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Jet engines

Jet engine test by NASA

Last updated: May 14, 2007.

Jet engines have put rockets into space and powered boats and automobiles to world speed records, but they are more familiar as the engines on airplanes such as Concorde and the Jumbo Jet. Unlike internal combustion engines in automobiles and trucks, which convert an up-and-down movement of pistons into rotary movement in a crankshaft, jet engines produce power by sucking in air at the front and blasting out hot exhaust gases at the back.

Photo: A jet engine taken apart during testing. You can clearly see the giant fan at the front. This spins around to suck air into the engine as the plane flies through the sky. Picture courtesy of US Navy.

A jet engine is a machine for turning fuel into thrust (forward motion). The thrust is produced by action and reaction—a piece of physics also known as Newton's third law of motion. The force (action) of the exhaust gases pushing backward produces an equal and opposite force (reaction) called thrust that powers the vehicle forward. Exactly the same principle pushes a skateboard forward when you kick backward with your foot. In a jet engine, it's the exhaust gas that provides the "kick". Let's take a closer look inside the engine...

How a jet engine works

Artwork showing how a jet engine works

  1. For a jet going slower than the speed of sound, the engine is moving through the air at about 1000 km/h (600 mph). We can think of the engine as being stationary and the cold air moving toward it at this speed.
  2. A fan at the front sucks the cold air into the engine.
  3. A second fan called a compressor squeezes the air (increases its pressure) by about eight times. This slows the air down to about 400 km/h (240 mph).
  4. Kerosene (liquid fuel) is squirted into the engine from a fuel tank in the plane's wing.
  5. Jet engine test by NASA
  6. In the combustion chamber, just behind the compressor, the kerosene mixes with the compressed air and burns fiercely, giving off hot exhaust gases. The burning mixture reaches a temperature of around 900°C (1650°F).
  7. The exhaust gases rush past a set of turbine blades, spinning them like a windmill. The turbine blades are connected to a long axle that runs the length of the engine. The compressor and the fan are also connected to this axle. So, as the turbine blades spin, they also turn the compressor and the fan.
  8. The hot exhaust gases exit the engine through a tapering exhaust nozzle. The tapering design helps to accelerate the gases to a speed of over 2100 km/h (1300 mph). So the hot air leaving the engine at the back is travelling over twice the speed of the cold air entering it at the front—and that's what powers the plane. Military jets often have an after burner that squirts fuel into the exhaust jet to produce extra thrust.
  9. The backward-moving exhaust gases power the jet forward. Because the plane is much bigger and heavier than the exhaust gases it produces, the exhaust gases have to zoom backward much faster than the plane's own speed.

Photo: Massive thrust! A Pratt and Whitney F-119 jet aircraft engine creates 156,000 newtons (35,000 pounds) of thrust during this US Airforce test in 2002. Picture courtesy of US Airforce.

Types of jet engines

British engineer Sir Frank Whittle (1907-1996) invented the jet engine in 1930 and his design, called a turbojet, is still widely used in airplanes today. It is not the only type of jet engine, however. Turboprop engines have a propeller at the front and are popular in small, economical passenger aircraft. Turbofan engines are much quieter than turbojets and are typically used in large airliners. A turbofan engine has a large fan that sucks in air at the front. Some of the air is blown into the compressor; the rest is blown around the outside of the combustion chamber and straight out of the back. This "bypass" arrangement cools the engine and makes it much quieter. It also produces much more thrust at both takeoff and landing. Ramjet engines, which are simple and compact and little more than gas-burning pipes, are used to power guided missiles.

Vertical takeoff

USAF osprey plane

An Osprey 7 plane during early testing Picture courtesy of the US Airforce.

Airplanes have to travel at high speeds to produce enough lift for takeoff, but because they are immensely heavy and often carry substantial cargoes, they can accelerate only very slowly. A typical runway for a large airliner such as a Boeing 747 is around 2 miles (3 km) long, simply because the plane has to travel this far before it has picked up enough speed to get off the ground.

Long runways may be fine for passenger aircraft, but military fighters need to take off in much more confined spaces (for example, from the deck of an aircraft carrier). Vertical Takeoff and Landing (VTOL) aircraft solve this problem by having jet engines whose nozzles can be swiveled in different directions. During takeoff and landing, the jets point straight downward so the plane can rise or fall on the spot or even hover like a helicopter. Once the plane is airborne, the jets swivel backward and shoot the plane forward like a conventional airplane.

The best known plane of this sort is the Harrier "jump-jet" extensively used by the UK Royal Navy and the US Marine Corps. The Joint Strike Fighter (JSF) currently being developed by Boeing and Lockheed for the US military will also be a VTOL aircraft. The US Airforce Osprey plane works in a similar way, but has tilting propellers instead of jet engines. To land vertically, like a helicopter, it tilts the propellers upward. To fly horizontally, it points them forward.

Text copyright © Chris Woodford 2000. All rights reserved. 2007.

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