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Rocket Engines

Rocket engines produce thrust by ejecting stored propellants at high speed in accordance with Newton's third law. The propellants undergo combustion in the combustion chamber, which must be sized appropriately for different propellants. Net thrust is equal to the gross thrust minus static back pressure. Rocket nozzles can achieve over 60% efficiency by approximating an ideal reversible process. Specific impulse depends on propellant mix but is typically around 300-450 seconds. Rockets can achieve very high thrust, exhaust speeds over 10 times the speed of sound, and thrust-to-weight ratios of around 100.

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Raajeswaran Baskaran
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73 views3 pages

Rocket Engines

Rocket engines produce thrust by ejecting stored propellants at high speed in accordance with Newton's third law. The propellants undergo combustion in the combustion chamber, which must be sized appropriately for different propellants. Net thrust is equal to the gross thrust minus static back pressure. Rocket nozzles can achieve over 60% efficiency by approximating an ideal reversible process. Specific impulse depends on propellant mix but is typically around 300-450 seconds. Rockets can achieve very high thrust, exhaust speeds over 10 times the speed of sound, and thrust-to-weight ratios of around 100.

Uploaded by

Raajeswaran Baskaran
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Rocket Engines

Michal Czerwien
25.10.2012r

* Introduction

”A rocket engine, or simply ”rocket”, is a jet engine that uses only stored propellant
mass for forming its high speed propulsive jet.”

Rocket engines are reaction engines and obtain thrust in accordance with Newton’s third
law (if one object A exerts a force FA on a second object B, then B simultaneously exerts a
force FB on A, and the two forces are equal and opposite: FA = −FB ).

Combustion Chamber

Rocket propellant is mass that is stored,


usually in some form of propellant tank, prior
to being ejected from a rocket engine in the
form of a fluid jet to produce thrust.
Chemical rocket propellants are most
commonly used, which undergo exothermic
chemical reactions which produce hot gas
which is used by a rocket for propulsive pur-
poses. Alternatively, a chemically inert reac-
tion mass can be heated using a high-energy
power source via a heat exchanger, and then
no combustion chamber is used. Figure 1: An image representing the rocket Motor

The dimensions of the cylinder are such that the propellant is able to combust thoroughly;
different propellants require different combustion chamber sizes for this to occur. This leads
to a number called L∗ :

L∗ = Vc
At

where:

I) Vc is the volume of the chamber

II) At is the area of the throat

III) L* is typically in the range of 25-60 inches (0.64-1.5 m).

1
Net thrust

Below is an approximate equation for calculating the net thrust of a rocket engine:

Fn = ṁ ve = ṁ ve−act + Ae (pe − pamb )

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
to the gross thrust (apart from static back pressure).

Energy Efficiency
Rocket engine nozzles are surprisingly ef-
ficient heat engines for generating a high
speed jet, as a consequence of the high
combustion temperature and high compres-
sion ratio. Rocket nozzles give an excel-
lent approximation to adiabatic expansion
which is a reversible process, and hence they
give efficiencies which are very close to that
of the Carnot cycle.Given the temperatures
reached, over 60 precent efficiency can be
achieved with chemical rockets.
Figure 2: Rocket energy efficiency as a function of vehi-
cle speed divided by effective exhaust speed

Specyfic Impulse
The specific impulse that can be achieved is primarily a function of the propellant mix, but
practical limits on chamber pressures and the nozzle expansion ratios reduce the performance
that can be achieved.

Typical performances of common propellants

Propellant mix Vacuum Isp(seconds) Effective exhaust velocity(m/s)


liquid oxygen 455 4462
liquid oxygen (RP-1) 358 3510
nitrogen tetroxide 344 3369

Overall rocket engine performance


Rocket technology can combine very high thrust (meganewtons), very high exhaust speeds
(around 10 times the speed of sound in air at sea level) and very high thrust/weight ratios
(around 100) simultaneously as well as being able to operate outside the atmosphere, and
while permitting the use of low pressure and hence lightweight tanks and structure.

Rockets can be further optimised to even more extreme performance along one or more of
these axes at the expense of the others.

2
Bibliography
.) http://en.wikipedia.org/wiki/Rockete ngine

.) http://www.howstuffworks.com/rocket.htm

.) http://www.space.com/15099-apollo-moon-rocket-engine-recovery-infographic.html

access to pages above 25.10.2012r

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