Rocket overview

A rocket is a vehicle, missile or aircraft which obtainsnuclear or solar heating as the power source to
thrust by the reaction to the ejection of fast movingvaporize water collected from around the solar
fluid from within a rocket engine.system.
OverviewRockets where the heat is supplied from other than
Usesthe propellant, such as steam rockets, are classed as
In military use, rockets generally use solid propellantexternal combustion engines. Other examples of
and are unguided. Rockets equipped with warheadsexternal combustion rocket engines include most
(representing a form of missile) can be fired bydesigns for nuclear powered rocket engines. Use of
ground-attack aircraft at fixed targets such ashydrogen as the propellant for external combustion
buildings, or can be launched by ground forces atengines gives very high velocities.
other ground targets. During the Vietnam era, thereRockets must be used when there is no other
were also air-launched unguided rockets that carried asubstance (land, water, or air) or force (gravity,
nuclear payload designed to attack aircraftmagnetism, light) that a vehicle may employ for
formations in flight. In military terminology, the wordpropulsion, such as in space. In these circumstances, it
missile is often preferred over rocket when theis necessary to carry all the propellant to be used.
weapon uses either solid or liquid propellant, and has aDelta-v
guidance system. (This distinction generally does notDelta-v is the theoretical total gain in speed that a
apply to civilian or orbital launch vehicles.)rocket can achieve without any external interference
Rockets remain the only way to leave the Earth and(without air drag or gravity or other forces).
are used to launch into orbit, to rapidly accelerateDue to their high exhaust velocity (mach ~10+),
vehicles, to change orbits, to de-orbit for landing, orrockets are particularly useful when very high speeds
for landing especially if there is no atmosphere, e.g.,are required, such as orbital speed (mach 25). The
for landing on the Moon, and sometimes to soften aspeeds that a rocket vehicle can reach can be
hard parachute landing immediately before touchdowncalculated by the rocket equation; which gives the
(see Soyuz spacecraft).speed difference ("delta-v") in terms of the exhaust
Operationspeed and ratio of initial mass to final mass ("mass
In all rockets, the exhaust is formed from propellant,ratio").
which is carried within the rocket prior to its release.The mass ratios that can be achieved with a single
Rocket thrust is due to the action of accelerating theset of fixed rocket engines and tankage varies
exhaust to very high speeds causing an equal anddepends on acceleration required, construction
opposite reaction on the vehicle.materials, tank layout, engine type and propellants
Typesused, but for example the first stage of the Saturn
There are many different types of rockets, and aV was able to achieved about a mass ratio of 10.
comprehensive list can be found in rocket engineStaging
— they range in size fromOften, the required velocity (delta-v) for a mission is
tiny models such as water rockets or small solidunattainable by any single rocket because the
rockets that can be purchased at a hobby store, topropellant, structure, guidance and engines weigh so
the enormous Saturn V used for the Apollo program.much as to prevent the mass ratio from being high
Most current rockets are chemically powered rocketsenough.
(internal combustion engines) that emit a hot exhaustThis problem is frequently solved by staging
gas. A chemical rocket engine can use solid propellant— the rocket sheds
(see Space Shuttle's SRBs), liquid propellant (seeexcess weight (usually tankage and engines) during
Space shuttle main engine), or a hybrid mixture oflaunch to reduce its weight and effectively increase
both. A chemical reaction is initiated between the fuelits mass ratio.
and the oxidizer in the combustion chamber, and theTypically, the acceleration of a rocket increases with
resultant hot gases accelerate out of a nozzle (ortime (if the thrust stays the same) as the weight of
nozzles) at the rearward-facing end of the rocket.the rocket decreases as propellant is burned.
The acceleration of these gases through the engineDiscontinuities in acceleration will occur when stages
exerts force ("thrust") on the combustion chamberburn out, often starting at a lower acceleration with
and nozzle, propelling the vehicle (in accordance witheach new stage firing.
Newton's Third Law). See rocket engine for details.Vehicles
Not all rockets use chemical reactions. Steam rockets,Rockets as a group have the highest thrust/weight
for example, release superheated water through aratio of any type of engine; and this helps vehicles
nozzle where it instantly flashes to high velocityachieve a high mass ratios, which improves the
steam, propelling the rocket. The efficiency of steamperformance of flight.
as a rocket propellant is relatively low, but it is simpleCommon mass ratios for vehicles are 20:1 for dense
and reasonably safe, and the propellant is cheap andpropellants such as liquid oxygen and kerosene, 25:1
widely available. Most steam rockets have been usedfor dense monopropellants such as hydrogen
for propelling land-based vehicles but a small steamperoxide, and 10:1 for liquid oxygen and liquid
rocket was tested in 2004 on board the UK-DMChydrogen. However, mass ratio is highly dependent
satellite. There are even proposals to use steamon many factors such as the type of engine the
rockets for interplanetary transport using eithervehicle uses and structural safety margins