Rocket Engines
Most rocket engines produce
thrust by the expulsion of a high-temperature, high-speed gaseous exhaust.
This is typically created by high pressure (10-200 bar) combustion of
solid or liquid propellants, consisting of fuel and oxidiser components,
within a combustion chamber.
A rocket engine is a reaction engine that takes all its reaction mass
from within tankage and forms it into a high speed jet, thereby obtaining
thrust in accordance with Newton's third law. Rocket engines can be
used for spacecraft propulsion as well as terrestrial uses, such as
missiles. Most rocket engines are internal combustion engines, although
non combusting forms also exist.
Liquid-fueled rockets typically
pump separate fuel and oxidiser components into the combustion chamber,
where they mix and burn. Solid rocket propellants are prepared as a
mixture of fuel and oxidising components and the propellant storage
chamber becomes the combustion chamber. Hybrid rocket engines use a
combination of solid and liquid or gaseous propellants. Alternatively,
a chemically inert reaction mass can be heated using a high-energy power
source.
The hot gas produced escapes
through a narrow opening (the "throat"), into a high expansion-ratio
'de Laval nozzle'. The nozzle dramatically accelerates the gas, converting
most of the thermal energy into kinetic energy. The large bell or cone
shaped expansion nozzle gives a rocket engine its characteristic shape.
Exhaust speeds as high as ten times the speed of sound at sea level
are not uncommon.
Rocket thrust is caused
by pressures acting in the combustion chamber and nozzleA portion of
the rocket engine's thrust comes from the unbalanced pressures inside
the combustion chamber but the majority comes from the pressures against
the inside of the nozzle. As the gas expands (adiabatically) the pressure
against the nozzle's walls forces the rocket engine in one direction
while accelerating the gas in the other.
The highest exhaust speed
possible is highly desirable for rocket engines to minimise propellant
usage. For aerodynamic reasons the flow goes sonic ("chokes") at the
narrowest part of the nozzle, the 'throat'. Since the speed of sound
in gases increases with the square root of temperature, the use of hot
exhaust gas greatly improves performance. By comparison, at room temperature
the speed of sound in air is about 340m/s while the speed of sound in
the hot gas of a rocket engine can be over 1700m/s; much of this performance
is due to the higher temperature, but additionally rocket propellants
are chosen to be of low molecular mass, and this also gives a higher
velocity compared to air.
Expansion in the rocket
nozzle then further multiplies the speed, typically between 1.5 and
4 times, giving a highly collimated hypersonic exhaust jet. The speed
increase of a rocket nozzle is mostly determined by its area expansion
ratio—the ratio of the area of the throat to the area at the exit, but
detailed properties of the gas are also important. Larger ratio nozzles
are more massive but are able to extract more heat from the combustion
gases, increasing the exhaust velocity.
Nozzle efficiency is affected
by operation in the atmosphere because atmospheric pressure changes
with altitude; but due to the supersonic speeds of the gas exiting from
a rocket engine, the pressure of the jet may be either below or above
ambient, and equilibrium between the two is not reached.
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