Lockheed Martin SR-72

Aircraft Engines Part 2: The Jet Age

Turbojet engines

In the 1920s an RAF cadet by the name of Frank Whittle wrote a short paper entitled Speculation which detailed the shortcomings of reciprocating engines but also showed how an aircraft could be powered by an engine which drew in air, heated it and then expelled it at high speed. Published in the RAF college journal, he also published the basic thermodynamic equations for this type of propulsion. Remarkably, in these equations, Whittle took his cruising altitude to be the unheard-of-altitude of 115,000ft.

It wasn’t until 1937 when Power Jets Ltd., along with Whittle, built and ran the first working jet engine, the U-Type jet engine. While this was not the first gas turbine built based on designs very similar to that of Whittle’s, it was the first ever to be built with the specific purpose of powering an aircraft.

Air Commodore Sir Frank Whittle
Air Commodore Sir Frank Whittle sits at his desk during WW2. He is one of the most influential figures in aerospace history.

Around the same time, the German engineer Hans von Ohain was independently developing the Heinkel HeS1 engine. This design was remarkably similar to Whittle’s engine. Heinkel, unlike Power Jets Ltd, had funding to further develop their ideas (Heinkel had contracts from the Luftwaffe to produce aircraft for the impending war). As a result, von Ohain’s ideas were refined, developed and implemented in the Heinkel He 178 – the first aircraft to fly under the power of a turbojet engine.

So the aircraft industry had entered another era. The introduction of the jet engine allowed aircraft to reach new heights – quite literally. It allowed supersonic flight to be developed, with the Reaction Motors XLR11 rocket engine (the engine which powered Chuck Yeager piloting the Bell X-1 to break the sound barrier for the first time) being closely derived from the jet engine. It is this large increase in performance which, I believe, is the real catalyst for the age of flight.

While most commercial airliners don’t fly at the speed of sound (the only supersonic commercial aircraft to regularly see service are the Concorde and Tupolev Tu-144), most military aircraft developed since the 1950s are supersonic. If I were to build an aircraft designed to cruise at low supersonic speeds, I would likely choose a turbojet to propel it. While nowadays the turbojet engine has been succeeded by the turbofan engine in commercial flight, most people would still refer to a jet engine or a jumbo jet aircraft. While technically incorrect, the use of these terms shows the broad success of the turbojet engine.

I believe that one benefit of the war was the aeronautical infrastructure developed and the technological advances made. I believe that the turbojet is the ultimate example of this – a technology born of war but swiftly adapted for commercial use.

The main factor, in my opinion, contributing to the development of the turbojet engine is the simple fact that the current reciprocating engines had more or less hit a glass ceiling in terms of their performance. Their cruising speeds and altitudes were vastly limited and there needed to be a way around it. The fact that two engineers working independently, either side of the channel were able to produce remarkably similar solutions shows that the turbojet was a very good solution. The fact the modern turbojet engine and turbofan engine is remarkably similar to those filed for patent by Whittle in the 1930s is a testament to the success of Whittle’s creation.

Turboprop engines

Turboprops are very similar to turbojets. In a turbojet, the turbine near the exhaust is driven by the hot gases passing through it.  This is connected to the compressor which draws more air in. In a turboprop engine, the hot exhaust gases are not enough to drive the aircraft by itself. Instead, the turboshaft connecting the turbine and the compressor is attached to a propeller which then drives the aircraft – the exhaust velocity is sacrificed in favour for torque in the shaft. Through the use of gears, the propeller spins much slower than the turbine. This rotation can also be converted into vertical rotation to power a helicopter. If you were to look at the cross-section of turbojets against turboprops you would see very little difference, apart from the noticeable addition of a propeller unit at the front of the engine.

The first working turboprop engine was designed and built by the Hungarian engineer György Jendrassik in the 1930s and the first turboprop-powered airliner, the Vickers-Armstrong Viscount, flew in July 1948. The turboprop, therefore, was developed in a very similar period to the turbojet. Yet surprisingly, when the average person sees a plane with a propeller they assume that the aircraft is old. They assume that it is not as efficient as an aircraft fitted with a turbojet engine. This is not the case.

