The Physics of War (30 page)

Another problem with radar is that microwaves in the atmosphere and even within the device itself can interfere with the signal. This interference superimposes itself on the radar signal and has to be reduced or “cleaned up” before the returning signal can be analyzed properly. The interfering waves may come from buildings, mountains, and other objects that reflect microwaves.

AN AMAZING DISCOVERY

In the late 1930s it became obvious that Germany was building up its military and would likely unleash an all-out attack on Britain in the near future. And it was also known that the Germans had close to three thousand planes compared to only eight hundred for Britain. As a result, the British set up an extensive system of radar stations, but radar still had serious problems at that time. It was low-powered and used radio waves that did not give a clear image. The British needed something better, and they needed it fast. The shortest wavelengths available were about 150 centimeters (59 inches) with the power of about 10 watts.
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Scientist began a search. It was soon noticed that a General Electric physicist, Albert Hall, at Schenectady, New York, had invented a simple device he called a magnetron in 1920. It looked like it had promise, but he couldn't think of any uses for the device at the time. Hall's device did not generate microwaves, but it was soon discovered that with a slight modification, it might be able to generate them, and, as a result, it attracted some attention. It wasn't until the late 1930s, however, when two engineers in England, Harry Boot and John Randall, decided to explore the device further that people got really excited. Hall's earlier device consisted of a cathode (negative terminal) and an anode (positive terminal) in a glass tube, quite similar to an ordinary vacuum tube. Boot and Randall modified it; they used a copper body, which acted like an anode. It was cylindrical with several cylindrical cavities around its inner edge. These cavities opened into a central vacuum chamber that contained the anode. A permanent magnet was used to create a magnetic field that ran parallel to the axis of the cylinder. The cathode was hooked to a high-voltage power supply. This produced electrons that streamed out toward the cylinder walls. These electrons, however, were deflected by the magnetic field into curved paths, and this caused them to set up small circular currents within the cavities. These currents produced microwave radiation that could be directed into a device called the waveguide, which channeled it to an outside device where it could be used. Of particular interest, the wavelength of the microwave radiation was related to the size of the cavity, and therefore it could be adjusted.

When Boot and Randall completed their device in February 1940, they tested it and were amazed that it produced microwaves with the power of nearly five hundred watts—fifty times what the earlier devices were capable of. Furthermore, the wavelength of the microwave radiation was only 10 centimeters (3.93 inches), which would give a much clearer picture of enemy objects. In addition, the device was small enough to fit into the palm of your hand. They were delighted, and over the next few months they worked to perfect their device.

The cavity magnetron.

By now, however, the war had started, and Britain was strapped for money. But the British needed the device; in fact, they needed a large number of them for their radar-defense systems against German planes. Churchill knew that Britain could not produce the large numbers needed, but the United States could, and he also knew that the United States was working on its own radar system and would be amazed at the device that Boot and Randall had devised. He therefore suggested that Henry Tizard, the chairman of the Aeronautical Research Committee, offer the magnetron, as it was called, to the United States in exchange for help in mass-producing it.

In a secret mission that took place in September 1940, Tizard went to United States. In a small box he carried a magnetron capable of generating 500 watts (while the most powerful magnetron in United States at the time could create only about 10 watts). And indeed, within a short time, a deal was reached.
American officials later described the device as “the most valuable cargo ever brought to our shores.”
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Scientists at Bell Labs made a copy of the device suitable for mass production before the end of 1940, and a lab was set up at MIT (Massachusetts Institute of Technology) to develop a more powerful radar system using it. Back in England, scientist at TRE (Telecommunications Research Establishment) developed a revolutionary new radar system that could be used by airplanes for ground mapping.

The magnetron, which is usually called a cavity magnetron because of the small cavities within it, allowed the detection of very small objects such as submarine periscopes. And since magnetrons were now small enough to be installed in airplanes, a squadron of airplanes could easily spot enemy subs and destroy them. The new device also proved valuable in detecting incoming German bombers well before they got to England, so the Royal Air Force could prepare for them. And it also improved the accuracy of Allied bombing raids over Germany. This will be discussed in much more detail in
chapter 16
.

We have discussed submarines briefly in previous chapters. In this chapter we will look at the submarine in more detail. It was improved over the years, and it didn't take long for nations to realize that it had considerable military potential.

