<< Page 11 Page 12: Page 13 >>   Jet Engines These can be divided into Turbojets, Turbofans, Turboprops, Ramjets, Scramjets, and Pulsejets.   Turbojets: These were the first jet engines, developed during the 30’s, pioneered by Whittle. Propellers move through the air faster than the airplane they are pulling. Propellers also work less and less well as they go faster than MACH 1 (much of the noise of a piston-engined plane is in fact the shockwaves made by the propeller, not the “vroom” of the engine). So it is almost impossible for a propeller plane to go faster than MACH 1. So piston engines impose a subsonic speed limit to the airplanes they power, but jet engines allow an airplane to travel supersonically. A jet engine has essentially three stages: The compressor, the combustor (or burner), and the turbine. In addition, the casing of the engine will often have a diffuser in front – which slows down the incoming air and increasing its pressure, helping the compressor – and a nozzle in the back – which speeds up the gases ejected out the back, improving thrust: Here is how a jet engine works: Air goes into the compressor. It is compressed, and thus heated. It enters the burner, where fuel is injected and burnt, thus further heating the air (then as hot as it ever gets). It then escapes into the turbine, where its expansion spins the turbine around. The turbine spins the shaft that powers the compressor, which compressed and partially heated the air in the first place. But because of the fuel combustion, the air is even hotter and more energetic than it was after the compressor, so it leaves the turbine still hot and expanding, and thus shoots out the back of the jet engine with some speed higher than the one it came in at. A well-designed nozzle is one that optimizes this expansion and acceleration. Above: A J85 turbojet, like the ones in Learjets. So the compressor is powered by the turbine, but air must be compressed (and ideally burn fuel) so that the turbine can turn… this begs the question, How is a jet engine started? In one of two ways: Small jet engines, and the jet engines in many military aircraft, are connected to an extremely powerful electric motor, which spins the engine on start-up (i.e. the compressor is spun by a motor, rather than by the turbine, during start-up). This requires a powerful electric power source to start the engines, a piece of equipment (“start cart”) usually specific to that aircraft type, so aircraft that are started electrically – like the F-86, F-16, and SR-71 – can only operate from a few bases, and must not turn off their engines when at a base without the appropriate electric equipment (otherwise the jet is stuck there until the equipment can be brought to start the engines again). The other way to start a jet engine, a method found in larger jets and in private and commercial jets, is to use an APU, or Auxiliary Power Unit. The APU is a tiny jet engine, easily started by a smaller motor driven by the airplane’s batteries (no need for a start cart – that’s the point of having an APU). Compressed air from this smaller engine’s compressor is channeled and piped into the turbine of the big main engine, which spins it up and makes the big engine’s compressor (and thus the big engine) work. Most airliners have the APU inside the tail tip of the fuselage. Doors usually open when the APU is in operation, so as to allow air into it, and these doors close during flight (when the APU is off) to reduce drag, almost eliminating any visible evidence of the APU’s presence. The APU’s exhaust leaves through a nozzle either at the very back tip of the fuselage, or just beside the back tip. The picture to the left shows a 767’s tail. The APU air intake is open, and the nozzle is the metallic part all the way on the left. The air distortion behind it reveals the APU is working. Some turbojets (the ones in fighters and other supersonic jets) have what are called afterburners. This is a section of pipe after the turbine, before the nozzle, where more fuel is injected and burned, so the nozzle has even more energy to work with in expanding and accelerating the air. So an afterburner is essentially a second combustor. Using afterburners is not a very efficient way of generating extra thrust (they at least double the fuel consumption and often do not increase thrust by more than 30%), it is a last resort to quickly accelerate away form trouble, and to get off the ground quickly during takeoff. Most airplanes running on full afterburners would empty their fuel tanks in 10-15 minutes. Exceptions to this are airplanes made to cruise at fast speeds with afterburner, such as the SR-71 and Tupolev-144 (the Russian imitation of the Concorde), which have well designed afterburners that double the fuel consumption and the thrust, and can fly for hours with afterburners on. An afterburning J85, like the ones in T-38s and F-5s: A J79 engine, like the ones in the F-104 and F-4: Of course, ideally one would want a jet engine that delivers enough thrust for a very fast cruise without afterburners. Cruising faster than the speed of sound without afterburners is an ability called supercruise. This is very rare, and can be done only by the Concorde and by some next-generation fighters such as the F-22 and Eurofighter. And here’s an interesting historical side-note, more for the sake of trivia than anything else. The very first attempts at making what today could be considered