A general derivation
of the thrust equation shows that the
amount of thrust generated depends on the mass flow through the
engine and the exit velocity of the gas. Different propulsion systems
generate thrust in slightly different ways. We will discuss four
principal propulsion systems: the propeller,
the turbine (or jet) engine, the ramjet,
and the rocket.
Why are there
different types of engines? If we think about Newton's first
law of motion, we realize that an airplane propulsion system
must serve two purposes. First, the thrust from the propulsion system
must balance the drag of the airplane
when the airplane is cruising. And second, the thrust from the propulsion
system must exceed the drag of the airplane
for the airplane to accelerate. In fact, the greater the difference
between the thrust and the drag, called the excess
thrust, the faster the airplane will accelerate.
like airliners and cargo planes, spend most of their life in a cruise
condition. For these airplanes, excess thrust is not as important
as high engine efficiency and low fuel usage.
Since thrust depends on both the amount of gas moved and the velocity,
we can generate high thrust by accelerating a large mass of gas
by a small amount, or by accelerating a small mass of gas by a large
amount. Because of the aerodynamic efficiency of propellers
and fans, it is more fuel efficient to
accelerate a large mass by a small amount. That is why we find high
bypass fans and turboprops on cargo planes and airliners.
like fighter planes or experimental high speed aircraft, require
very high excess thrust to accelerate quickly and to overcome the
high drag associated with high speeds. For these airplanes, engine
efficiency is not as important as very high thrust. Military aircraft
typically employ afterburning turbojets.
Future hypersonic aircraft will employ some type of ramjet
or rocket propulsion.
The site was
prepared at NASA Glenn by the Learning Technologies Project (LTP)
to provide background information on basic propulsion for
secondary math and science teachers. The pages were originally
prepared as teaching aids to support
an interactive educational computer program that allows students to
design and test jet engines on a personal computer. Other slides
were prepared to support LTP videoconferencing workshops
(http://www.grc.nasa.gov/WWW/K-12/CoE/Coemain.html) for teachers
There is a special section of the Beginner's Guide which deals with
or high speed, aerodynamics. This section is intended for undergraduates who
shock waves or
and contains several
calculators and simulators
for that flow regime.
We have intentionally
organized this site to mirror the unstructured nature of the world
wide web. There are many pages here connected
to one another through hyperlinks. You can then navigate through
the links based on your own interest and inquiry. However, if you
prefer a more structured approach, you can also take one of our
Guided Tours through the site. Each tour
provides a sequence of pages dealing with some aspect of propulsion.
The site has recently been modified to support Section 508 of the Rehabilitation Act.
Many of the pages contain mathematical equations which have been produced graphically
and which are too long or complex to provide in an "ALT" tag. For these pages, we
have retained the (non-compliant) graphical page and have provided a separate
(compliant) text only page which contains all of the information of the original page.
The two pages are connected through hyperlinks.
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