Jet engine

A jet engine is any engine that accelerates and discharges a fast moving jet of fluid to generate thrust in accordance with Newton's . This broad definition of jet engines includes turbojets, turbofans, turboprops, rockets and ramjets, but in common usage, the term generally refers to a gas turbine used to produce a jet of high speed exhaust gases for propulsive purposes.

Types

There are a large number of types of jet engines, which get propulsion from a high speed exhaust jet. Some examples are as follows:

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Type

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Description

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Advantages

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Disadvantages

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

water jet

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Squirts water out the back of a boat

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Can run in shallow water, powerful, less harmful to wildlife

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Can be less efficient than a propeller

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Thermojet

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Most primitive airbreathing jet engine

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Very inefficient and underpowered

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Turbojet

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Generic term for simple turbine engine

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Simplicity of design

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Basic design, misses many improvements in efficiency and power

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Turbofan

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Power tapped off exhaust used to drive bypass fan

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Quieter due to greater mass flow and lower total exhaust speed, more efficient for a useful range of subsonic airspeeds for same reason

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Greater complexity (additional ducting, usually multiple shafts), large diameter engine, need to contain heavy blades. More subject to FOD and ice damage. Different degrees of bypass are possible - this is the design most commonly used on commercial airliners

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Rocket

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Carries own propellant onboard, emits jet for propulsion

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Very few moving parts, Mach 0 to Mach 25+, efficient at very high speed (> Mach 10.0 or so), thrust/weight ratio over 100, relatively simple, no air inlet, doesn't require atmosphere, high compression ratio, very high speed exhaust

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

very low specific impulse- typically 100-450 seconds. Typically requires carrying oxidiser onboard which increases risks.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Ramjet

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Intake air is compressed entirely by speed of oncoming air and duct shape (divergent)

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Very few moving parts, Mach 0.8 to Mach 5+, efficient at high speed (> Mach 2.0 or so), lightest of all airbreathing jets (thrust/weight ratio up to 30 at optimum speed)

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Must have a high initial speed to function, inherently inefficient at slow speeds due to poor compression ratio, difficult to arrange shaft power for accessories, difficult to engineer to be efficient over a wide range of airspeeds.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Turboprop (Turboshaft similar)

Related Topics:
Turboprop - Turboshaft

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Strictly not a jet at all- a gas turbine engine is used as powerplant to drive (propeller) shaft

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

High efficiency at lower subsonic airspeeds(300 knots plus), high shaft power to weight

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Limited top speed (aeroplanes), somewhat noisy, complexity of propeller drive, very large yaw (aeroplane) if engine fails

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Propfan

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Turboprop engine drives one or more propellers. much like a turbofan but without ductwork

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Higher fuel efficiency, some designs are less noisy than turbofans, could lead to higher-speed commercial aircraft, popular in the 1980s during fuel shortages,

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Development of propfan engines has been very limited, typically more noisy than turbofans, complexity

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Pulsejet

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Air enters a divergent-duct inlet, the front of the combustion area is shut, fuel injected into the air ignites, exhaust vents from other end of engine

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Very simple design, commonly used on model aircraft

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Noisy, inefficient (low compression ratio), works best at small scale, valves need to be replaced very often

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Pulse detonation engine

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Similar to a pulsejet, but combustion occurs as a detonation instead of a deflagration, may or may not need valves

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Maximum theoretical engine efficiency

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Extremely noisy, parts subject to extreme mechanical fatigue, hard to start detonation, not practical for current use

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Integral rocket ramjet

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Essentially a ramjet where intake air is compressed and burnt with the exhaust from a rocket

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Mach 0 to Mach 4.5+ (can also run exoatmospheric), good efficiency at Mach 2 to 4

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Similar efficiency to rockets at low speed or exoatmospheric, inlet difficulties, a relatively undeveloped and unexplored type, cooling difficulties

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Scramjet

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Intake air is compressed but not slowed to below supersonic, intake, combustion and exhaust occur in a single constricted tube

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

can operate at very high Mach numbers (Mach 8 to 15)http://www.dod.mil/ddre/downloads/ddre_briefings/Merging_Air_and_Space071603.pdf

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

still in development stages, must have a very high initial speed to function (Mach >6), cooling difficulties, inlet difficulties, very poor thrust/weight ratio (~2), airframe difficulties, testing difficulties

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Turborocket

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

An additional oxidizer such as oxygen is added to the airstream to increase max altitude

Related Topics:
Oxidizer - Oxygen

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Very close to existing designs, operates in very high altitude, wide range of altitude and airspeed

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Airspeed limited to same range as turbojet engine, carrying oxidizer like LOX can be dangerous

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Precooled jets / LACE

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Intake air is chilled to very low temperatures at inlet

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Very high thrust/weight ratios are possible (~14) together with good fuel efficiency over a wide range of airspeeds, mach 0-5+

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Exists only at the lab protoyping stage. Examples include RB545, SABRE, ATREX

Related Topics:
RB545 - SABRE - ATREX

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~