The Physics of War (33 page)

In the east, the Soviets, who had beaten back a German invasion, were also pushing toward Berlin. Although it was now almost certain that Germany would soon be defeated, the Germans didn't give up easily, and in December 1944 they launched a massive counterattack in the Ardennes Forest that caught the Allies
off guard. This engagement became known as the Battle of the Bulge because of the large bulge it created in the Allied lines. By late January, however, with large numbers of Allied reinforcements rushing to the front, the German offensive was stopped. And in March, Allies crossed the Rhine River and began a final push toward Berlin. The remaining German forces were now being squeezed from the east and the west. On May 2, 1945, the Germans surrendered.

ADVANCES IN AVIATION

Let's go back now and look at some of the important advances that were made during the war, many of which depended on physics. Major advances in aircraft design occurred, with the most important being the building of the first jet plane. Aside from the first jet plane, however, there were significant advances in traditional aircraft. Let's begin by looking at some of the major planes that were used in the war, and there capabilities. The British Spitfire was, without a doubt, one of the best. It was used very successfully against the Luftwaffe in the Battle of Britain. It had a maximum speed of approximately 350 miles per hour, and it performed well in climbs; furthermore, it was relatively easy to fly. The British Hurricane was also an excellent plane, and it was also used extensively in the Battle of Britain.
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The German Messerschmitt 109 was the only German plane comparable to the Spitfire. It had a maximum speed slightly less than that of the Spitfire, and it was less maneuverable, but it was faster in a dive.

The Japanese Mitsubishi Zero was the primary Japanese naval plane. It was used in the attack on Pearl Harbor and throughout the Pacific war. In the early years no American plane was a match for it. By late in the war, however, it was no match for most American planes.

The P-51 Mustang was one of the best American planes. It had a top speed of 370 miles per hour and was a favorite among American pilots. Many considered it the best fighter plane of the war. Its speed, maneuverability, and range made it an excellent aircraft. Another of the American planes was the Lockheed P-38 Lightning. It is said to have shot down more Japanese planes than any other American fighter during the war. It had a top speed of 414 miles per hour. Another excellent American plane was the F4U Corsair, which was used by US naval and marine pilots. It was the first plane to finally give Americans superiority over the Japanese zero, as it was much faster and had a better roll rate. Its maximum speed was 435 miles per hour.

The fastest and most interesting plane of the war, however, was the Messerschmitt Me 262, which was the world's first jet plane. It had a maximum
speed of about 530 miles per hour, which was 93 miles per hour faster than the swiftest Allied fighters. Fortunately for the Allies, it came into the war relatively late, and only a few were built, so it had little impact. Nevertheless, German pilots of the Messerschmitt Me 262 shot down approximately 540 Allied planes, and they were so fast that they were difficult targets. They were so fast, in fact, that German pilots had to learn new tactics when using them in combat. Allied pilots soon found that the best way to deal with them was to attack them on the ground or during takeoff or landing. Airfields in Germany that were identified as jet bases were therefore heavily bombed. The Me 262 did have a number of drawbacks, however; it used twice as much fuel as a conventional aircraft, and near the end of the war, Germany was running short on fuel. Furthermore, there were engine reliability problems.

The jet engine was invented by two different inventors at about the same time: Hans von Ohain and Frank Whittle. Frank whittle was the first to patent a turbojet engine; in fact, his patent came in 1930, six years before Ohain's. But neither man knew anything about the other's work. But it was Ohain who was first to build a workable jet plane.

Whittle was a pilot and an English aviation engineer who joined the RAF in 1928. At the age of twenty-two he came up with the idea of using a jet turbine to power an aircraft, and he began construction of a jet engine in 1935. It was tested in 1937, and an airplane using his engine first flew in 1941.

Like Whittle, Ohain was only twenty-two when he conceived the idea of a jet-propelled aircraft. His design was similar to Whittle's, but it differed in the internal arrangement of the parts. An airplane using his design for an engine was first flown in 1939. So both Germany and England actually had jet engines before the beginning of the war. But only Germany used the technology for a new type of fighter before the end of the war.

Details of a jet engine.

Jet engines operate as a result of Newton's third law, which states that for every action there is an equal and opposite reaction. The opposite reaction is what gives the thrust that pushes the jet plane forward. The easiest way to visualize this is to blow up a rubber balloon and let it go. You see immediately that it flies off in an array of flips and loops as the air forces its way out of the balloon. In short, as the air pushes its way out, it forces the deflating balloon in the opposite direction. This is basically what happens in a jet engine.

