A self-propelling device in which the fuel substances needed to produce the propulsion are carried internally. The term most commonly refers to space vehicles, although it can also apply to distress rockets and fireworks. In addition, rockets are used to power missiles, and for supersonic and assisted-take-off aeroplane propulsion. Rockets work by burning fuel inside a combustion chamber. Both the fuel and the oxygen (oxidant) needed to burn it are carried inside the rocket itself. When the fuel is burnt, a large volume of hot gas is produced, which exerts great pressure on the inside surface. The upward pressure is much greater than the downward pressure, because the gases are allowed to escape through a nozzle at the bottom. The stronger pressure at the top results in an upward force (thrust) that makes the rocket rise. The force and the upward movement continue until all the fuel has been exhausted. Solid fuel rockets (like fireworks) commonly use a mixture of nitrocellulose and nitroglycerine as the fuel source. The solid rocket boosters of the space shuttle use a mixture of powdered aluminium in a synthetic rubber binder. The more efficient liquid fuel rockets use hydrogen and kerosene (fuel) and liquid oxygen (oxidant). Other means of propulsion, such as nuclear furnaces, are being developed. No single rocket is powerful enough to lift itself into orbit. A space-launcher is made up of several rockets creating a multi-stage (step) rocket. Once the fuel for one stage is exhausted, that stage is dumped, and the next stage is ignited. The rocket launcher becomes progressively lighter and faster as it climbs into space. One of the largest rockets was the Saturn V Moon rocket (a three-stage rocket) which weighed 2700 tonnes at launch.
The traditional definition of a rocket is a vehicle, missile or aircraft which obtains thrust by the reaction to the ejection of fast moving fluid from within a rocket engine. Often the term is also used to refer to a rocket engine.
Overview
Uses
In military use, rockets generally use solid propellant and are unguided. Rockets equipped with warheads (representing a form of missile) can be fired by ground-attack aircraft at fixed targets such as buildings, or can be launched by ground forces at other ground targets. In military terminology, the word missile is often preferred over rocket when the weapon uses either solid or liquid propellant, and has a guidance system. (This distinction generally does not apply to civilian or orbital launch vehicles.)
Rockets are used to accelerate, change orbits, de-orbit for landing, for the whole landing if there is no atmosphere, e.g., for landing on the Moon, and sometimes to soften a hard parachute landing immediately before touchdown (see Soyuz spacecraft).
Operation
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 very fast release of exhaust gases (for more info see Newton's 3rd Law of Motion).
Types
There are many different types of rockets, and a comprehensive list can be found in spacecraft propulsion — they range in size from tiny models such as water rockets or small solid rockets that can be purchased at a hobby store, to the enormous Saturn V used for the Apollo program.
Most current rockets are chemically powered rockets (internal combustion engines) that emit an exhaust gas. A chemical rocket engine can use solid propellant (see Space Shuttle's SRBs), liquid propellant (see Space shuttle main engine), or a hybrid mixture of both. 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. See rocket engine for details.
Not all rockets use chemical reactions. Steam rockets, for example, release superheated water through a nozzle where it instantly flashes to high velocity steam, propelling the rocket. The efficiency of steam as a rocket propellant is relatively low, but it is simple and reasonably safe, and the propellant is cheap and widely available. Most steam rockets have been used for propelling land-based vehicles but a small steam rocket was tested in 2004 on board the UK-DMC satellite.
Rockets where the heat is supplied from other than the propellant, 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.
Due to their high exhaust velocity (mach ~10+), rockets are particularly useful when very high speeds are required, such as orbital speed (mach 25). The speeds that a rocket vehicle can reach can be calculated by the rocket equation;
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.
Vehicles
Rockets as a group have the highest thrust/weight ratio of any type of engine;
Velocities
Often, the required velocity (delta-v) for a mission is unattainable by any single rocket because the propellant, structure, guidance and engines weigh so much as to prevent the mass ratio from being high enough. This problem is frequently solved by staging — the rocket sheds excess weight (usually tankage and engines) during launch to reduce its weight and effectively increase its mass ratio.
Typically, the acceleration of a rocket increases with time (even if the thrust stays the same) as the weight of the rocket decreases as fuel is burned.
History
Origins of rocketry
The origin of rockets as most people think of them dates back over 2,000 years ago when people of the Han Dynasty in China (c.