a jet engine worked a little differently. They had the compressor and the combustor, but no turbine. The air was compressed, fuel was injected and burned, and the exhaust came out the back. So what powered the compressor? An external piston engine. Realizing that the compressor-combustor-nozzle set-up did not have the speed limitations that propeller planes had, some engineers’ airplanes (Coanda’s 1910 biplane and Campini’s 1931 Caproni CC.2/N-1) had their piston engines power a compressor (instead of a prop), and then fuel was injected into the compressed air, and the hot high-pressure exhaust expanded out the back. This did not provide more thrust than contemporary propellers, however, and required more fuel than if the engine were powered by a prop (or, as we now know, by a turbine that extracted energy from the compressed and heated air). Still, these designs showed that efficient compressor design was to play a huge role in future aircraft engines. 2) Turbofans: These are a lot like turbojets, but the first compressor stage is much wider than the rest of the engine. So air is ingested by the engine, going through all of the combustor and into the burner to burn fuel and into the turbine to make the whole thing work, like in a regular turbojet. However, much of the air that goes through the first compressor fan is simply blown backwards, around the outside of the engine, rather than through the core (combustor & turbine). Above; A TF39: Most of the air blown by the fan (wide part at left side) goes around the outside, rather than through, the rest of the engine Turbofans are more efficient because, instead of accelerating a little bit of air by a lot (i.e. very high speeds out the nozzle), they accelerate a lot of air by a little (i.e. lots of air through the fan). Why is it better to move more air less than less air more? Because you need to spend less energy to get a certain amount of thrust. Thrust force says how much momentum is given to the air through the engine, per second. This momentum given to the air is the mass of the air through the engine per second, times the change in speed of the air (i.e. times how fast it is when blown out the back). However, to do this you must convert energy from the fuel chemistry to kinetic energy in the air. Kinetic energy you use per second (power) to give the air is ˝ of the mass of air blown per second times its speed SQUARED. This means that, say, if mass flow doubles and speed change goes to half (i.e. fatter engine, bigger fan, smaller core), you get the same thrust but only spend half the energy (i.e. about half the fuel). This is also the reason why big propellers are more efficient than small propellers – they convert the engine’s power (horsepower = energy used per second) into more thrust (Newtons or pounds of force), which is why helicopters have huge rotors (they need all the thrust – which for them is lift – that they can get) and why WW2 airplanes had their props high off the ground (allows for bigger radius before the prop hits the ground). In modern turbofans, about 5/6 of the air goes around the outside, only 1/6 makes it into the “core” of the engine (into the compressor) to burn fuel. The ratio of the amount of air blown outside to the amount of air ingested by the core is the bypass ratio. Because a higher bypass ratio means “moving more air less”, it means a more efficient engine, and much quieter too. (Noise requirements are progressively more stringent in airports near populated areas. The noise a turbofan generates is roughly proportional to the bypass ratio to the eighth power. So a high bypass ratio engine will make it much more likely that the airplane carrying it will be allowed to, say, land at Heathrow at night. A 777 landing produces more noise due to the air blowing by its flaps and landing gear than due to the two GE90s under its wings – the largest and most powerful, and so far the quietest, turbofans ever). The challenge is one for material scientists and engineers: how long and how thin can those fans and compressor blades be made? In 1970, the 747 was the first airplane to incorporate turbofan technology in a big way, with the GE CF6. The four huge engines allowed for an airplane of unprecedented size, and also for very low fuel consumption. The 747 proved that these very fuel-efficient engines would bring air travel to the masses, and high-bypass-ratio turbofans have been used in every airliner designed since, and retrofitted into every older jet liner still in service. Fighter jet engines have very low bypass ratios (usually less than 1, no more than 1.2), so in efficiency and performance they still approximate the older turbojets. But, technically, they are still turbofans (and they too have afterburners). Above, the extremely high-bypass-ratio turbofans found in 747s (GE CF6, above) and 777s (GE 90, top left). Below, the low-bypass-ratio turbofans found in F-15s (P&W F100) and F-22s (P&W F119). Check out page 162 for some awesome specs on these engines (for example, the GE90 can ingest 2000 cubic meters of air PER SECOND). 