Science Fair Project Encyclopedia
A rocket is a vehicle, missile or aircraft which obtains thrust by the reaction to the ejection of fast moving exhaust gas from within a rocket engine. Often the term rocket is also used to mean a rocket engine.
In military terminology, a rocket generally uses solid propellant and is unguided. These rockets can be fired by ground-attack aircraft at fixed targets such as buildings, or can be launched by ground forces at other ground targets. During the Vietnam era, there were also air launched unguided rockets that carried a nuclear payload designed to attack aircraft formations in flight.
In all rockets the exhaust is formed from propellant which is carried within the rocket prior to its release. Rocket thrust is due to the exhaust gases applying pressure on the inside surfaces of the rocket engine as they accelerate (see Newton's 3rd Law of Motion).
There are many different types of rockets, and a comprehensive list can be found in spacecraft propulsion- they range in size from tiny models that can be purchased at a hobby store, to the enormous Saturn V used for the Apollo program.
Rockets are also used for deceleration, to transfer to a lower-energy orbit, for example to enter into a circular orbit from outside, to de-orbit for landing, for the whole landing if there is no atmosphere (e.g. for landing on the Moon, the rocket of the descent stage of the Apollo Lunar Module was applied), and sometimes to soften a parachute landing.
Most current rockets are chemically powered rockets (internal combustion engines). A chemical rocket engine may use solid propellant, such as the Space Shuttle's SRBs, or liquid propellant, like the Space Shuttle's main engines, or a hybrid. A chemical reaction is initiated between the fuel and the oxidizer in the combustion chamber, and the resultant hot gases accelerate out of a nozzle (or nozzles) at the rearward facing end of the rocket. The acceleration of these gases through the engine exerts force ('thrust') on the combustion chamber and nozzle, propelling the vehicle (in accordance with Newton's Third Law). See rocket engine for details.
However not all rockets use chemical reactions. Steam rockets have also been used, e.g. drag racing. Steam rockets store superheated water under high pressure in their propellant tanks. The water may be at any temperature from 200 C to 500 C or more. When the water is released through a nozzle it instantly flashes to high velocity steam, propelling the rocket as described above for chemical rockets. Generally, the attainable exhaust velocity of steam is relatively low, but is simple and nevertheless effective. To date, most steam rockets have been used for propelling land-based vehicles but there are serious proposals to use them for interplanetary spacecraft using either nuclear or solar heating as the power source. A small steam rocket was tested in 2004 on board the UK-DMC satellite.
Rockets where the heat is supplied from other than the propellent, such as steam rockets, are classed as external combustion engines. Other examples of external combustion rocket engines include most designs for nuclear powered rocket engines. Use of hydrogen as the propellent for external combustion engines gives very high velocities.
Rockets are particularly useful when very high speeds are required, such as orbital speed (mach 25 or so). The speeds that a rocket vehicle can reach can be calculated by the rocket equation; which gives the speed difference ('delta-v') in terms of the exhaust speed and ratio of inital mass to final mass ('mass ratio').
Rockets must be used when there is no other substance (land, water, or air) or force (gravity, magnetism, light) that a vehicle may employ for propulsion, such as in space. In these circumstances, it is necessary to carry all the propellant within the vehicle, until use.
Common mass ratios for vehicles are 20/1 for dense propellants such as liquid oxygen and kerosene, 25/1 for dense monopropellants such as hydrogen peroxide, and 10/1 for liquid oxygen and liquid hydrogen. However, mass ratio is highly dependent on many factors such as the type of engine the vehicle uses and structural safety margins.
Sometimes, particularly in launch scenarios, the required velocity (delta-v) for a mission is unattainable because the propellant, structure, guidance and engines weigh so much as prevent the mass ratio from being high enough. This problem is frequently solved by staging - the rocket sheds excess weight (usually tankage and engines) to attain a higher effective mass ratio thus permitting a higher delta-v.
Typically, the acceleration of a rocket increases with time, even when applying the same thrust- due to decreasing fuel mass. Discontinuities in acceleration will occur when stages burn out, often starting at a lower acceleration with each new stage firing.
Beginnings of rocketry
Historically, rockets were first developed by the Chinese as early as B.C. 300, using gunpowder. These were initially developed for entertainment, the precursors to modern fireworks, but were later adapted for warfare in the 11th century. Because the pressures on the rocket walls are lower, the use of rockets in warfare preceded the use of the gun, which required a higher level of metal technology. It was in this role that rockets first became known to Europeans following their use by Ottomans at the siege of Constantinople in 1453. For several more centuries they remained curiosities to those in the West.
Since mid-17th century, for over two centuries the work of Polish-Lithuanian Commonwealth nobleman Kazimierz Siemienowicz, "Artis Magnae Artilleriae pars prima" ("Great Art of Artillery, the First Part". also known as "The Complete Art of Artillery"), was used in Europe as a basic artillery manual. The book provided the standard designs for creating rockets, fireballs, and other pyrotechnic devices. It contains a large chapter on caliber, construction, production and properties of rockets (for both military and civil purposes), including multi-stage rockets, batteries of rockets, and rockets with delta wing stabilizers (instead of the common guiding rods ).
At the end of the 18th century, rockets were used militarily in India against the British by the Mahrattas. The British then took up the practice and developed them further during the 19th century. The major figure in the field at this time was William Congreve. From there, the use of military rockets spread throughout Europe. The rockets' red glare helped to inspire the US national anthem.
