Thrust is the force which moves the
rocket through the air, and through space. Thrust is generated by the
propulsion system of the rocket through the
application of Newton's third law of motion;
For every action there is an equal and opposite re-action.
In the propulsion system, an engine does
gas or liquid,
called a working fluid, and accelerates
the working fluid through the propulsion system.
The re-action to the acceleration of the working fluid produces the
thrust force on the engine.
The working fluid is expelled from the engine in one direction
and the thrust force is applied to the engine in the opposite direction.
having both a magnitude and a direction. When describing the
action of forces, one must
both the magnitude and the direction.
The direction of the thrust is normally along the longitudinal axis of the rocket
through the rocket
center of gravity.
But on some rockets, the exhaust nozzle and the thrust direction
can be rotated, or
The rocket can then be maneuvered by using the
about the center of gravity.
The magnitude of the thrust can be determined by the general
The magnitude of the thrust depends on the mass flow rate
of the working fluid through the engine
and the exit velocity and pressure of the working fluid.
The efficiency of the propulsion system is characterized by the
the ratio of the amount of thrust produced to the
weight flow of the propellants.
All rocket engines produce thrust by accelerating a working
fluid. But there are many different ways to produce the acceleration,
and many different available working fluids. Let's look at some of the
various types of rocket engines and how they produce thrust.
The simplest rocket engine uses air as the working fluid, and
pressure produced by a pump to accelerate the air. This is
the type of "engine" used in a toy balloon or a
Because the weight flow of air is so small, this type of rocket engine
does not produce much thrust. A
uses water as the working fluid and
pressurized air to accelerate the working fluid.
Because water is much heavier than air,
bottle rockets generate more thrust than stomp rockets.
full scale rockets
use chemical rocket engines.
Chemical rocket engines use the
of propellants to produce exhaust gases as the working
fluid. The high pressures and temperatures of combustion
are used to accelerate the exhaust gases through a
to produce thrust.
There are two important parts of a chemical rocket engine;
the nozzle, and the propellants.
The nozzle design
the mass flow rate, exhaust velocity, and exit pressure
for a given initial pressure and temperature.
The initial pressure and temperature are determined by
the chemical properties of the propellants.
Propellants are composed of a fuel to be burned and
an oxidizer, or source of oxygen, for combustion.
Under normal temperature conditions,
propellants do not burn, but require some source of heat, or
igniter, to initiate combustion.
Chemical rocket engines do not typically rely on the surrounding
as a source of oxygen. Therefore, chemical rocket engines can
be used in space, where there is no atmosphere present.
There are two main types of chemical rocket engines;
liquid rockets and solid rockets. In a
the fuel and the oxidizer
are stored separately and pumped into the combustion chamber of the nozzle
where the burning occurs. In a
the fuel and oxidizer are mixed together into a solid propellant
which is packed into a cylinder.
The propellant only burns on the surface. So,
as the propellant burns, a
is produced which moves into the propellant.
Once the burning starts,
it will proceed until all the propellant is consumed.
With a liquid rocket, you can stop the thrust by turning off the flow of
fuel or oxidizer; but with a solid rocket, you must destroy the casing to stop
the engine. Liquid rockets tend to be heavier and more
complex because of the pumps used to move the fuel and oxidizer,
and you usually load the fuel and oxidizer into the rocket just
before launch. A solid rocket is much easier to handle and can sit for years
A new type of rocket engine is the electric engine, also called an
The working fluid for an electric engine
is composed of very many, but very small, charged particles called ions.
The acceleration of the working fluid is produced by electrostatic forces,
not by combustion. Ion engines produce very small amounts of thrust, but can
produce the thrust for long periods of time because the mass flow rates are
very small. Ion engines have very high specific impulse when compared to chemical
Another new type of rocket engine is the
nuclear thermal engine.
In the nuclear thermal engine, a nuclear reactor provides a
continuous source of heat which is used to accelerate the working fluid.
The working fluid can be any gas which is heated as
it is passed over or through the reactor
and exited through the nozzle. The temperature of
the exhaust, and the resulting exit velocity, can be much higher than
for the typical chemical rocket. The only propellant is the
single working fluid, so nuclear rockets are predicted to have very high
Nuclear thermal engines are being developed under
Forces on a Rocket:
Rocket Propulsion Activity: Grade 9-10
Fundamental Terminology: Grade 10-12
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