When Sir Isaac Newton formulated his third law of motion in the 17th century (stated simply: Every action has an equal and opposite reaction) he made possible an invention that would ultimately result in the high-speed travel we now take for granted: The jet engine.
The jet engine is simply a form of gas turbine that produces thrust by igniting a fuel/compressed air mixture. The action of expanding gases exhausting through the rear of the engine produces a reaction resulting in forward motion.
The first plans for a jet engine were outlined in 1928 by a young British RAF cadet, Frank Whittle. He suggested the possibility of using gas turbines or rockets to power airplanes. The British authorities were not interested in his plans, describing them as unworkable, but Whittle carried on with his work and in 1930 applied for a patent to his basic design.
Six years later, with two ex-RAF officers backing him financially, he set up a company, Power Jets Limited, to develop his design and in April of the following year the first static bench-test of his engine was successfully carried out. Jet engines were now showing promise and the previously indifferent British government took a second look at Whittle’s work.
Meanwhile, in Germany a young engineer working for the Heinkel company, Dr Hans von Ohain, was making progress with his own turbine engine. In 1937 it ran for a sustained period using gaseous hydrogen and this success caused Heinkel to urge von Ohain to develop a practical version that would run on liquid hydrocarbon fuel.
By 1939 von Ohain had a diesel-powered turbine engine and a prototype airplane for it to power: The Heinkel He 178. On August 27th 1939 (just days before the outbreak of WWII) this little airplane successfully took to the skies. It was the world’s first jet-powered flight.
Back in England, Frank Whittle’s work was progressing well and the British government contracted his company, Power Jets, to supply an engine to be used for flight testing. The Gloster Aircraft Company was contracted to supply an airframe. Engine and airframe successfully came together on May 15th 1941 when the Gloster E.28/39 (Whittle) flew for the first time.
Work carried out with the E.28/39 led to the development of the Allied powers’ only jet-powered airplane to fly combat missions in WWII: The Gloster Meteor. In the Summer of 1941 a Whittle W.1 engine was dispatched across the Atlantic where the General Electric Company built an American version. In the Fall of 1942 the Bell XP-59A became the first American jet airplane to fly.
Only Germany had produced jet airplanes in large numbers by the end of WWII, although not in sufficient numbers to affect the outcome. However, by 1945 no-one was in any doubt that jet-propulsion would be central to aviation development.
Yet those early jet engines (or turbojets) were problematic. They burned a lot of fuel, produced limited thrust and accelerated sluggishly. The answer would come in the shape of the ‘twin spool’ engine.
Early turbojets had just one compressor powered by an exhaust-driven turbine. The twin-spool engine had two compressors each powered by their own turbine. A low pressure compressor at the front of the engine was linked to a low pressure turbine at the rear. A high pressure compressor behind the first compressor was linked to its own turbine. This configuration split the power load and increased compression thereby improving performance, acceleration, thrust and fuel efficiency.
The first twin-spool engine to be built was the Pratt & Whitney J-57 in the early 1950’s. It developed over 10,000 pounds of thrust (double the thrust of other contemporary engines) and made long range jet flights possible. Strategic bombers and airliners were developed, as were fighter aircraft capable of sustained supersonic flight.
However, increased performance led to another problem: Compressor stall. This happens at low speeds and during acceleration when the airflow becomes unstable and the compressor blades can’t operate efficiently. This can result in loss of pressure and cause a ‘blowback’ of hot air through the engine with the possibility of compressor damage due to overheating and even a ‘flameout’ where the engine loses all power.
The answer was provided by the variable-stator jet engine, developed by a German-born engineer, Gerhard Neumann, who worked for the General Electric Company. Small blades between the compressor blades were able to change their angle of attack to suit different airflow rates thereby optimizing and regulating airflow and solving stall problems.
The ‘variable stator’ allowed General Electric to developed the famous J79 engine in the mid 1950’s. The J79 powered the Lockheed F 104 Starfighter, the first jet to exceed Mach 2 (twice the speed of sound). It also powered America’s most successful jet-powered combat aircraft, the McDonnell Douglas F 4 Phantom II. Almost 17,000 J79 engines were built in the US and under license around the world.
By the 1960’s work was being carried out to develop the engine that most aircraft use today: The turbofan. A turbofan consists of a large ducted fan behind which is a turbojet of smaller diameter. Part of the airstream passes through the combustion chamber but most goes round the outside to exit ‘cold’. A turbofan greatly increases thrust and efficiency and is significantly quieter than a turbojet. Modern military jets use complex turbofans coupled with afterburners (an afterburner works by injecting fuel into the hot exhaust gases to produce additional surges of thrust).
So, where next for the jet engine? Perhaps the next step will be to significantly increase speed. In the 1950’s and 60’s rocket-powered research airplanes achieved increasingly impressive speeds. The American X-program culminated in the X-15, a sleek black bullet that reached speeds in excess of Mach 6.6 (4,500 mph). One of the test pilots that flew the X-15 was none other than Neil Armstrong, the first man on the moon.
However, since the 1960’s the main concerns within aviation have been to increase fuel efficiency and noise reduction in civil aircraft and maneuverability in military ones. For fifty years the relative airspeed of aircraft has remained static.
A possible way forward might be with the scramjet. A scramjet is essentially an engine with no moving parts that can theoretically travel at hypersonic speeds (in excess of Mach 5). It resembles two narrow cones facing point to point. The air is collected in one end, compressed by the narrowing funnel, combusted and then exhausted. A scramjet needs supersonic airflow so in order to work it must travel at high speed. This is extremely difficult to achieve.
In the 1980’s the United States ran a program to develop a scramjet-powered airplane called the X-30. It could theoretically attain speeds of Mach 25 but the program was canceled in the early 90’s due to ongoing technical problems and budget restraints.
Yet engines WILL evolve. Perhaps we are now in the same position we were in throughout the 1920’s when theories abounded about fantastic engines that would transform travel, theories that were largely ignored by a skeptical world. The jet engine was one such theory and it DID work. Nowadays many theories abound and who knows which of them will work? Dare we dream that one of them WILL?