The Physics of War

by Barry Parker

  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.12

  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 Fthrust − Fdrag − 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. − Fdrag) / 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.

  In flight a rocket can tumble around one or more of three different axes, referred to as the roll, pitch, and yaw axes. Spin around the roll axis is no problem, but we want to avoid tumbling around either of the other axes. Gyroscopes are used for this, and also to assist in guidance. The vanes on the lower end of the rocket also help stabilize it.

  The V-2 was to be Hitler's vengeance weapon, and in early September 1944 he declared that V-2 attacks would begin, and London was to be a major target. Over the next few months over fourteen hundred were directed at London. But their accuracy was poor and they were unable to hit vital targets. For the most part, the V-2 was a terror weapon, and it did, indeed, create a lot of terror as it shot across the English sky. Because of the speed of V-2 rockets (approximately 2,200 miles per hour) and their high-altitude flight, they we
re almost impossible to shoot down. In all, about 2,550 civilians were killed in London by V-2s, and another 6,500 were injured.

  The Germans also built another, similar weapon called the V-1 “buzz bomb.” It was smaller than the V-2, with a length of twenty-seven feet, compared with the V-2's forty-six feet, and it was much slower. A pulsed jet engine powered it; air entered the intake of the engine where it was mixed with fuel and ignited by spark plugs. Shutters opened and closed at the rear of the device about fifty times per second, giving it the buzzing sound that inspired its nickname.

  The V-1 was developed at Peenemünde at the same time that the V-2 was being built. It was not a ballistic rocket; rather, it was launched from ground sites using a ramp and catapult. It is therefore referred to as a cruise missile. The first V-1 attack took place in mid-June 1944, just before the V-2 attacks began, and the V-1 attacks were also directed toward London. Like the V-2, the V-1 could not attack specific targets, so it, too, was mainly meant as a terror weapon. But unlike the V-2, there was considerable defense against it. Some of the faster airplanes could knock it down in flight, and it was quite vulnerable to coastal artillery. In fact, by late August 1944, almost 70 percent of incoming V-1s were being destroyed by coastal artillery. In all, about ten thousand V-1s were fired at England. About 2,420 reached London, killing approximately 6,180 people and injuring 17,780.

  OTHER WEAPONS AND SMALL ARMS

  Tanks played a large role in World War II. During the German blitzkrieg, in fact, they seemed to be unstoppable, and the Allies were soon looking for weapons that could counter them. Over the next few years several types of warheads were developed that were able to penetrate the armor of a tank, and they employed an important physics principle. They were based on the idea of a shaped charge. A shaped charge is an explosion that has a shape that focuses the energy of the shell. It is based on what is called the Munroe effect, discovered by the American chemist Charles Munroe. Munroe showed that a hollowed end on a charge produces a much more powerful wave that concentrates the explosion along the axis of the charge. This is because the shock waves from the explosion are reinforced in this case.

  When applied to stopping tanks, the warheads are referred to as HEAT warheads (high- explosive, anti-tank warheads). They create a high-velocity stream of metal that can push through relatively heavy tank armor. This stream actually moves at nearly twenty-five times the speed of sound. HEAT warheads are less effective if they spin, so they usually are fin-stabilized.

  HEAT rounds caused a significant change in tank warfare when they were first introduced late in the war. A single soldier could now destroy a tank using a hand-held weapon. The search was soon underway for a protection from the new shells, and the Germans began protecting their tanks with armored or mesh skirts, which caused the HEAT shells to detonate prematurely.

  Another type of shell was also used quite effectively against tanks. It was called the HESH warhead (high-explosive squashed head). It was originally developed for penetrating concrete buildings, but it was also found to be effective against tanks. In this case the explosive material is “squashed” when it hits the target so that it spreads out over a large area. A detonating fuse triggers it at this point, creating a larger shockwave due to its larger area. This shockwave moves through the metal to the interior of the tank, causing pieces of metal to fly off the interior wall at high speed. These metal pieces could injure or kill the crew and ignite ammunition or fuel inside the tank.

  Both HESH and HEAT warheads were delivered against armored vehicles using bazookas. A bazooka is a rocket-powered, recoilless weapon originally developed by Robert Goddard while he was working on rocket propulsion. He and a coworker Clarence Hickman developed and demonstrated it to the US Army at the Aberdeen Proving Ground in Maryland in November 1918. At this point, however, it didn't use a shaped charge. It was teamed up with shaped charges in 1942, and it was first used in North Africa and by the Russians on the Eastern front at about the same time. The early models were not too reliable, however, and some of them were captured by the Germans. The Germans quickly copied and improved on the early bazookas, and much to the surprise of the Allies, the German bazookas were more powerful than theirs and had greater armor penetration.

  Another important development in which physics was involved was the proximity fuse. At the beginning of the war detonation of a warhead occurred when it hit the target, or after a certain time set on a timer. Both of these had disadvantages, and the full effect of most exploding shells was not realized. With the proximity fuse, the device detonates automatically when the distance between the target and the projectile is smaller than some predetermined value. Shells could therefore be made to detonate before they hit the ground—in particular, over the heads of enemy troops—which improved their effectiveness.

  The fuse was based on electromagnetic principles; it contained an oscillator connected to an antenna that functioned as both the transmitter and receiver. As the shell closed in on the target it could determine how far it was away by analyzing the reflected signal. It was used quite effectively against V-1 buzz bomb attacks on England as well as during the Battle of the Bulge. It was also helpful in the defense against Japanese kamikaze attacks in the Pacific.