The efficiency of turboprops is far superior to that of turbojet engines (and indeed the later turbofan engines) at very low speeds. Its peak efficiency speed is about Mach 0.4. This, combined with the decreased fuel consumption over relatively short distances, has led the turboprop engine to be widely used in short distance flights.

On the face of it, therefore, the turboprop engine and reciprocating engines are used in the same circumstances – light, subsonic aircraft for relatively short journeys. Yet one could find both types of aircraft engine in many modern light aircraft. I have mentioned the Lycoming O-Series of reciprocating engines found in aircraft like the Cessna 172 in part 1, and an example of a turboprop engine in an aircraft is Piper M600, driven by the Pratt & Whitney PT6A-42A. The main distinguishing factors of the two is that turboprops have superior performance and are more expensive, but cheaper to maintain. While generally powering relatively small aircraft, they have also been used to power larger civil aircraft, and indeed some military aircraft such as the Airbus A400M Atlas and the legendary Lockheed C-130 Hercules.

The primary role of the turboprop, therefore, was to provide an alternative engine for light civil aircraft, despite its inclusion in many military aircraft. Over the past decades we have seen that the turbojet sales have dominated the turboprop sales, although in smaller capacity aircraft, typically under 100 passengers, the primary choice of engine was turboprop. I believe that the turboprop was born out of a need for efficient, economic engines for low-end performance.

Turbofan engines

Ever since the dawn of the turbojet engine, it has been evolving. If you were to compare the engines of a Boeing 737 from 1970 to the engine of a 1994 Boeing 777 the obvious difference is the engine dimensions. The Pratt & Whitney JTD8 powering the 737 is nearly 4m long and a diameter of just 1.25m.

Boeing 777
Dawn of the jumbo jet: the development of turbofan engines allowed the first commercial jumbo jets to be developed. Pictured is a Boeing 777 in Air France livery.

Compare that to the Rolls-Royce Trent 800 engine in the Boeing 777 the engine has a length of 4.37m and a diameter of just less than 3m (this is very nearly the diameter of the fuselage of a Boeing 737).  Given the size of the 777 (the cabin length is over double that of the 737 and its wingspan is over 2m longer) proportionally the engine is much shorter. The height of the engine, however, is a testament to the evolution of the turbojet engine.

Strictly speaking, the two engines above are turbofan engines. Indeed, for the past 50 years the aeronautical industry, specifically in the civil aviation scene, has been in the fan age, rather than the jet age. As mentioned before, the Concorde and Tupolev Tu-144 are the only supersonic airliners ever to have seen service – every modern airliner flies subsonically (their maximum speeds are typically around Mach 0.98 and below). Whereas a turbojet ejects a high velocity column of air, a turbofan directs air around the engine. This bypassed air means that instead of a relative low-mass, high-velocity thrust, turbofans produce high-mass, low-velocity thrust. Air is deflected passed the core of the engine where the compressed air is mixed with fuel and igniting. This has two main advantages. Firstly, the less air compressed in the core leads to an increased efficiency as less air needs to be compressed. This means that aircraft with turbofan engines have significantly lower running costs compared to the same aircraft if it were fitted with a turbojet engine. Secondly, the fact that the air has a lower velocity means that the noise of the engine is reduced (exhaust noise is a function of exhaust velocity raised to the seventh power.) So the main advantages of turbofans over turbojets are lower noise and higher efficiency. These are indeed also some of the key factors in the popularisation of air travel. The lower noise helped to make it more socially acceptable and the increased efficiency making it much more economically viable.

But the general design of the turbofan engine had been patented nearly 40 years before the Lockheed Tristar, one of the world’s first jumbo jets powered by Rolls Royce’s RB211 turbofan engine (in actual fact this was one of the first turbofan engines and the first 3-shaft engine of that type. It was designed to compress more air than was necessary and expel it, creating a cold jet). The patent was held by a certain Frank Whittle. His very first engine designs were patented and were very similar to the basic design of the turbofan engine – not too dissimilar from a turbojet, with a large bypass ratio and a fan to draw in the air.