Early rockets were highly inaccurate. Without any spinning up of the rocket, nor any gimballing of the thrust, they had a strong tendency to veer sharply off course. The early British Congreve rockets reduced this tendency somewhat by attaching a long stick to the end of a rocket (similar to modern bottle rockets) to make it harder for the rocket to change course. The largest of the Congreve rockets was the 32 pound (14.5 kg) Carcass, which had a 15 foot (4.6 m) stick. Originally, sticks were mounted on the side, but this was later changed to mounting in the center of the rocket, reducing drag and enabling the rocket to be more accurately fired from a segment of pipe.
The accuracy problem was mostly solved in 1844 when William Hale modified the rocket design so that thrust was slightly vectored to cause the rocket to spin along its axis of travel like a bullet. The Hale rocket removed the need for a rocket stick, travelled further due to reduced air resistance, and was far more accurate.
In 1903, high school mathematics teacher Konstantin Tsiolkovsky (1857-1935) published Исследование мировых пространств реактивными приборами (The Exploration of Cosmic Space by Means of Reaction Motors), the first serious scientific work on space travel. The Tsiolkovsky rocket equation—the principle that governs rocket propulsion—is named in his honor. His work was apparently unknown outside Soviet Russia, where it inspired further research, experimentation, and the formation of the Cosmonautics Society. It remained for Robert Goddard and Hermann Oberth to independently discover the same principles.
Early rockets were also remarkably inefficient. Modern rockets were born when, after receiving a grant in 1917 from the Smithsonian Institution, Robert Goddard attached a de Laval nozzle to a rocket engine's combustion chamber, doubling the thrust and enormously raising the efficiency, giving the real possibility of practical space travel.
In 1923, Hermann Oberth (1894-1989) published Die Rakete zu den Planetenraumen ("The Rocket into Planetary Space"), a version of his doctoral thesis, after the University of Munich rejected it. This book is often credited as the first serious scientific work on the topic that received international attention. Among other contributions, Oberth suggested that stages would be more effective than carrying dead weight.
In the mid-1920s, German scientists had begun experimenting with rockets which used liquid propellants capable of reaching relatively high altitudes and distances. A team of amateur rocket engineers had formed the Verein für Raumschiffahrt (German Rocket Society, or VfR) in 1927, and in 1931 launched a liquid propellant rocket (using oxygen and gasoline).
In 1932, the Reichswehr (which in 1935 became the Wehrmacht) began to take an interest in rocketry, seeing the possibility of using rockets as long-range artillery fire. The Wehrmacht initially funded the VfR team, but seeing that their focus was strictly scientific, created its own research team, with Hermann Oberth as a senior member. At the behest of military leaders, Wernher von Braun, at the time a young aspiring rocket scientist, joined the military (followed by two former VfR members) and developed long-range weapons for use in World War II by Nazi Germany, notably the A series of rockets, which led to the infamous V-2 rocket (initially called A4).
In 1943, production of the V-2 rocket began. The V-2 had an operational range of 300 km (185 miles) and carried a 1000 kg (2204 lb) warhead, with an amatol explosive charge. Thousands were fired at various Allied nations, mainly England, as well as Belgium and France. While uninterceptible, their crude guidance systems and single conventional warhead meant that the V-2's were largely militarily ineffective. They did kill 2,754 people in England alone, and wounding another 6,523 until the termination of the launches, and provided a lethal demonstration of the potential for guided rockets as weapons.
At the end of the war, competing Russian, British, and U.S. military and scientific crews raced to capture technology and trained personnel from the German rocket program at Peenemünde. Russia and Britain had some success, but the United States benefited most, taking a large number of German rocket scientists—many of whom were members of the Nazi Party, including von Braun—from Germany to the United States as part of Operation Paperclip. There the same rockets which would have been destined to rain down on Britain had the war continued were used by scientists for other uses.
After the war, rockets were used to study high-altitude conditions, by radio telemetry of temperature and pressure of the atmosphere, detection of cosmic rays, and further research. This continued under von Braun and the others, who were destined to become part of the U.S. scientific complex.
Rockets remain a popular military weapon. The use of large battlefield rockets of the V-2 type has given way to guided missiles, but rockets are often used by helicopters and light aircraft for ground attack, being more powerful than machine guns, but without the recoil of a heavy cannon. In the 1950s there was a brief vogue for air-to-air rockets, including the formidable AIR-2 'Genie' nuclear rocket, but by the early 1960s these had largely been abandoned in favor of air-to-air missiles.
- Nuclear thermal rockets have also been developed, but never deployed, they are particularly promising for interplanetary use.
- Neofuel - Nuclear/solar steam rockets for interplanetary use, using abundant extraterrestial ice
- Nuclear pulse propulsion rocket concepts give very high thrust and exhaust velocities.
Another class of rocket-like thrusters in increasingly common use are ion drives, which use electrical rather than chemical energy to accelerate their reaction mass.
- Timeline of rocket and missile technology
- List of rockets
- Model rocket
- Pulse jet engine
- Rocket fuel
- Rocket launch
- Rocket propelled grenade
- Rocket sled
- Sounding rocket
- Spacecraft propulsion
- Stalin Organ
- Water rocket
- Tsiolkovsky rocket equation
- National Association of Rocketry
- Tripoli Rocketry Association
- Canadian Association of Rocketry
- Hobby Industry Association
- Radio Control Hobby Trade Association
- Japan Association of Rocketry (site in Japanese)
- Indian Space Research Organisaion
- Encyclopedia Astronautica - Rocket and Missile Alphabetical Index
- Gunter's Space Page - Complete Rocket and Missile Lists
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