A consquence of Newton's laws of motion is that for any object, or collection of objects, forces which only involve those objects and nothing else ("internal forces") cannot shift the center of gravity. For example, an astronaut floating in a space suit cannot shift his position without involving something else, e. g. pushing against his spacecraft. The center of gravity--or "center of mass"--is a fixed point, which cannot be moved without outside help (turning around it, however, is possible).
By throwing a heavy tool in one direction, the astronaut could get moving in the opposite direction, though the common center of gravity of the two would always stay the same. Given a bottle of compressed oxygen, the same result follows from squirting out a blast of gas (a scene that appeared in an early science fiction film). A rocket does much the same, except that the cold gas is replaced by the much faster jet of glowing gas produced by the burning of suitable fuel. At present, rockets are the only means capable of achieving the altitude and velocity necessary to put a payload into orbit.
A rocket engine is a machine that develops thrust by the rapid expulsion of matter. Most rockets today operate with either solid or liquid propellants. The word propellant does not mean simply fuel, as you might think; it means both fuel and oxidizer. The fuel is the chemical rockets burn but, for burning to take place, an oxidizer (oxygen) must be present. Jet engines draw oxygen into their engines from the surrounding air. Rockets do not have the luxury that jet planes have; they must carry oxygen with them into space, where there is no air.
There are a number of terms used to describe the power generated by a rocket.
Solid Propellant Rockets:
A solid-propellant rocket has the simplest form of engine. Solid propellant rockets are basically combustion chamber tubes packed with a propellant that contains both fuel and oxidizer blended together uniformly. It has a nozzle, a case, insulation, propellant, and an igniter. The case of the engine is usually a relatively thin metal that is lined with insulation to keep the propellant from burning through. The propellant itself is packed inside the insulation layer.
Solid rocket propellants, which are dry to the touch, contain both the fuel and oxidizer combined together in the chemical itself. Usually the fuel is a mixture of hydrogen compounds and carbon and the oxidizer is made up of oxygen compounds. The principal advantage is that a solid propellant is relatively stable therefore it can be manufactured and stored for future use. Solid propellants have a high density and can burn very fast. They are relatively insensitive to shock, vibration and acceleration. No propellant pumps are required thus the rocket engines are less complicated.
Disadvantages are that, once ignited, solid propellants cannot be throttled, turned off and then restarted because they burn until all the propellant is used. The surface area of the burning propellant is critical in determining the amount of thrust being generated. Cracks in the solid propellant increase the exposed surface area, thus the propellant burns faster than planned. If too many cracks develop, pressure inside the engine rises significantly and the rocket engine may explode. Manufacture of a solid propellant is an expensive, precision operation. Solid propellant rockets range in size from the Light Antitank Weapon to the 100 foot long Solid Rocket Boosters (SRBs) used on the side of the main fuel tank of the Space Shuttle.
Many solid-propellant rocket engines feature a hollow core that runs through the propellant. Rockets that do not have the hollow core must be ignited at the lower end of the propellants and burning proceeds gradually from one end of the rocket to the other. In all cases, only the surface of the propellant burns. However, to get higher thrust, the hollow core is used. This increases the surface of the propellants available for burning. The propellants burn from the inside out at a much higher rate, and the gases produced escape the engine at much higher speeds. This gives a greater thrust. Some propellant cores are star shaped to increase the burning surface even more.
To fire solid propellants, many kinds of igniters can be used. Fire-arrows were ignited by fuses, but sometimes these ignited too quickly and burned the rocketeer. A far safer and more reliable form of ignition used today is one that employs electricity. An example of an electrically fired rocket is the space shuttle's SRM. An electric current, coming through wires from some distance away, heats up a special wire inside the rocket. The wire raises the temperature of the propellant it is in contact with to the combustion point.
The nozzle in a solid-propellant engine is an opening at the back of the rocket that permits the hot expanding gases to escape. The narrow part of the nozzle is the throat. Just beyond the throat is the exit cone. The purpose of the nozzle is to increase the acceleration of the gases as they leave the rocket and thereby maximize the thrust. It does this by cutting down the opening through which the gases can escape.
To see how this works, you can experiment with a garden hose that has a spray nozzle attachment. This kind of nozzle does not have an exit cone, but that does not matter in the experiment. The important point about the nozzle is that the size of the opening can be varied. Start with the opening at its widest point. Watch how far the water squirts and feel the thrust produced by the departing water. Now reduce the diameter of the opening, and again note the distance the water squirts and feel the thrust. Rocket nozzles work the same way.
As with the inside of the rocket case, insulation is needed to protect the nozzle from the hot gases. The usual insulation is one that gradually erodes as the gas passes through. Small pieces of the insulation get very hot and break away from the nozzle. As they are blown away, heat is carried away with them.
