Ask A Scientist

How do jet engines work?

Asked by: Andrew Landon
School: Maine Endwell Middle School
Grade: Kevin Wags
Teacher: Kevin Wagstaff
Hobbies/Interests: Airplanes, drawing, baseball, football and animals.
Career Interest: Comic writer

Answer from George Catalano

Professor of Mechanical Engineering, Director of t

Research area:
Turbulence, Fluid Mechanics, Aerodynamics, Environmental Ethics, and Modeling Ecosystems

PhD school:
University of Virginia, Aerospace Engineering, 1977

Interests/hobbies:
All things Italian, Creative Arts, Model trains & cars

Family:
Wife, Karen, is a registered yoga teacher at Yoga for Everybody at the Orthopedic Associates; lives with 3 Alaskan Malamutes, two more in our hearts

Did you know that there are hundreds of fully operational jet engines located in the Southern Tier of New York each and every summer? There are hundreds located here in the Binghamton area and hundreds more located in each and every county in New York. Surely, I must be joking, right? If there are so many jet engines located throughout our state (and all the others), how come we cannot hear them all? Perhaps before I answer that question, it might useful to explain the basics of jet engines and jet propulsion. Jet engines are governed by principles first formally described by Sir Isaac Newton. Newton's 3rd Law states that for every action, there is an equal and opposite reaction. The principle behind the jet engine is that if we can somehow create a fast moving stream of air or gas or water or some mixture, we can use its speed and its mass to generate what is commonly referred to as thrust. Thrust is generated when a fast moving stream exhausts into a slower moving or still stream. We have all seen photographs or videos of those huge rocket boosters lifting the NASA Space Shuttle off into space. Those rocket boosters are jet engines. Hot exhaust gases are leaving the end of the rockets at incredibly high speed as well as high temperatures. The gases are exhausted into still air. Fast gases exhausting into slower moving air creates thrust--- in this case, the thrust that lifts that incredibly heavy space shuttle into Earth's orbit. There is a wide variety of jet engines in use today. Some of the more common types include turbojets, turbopropos, rockets, ramjets and gas turbine engines. All use the same principle: hot and fast moving exhaust gases moving into slow or still air. In order to get the exhaust gases moving at a relatively high speed, we typically need some type of combustion or controlled explosion. The energy released from the combustion generates a large pressure difference between the gases inside the engine and those on the outside. This large pressure difference or gradient then creates a jet "wind" in the same way that high and low pressure areas generate wind in our atmosphere. It is this "wind" that creates the thrust. Let's go back to my insistence that we are overrun in the summer with jet engines everywhere much like a plague of locusts. How can that possibly be true? It seems hard to imagine now as the temperature hovers near zero and the landscape is covered with snow but in a few months or so we will probably be outside in our backyards or at a park. Take a moment to look at the rotating lawn sprinkler you see before you. What is going on there? There is relatively high speed water exiting the nozzle and moving into still air with the resulting effect (in addition to watering the lawn!), a force or thrust generated which turns the nozzle assembly. Looks like a jet engine to me!

Last Updated: 9/18/13