Engines are used not only in commercial airlines but also in military aircraft, missiles and for other machines flying at very high speeds. There is no worry for fuel efficiency in these applications (generally). There’s only a need for speed.
Turbojet engines are Turbofan engines without a bypass duct. Or, I should rather say that Turbofan engines are Turbojet engines with a bypass duct. This means that all air entering the engine passes through the central core of the engine. It works similar to what you read in the Primary Flow section of the first article. Don’t worry, I’ll repeat it. The large fan at the front of the engine sucks in air. This air passes through a set of compressors. These compressors compress the air resulting in an increased air pressure and heat it up to a temperature that is optimal for combustion. This is done by blades spinning at high speeds which squeeze the incoming air. Then it passes through a constant stream of atomized fuel, which is pumped into the chamber. This fuel upon contact with the hot air ignites, combining exothermically with the oxygen inside the stream. This creates an incredibly high pressure mixture, which is then passed through the turbine which runs the compressor blades. After which it escapes out through the nozzle.
There is no bypass duct present in a Turbojet engine. Which means that a lot of fuel is required. This is because the majority of thrust was being produced by the secondary flow in the Turbofan engine, and hence very less amount of fuel was required making it fuel efficient. Since all of the air passes through the gas generator (the combination of the compressor, combustion chamber and the turbine), the amount of fuel required is extremely high, as compared to a Turbofan engine. But the amount of thrust generated is very high. This is because, the secondary flow of the Turbofan engine generates thrust by increasing the total mass flow of the air. The total energy supply can be increased by increasing the mass or the velocity. But the amount of energy supplied increases more with an increase in velocity as compared to an increase in mass of the air. This immensely increases the thrust, and hence allows the aircraft to fly at very high speeds.
What if you need even more thrust though? Without better materials, we’re kind of stuck if we try to increase thrust by pushing more air and fuel through the combustion chamber. The turbine is already running so close to its melting point that adding any more heat to the exhaust will cause it to melt, which is not a good thing for healthy functioning engines. Well, what if we added more fuel, but inject it into the stream after the turbine? The exhaust gas has plenty of oxygen left even though it’s coming through the combustion chamber, and since there are no parts in the way of the airflow after the turbine, there’s nothing to be adversely affected by increased heat. This is essentially what an afterburner is – a second combustion chamber after the turbine that injects fuel into the exhaust, reheating it and raising its pressure. This causes the gasses to leave the nozzle at an even higher velocity, thus producing more thrust.
The engine of an SR-71 Blackbird in wet mode – notice how with afterburner active, the exhaust from the engine is actually visible due to the incredibly high temperature of the escaping gasses, and also exhibits a “pulsed” look due to the formation of Mach diamonds
As you can imagine, however, this method of thrust generation is extremely, extremely inefficient. So inefficient in fact, that in order to increase the amount of generated thrust by 50%, a jet engine with afterburner active (also known as operating in “wet” mode) can consume nearly four times the amount of fuel the same engine would normally consume at full thrust. This means that usage of afterburners is usually restricted to very short burst periods where an aircraft needs the additional thrust like during combat maneuvers or to quickly accelerate to supersonic speeds. The Concorde, one of the only airliners to have afterburning engines, only activated its afterburners during takeoff and for accelerating through transonic speeds. In cruise, it actually flew with its afterburners off since flying with them on would greatly reduce its range.
Even though ramjets generate the most thrust out of any other in-use engine type out there, they are still counterintuitively one of the simplest types of engine. The main reason for this is the lack of moving parts. Whereas a standard jet engine relies on a set of compressors to raise the incoming air’s pressure and bring it to a state optimal for combustion, ramjets use the forward motion of the vehicle to compress the air via a specially designed intake. The air is then ignited by spraying it with atomized fuel similar to how a combustion chamber works in normal engines, and then the exhaust gasses are pushed out of the nozzle at high speed, generating thrust.
As they rely on the forward motion of the vehicle to work, they cannot generate any thrust when stationary. This means that ramjets alone cannot power an aircraft, and need something else that brings it up to supersonic speed before it will start generating thrust. This makes them unfit for use on most combat aircraft since having the weight of an extra engine that is inactive most of the time is a big no for aircraft that need every last bit of maneuverability and acceleration they can get. So, they are mostly used on missiles, with the only real use on operational aircraft being the ramjet-style compression that the Blackbird’s engines used at supersonic speeds, even though the engines were and worked like traditional turbojets at lower speeds.
Whereas turbojets won’t be able to generate thrust much further beyond Mach 3 (the Blackbird was right at the limits of what its engines had to offer), ramjets generate reasonably amounts of thrust all the way up to Mach 6 and are most efficient around Mach 3. This makes them very attractive for potential hypersonic aircraft designs, with variants that don’t decelerate the oncoming air to subsonic velocities (called scramjets) achieving nearly Mach 10 in experimental flight. However, the main problem right now with reaching speeds that high is the airframe of the aircraft heating up due to atmospheric friction at high speeds, and also the extremely low control error margins due to extremely high dynamic pressure.
Concept art for NASA’s X-43A “aircraft” that achieved a staggering speed of Mach 9.6 or 12,000 km/h
For now though, high performance aircraft propulsion has taken a hit over the last few years owing due to the lessening importance of having higher speed aircraft when rockets are available. The newest generation of fighters actually have lower top speeds than much older designs; the F-22 for example can only manage about Mach 2.2 at full tilt whereas the much older F-15 can reach Mach 2.5. This is because as it turns out, sustained supersonic flight is too inefficient and too restrictive to be of much use, even in military applications. In a combat situation, a pilot is much more likely to want a tighter turn radius than a higher speed, and even going point to point, high speed flight doesn’t offer enough of an advantage to be worth making sacrifices on other design aspects of the aircraft.
Although the future for high speed flight is bright with newer materials and engine designs allowing experimental aircraft to continue breaking world records and concept designs for unmanned hypersonic vehicles like the SR-72 floating around, it seems very unlikely the world is ever going to see another manned high speed vehicle like the Blackbird – there is simply no need for something like that in the age of computers.
Assignments on high performance engines
- Turbojets are said to be more efficient than turbofans at high speeds. Why?
- What modifications besides fuel injectors do engines require for afterburners?
- What is the SABRE engine? How is it different from conventional jet engines?