-Bomb,” the hydrogen bomb differs from the original atomic bomb in using fusion, rather than fission, to create a thermonuclear explosion almost a thousand times more powerful. The people behind the invention: Edward Teller (1908- ), a Hungarian-born theoretical physicist Stanislaw Ulam (1909-1984), a Polish-born mathematician Crash Development Afew months before the 1942 creation of the Manhattan Project, the United States-led effort to build the atomic (fission) bomb, physicist Enrico Fermi suggested to Edward Teller that such a bomb could release more energy by the process of heating a mass of the hydrogen isotope deuterium and igniting the fusion of hydrogen into helium. Fusion is the process whereby two atoms come together to form a larger atom, and this process usually occurs only in stars, such as the Sun. Physicists Hans Bethe, George Gamow, and Teller had been studying fusion since 1934 and knew of the tremendous energy than could be released by this process—even more energy than the fission (atom-splitting) process that would create the atomic bomb. Initially, Teller dismissed Fermi’s idea, but later in 1942, in collaboration with Emil Konopinski, he concluded that a hydrogen bomb, or superbomb, could be made. For practical considerations, it was decided that the design of the superbomb would have to wait until after the war. In 1946, a secret conference on the superbomb was held in Los Alamos, New Mexico, that was attended by, among other Manhattan Project veterans, Stanislaw Ulam and Klaus Emil Julius Fuchs. Supporting the investigation of Teller’s concept, the conferees requested a more complete mathematical analysis of his own admittedly crude calculations on the dynamics of the fusion reaction. In 1947, Teller believed that these calculations might take years. Two years later, however,the Soviet explosion of an atomic bomb convinced Teller that America’s ColdWar adversary was hard at work on its own superbomb. Even when new calculations cast further doubt on his designs, Teller began a vigorous campaign for crash development of the hydrogen bomb, or H-bomb. The Superbomb Scientists knew that fusion reactions could be induced by the explosion of an atomic bomb. The basic problem was simple and formidable: How could fusion fuel be heated and compressed long enough to achieve significant thermonuclear burning before the atomic fission explosion blew the assembly apart? A major part of the solution came from Ulam in 1951. He proposed using the energy from an exploding atomic bomb to induce significant thermonuclear reactions in adjacent fusion fuel components. This arrangement, in which the A-bomb (the primary) is physically separated from the H-bomb’s (the secondary’s) fusion fuel, became known as the “Teller-Ulam configuration.” All H-bombs are cylindrical, with an atomic device at one end and the other components filling the remaining space. Energy from the exploding primary could be transported by X rays and would therefore affect the fusion fuel at near light speed—before the arrival of the explosion. Frederick de Hoffman’s work verified and enriched the new concept. In the revised method, moderated X rays from the primary irradiate a reactive plastic medium surrounding concentric and generally cylindrical layers of fusion and fission fuel in the secondary. Instantly, the plastic becomes a hot plasma that compresses and heats the inner layer of fusion fuel, which in turn compresses a central core of fissile plutonium to supercriticality. Thus compressed, and bombarded by fusion-produced, high-energy neutrons, the fission element expands rapidly in a chain reaction from the inside out, further compressing and heating the surrounding fusion fuel, releasing more energy and more neutrons that induce fission in a fuel casing-tamper made of normally stable uranium 238. With its equipment to refrigerate the hydrogen isotopes, the device created to test Teller’s new concept weighed more than sixty tons. During Operation Ivy, it was tested at Elugelab in the Marshall Islands on November 1, 1952. Exceeding the expectations of all concerned and vaporizing the island, the explosion equaled 10.4 million tons of trinitrotoluene (TNT), which meant that it was about seven hundred times more powerful than the atomic bomb dropped on Hiroshima, Japan, in 1945. A version of this device weighing about 20 tons was prepared for delivery by specially modified Air Force B-36 bombers in the event of an emergency during wartime. In development at Los Alamos before the 1952 test was a device weighing only about 4 tons, a “dry bomb” that did not require refrigeration equipment or liquid fusion fuel; when sufficiently compressed and heated in its molded-powder form, the new fusion fuel component, lithium-6 deutride, instantly produced tritium, an isotope of hydrogen. This concept was tested during Operation Castle at Bikini atoll in 1954 and produced a yield of 15 million tons of TNT, the largest-ever nuclear explosion created by the United States. Consequences Teller was not alone in believing that the world could produce thermonuclear devices capable of causing great destruction. Months before Fermi suggested to Teller the possibility of explosive thermonuclear reactions on Earth, Japanese physicist Tokutaro Hagiwara had proposed that a uranium 235 bomb could ignite significant fusion reactions in hydrogen. The Soviet Union successfully tested an H-bomb dropped from an airplane in 1955, one year before the United States did so. Teller became the scientific adviser on nuclear affairs of many presidents, from Dwight D. Eisenhower to Ronald Reagan. The widespread blast and fallout effects of H-bombs assured the mutual destruction of the users of such weapons. During the Cold War (from about 1947 to 1981), both the United States and the Soviet Union possessed H-bombs. “Testing” these bombs made each side aware of how powerful the other side was. Everyone wanted to avoid nuclear war. It was thought that no one would try to start a war that would end in the world’s destruction. This theory was called deterrence: The United States wanted to let the Soviet Union know that it had just as many bombs, or more, than it did, so that the leaders of the Sovet Union would be deterred from starting a war.Teller knew that the availability of H-bombs on both sides was not enough to guarantee that such weapons would never be used. It was also necessary to make the Soviet Union aware of the existence of the bombs through testing. He consistently advised against U.S. participation with the Soviet Union in a moratorium (period of waiting) on nuclear weapons testing. Largely based on Teller’s urging that underground testing be continued, the United States rejected a total moratorium in favor of the 1963 Atmospheric Test Ban Treaty. During the 1980’s, Teller, among others, convinced President Reagan to embrace the Strategic Defense Initiative (SDI). Teller argued that SDI components, such as the space-based “Excalibur,” a nuclear bomb-powered X-ray laser weapon proposed by the Lawrence- Livermore National Laboratory, would make thermonuclear war not unimaginable, but theoretically impossible.