  Radio-guided missiles were also used for the first time in World War II. The Germans developed an antiship guided bomb called the Fritz X. It was delivered by aircraft and was radio-controlled from the delivering plane. Signals were picked up by a receiver in the missile. Fritz X was not considered to be very successful, however. Similar guided bombs were also developed in England. Called GB-1s, they were dropped on Cologne, Germany. Another German guided bomb was the Kraus X-1; several Allied warships were heavily damaged by it. And the V-1 and V-2 were also radio guided.

  Another of the ingenious devices to come out of the war was the Norden bombsight.13 One of the major problems during the early part of the war was accurate bombing from high altitudes. In 1943 a plane dropping a bomb from a high altitude had a CEP (circular error probability) of twelve hundred feet, which made the likelihood of hitting a target extremely low. It was so low, in fact, that both the air force and the navy had given up on pinpoint bombing attacks. Over several years, however, Carl Norden, a Dutch engineer who had immigrated to the United States, had been working on a bombsight. One of the main problems in using bombsights was leveling the aircraft so that the sight could be pointed straight down. Wind was also a serious problem. Norden's bombsight allowed bombs to be dropped at exactly the right time for hitting a given target. It used an analog computer consisting of gyros, motors, gears, mirrors, levels, and a telescope. The bombardier would program the airspeed, wind speed, direction, and altitude into the device. The computer would then calculate the trajectory needed for the bomb to hit the target. Then, as the plane approached the target, the pilot would turn the plane over to autopilot so that it would fly to the precise point needed for the drop. It is said that with this device a bomb could be placed within a one-hundred-foot circle from a height of four miles.

  The Norden bombsight was one of the major secrets of the war, and its existence was carefully guarded for the duration of the war. It was particularly effective in the bombing of Germany during the later parts of the war.

  Finally, let's look at the small arms and infantry weapons that were used during the war. They were much more powerful, accurate, and lethal than those used in World War I. At the beginning of the war, however, some of the same weapons were used. The bolt-action rifles used in World War I were also used at the beginning of World War II. Later on they were used as sniper rifles, mostly because of their long range and high accuracy. A bolt-action rifle equipped with a telescopic sight was an excellent sniper weapon, but for close-up fighting soldiers needed a much faster rate of fire, and because of this, semiautomatic rifles were soon developed. One of the best American semiautomatics was the M1 Garand, and it soon became the standard American rifle of the war.

  The submachine gun also played a larg
e role in the war. It was the small, relatively light equivalent of the regular machine gun. Its ammunition, however, was much smaller and lighter, and this meant that it had a relatively short range, and its accuracy was not as high. But it was quite effective in short-range combat. The Germans used it extensively; their best-known submachine gun was the MP-18. The American equivalent was the Thompson submachine gun.

  The major problem with the submachine gun was its inaccuracy and short range. In most battlefield situations soldiers needed both rapid fire and accuracy at a distance. The accuracy did not need to be as great as that of a standard bolt-action rifle, such as the Lee-Enfield or the Springfield, but a range greater than that of the submachine gun was desired. Because of this, the assault rifle was developed. It was first used by the German army; their MP-43 came into service in 1943 and was clearly a superior weapon. The American M-16 and Russian AK-47, which came into being after the war, were based on it.

  Basic machine guns were still used, as they were in World War I, but they were now much lighter so that they could be handled by a single soldier. In most cases, however, a second soldier was needed for carrying ammunition and to help set it up and feed it during firing. Finally, other weapons such as hand grenades, flamethrowers, and light mortars of various types were also used. And most were more lethal because of technical advances.

  COMPUTERS AND INTELLIGENCE

  Another area in which tremendous advances were made as a result of the war was that of computers. World War I was perhaps the first war in which a large amount of information had to be moved as quickly as possible, and for this, a good communication system was needed. And of course, the need became even greater in World War II. Not only was there a need for communication about the movement and direction that various troops, squadrons, and so on should take, but it was also important to keep this information from the enemy. This meant that it had to be enciphered, which soon set off a race between code breakers and code makers. Codes became more and more complicated, and soon they could only be deciphered by machines, namely computers. Work on computers had begun before the war, much of it in Germany. The German engineer Konrad Zuse had built a simple computer that he called the Z1 in 1936. He continued to work on it during the war, improving it significantly. A similar device, eventually called Mark I was being built in the United States.14 The war, and particularly a need for decoding enemy ciphers, soon created a demand for larger and faster computers. The Germans had begun using a coding machine called Enigma. Enigma allowed an operator to type a message then scramble it using notched wheels or rotors, each of which contained the letters of the alphabet. There were twenty-six electrical contacts on each side of the wheels corresponding to the letters of the alphabet. When a message was typed in, it was sent to the second wheel via electrical contacts, but contact was made at a different position on the second wheel, so a given letter, such as C, would be given a different designation, such as Z. Contact was then passed from this wheel to a third wheel, and again contact was made at a different position. In the earliest models, three wheels were used, but more wheels were added, making it even more complicated. With such a setup, it was almost impossible for someone to decode its messages. Furthermore, the codes could be changed each time the machine was used. Decoding was simple for the receiver, however; he merely had to set his machine up in the same way as the sender's machine.