An Introduction to Jet Engines

Estimated Reading Time: 7 minutes

TL;DR

In last week’s free issue we learned about supersonic flight and low boom technology. Today, we will learn more about the powerful jet engines which enable supersonic (and hypersonic) flight. We’ll also briefly explain how rocket engines operate.

Jet Engines

Jet engines take in air, compress it, mix it with fuel, perform combustion, and use produced exhaust gases to produce thrust. The total cycle is called the Brayton process and is illustrated below. There are several types of jet engines including turbofans, turbojets, ramjets, and scramjets which we will learn about in the rest of the post. These jets operate with different efficiencies at different speeds.

The Brayton cycle takes in fresh air, compresses it, mixes it with fuel, combusts it, and then uses the released energy to do work. In a jet engine, the turbine powers the compressor while the exhaust gases are used to propel the aircraft (source).

Turbojets

Breaking through the sonic boom requires considerable thrust. Supersonic planes often use turbojets (see diagram below), which compress cold intake air with fuel, combust the mixture, then use the hot exhaust to provide forward thrust. Turbojets are most efficient for supersonic aircraft, but are less efficient for aircraft traveling at subsonic speeds.

A turbo jet engine intakes cold air from the front and compresses it with fan blades. This compressed air is combined with fuel and burned in the combustion section. The hot exhaust provides forward thrust and the power necessary to continue running the compressor. (source).

Turbofan

In a turbofan, some of the output from the turbojet is used to drive a fan which also contributes to thrust (see image below). As a result, turbofans are more efficient at subsonic speeds. Most passenger jets today use turbofan engines.

Like a turbojet, a turbofan also takes in cold air, but not all the cold air is compressed for combustion. Some portion of it is directly expelled by the fan to produce thrust. The bypass ratio is the ratio of air that bypasses compression. (source).

Ramjets

A ramjet uses the engine’s forward motion to directly compress input air without a compressor. As a result, the ramjet cannot operate at zero speed. Ramjets require assisted takeoff where another engine is used to accelerate the ramjet to the speed where it can propel itself. Ramjets are most efficient at supersonic speeds of Mach 3 and higher.

A diagram of ramjet operation. The ramjet uses the speed of the incoming air to compress itself instead of using a compressor. As a result, the ramjet cannot operate at zero airspeed and requires assisted takeoff. Ramjets are most commonly used in missiles (source).

Scramjets

A scramjet is a ramjet in which combustion happens in a supersonic airflow. Incoming air is not compressed as heavily and as a result passes through the fuel injection region at supersonic speeds. Scramjets can operate at much higher speeds than ramjets and are a leading jet choice for hypersonic jets (hypersonic refers to speeds of Mach 5 and above). Due to their design, scramjets can only start operating near hypersonic speeds. Scramjets have drawn considerable interest due to their use for designing hypersonic aircraft and cruise missiles (see the second illustration below).

A scramjet is a variant of a ramjet in which air is travelling at supersonic speeds through the fuel injection. Scramjets are most commonly used for hypersonic travel (source).

An artist’s conception of the X-43, an experimental aircraft powered by a scramjet. An X-43 achieved Mach 9.6 for a brief time (source).

Rocket Engines

Unlike the jet engines we’ve seen so far, rocket engines don’t take in air for combustion. As a result, rocket engines can operate in space, but must store some oxidizer as part of their fuel as the diagram below illustrates.

A simplified diagram of a liquid-fuel rocket.1. Liquid rocket fuel.2. Oxidizer.3. Pumps carry the fuel and oxidizer.4. The combustion chamber mixes and burns the two liquids.5. The hot exhaust is choked at the throat, which, among other things, dictates the amount of thrust produced.6. Exhaust exits the rocket (caption and image from source).

Discussion

Jet engines are powerful pieces of technology useful for constructing supersonic planes, military aircraft and missiles. Their close cousins, rocket engines, are critical for building spacecraft. In today’s post, we learned about the basic operation of these engines. Jet and rocket engines feature heavily in cutting edge defense technology such as hypersonic cruise missiles, which we will learn more about in future issues.

Highlights for the Week

• https://www.wsj.com/articles/senate-approves-250-billion-bill-to-boost-tech-research-11623192584: The senate has just approved a $250 billion dollar bill boosting research funding.

• https://www.nature.com/articles/s41586-021-03544-w: Google used reinforcement learning to design the layout of its TPUv4 chip

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Deep Into the Forest is a newsletter by Deep Forest Sciences, Inc. We’re a deep tech R&D company specializing in the use of AI for deep tech development. We do technical consulting and joint development partnerships with deep tech firms. Get in touch with us at partnerships@deepforestsci.com! We’re always welcome to new ideas!

Credits

Author: Bharath Ramsundar, Ph.D.
Editor: Sandya Subramanian