The ancient Chinese invention of gunpowder by Taoist alchemists, and their use of it in various forms of weapons (fire arrows, bombs, and cannons) resulted in the development of the rocket. They were the precursors to modern fireworks, and after extensive research, were adapted for use as artillery in warfare during the 10th century to 12th century which is when the earliest documented solid rockets are found.
Spread of rocket technology
Rocket technology first became known to Europeans following their use by the Mongols Genghis Khan and Ögedei Khan when they conquered Russia, Eastern Europe, and parts of Central Europe, i.e., Austria.
Additionally, the spread of rockets into Europe was also influenced by the Ottomans at the siege of Constantinople in 1453, although it is very likely that the Ottomans themselves were influenced by the Mongol invasions of the previous few centuries. It contained 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, iron-cased rockets were successfully used militarily in India against the British by Tipu Sultan of the Kingdom of Mysore during the Anglo-Mysore Wars. At the Battle of Baltimore in 1814, the rockets fired on Fort McHenry by the rocket vessel HMS Erebus were the source of the rockets' red glare described by Francis Scott Key in The Star-Spangled Banner.
Early rockets were very inaccurate. The early British Congreve rockets reduced this 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. 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. The Tsiolkovsky rocket equation—the principle that governs rocket propulsion—is named in his honor. Among other ideas, Tsiolkovsky accurately proposed to use liquid oxygen and liquid hydrogen as a nearly optimal propellant pair and determined that building staged and clustered rockets to increase the overall mass efficiency would dramatically increase range.
Modern rocketry
Modern rockets were born when Robert Goddard attached a supersonic (de Laval) nozzle to a rocket engine's combustion chamber. In 1920, Goddard published A Method of Reaching Extreme Altitudes, the first serious work on using rockets in space travel after Tsiolkovsky.
In 1923, Hermann Oberth (1894-1989) published Die Rakete zu den Planetenräumen ("The Rocket into Planetary Space"), a version of his doctoral thesis, after the University of Munich rejected it.
In 1926, Robert Goddard launched the world's first liquid-fueled rocket in Auburn, Massachusetts.
During the 1920s, a number of rocket research organizations appeared in America, Austria, Britain, Czechoslovakia, France, Italy, Germany, and Russia. 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).
From 1931 to 1937, the most extensive scientific work on rocket engine design occurred in Leningrad, at the Gas Dynamics Laboratory. 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.
At the end of World War II, competing Russian, British, and U.S. military and scientific crews raced to capture technology and trained personnel from the German rocket program at Peenemünde. There the same rockets that were designed to rain down on Britain were used instead by scientists as research vehicles for developing the new technology further. The V-2 evolved into the American Redstone rocket, used in the early space program.
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.
Rockets became extremely important militarily in the form of intercontinental ballistic missiles (ICBMs) when it was realised that nuclear weapons carried on a rocket vehicle were essentially not defensible against once launched, and they became the delivery platform of choice for these weapons.
Fuelled partly by the Cold War, the 1960s became the decade of rapid development of rocket technology in the Soviet Union (Vostok, Soyuz, Proton) and in the United States (e.g.
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 AIR-2 'Genie' nuclear rocket, but by the early 1960s these had largely been abandoned in favor of air-to-air missiles.
However, in the minds of much of the public, the most important use of rockets is manned spaceflight.
Net thrust
Below is an approximate equation for calculating the Gross Thrust of a rocket:
where:
exhaust gas mass flow
jet velocity at nozzle exit plane
flow area at nozzle exit plane
static pressure at nozzle exit plane
ambient (or atmospheric) pressure
Since, unlike a jet engine, a conventional rocket motor lacks an air intake, there is no Ram Drag to deduct from the Gross Thrust. Consequently the Net Thrust of a rocket motor is equal the Gross Thrust. At full throttle, the net thrust of a rocket motor improves slightly with increasing altitude, because the reducing atmospheric pressure increases the pressure thrust term.
It is however very usual to rearrange the above equation slightly:
Where: the effective exhaust velocity in a vacuum of that particular engine.
In the US any rocket launch that is not classified as amateur, and also is not "for and by the government," must be approved by the Federal Aviation Administration's Office of Commercial Space Transportation (FAA/AST), located in Washington, DC.
Accidents
Because of the enormous chemical energy in all useful rocket fuels (greater energy per weight ratio than explosives, but lower than gasoline), accidents can and have happened.
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