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Rocket 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. A missile, by contrast, can use either solid or liquid propellant, and has a guidance system. 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. 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. |