But if the turbofan engine is at such an obvious advantage, both practically and economically, and it had been conceived even before the turbojet, why did it take such a long time to be engineered? As I have already explained, the turbine is rotated by the rapid expulsion of hot gases passing through it. Connected via the turboshaft, the fan at the front of the engine is also rotated with this and air is hence drawn in. The problem with a bypass is that more air is required which, in turn, demands a greater revolution of the fan. The way to accelerate the angular velocity of the fan is to accelerate the high-pressure gases and thus turn the turbine much quicker.

However, in the mid 20th Century the technology was not available to increase the performance of combustors. The material science of the era was not suitable. The typical turbofan engine today has an internal temperature of about 2250K (1976°C), but the melting point of titanium is just 1943K. You can see then that the performance materials of the 1960s would not have been able to withstand the high temperatures required for bypass engines. Moreover, the energy used to compress the air must come from the air itself. The technology was not available to compress large quantities of air and pressurise it.

It is no wonder then that with the dawn of state-of-the-art material science, engine components today are made from alloys with many different metallic components. It is why, I believe, that the turbofan engine took so long to replace the turbojet but, once it did, why it did it in such a profound way. While technology has, of course, developed the configuration is relatively unchanged from the aforementioned Pratt & Whitney JT8D. It remains the single most common jet engine in commercial service. It has sold more than 13,000 units and powering almost all civil aircraft today is a testament of the magnificence of turbofan engines.

Ramjets

Published posthumously in 1657, Cyrano de Bergerac’s novel L’Autre Monde: ou les États et Empires de la Lune detailed a machine which drew in air, compressed it and expelled it at a high velocity. He described an engine with no moving parts but instead propelled a vehicle through a clever application of its icosahedral shape. Arthur C Clarke believes that Bergerac should be credited with, not only the conception of the rocket but of the ramjet, also.

Science-fiction aside, however, the ramjet patent is held by René Lorin, filed in the early 20th Century. He detailed how the exhaust gases could be channelled through a nozzle and recreate the standard compression-combustion-expansion cycle of the turbojet, but without any moving parts. While this would have been far more efficient, there were no means of propulsion in the early decades of the 20th Century of accelerating an aircraft to operational speeds, thus it was never built.

The modern ramjet is not too dissimilar. Air is rammed (hence the term ramjet) by a diffuser. Fuel is sprayed into the compressed air and the mixture is ignited before its expulsion at high velocity. Ramjets have to be used in tandem with other engines. Due to the fact that the air needs to be travelling at a velocity relative to the aircraft for the engine to work, it cannot be used from rest. Moreover, ramjets are relatively ineffective at subsonic speeds. These two factors have led to no subsonic aircraft being fitted with ramjets. One of the most famous applications of the ramjet is on the Lockheed SR-71 Blackbird, although this has a turbojet engine which behaves as a ramjet engine at supersonic speeds. More research and development in recent years has led to prototype ramjets being developed which can, indeed, operate at very low speeds and quite possibly from rest, according to their manufacturers. I do, however, believe that these engines would be very similar the Pratt & Whitney J58 turbo-ramjet engine of the SR-71; a turbojet engine which behaves as a ramjet at high speeds.

Could you call an engine like this a true ramjet? Possibly not, although there is no doubt that the ramjet engines are the most efficient at supersonic speeds and, indeed, at any speed greater than Mach 0.5. Moreover, newer technology has led to the invention of scramjets (supersonic combustion ramjets) which are specifically designed to fly at supersonic speeds, and indeed at hypersonic speeds. This is because even ramjets tend to lose efficiency at around Mach 4.5 and so scramjets are used to accelerate aircraft to speeds up to a theoretical Mach 15 (an incredible 3.2 miles per second).

Lockheed Martin SR-72
The successor to the Blackbird, the Lockheed Martin SR-72 is a hypersonic UAV concept capable of reaching Mach 7.

Ramjets and scramjets alike, therefore, have been born out of a desire for speed. I do not believe that there will ever be a necessity for hypersonic air travel. Nevertheless, there is a means and, like in all other areas, this ground-breaking technology will trickle down into everyday aeronautical engineering and manufacturing. This hypersonic flight will indirectly impact the design of regular turbofan and turbojet engines in the future, in its refinement and development. I predict that, while it has had a lack of impact so far in its life, the ramjet will become a key factor in the future development of the aircraft engine.

This is part 2 of a 3-part editorial.

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