Rocket
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A Redstone rocket, part of the Mercury program
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. A missile, by contrast, can use either solid or liquid propellant, and has a guidance system. This distinction generally applies only in the case of weapons, though, and not to civilian or orbital launch vehicles.
In all rockets the exhaust is formed from propellant which is carried within the rocket prior to its release. Rocket thrust is due to accelerating the exhaust gases (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 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 parachute landing immediately before touchdown (see Soyuz spacecraft).
Most current rockets are chemically powered rockets (internal combustion engines). 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. 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.
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. There are proposals to use steam rockets for interplanetary transport using either nuclear or solar heating as the power source to vaporize water collected from around the solar system.
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. Use of hydrogen as the propellant for external combustion engines gives very high velocities.
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; which gives the speed difference ('delta-v') in terms of the exhaust speed and ratio of initial 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 to be used.
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.
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. Discontinuities in acceleration will occur when stages burn out, often starting at a lower acceleration with each new stage firing.