Several different kinds of jet engines now exist, but we'll restrict our discussion to the turbojet. At the front of the turbojet is an inlet that allows air to enter. Once inside, the air is compressed by blades that squeeze it into a much smaller volume. From here it is forced into what is called the combustion chamber. With the increase in pressure, the temperature of the gas goes up until it reaches over a thousand degrees Fahrenheit. Fuel is then sprayed into the air, and the mixture is ignited. This causes it to heat even more dramatically, and it leaves the combustion chamber, or combustor, with a temperature of about three thousand degrees Fahrenheit. The resulting heated gas exerts a large force in all directions, but it exits only at the rear of the engine, and this gives the plane a tremendous forward thrust. As the gas leaves the engine it passes through a series of blades that constitute the turbine, which rotates the turbine shaft. The turbine shaft, in turn, rotates a compressor that brings in a new supply of air. Thrust can be increased with the use of what is called an afterburner, where extra fuel is sprayed into the exiting gases, which burn to provide additional thrust.

THE FIRST ROCKETS IN WAR

Not only was the first jet introduced in World War II, but so was the first large ballistic rocket. Much of the technology, however, had already been developed by the physicist Robert Goddard. Goddard is now often referred to as the father of modern rocket propulsion, and the NASA Goddard space Center in Maryland is named after him. Most of his work took place at Clark University at Worcester, Massachusetts, where he was head of the physics department. In 1926 he constructed and launched the first rocket using liquid fuel. Earlier, in 1914, he had patented both liquid rocket fuel and solid rocket fuel. He made many contributions to rocketry, including a gyroscope control, power-driven fuel pumps, and vanes on the exterior of the rocket to help in its guidance. And he was the first to show that a rocket would work in vacuum and that it didn't need air to push against.

Early in World War II the Germans became interested in the possibility of
using rockets as weapons. Artillery Captain Walter Dornberger was assigned the job of determining how effective they would be. While looking into the problem, a young engineer by the name of Wernher von Braun came to his attention, and he hired him as head of his rocket artillery unit. By 1934 von Braun had a team of eighty engineers working for him, and operations were moved to Peenemünde, on the Baltic coast. Hitler now began taking an interest in the project.

Wernher von Braun.

Von Braun and his team had many problems to overcome. Rockets look rather simple, but a lot of science, particularly physics, is needed to make them work properly. The V-2 that von Braun was building could reach an altitude of almost seventy miles, and at this altitude there is almost no air. And the rocket fuel needed an ample supply of oxygen for it to burn. This meant that oxygen had to be added to the propellant. The V-2 used a 75 percent ethanol-water mixture for fuel and liquid oxygen as an oxidizer.
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Rockets are propelled in the same way jets are propelled. They also work because of Newton's third law, and again it's the reactive force that produces the thrust. It's also important to note that the rocket flight consists of several phases: launch, thrust, cruise, and crash. Actually, the first phase (launch) is when the rocket is sitting on the launch pad, so it's not moving. At this point there are two forces acting on it: the weight of the rocket downward, and its reaction force acting back from the pad. These two forces are equal and opposite.

The thrust phase begins when the rocket engine begins firing. At this time
there will be three forces acting on the rocket: the weight of the rocket, the thrust provided by the engine, and a drag force that is a result of air resistance. If we now apply Newton's second law, which states that force equals mass times acceleration, we get F
_thrust
− F
_drag
− wt. = ma, where m is mass, a is acceleration, and wt. is the rocket's weight. There is a small problem here, however: the mass of the rocket changes as a rocket moves upward because fuel is being burned. But this was easily overcome by early engineers.

Rocket, showing thrust, drag, and weight.

The blast from the engine will eventually stop at some point, and the rocket will enter the cruise phase. During this time there is no longer an upward thrust on the rocket, and it is on its own. It will continue gaining altitude for some time after its engines are shut down because of its velocity, but eventually it will reach its maximum altitude and begin falling back to earth, and because of gravity it will accelerate according to the formula a = (wt. − F
_drag
) / m. Thus, except for drag, it will drop like a falling stone. In reality, of course, the rocket is not going straight up and down, it is also moving horizontally, so its path will generally be similar to that of an artillery shell.

In a liquid-fueled rocket, the propellant and oxidizer have to be kept in separate tanks before the combustion. Oxygen is then combined with the fuel, with mixing taking place when the oxygen and fuel are sprayed into the combustion chamber. The ignition gases exit through a nozzle at the lower end, producing the thrust. These gases are at a very high temperature, so the nozzle has to be cooled. In early rockets the exhaust was cooled using alcohol and water.

The rocket also had to be stabilized once it was in flight, otherwise it would tumble uncontrollably. Two types of systems have been used for this: active and passive. Active elements are movable and passive are fixed. Of critical importance is the center of gravity of the rocket. It is important because all objects, including rockets, move around their center of gravity when they tumble. The center of gravity is the same as a center of mass, namely the point where all the mass can be considered to be concentrated.