Although several crude designs for submarines appeared before the 1700s, one of the first to build an operational model was the American engineer Robert Fulton. Between 1793 and 1797 he built the first working submarine while living in France. It could stay underwater for seventeen minutes, and it was about twenty-four feet long. He called it the
Nautilus
. Submarines were also used in the American Civil War. The Confederates built four submarines, the most famous of which was the
H. L. Hunley
. After the war, research on submarines continued, and this research is usually associated with two names: Simon Lake and John Holland. Lake began experimenting with the idea of using buoyancy to submerge and surface a submarine. Holland worked on various methods of propulsion. The US Navy's first commissioned submarine, the USS
Holland
, was built by Holland in 1898. It was fifty-three feet long, weighed seventy-five tons, and it had an internal combustion engine for running on the surface and an electric motor for use while submerged.

All submarines depend on a principle that was formulated many years earlier by Archimedes of Syracuse, Sicily. We discussed it briefly earlier; let's look at it now in more detail.

ARCHIMEDES' PRINCIPLE

Archimedes' principle is related to the pressure on an object in water or another liquid, or more exactly the buoyancy on an object in a fluid.
¹
To understand it, let's begin with the concept of pressure; it is defined as force per unit area, or algebraically, as P = F/A, where P is pressure, F is force, and A is area. If you're considering the pressure on a given surface under a certain amount of
water, it's easy to see that the pressure comes from the weight of the column of water above it acting on the surface. And the weight of this water depends on its density, which is defined as its weight per unit volume. The density of water is sixty-two pounds per cubic foot.

But we're mainly interested in buoyancy, so let's consider a solid cube within a tank of water and determine the buoyant force on it. This is the force pushing it upward. Archimedes' principle states that
the upward force on any object in water or other fluid is equal to the weight of the fluid displaced
. Archimedes (278–212 BCE) arrived at his principle when he was asked by the king of Syracuse to find out if a blacksmith had stolen some of the gold he had been given to make a crown by substituting silver for it. And, as it turned out, he had.

Archimedes' principle is valid if the body is totally submerged, or if it is floating. In fact, it's easy to see that if the weight of the body is less than the weight of the water displaced (when it is totally submerged), the body will float. This means that if its density is less than that of water, it will float.

Let's look now at our solid cube in the tank of water. Assume it is totally submerged. The pressure on all sides will equalize because there is an equal opposing force across from any force acting on it. But the force on the top and the bottom will be different because the upward force on the bottom is greater than that of the force on the top, since the bottom of the cube is deeper. The difference will, in fact, be equal to the weight of the water the cube displaces. This is the buoyant force. And if it is greater than the weight of the cube, the cube will move upward and float. This is, of course, what Archimedes' principle tells us. It occurs for any object in water when the object's density is less than in that of water, and this is, in fact, why ships, which are made of heavy steel, can float. Steel has a high density, but the ship is made up mostly of air, which has a density much less than that of water, so it has an average density less than that of water.

PHYSICS OF SUBMARINES

A submarine can, of course, float on the surface, and it can also dive under the water. And when it is floating its average density has to be less than that of water, but when it dives its average density has to be greater. So it obviously has to change its density, and it does this using ballast tanks that are on its outer surface. When these tanks are full of air the average density of the submarine is less than that of water, so the submarine floats. To submerge, the submarine releases the air through small vents and allows the tanks to fill with water. When they are full (or partially full), the average density of the submarine is sufficient for it to sink. To surface, air is pumped into the ballast tanks from a compressed air tank. It forces the water out.
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Hydroplanes are also used to assist in the process of diving and resurfacing. They are at the rear of the submarine and look like the wings of an airplane. They help steer the submarine up and down in the same way that rudders on an airplane do.

It is also important to keep the submarine level and steady at various depths when it is underwater. In practice there are several problems. For example, water density increases with depth, so buoyancy increases as depth increases. The temperature of the water also has a small effect. Because of these and other problems, the submarine is in unstable equilibrium when it is submerged, and it therefore has a tendency to rise or sink at one end or the other unless adjustments are made continuously. This is referred to as maintaining trim. To achieve it, submarines use smaller forward and aft tanks. Pumps move water back and forth between them, changing the weight distribution almost continuously. A similar system is also used for stability.
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Ballast tanks on a submarine.