3) Turboprops: These are like turbofans, but proportionally different: The core is very, very small, and powers a propeller instead of a turbofan. The main difference in the way they work, compared to a turbojet, is that they exist not to accelerate air backwards, but to provide shaftpower to a propeller. So as to extract all this power from the air as it expands at the back of the engine, the turbine in a turboprop engine is usually much bigger, proportionally. Turboprop engines are much smaller than turbofan engines, and power much smaller aircraft. In an ideal world, the shaft of the jet engine inside a turboprop engine would stick right out and power the prop (left), like a turbofan. However, because prop RPMs are much lower than those in the jet engine, a gearbox must be in between. A real turboprop: The jet engine (top right – notice the two centrifugal compressors, the red combustor, and the three turbine stages) powers a gearbox that in turns spins a propeller (left end). In the engine below, the prop IS in line with the jet engine shaft, but there are still gears in between: Starting around the 1970s, all large-ish propeller-driven planes (i.e. anything larger than a 6-person Cessna or Piper, including small airliners, patrol planes, military trainers, etc) have been powered by turboprop engines. Some old airplanes still in service, like DC-3s and Goose flying boats and firefighting Grumman S-2s, have been retrofitted with newer, safer turboprop engines to replace their old radial piston engines. While turbofans offer better mileage at high altitudes and near-sonic speeds, airplanes that make shorter-range flights (and thus don’t have time to go so high), and slower airplanes, are most efficiently powered by turboprops instead. These slower aircraft, if fitted with turbofan engines, would see increases in weight and fuel consumption but no real gain in fuel economy. Not every airplane can fly at 30,000 feet and 650 mph, where turbofan use becomes really advantageous. Turboprops come in all shapes and sizes, from military trainers to regional airliners, from huge cargo planes and patrol planes to small general-aviation aircraft. And then the exotic engines types (not at all common). You will probably never fly in one of these, and would be very lucky to ever see one in flight, but for the sake of completeness, here you go: 4) Ramjets and Scramjets: As we have seen when studying drag, a shockwave slows down, heats, and compresses air. This is what the compressor does in a jet engine. So if you are going at the speed of sound or faster, you can generate shockwaves to do a lot of that work for you. In fact, many supersonic jets do this in order to squeeze some more thrust out of the engines. Below; How a ramjet intake uses shockwaves to compress the air. Streamlines of air are blue, shockwaves (the places where a high pressure difference compresses the air and suddenly make it change direction) are red. If you are flying fast enough, these shockwaves could heat the air enough (no compressor needed) for fuel to be injected and burned. Then, because no turbine is needed to spin a compressor that is not there, the hot air can just be accelerated out the back of the engine after being burnt! The engine is a tube with no moving parts! This is a ramjet. Ramjets can be built which work at subsonic speeds (see below); The intake can compress the air but will just have a harder time heating it. Subsonic ramjets have been added to the wingtips of small airplanes and have fully powered small helicopters. Temperatures in supersonic ramjets become extremely high, and you do have to get to supersonic speeds before the shockwave effect kicks in. This is why supersonic ramjets have only been flown experimentally (except for the D21 spy drone, above), and with very limited success: they must be boosted to supersonic speeds by conventional jet engines before they can work. With one major exception: The J58 engine that powers the SR-71 is a hybrid. It can, under certain conditions, work only as a ramjet. But this is more of an accident and a symptom of a limitation than it is an innovation. How much fuel is injected into the burner of a jet engine depends largely on how hot the air can get before it melts the turbine blades. This is true of any jet, so unless a jet is flying at high altitudes and taking in very thin air, it is probably ingesting more air than it needs (more on this in the Envelope section of this book). Anyways, as the SR-71 approaches MACH 3, the air at the back end of the combustor gets hotter and hotter, until it is as hot as the turbine can take it before melting. At this point, any fuel injected into the burner would heat the air too much, so the burner is turned off, and the air goes straight into the turbine with no fuel, and is finally used to burn fuel only in the afterburner. At this point, the compressor and turbine are just sitting there, spinning around but doing no good. So the Blackbird engine then can be seen as a weird ramjet, with a turbine-compressor assembly in the way of the air, and all the thrust coming from the afterburner. As an airplane goes faster and faster, though, it is harder and harder to slow down air to subsonic speeds in the engine without compressing the air enough to melt the engine. Air coming into the engine at MACH 6 or so would get VERY hot if slowed down to below MACH 1. However, all current jet engines – all compressors, burners and turbines, or even ramjet burners by themselves – need subsonic air to work. But this means the air coming in can’t be too fast, which means the plane can’t be too fast. Burners that can only burn fuel in subsonic air thus impose a speed limit on the plane (about MACH 3 ˝ to 4), even in a ramjet. A burner that can deal with supersonic air would not impose this kind