19 July 2009

Hovercraft

The invention: A vehicle requiring no surface contact for traction that moves freely over a variety of surfaces—particularly water—while supported on a self-generated cushion of air. The people behind the invention: Christopher Sydney Cockerell (1910- ), a British engineer who built the first hovercraft Ronald A. Shaw (1910- ), an early pioneer in aerodynamics who experimented with hovercraft Sir John Isaac Thornycroft (1843-1928), a Royal Navy architect who was the first to experiment with air-cushion theory Air-Cushion Travel The air-cushion vehicle was first conceived by Sir John Isaac Thornycroft of Great Britain in the 1870’s. He theorized that if a ship had a plenum chamber (a box open at the bottom) for a hull and it were pumped full of air, the ship would rise out of the water and move faster, because there would be less drag. The main problem was keeping the air from escaping from under the craft. In the early 1950’s, Christopher Sydney Cockerell was experimenting with ways to reduce both the wave-making and frictional resistance that craft had to water. In 1953, he constructed a punt with a fan that supplied air to the bottom of the craft, which could thus glide over the surface with very little friction. The air was contained under the craft by specially constructed side walls. In 1955, the first true “hovercraft,” as Cockerell called it, was constructed of balsa wood. It weighed only 127 grams and traveled over water at a speed of 13 kilometers per hour. On November 16, 1956, Cockerell successfully demonstrated his model hovercraft at the patent agent’s office in London. It was immediately placed on the “secret” list, and Saunders-Roe Ltd. was given the first contract to build hovercraft in 1957. The first experimental piloted hovercraft, the SR.N1, which had a weight of 3,400 kilograms and could carry three people at the speed of 25 knots, was completed on May 28, 1959, and publicly demonstrated on June 11, 1959. Ground Effect Phenomenon In a hovercraft, a jet airstream is directed downward through a hole in a metal disk, which forces the disk to rise. The jet of air has a reverse effect of its own that forces the disk away from the surface. Some of the air hitting the ground bounces back against the disk to add further lift. This is called the “ground effect.” The ground effect is such that the greater the under-surface area of the hovercraft, the greater the reverse thrust of the air that bounces back. This makes the hovercraft a mechanically efficient machine because it provides three functions. First, the ground effect reduces friction between the craft and the earth’s surface. Second, it acts as a spring suspension to reduce some of the vertical acceleration effects that arise from travel over an uneven surface. Third, it provides a safe and comfortable ride at high speed, whatever the operating environment. The air cushion can distribute the weight of the hovercraft over almost its entire area so that the cushion pressure is low. The basic elements of the air-cushion vehicle are a hull, a propulsion system, and a lift system. The hull, which accommodates the crew, passengers, and freight, contains both the propulsion and lift systems. The propulsion and lift systems can be driven by the same power plant or by separate power plants. Early designs used only one unit, but this proved to be a problem when adequate power was not achieved for movement and lift. Better results are achieved when two units are used, since far more power is used to lift the vehicle than to propel it. For lift, high-speed centrifugal fans are used to drive the air through jets that are located under the craft. A redesigned aircraft propeller is used for propulsion. Rudderlike fins and an air fan that can be swiveled to provide direction are placed at the rear of the craft. Several different air systems can be used, depending on whether a skirt system is used in the lift process. The plenum chamber system, the peripheral jet system, and several types of recirculating air systems have all been successfully tried without skirting. Avariety of rigid and flexible skirts have also proved to be satisfactory, depending on the use of the vehicle. Skirts are used to hold the air for lift. Skirts were once hung like curtains around hovercraft. Instead of simple curtains to contain the air, there are now complicated designs that contain the cushion, duct the air, and even provide a secondary suspension. The materials used in the skirting have also changed from a rubberized fabric to pure rubber and nylon and, finally, to neoprene, a lamination of nylon and plastic. The three basic types of hovercraft are the amphibious, nonamphibious, and semiamphibious models. The amphibious type can travel over water and land, whereas the nonamphibious type is restricted to water travel. The semiamphibious model is also restricted to water travel but may terminate travel by nosing up on a prepared ramp or beach. All hovercraft contain built-in buoyancy tanks in the side skirting as a safety measure in the event that a hovercraft must settle on the water. Most hovercraft are equipped with gas turbines and use either propellers or water-jet propulsion. Impact Hovercraft are used primarily for short passenger ferry services. Great Britain was the only nation to produce a large number of hovercraft. The British built larger and faster craft and pioneered their successful use as ferries across the English Channel, where they could reach speeds of 111 kilometers per hour (160 knots) and carry more than four hundred passengers and almost one hundred vehicles. France and the former Soviet Union have also effectively demonstrated hovercraft river travel, and the Soviets have experimented with military applications as well. The military adaptations of hovercraft have been more diversified. Beach landings have been performed effectively, and the United States used hovercraft for river patrols during the Vietnam War. Other uses also exist for hovercraft. They can be used as harbor pilot vessels and for patrolling shores in a variety of police-and customs- related duties. Hovercraft can also serve as flood-rescue craft and fire-fighting vehicles. Even a hoverfreighter is being considered. The air-cushion theory in transport systems is rapidly developing. It has spread to trains and smaller people movers in many countries. Their smooth, rapid, clean, and efficient operation makes hovercraft attractive to transportation designers around the world.