Jet Propulsion

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Introduction; Atmospheric Jet Engines; History

Introduction

Jet Propulsion, thrust imparting forward motion to an object, as a reaction to the rearward expulsion of a high-velocity liquid or gaseous stream.

A simple example of jet propulsion is the motion of an inflated balloon when the air is suddenly discharged. While the opening is held closed, the air pressure within the balloon is equal in all directions; when the stem is released, the internal pressure is less at the open end than at the opposite end, causing the balloon to dart forward. Not the pressure of the escaping air pushing against the outside atmosphere but the difference between high and low pressures inside the balloon propels it.

An actual jet engine does not operate quite as simply as a balloon, although the basic principle is the same. More important than pressure imbalance is the acceleration to high velocities of the jet leaving the engine. This is achieved by forces in the engine that enable the gas to flow backward forming the jet. Newton's second law (see Mechanics) shows that these forces are proportional to the rate at which the momentum of the gas is increased. For a jet engine, this is related to the rate of mass flow multiplied by the rearward-leaving jet velocity. Newton's third law, which states that every force must have an equal and opposite reaction, shows that the rearward force is balanced by a forward reaction, known as thrust. This thrusting action is similar to the recoil of a gun, which increases as both the mass of the projectile and its muzzle velocity are increased. High-thrust engines, therefore, require both large rates of mass flow and high jet-exit velocities, which can only be achieved by increasing internal engine pressures and by increasing the volume of the gas by means of combustion.

Jet-propulsion devices are used primarily in high-speed, high-altitude aircraft, in missiles, and in spacecraft (see Airplane; Guided Missiles; Space Exploration). The source of power is a high-energy fuel that is burned at intense pressures to produce the large gas volume needed for high jet-exit velocities. The oxidizer required for the combustion may be the oxygen in the air that is drawn into the engine and compressed, or the oxidizer may be carried in the vehicle, so that the engine is independent of a surrounding atmosphere. Engines that depend on the atmosphere for oxygen include turbojets, turbofans, turboprops, ramjets, scramjets, and pulse jets (see below). Nonatmospheric engines are usually called rocket engines (see Rocket).

Atmospheric Jet Engines

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All atmospheric engines depend on the flow of a large mass of air that is first compressed, then used to oxidize fuel, and finally expanded to low pressures through a nozzle in order to achieve a high jet-exit velocity.

Turbojet Engines

The most widely used atmospheric engines are turbojets. After air has been drawn into the engine through an inlet, the air pressure is increased by a compressor before it enters the combustion chamber (see Air Compressor). The power required to drive the compressor is provided by a turbine that is placed between the combustion chamber and the nozzle.

Practically all airborne jet engines use an axial-flow compressor, in which the air flows generally in the direction of the shaft axis through alternate rows of stationary and rotating blades, called stators and rotors. The blades are arranged so that the air enters each row at a high velocity. As it flows through the blade passage the air is decelerated to a lower velocity, thereby increasing the pressure. Modern axial-flow compressors can increase the pressure 24 times in 15 stages, with each set of stators and rotors making up a stage.

The compressed air then enters the combustion chamber where it is mixed with fuel vapor and then burned. For best performance, the combustion temperature should be the maximum obtainable from the complete combustion of the oxygen and the fuel. This temperature, however, would make the turbine too hot; turbine inlet temperatures, which currently limit turbojet performance, cannot exceed about 1100° C (about 2000° F) because of the thermal limitations of the materials. To reduce the temperature of the turbine inlet, only part of the compressed air is burned. This is achieved by dividing the air as it enters the combustion chamber. Part of the air is mixed with the fuel and ignited; the remainder is used to cool the turbine.

In the turbine, which acts in opposite fashion to the compressor, the gases are partially expanded through alternate stator and rotor passages. At the entry to each blade row, the velocity is low, allowing the gas to expand and speed up in the passage while it turns the rotor. The turbine provides the power to drive the compressor, to which it is connected by a shaft through the center of the engine, and it also provides the power for the fuel pump, generator, and other accessories.

The gases, which are now at an intermediate pressure, are finally expanded through the rearward-facing nozzle to reach the desired high jet-exit velocity. The greatest thrust would be obtained if the nozzle expanded the gases to the pressure of the surrounding atmosphere. In practice, however, such nozzles would be too large and too heavy. Actual nozzles are made shorter in order to provide higher exit pressures and a somewhat reduced engine performance.

A turbojet engine cannot start directly from rest; the engine must first be induced to spin by an external starting motor. The fuel is then ignited by a heated plug. Once the engine is running, however, combustion is maintained without spark plugs.

The thrust delivered by a turbojet decreases as the surrounding air temperature increases because the decreased density of the hot air reduces the mass flow through the engine. On hot days, takeoff thrust can be increased by injecting water at the compressor inlet and allowing the evaporating water to cool the air.

In military engines, bursts of speed or additional thrust for takeoff and climb can be provided by a second burner, or afterburner, installed between the turbine and the nozzle. In the afterburner, more fuel is added to burn the oxygen in the air that is not used in the combustion chamber; this process increases both the air volume and the jet velocity. The low efficiency of an afterburner, however, restricts its use to situations requiring a great burst of speed.

Turbofans or Bypass Engines

The turbofan engine is an improvement on the basic turbojet. Part of the incoming air is only partially compressed and then bypassed in an outer shell beyond the turbine. This air is then mixed with the hot turbine-exhaust gases before they reach the nozzle. A bypass engine has greater thrust for takeoff and climb, and increased efficiency; the bypass cools the engine and reduces noise level.

In some fan engines the bypass air is not remixed in the engine but exhausted directly. In this type of bypass engine, only about one-sixth of the incoming air goes through the whole engine; the remaining five-sixths is compressed only in the first compressor or fan stage and then exhausted. Different rotational speeds are required for the high- and low-pressure portions of the engine. This difference is achieved by having two separate turbine-compressor combinations running on two concentric shafts or twin spools. Two high-pressure turbine stages drive the 11 high-pressure compressor stages mounted on the outer shaft, and 4 turbine stages provide power for the fan and 4 low-pressure compressor stages on the inner shaft. An example of an engine of this type is the JT9D-3 jet engine, which weighs about 3850 kg (about 8470 lb) and can develop a takeoff thrust of about 20,000 kg (about 44,000 lb). This is more than double the thrust available for the largest commercial planes before the Boeing 747.

Current research in turbojet and turbofan engines is largely directed to achieving more efficient operation of the compressors and turbines, to devising special turbine-blade cooling systems to permit higher turbine-inlet temperatures, and to reducing jet noise.

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Introduction

Atmospheric Jet Engines

Turbojet Engines

Turbofans or Bypass Engines