POWER FOR THE PROPELLERS

A submarine needs power to turn the propellers, and over the years the power source has changed. The earliest submarines were powered by human muscle. A number of men actually cranked the propeller by hand. But it wasn't long
before engines of various types were introduced to do this. By about 1900 gas-powered engines were used on the surface, and electric motors were used when the submarines submerged. Gasoline engines were, however, soon replaced by diesel engines. In the first submarines of this type, the diesel and electric motors were separated by a clutch, so they were both on the same drive shaft to the propeller. This allowed the engine to drive the electric motor as a generator that could be used to recharge the battery that was used for electric power. One of the main problems with the submarines of World War I and World War II was that they had to resurface to recharge their batteries quite frequently. Eventually a snorkel device was invented so they could recharge while still submerged, but they still had to be quite close to the surface.

SHAPE AND PERISCOPES

One of the major problems associated with submarines is hydrodynamic drag. Drag is also a problem in relation to cars, and they are therefore designed with a shape that minimizes it. In the case of submarines, the medium through which they travel is water, and water creates a much greater drag than air does. A teardrop shape for the front was used to keep drag to a minimum on most submarines deployed during World War I and World War II; more recently, however, a slightly different surface shape has been used, although the teardrop shape is still used to some extent.

Submarine showing periscope, sail, and rudder.

On the top of the submarine is a raised tower, known as the conning tower, which accommodates the periscope, various electronic devices, and the radio. In many early submarines the control room was also located here. The control room is now located within the submarine and the raised tower is now called the sail. The periscope allows an observer in the submarine to see what is happening on the surface when the submarine is submerged. It consists of a system of mirrors and lenses that bend and reflect images down a long tube. In newer submarines photon masts have superseded periscopes. They are high-resolution, color cameras that send images via fiber optics to a large screen (in fiber optics, pulses of light are sent along a long optical fiber).

NAVIGATION

A modern submarine can use GPS (the global-positioning system) to help guide it while it is on the surface, but when it is submerged GPS does not work. Newer submarines therefore have underwater inertial guidance systems that keep track of their position by noting their motion away from a fixed stationary point. These systems are quite complex, and they generally use gyroscopes to track the location of the submarine. American submarines use a system called
SINS (ships inertial navigation system); it keeps track of the location of the submarine by following its course changes using gyroscopes. Numbers are fed to a computer and compared to the starting coordinate. With this system submariners can quickly determine where they are at any time.

Gyroscopes are not only useful for underwater navigation, but, as we will see, they are also used to guide torpedoes to a target. A gyroscope is helpful because it exhibits a fundamental property called gyroscopic inertia, which gives it rigidity in space. As we saw earlier, this is a consequence of Newton's first law of motion, which states that a body tends toward a continuous state of rest or uniform motion unless subjected to an outside force. This means that when a gyroscope is set spinning in a particular direction, it takes a force to change it. Gyroscopes are, in fact, not only used in submarines and torpedoes; they are also used in spacecraft, rockets, guided missiles, and ships, so they obviously play an important role in warfare.

SONAR

Also important in relation to underwater navigation is sonar. When a submarine is submerged it is generally cut off from the region around it because light does not penetrate very far into water. So even if it had video cameras attached to its exterior, they would be of little use. Sonar is similar to radar except that it uses sound waves rather than microwaves. In the
last chapter
we saw that radar systems send out an electromagnetic pulse then look for an echo, or reflection, of the pulse. By analyzing the echo, radar systems can determine what is around them, even if the objects can't be seen directly. In the same way, sonar allows submariners to see what is in the water around them.
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Two types of sonar are used in submarines: active and passive. Active sonar is an analog to radar in that the travel time for a reflected wave is recorded along with any change in frequency of the initial signal. An active signal transmitter, or signal generator, creates a pulse of sound that is referred to as a ping. This pulse is concentrated into a relatively narrow beam, so that it is going in a particular direction. It is used mainly for detecting other submarines, ships, or other objects around the submarine. Analysis of the echo gives information about the distance to the object and the direction and speed at which it is traveling. Its distance can easily be determined from the time between the release of the signal and the return of the echo. Its speed can be determined from the Doppler Effect.
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One of the problems with active sonar in warfare is that any ship or submarine in the neighborhood can easily pick it up, which could allow the enemy to determine the vessel's position. Because of this, passive sonar is used in many situations. It is simply a very sensitive underwater microphone that is used to listen for noises in the water around the submarine. The problem, of course, is identifying the sound that the microphone picks up. In most cases, however, this is left to a computer. A large database of different sounds, along with the things that cause them, is stored in the computer. When a particular sound is detected, it is fed to the computer for identification. In general, passive sonar has a greater range, and it has the advantage of being undetectable.