Liquid Propellant Rockets:
The other main kind of rocket engine is one that uses liquid propellants. This is a much more complicated engine, as is evidenced by the fact that solid rocket engines were used for at least seven hundred years before the first successful liquid engine was tested. Liquid propellants have separate storage tanks - one for the fuel and one for the oxidizer. They also have pumps, a combustion chamber, and a nozzle. The fuel of a liquid-propellant rocket is usually kerosene or liquid hydrogen; the oxidizer is usually liquid oxygen. They are combined inside a cavity called the combustion chamber. High pressure turbopumps provide an example of the rocket engine. Here the propellants burn and build up high temperatures and pressures, and the expanding gas escapes through the nozzle at the lower end. To get the most power from the propellants, they must be mixed as completely as possible. Small injectors (nozzles) on the roof of the chamber spray and mix the propellants at the same time. Because the chamber operates under high pressures, the propellants need to be forced inside. Powerful, lightweight turbine pumps between the propellant tanks and combustion chambers take care of this job.
The major components of a chemical rocket assembly are a rocket motor or engine, propellant consisting of fuel and an oxidizer, a frame to hold the components, control systems and a cargo such as a satellite. A rocket differs from other engines in that it carries its fuel and oxidizer internally, therefore it will burn in the vacuum of space as well as within the Earth's atmosphere. The cargo is commonly referred to as the payload. A rocket is called a launch vehicle when it is used to launch a satellite or other payload into space. A rocket becomes a missile when the payload is a warhead and it is used as a weapon.
Many different types of rocket engines have been designed or proposed. Currently, the most powerful are the chemical propellant rocket engines. Other types being designed or that are proposed are ion rockets, photon rockets, magnetohydrodynamic drives and nuclear fission rockets; however, they are generally more suitable for providing long term thrust in space rather than launching a rocket and its payload from the Earth's surface into space.
A cryogenic propellant is one that uses very cold, liquefied gases as the fuel and the oxidizer. Liquid oxygen boils at -297 F and liquid hydrogen boils at -423 F. Cryogenic propellants require special insulated containers and vents to allow gas from the evaporating liquids to escape. The liquid fuel and oxidizer are pumped from the storage tanks to an expansion chamber and injected into the combustion chamber where they are mixed and ignited by a flame or spark. The fuel expands as it burns and the hot exhaust gases are directed out of the nozzle to provide thrust.
A hypergolic propellant is composed of a fuel and oxidizer that ignite when they come into contact with each other. No spark is needed. Hypergolic propellants are typically corrosive so storage requires special containers and safety facilities.
Monopropellants combine the properties of fuel and oxidizer in one chemical. By their nature, monopropellants are unstable and dangerous. Monopropellants are typically used in adjusting or vernier rockets to provide thrust for making changes to orbits once the payload is in orbit.
Advantages of liquid propellant rockets include the highest energy per unit of fuel mass, variable thrust, and a restart capability. Raw materials, such as oxygen and hydrogen are in abundant supply and a relatively easy to manufacture. Disadvantages of liquid propellant rockets include requirements for complex storage containers, complex plumbing, precise fuel and oxidizer injection metering, high speed/high capacity pumps, and difficulty in storing fueled rockets.
The Rocket Pioneers :
Authors Jules Verne and H. G. Wells wrote about the use of rockets and space travel and serious scientists soon turned their attention to rocket theory.
It was, of course, the 20th century that witnessed an explosion in the field of rocketry. By the end of the 19th century, the three men considered to be the primary pioneers of modern rocketry had been born and begun their studies, Konstantin Tsiolkovsky (Russian), Robert Goddard (American) and Hermann Oberth (German).
In 1898, a Russian schoolteacher, Konstantin Tsiolkovsky (1857-1935), proposed the idea of space exploration by rocket. In a report he published in 1903, Tsiolkovsky suggested the use of liquid propellants for rockets in order to achieve greater range. Tsiolkovsky stated that the speed and range of a rocket were limited only by the exhaust velocity of escaping gases. For his ideas, careful research, and great vision, Tsiolkovsky has been called the father of modern astronautics.
Hermann Oberth, a German scientist, also contributed to the theory and design of rockets. In 1923 he published a work in which he proved flight beyond the atmosphere is possible. In a 1929 book called "The Road to Space Travel" Oberth proposed liquid-propelled rockets, multistage rockets, space navigation, and guided and re-entry systems. He also advanced the idea of a transatlantic postal rocket for quick mail delivery. It was taken seriously at the time but never attempted.
From 1939 to 1945 he worked on German war rocket programs with such notables as Wernher von Braun. After the war he came to the United States where he again worked with von Braun. During the war one of the weapons the scientists were designing was reminiscent of Oberth's postal rocket. The German's wanted to build a rocket which would carry a bomb from Europe to strike New York City.
Most historians call Oberth and Tsiolkovsky the fathers of modern rocket theory. If that is so, an American, Dr. Robert H. Goddard, can be called the father of the practical rocket. His designs and working models eventually led to the German big rockets such as the V-2 used against the Allies in World War II. All three men are enshrined in the International Space Hall of Fame in Alamogordo, N.M.
