All About Jet Propulsion

Apple | Spotify | Amazon | Player.FM | TuneIn
Castbox | Podurama | Podcast Republic | RSS | Patreon

Podcast Transcript

When humans first managed to harness heavier-than-air flight, they did so by using piston engines and propellers. 

Several decades later, a more efficient means of propulsion was developed. It could make planes travel faster and higher and could use less fuel over longer distances. 

In addition to transforming civilian and military aviation, new modes of it still have the potential to revolutionize the world once again.

Learn more about jet propulsion, how it works, and how it was developed on this episode of Everything Everywhere Daily. 

Most of you would probably recognize a jet engine if you saw one on an airplane. However, most people probably couldn’t really explain how a jet engine works. A propeller is somewhat intuitive in how it works, but a jet is a bit more tricky because much of it is encased, and you can’t see what is happening. 

So, let’s start with the basics: What is a jet and jet propulsion in the most generic sense of the word? 

The dictionary definition of a jet is “A high-velocity fluid stream forced under pressure out of a small-diameter opening or nozzle.” 

The fluids usually used in jets would be air or water. 

Jet propulsion, therefore, is the propulsion of an object in one direction, produced by ejecting a jet of fluid in the opposite direction.”

Jet propulsion is nothing more than an application of Newton’s third law of motion, which states that for every action, there is an equal and opposite reaction. 

You throw something out backward at a high velocity to make something go forward. 

This might sound very familiar. If you remember a previous episode, rockets use the exact same principle. They create a superheated gas that is expelled out of the nozzle to produce thrust. 

So, are jets the same as rockets?

No, but they are much more similar than most people realize. Both are considered reaction engines.

Both rockets and jets oxidize a propellent with the resulting gas from the combusting, providing the thrust. The difference is that rockets carry the oxidizing agent with them, often in the form of liquid oxygen. 

A jet, on the other hand, gets its oxygen from the atmosphere. A rocket can operate in the vacuum of space, whereas a jet can only operate in the atmosphere. 

So, what exactly is happening inside of a jet engine?

Most jet engines you’ve probably seen on commercial aircraft are cylindrical, but they don’t have to be. All that is required is that there is an opening for air intake to enter and an exit for exhaust to leave. 

In most jet engines, air will be brought into the engine via a fan. The fan allows for air to enter the jet at lower speeds. 

Once air enters the air intake, it is then compressed in stages, sometimes by other fans called rotors and sometimes by stationary blades called stators. 

Once the air is compressed, it is mixed with fuel, and it is ignited. 

The product of the combustion is a hot gas that dramatically expands in volume. With the air entering in one direction, the easiest path out is in the other direction, which provides the thrust. 

There are roughly two types of jets that are used in aviation today, although there are more, which I’ll get to in a bit. 

The two are turbojets and turbofans.

A turbojet is basically what I just described. All of the incoming air goes through the combustion chamber. At lower speeds, it is less efficient and is much louder. 

A turbofan is where only part of the air is compressed and enters the combustion chamber. The exhaust from the combustion chamber is used to turn the fan at the air intake section. 

The rest of the air goes around the combustion chamber and provides thrust just like a propeller does on an airplane. Turbofan engines tend to be more fuel-efficient and quieter.  

Most jets you see on commercial aircraft are turbofan engines.

You might have heard of an engine called a turboprop. These are not jet engines, These are propeller engines where the exhaust from the engine that drives the propeller is expelled to provide more thrust and more efficiency. The exhaust is not the primary source of thrust, but it does increase fuel efficiency.

So, why are jets better than propellers for powering an aircraft?

Propellers have some very hard limits. 

For starters, it is extremely difficult for a propeller-driven aircraft to achieve very high speeds. The fastest speed ever recorded for a propeller-driven aircraft is March 0.82, or 82% the speed of sound. 

The reason why propeller-driven aircraft are limited has to do with the tips of the propellers. Because a propeller moves in a circle, the tip of the propeller is moving much faster than the rest of the propeller because it has to travel a longer distance around a circle. 

The tips of the propeller will break the speed of sound first, causing a great deal of turbulence, which slows the airplane. 

The other problem has to do with altitude. The higher the altitude, the thinner the air, and the less efficiently propellers work. 

However, propellers work very well at lower speeds and can take off from shorter runways. I remember being picked up in northern Manitoba by a bush plane called a Beaver. The plane landed on a stretch of sand, and we literally had to dig it out of the sand when it got stuck like it was a car stuck in a snowbank. 

It managed to take off on the very short stretch of sand it landed on.

So, a jet engine allows planes to fly higher and faster with greater fuel efficiency. 

How did jet engines develop?

Jets, again, generally defined as a high-velocity fluid stream under pressure, can be found in nature. Certain species of cephalopods will squirt out a jet of water to flee from predators. 

If you remember back to my episode on the steam engine, the first thing that could be called a steam device was created by Hero of Alexandria in the first century. 

He heated a sphere filled with water, and when it was heated, jets of steam came out of two exhaust valves. It isn’t usually considered a jet, but it was a jet of steam, even though it had no practical application.

