Thrust is the force which moves a
rocket through the air, or through space. Thrust is generated by the
propulsion system
of the
rocket.
How is thrust generated?
Thrust is a mechanical force which is generated through the
reaction of accelerating a mass of gas,
as explained by Newton's third law
of motion.
A gas or working fluid is accelerated to the
rear and the engine and rocket
are accelerated in the opposite direction. To accelerate the gas, we
need some kind of propulsion system. We will
discuss the details of various propulsion systems on some other
pages. For right now, let us just think of the propulsion system as
some machine which accelerates a gas.
From Newton's second law of motion, we
can define a force F to be the change in momentum of an object with a
change in time. Momentum is the object's mass m times the
velocity V. So, between two times t1 and t2, the force is given by:
F = ((m * V)2  (m * V)1) / (t2  t1)
If we keep the mass constant and just change the velocity
with time we obtain the simple force equation  force equals mass
time acceleration a
F = m * a
If we are dealing with a
solid,
keeping track of the mass is relatively easy; the molecules of a solid are
closely bound to each other and a solid retains its shape. But if
we are dealing with a fluid (liquid or gas) and particularly if we
are dealing with a moving fluid, keeping track of the mass gets
tricky. For a moving fluid, the important parameter is the mass
flow rate. Mass flow rate is the amount of mass moving through a
given plane over some amount of time. Its dimensions are mass/time
(kg/sec, slug/sec, ...) and it is equal to the
density
r times the
velocity
V times the
area
A. Aerodynamicists denote this parameter
as m dot (m with a little dot over the top).
m dot = r * V * A
Note:
The "dot" notation is used a lot by mathematicians, scientists, and
engineers as a symbol for "d/dt", which means the variable changes
with a change in time. For example, we can write Newton's second law
as either
F = d(mv)/dt or F = (mv)dot
So "m dot" is not simply the mass of the fluid, but is
the mass flow rate, the mass per unit time.
Since the mass flow rate already contains the time dependence
(mass/time), we can express the change in momentum across the
propulsion device as the change in the mass flow rate times the
velocity. We will denote the exit of the device as station "e" and
the free stream as station "0". Then
F = (m dot * V)e  (m dot * V)0
A units check shows that on the right hand side of the equation:
mass/time * length/time = mass * length / time^2
This is the dimension of a
force. There is an additional effect which we must account for if the
exit pressure p is different from the free stream pressure. The fluid
pressure is related to the momentum of
the gas molecules and acts perpendicular to any boundary which we
impose. If there is a net change of pressure in the flow there is an
additional change in momentum. Across the exit area we may encounter
an additional force term equal to the exit area Ae times the exit
pressure minus the free stream pressure. The general thrust equation
is then given by:
F = (m dot * V)e  (m dot * V)0 + (pe  p0) * Ae
Normally, the magnitude of the pressurearea term is small
relative to the m dotV terms.
Looking at the thrust equation very carefully, we see that there
are two possible ways to produce high thrust.
One way is
to make the engine flow rate (m dot) as high as possible. As long as the
exit velocity is greater than the free stream, entrance velocity, a
high engine flow will produce high thrust. This is the design
theory behind propeller aircraft and
highbypass turbofan engines.
A large amount of air is processed each
second, but the velocity is not changed very much. The other way
to produce high thrust is to make the exit velocity very much greater
than the incoming velocity. This is the design theory behind pure
turbojets, turbojets with afterburners, and rockets.
A moderate amount of flow is accelerated to a high velocity in
these engines. If the exit velocity becomes very high, there are
other physical processes which become
important and affect the efficiency of the engine. These effects are
described in detail on other pages at this site.
There is a simplified version of the general thrust equation that
can be used for gas turbine engines.
The nozzle of a turbine engine
is usually designed to make the exit pressure equal to free stream.
In that case, the pressurearea term in the general equation is equal to
zero.
The thrust is then equal to the exit mass
flow rate times the exit velocity minus the free stream mass flow
rate times the free stream velocity.
F = (m dot * V)e  (m dot * V)0
There is a different simplified version of the general thrust equation that
can be used for
rocket engines.
Since a rocket carries its own oxygen on board,
the free stream mass flow rate is zero and the second term of the
general equation drops out.
F = (m dot * V)e + (pe  p0) * Ae
We have to include the pressure correction
term since a rocket nozzle produces a fixed exit pressure
which in general is different than free stream pressure.
There is a useful rocket performance parameter called the
specific impulse Isp,
that eliminates the mass flow dependence in the analysis.
Isp = Veq / go
where Veq is the equivalent velocity, which is equal to the
nozzle exit velocity plus the pressurearea term, and g0 is
the gravitational acceleration.
For both rockets and turbojets, the nozzle performs two
important roles. The design of the nozzle determines the
exit velocity
for a given pressure and temperature.
And because of
flow choking
in the throat of the nozzle, the nozzle design also sets the mass flow rate
through the propulsion system. Therefore, the nozzle design determines the
thrust of the propulsion system as defined on this page.
You can investigate nozzle operation with our
interactive thrust simulator.
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