Although rockets were used during World War I, they were of limited value. As was the case during the U.S. Civil War, rockets were simply not as effective as artillery weapons of the day. Rockets sometimes were employed both on land and at sea to lay smoke screens. Allied forces also used rockets as a method of illuminating battlefields. Rockets were exploded in a brilliant flash that could illuminate a battlefield for several seconds. Some rockets carried a parachute with a flare attached. As the parachute and flare dropped toward the ground, a battlefield could be illuminated for about 30 seconds.
Robert Goddard:
Robert Hutchings Goddard was born on October 5, 1882 in Worcester, Massachusetts. Early in his life, Goddard was inspired by works of science fiction, primarily "War Of The Worlds" by H.G. Wells and "From The Earth To The Moon" by Jules Verne. Completely independent of Tsiolkovsky, Goddard realized that the reaction principle would provide a foundation for space travel. But rather than focus entirely on theory, Goddard set out at an early age to become equipped to build and test the hardware he believed was necessary to best demonstrate the reaction principle. Again independent of Tsiolkovsky, he too theorized that a combination of liquid hydrogen and liquid oxygen would make an ideal propellant.
Considered a staunch patriot until his death, Goddard went to work for the Army in 1917 with the goal of designing rockets that would aid in the war effort. The work was conducted in California, and yielded the development of a small, hand-held rocket launcher similar to what was later called the bazooka. In 1919, Goddard published a work entitled "A Method Of Reaching Extreme Altitudes", which contained a detailed compilation of much of the research he had completed to date. It also included speculation on the possibilities of spaceflight. Goddard concluded that a combination of liquid oxygen and gasoline were the only practical fuels that could be used in his continuing research in the development of liquid-fueled rocket motors.
By 1924, Goddard had developed and tested a liquid oxygen pump and engine that functioned. The unit, however, was too small to actually be employed on a working rocket. But, with a working design, he began to plan more elaborate research. Goddard successfully test fired a pressure-fed liquid oxygen engine inside the Clark University physics laboratory on December 6, 1925. The engine was attached to a small test rocket housed inside a fixed stand. The engine was fired for about 24 seconds and lifted the rocket for about 12 seconds within its stand. On March 16, 1926 Goddard launched a 10-foot long rocket from a 7-foot long frame. The rocket reached a maximum altitude of 41 feet at an average velocity of 60 m.p.h. The rocket remained in the air for 2.5 seconds and flew a distance of 184 feet. While this flight did not even come close to matching the performance of gunpowder propelled rockets of years past, it remains one of the most significant events in the history of rocketry. Powered by a combination of liquid oxygen and gasoline, the rocket launched by Goddard on March 16, 1926 was the first to ever be launched using liquid fuel.
Following this flight, Goddard realized that his rocket was too small to be refined. He decided to develop larger rockets for further tests. Work was also begun on the development of a more elaborate launch tower. The new rockets incorporated innovative technology like flow regulators, multiple liquid injection, measurement of pressure and lifting force and an electrically fired igniter to replace a gunpowder fired igniter used previously. A turntable was also designed to produce spin stabilization.
The fourth launch of a liquid-fueled rocket occurred on July 17, 1929. Considered much more elaborate than the first three, Goddard equipped the rocket with a barometer, thermometer and a camera to record their readings during flight. The rocket achieved a maximum altitude of 90 feet in an 18.5-second flight covering a distance of 171 feet. The scientific payload was recovered safely via parachute. However, the launch was so noisy and bright that it captured much public attention. Many eyewitnesses believed an aircraft had crashed in the area. Local fire officials quickly forced Goddard to discontinue his launch operations at the Auburn site.
Goddard then made a large move after deciding to embark on his first full-time effort at constructing and testing rockets. He set up shop at the Mescalero Ranch near Roswell, New Mexico in July, 1930. The relocation was initially financed through the Guggenheim grant. The first Roswell launch occurred on December 30, 1930 using a rocket 11 feet long by 12 inches wide and weighing 33.5 pounds empty. The test was impressive as the rocket reached a maximum altitude of 2,000 feet and maximum speed of 500 m.p.h. The rocket employed a new gas pressure tank to force the liquid oxygen and gasoline into the combustion chamber.
In the years approaching World War II, Goddard had agreed to allow military officials to review his research. On May 28, 1940 Goddard and Harry F. Guggenheim had met with a joint committee of Army and Navy officials in Washington, D.C. A complete report was given to these officials by Goddard which outlined his advances in both solid-fueled and liquid-fueled rockets. The Army rejected the prospect of long-range rockets altogether. The Navy expressed a minor interest in liquid-fueled rockets. Goddard later characterized these responses as negative. Neither branch of service was interested in an innovative rocket aircraft that had been patented by Goddard on June 9, 1931. The lack of military interest in rocketry had confounded Goddard for years, since he understood that only the government had adequate resources to fund proper research.
