Alternative ways to make a jet engine

You might have heard jet engines called "turbojets" or "turbofans".

How necessary is that "turbo", really? Could we use something else? And are there alternatives to jet fuel?

As it happens, in some aircraft, we already don't use turbines! But simple furnaces and pistons don't cut it in these places - and you're unlikely to see them on commercial aircraft anytime soon.

Dr Maciej Mazur, a senior lecturer at RMIT University's School of Engineering, says that turbojet and turbofan engines are the two most common types of things that we refer to as "jet engines".

"As the name implies, these engines feature turbines," says Mazur.

In both turbojet and turbofan engines, the turbine's first job is to take energy from the high-speed flow of hot gases, and convert it into rotary motion.

This drives a compressor: air in a jet engine has to be compressed, so that it's hot and explosive enough to generate the right amount of thrust when combusted with fuel.

You could write a whole extra article on how these engines work in more detail (and we have), but in short: the hot, combusted gases generate thrust, or "pushing power", when they're forced out the end of the jet engine.

"In the case of the turbojet engine, all air flows through the engine itself, and thrust is generated only by the flow of exhaust gases," says Mazur.

"In the case of a turbofan engine, in addition to driving the compressor, the turbine also drives a fan at the front of the engine which passes some air through the engine and some outside the engine. Thrust is generated by a combination of the engine exhaust gases and the air bypassed around the engine by the fan."

This addition of bypassed air makes the turbofan engine more efficient, and increases its thrust.

"For both turbofan and turbojet engines the compression of incoming air into the engine prior to mixing with fuel and combustion is a key factor which enables high power and efficiency," says Mazur.

So we don't need a turbine, exactly - we just need something that compresses air. Could we use something else? Mazur says that other means are possible.

"In fact, several different designs have been developed over the years including ramjet and scramjet engines. Instead of using a turbine driven compressor system, these engines rely on the rapid forward motion the engine to compress the incoming air using a ram compression effect," says Mazur.

"The air is then passed into the engine combustion chamber where it is mixed and combusted with fuel to produce hot gases that generate thrust when exiting the engine. Due to the high speed of the air required to generate sufficient compression of air, ramjet and scramjet engines can only operate at high speeds typically at supersonic or hypersonic speeds."

Because they can only operate at very high speeds, ramjet and scramjet engines can't take off or accelerate on their own - they need a rocket or some other means of assistance to get fast enough to start working.

"They have the advantage of having a simpler mechanical construction due to very few moving parts," says Mazur. "However, they present other challenges associated with managing supersonic airflow, effective mixing with fuel, achieving rapid fuel combustion, and dealing with the high temperature in the engine and aircraft."

Other jet engines can do away with the compression problem entirely, but they're not as efficient.

"There are also jet propulsion engines which do not require inlet air compression and can operate at static or low speeds such as a pulsejet engines," says Mazur.

Pulsejet engines "operate on the principle of a periodically igniting an air-fuel mixture in combustion chamber and creating a pulsating exhaust jet that produces thrust intermittently".

"These engines are also simple in construction but typically exhibit low efficiency," says Mazur.

And there are other types of jet engines in the works. One, called the Rotating Detonation Engine, works by detonating fuel rather than combusting (deflagrating) it - that is, burning it with a flame that expands so quickly that it breaks the sound barrier, becoming supersonic.

RDEs inject fuel and oxidisers into a ring-shaped (annular) chamber, causing a rotating detonation wave.

"RDEs are mechanically simple, however they pose many engineering challenges around maintaining sustained operation, combustion stability and acceptable sustainable operating temperatures and are subject to ongoing research," says Mazur.

Incidentally, pulsejet engines could also be made more efficient by using detonations, but that throws up a raft of other engineering challenges, so there's much less research interest in them. So yes, turbine-free jet engines are possible - but you're unlikely to see them on the Sydney-to-Melbourne anytime soon.

What about fuels? At the moment, turbojet and turbofan engines use fuel derived from kerosene, gasoline or naphtha - chemically, all very similar to kerosene. Theoretically, they could use other fuels pretty easily - although Mazur points out that kerosene is a useful lubricant as well, making the engine run more smoothly.

Dr Alan Finkel, former Chief Scientist and current Special Adviser to the government on low emissions technology, says that kerosene-derived fuel is popular because it's a "high energy density fuel with excellent characteristics, such as ease of handling and ability to operate at very low and high ambient temperatures".

But emissions-intensive kerosene may not always be the best option.

"There is a constant search to find zero carbon alternatives," says Finkel. "Bio fuels are most commonly attempted, but they are not zero emissions and they are costly. There is talk of synthetic aviation fuels made from zero emissions hydrogen and carbon dioxide captured from industrial processes. That would be a zero emissions fuel, but very expensive.

"For short range aeroplanes, batteries to drive electric motors to turn the turbine are being used in some prototypes."

Finkel also points out that hydrogen is being explored in both propeller motors and jet turbines. Last month, Airbus announced they'd be testing hydrogen to fly an A380, with plans to launch hydrogen flights by 2035. But for now, if you're not travelling at the speed of sound, turbines and kerosene remain supreme.