The origins of what we would recognize as a jet engine began with the development of the gas turbine engine. A gas turbine is a continuous-flow engine that turns a turbine, as opposed to a piston-driven engine.  The idea for the gas turbine engine was first developed by English inventor John Barber in 1791.

In 1921, a French agricultural engineer named Maxime Guillaume first suggested using a gas turbine in an aircraft. Unfortunately, given the state of technology at the time, his idea wasn’t practical.

However, by the 1930s, aviation engineers in the UK and Germany both began working on jet engines. In Germany, Hans von Ohain led the German effort to create a working jet aircraft and in England Frank Whittle was doing the same thing independently. 

Both men are credited as the co-inventors of the jet engine.

Whittle created his first working prototype in 1937, as did Ohain, who built his engine, the Heinkel HeS 1, that same year. 

Whittle lacked funding and government support, so it was Ohain who wound up creating the first jet aircraft. The Heinkel He 178 flew for the first time on August 27, 1939, just days before the start of the Second World War.

Whittle’s engine eventually led to the flight of the Gloster E.28/39 on  May 15, 1941, Britain’s first jet aircraft. 

The war accelerated the development of jet engines and jet aircraft. Jet-powered aircraft had a significant advantage over propeller aircraft in terms of speed and maneuverability. 

However, they saw very limited action in the war. The German Messerschmitt Me 262 flew in 1942, and the British Gloster Meteor flew in 1943. 

Both aircraft saw limited action because they weren’t able to be mass-produced by the end of the war. 

However, after the war, all aviation engineering efforts went heavily into developing jet aircraft.

While the initial push was for military aircraft, civilian jet aircraft was close behind. 

The de Havilland Comet, introduced in 1952, was the world’s first commercial jet airliner. The Comet had serious problems that were unrelated to its jet engines. Within the first year of service, three Comets were involved in accidents, two of which occurred in mid-air. The problem was with the metal in the airframe. 

It wasn’t until 1958, when Pan Am introduced flights on the Boeing 707, that the Jet Age truly began in commercial air travel. The 707’s inaugural route was between New York and Paris, offering the fastest way to cross the Atlantica. 

Jet aircraft proved to be so popular that by 1959, for the first time in history, more passengers crossed the Atlantic by air than by sea. 

Jet engines, in particular the adoption of turbofan engines, allowed for larger aircraft that could fly higher and further, most notably the Boeing 747, on which I’ve done a previous episode. 

These aircraft allowed people to travel internationally quickly and created a group of wealthy people who became known as the Jet Set. 

Jet aircraft were so popular that the New York City American Football League team, the Titans, changed their name to the Jets in 1963. 

Jet engines became dominant in commercial and military aviation by the 1960s, but there was still a role for propeller-driving aircraft as propeller engines are cheaper to produce and are more efficient at lower speeds and shorter distances.

Jet engines have seen constant incremental improvements in efficiency and power over the last 60 years. 

However, there is still more innovation on the horizon using jet engines in aircraft. 

One of the biggest desires in aviation is to see a return of supersonic commercial travel. The only supersonic commercial aircraft was the Concorde. The Concorde used turbojet engines rather than turbofan engines because of the high speeds it flew at. 

If aircraft are going to fly faster in the future, they are going to need advanced engines. One of the proposed engine designs is called a ramjet. 

The principle behind a ramjet is the same as a turbojet. The only difference is that that ramjet doesn’t have any moving parts. A ramjet relies on the speed of the aircraft to bring air into the combustion chamber and uses the speed of the air and the geometry of the engine to compress the air. 

Computer modeling shows that the ramjets would work most efficiently at around Mach 3. 

However, if you want to go even faster, there is a version of a ramjet known as a scramjet. A scramjet is designed for combustion at hypersonic speeds.

NASA has been working on scramjet technology, and in 2004, they flew the unmanned X-43A, which managed to reach speeds of Mach 9.6. 

There is still a lot of work to be done on jet aircraft that fly at hypersonic speeds. One of the biggest problems is that the engines that work well at high speeds don’t work very well at low speeds. 

I should also note that jet propulsion isn’t limited to just aircraft. Watercraft can be powered by water jets as well. The method of creating the water jet is very different than how an aircraft jet engine works, but the physics are the same. 

A watercraft, like a jet ski, simply uses a water pump at high speeds to create a jet of water to provide thrust. Pumpjet engines tend to be safer, as there is no exposed propeller. They can operate in shallower waters, have high power density, and are more maneuverable. 

I’ll close by making one observation. Most people, even if they have flown in jet aircraft, really don’t viscerally understand the raw power of jet engines. In the cabin of a plane, you are safely away from the engines.

However, there is one place I know of where you can experience the power of large jet engines. On the island of St. Thomas in the Caribbean, the end of the airport runway is right next to a beach. When large jumbo jets are ready for take-off, they sit at the end of the runway and rev up their jet engines, blowing the exhaust right into the beach and everyone on it.

I was there for several jet take-offs, and the air coming out of the jet engines was so powerful that it would knock most people off their feet. 

Jet propulsion has been one of the biggest innovations in transportation in the second half of the 20th century. They have allowed aircraft to fly faster, longer, and higher, Despite having been introduced over 80 years ago, all of us are still living in the jet age.