of speed limit on the airplane it is in. This is why much research is currently being done to achieve the scramjet, or supersonic-combustion ramjet. The X-43 (above) is a flying jet engine. Unmanned, carried under the wing of a B-52, boosted by an enormous Pegasus rocket to supersonic speeds, it then separates from the rocket and flies up to MACH 7 or so before it runs out of fuel and crashes into the Pacific. So each prototype is only flown once. Theoretically, that is. In the first launch, the Pegasus malfunctioned and the whole thing had to self-destruct. The remaining two X-43s have not flown yet, so NASA still cannot claim to have a successful scramjet. Other teams in the UK and Australia – one with a similar rocket-boosted jet engine, and one with a jet engine shot from a cannon like a bullet – claim to have achieved supersonic combustion in a ramjet for fractions of a second. This technology is at its very earliest stages, but keep an eye out for developments. It is likely that over the next year or two, a couple scramjets (one being the X-43) will fly and take good data to prove their accomplishments. A successful scramjet is the only air-breathing (non-rocket) way to get an airplane past MACH 3 or 4, so the future of very fast airplanes relies on these experiments. Let’s go back to the theory of fluid flow so we can better understand how subsonic ramjets, supersonic ramjets, and scramjets, work. This is actually very simple, and requires only one principle, or rather, an addition to a principle we already discussed. In the Lift section, I said when the path of some airflow narrows, it speeds up and the pressure goes down, but with no change in density. And when the path widens, it slows down and the pressure goes up, and again density does not change much. Well, that’s actually only true for air moving at less than the speed of sound. When air is moving faster than the speed of sound, a narrower path will cause it to slow down and increase in pressure (in which case it will actually become denser), and a widening path will cause it to speed up and decrease its pressure and density. (So how do wings generate lift at supersonic speeds? During the 40’s, people thought they couldn’t, and Bell and Nasa built the X-1 to find out. The answer (beyond “They can”), is very complicated and involves all kinds of pressure waves that I’m not getting into here). So a jet engine that takes in subsonic air and spits out subsonic air will have a diffuser (that slows down the air and increases its pressure, to help the compressor) and a nozzle (that speeds up the air before blowing it out the back) shaped like this: (Note: “M” = Mach number = ratio of airspeed to speed-of-sound) But most compressors and combustors don’t work with supersonic air. So if an engine needs to take in supersonic air and spit out supersonic air, as in any supersonic jet, it must have a nozzle like the one below, and either a diffuser like the one below or (most often) a ramjet intake like the one near the top of page 68. The intake below slows down the supersonic air by narrowing its path, while the ramjet-style intake from page 68 creates shockwaves across the path of the air (these shockwaves usually bounce around the inside of the intake, so the air passes the shockwave from each side of the intake and then one or more reflections from each shockwave). In either case, the air is slowed down and heated, so that it is below MACH 1 by the time it reaches the compressor. The nozzle on the right, called a “converging-diverging nozzle”, is found on most rockets and on all supersonic jets. So here is how the insides of a subsonic ramjet work: And here’s how the insides of a supersonic ramjet work: Of course, if you can pull off supersonic combustion (SCramjet), it’s even simpler: As an additional note on Ramjets… Many experimental helicopters were, and still are, powered by ramjets at the tips of the rotor blades. The most nearly-successful was probably the Hiller HJ1 and HJ2 (known to the US Army as the H-32): Other ramjet-tip designs include this Gluhareff helicopter and many kit-built helicopters, like the following two available from a company called Windspire. Lastly, the Windspire guy claims to have built a helicopter “backpack” with rotor-tip-jet power, but I’m not sure I believe him… Above, the Gluhareff. Below, Windspire kits: 5) Pulsejets: These are very unimportant engines, used first in the German V-1 Buzz-bomb, and now in small remote-controlled jets. Most pulsejets today are only made by hobbyists in garages, because 1) they are very loud and inefficient and 2) they are very cheap and easy to make. They consist of a tube with a valve at one end (with a carefully-enough-shaped intake, they can actually be valveless), making them almost as simple as a ramjet (essentially just a tube), without the need for supersonic speeds or shockwaves, and no spinning parts. Lately, though, big jet engine companies and research labs have been putting a lot of money into researching pulse-detonation engines, which may end up being more efficient while working at high speeds and altitudes than ramjets. Above: Top left: The V-1 Buzzbomb from World War 2. Below it, Pratt & Whitney’s 5-tube pulsejet. Above, GE’s experimental pulsejet. And to the left, a hobbyist runs a very loud pulsejet in his RC model plane. How do pulsejets work? A pulsejet is a big burner with a one-way valve at the inlet. When the pressure inside is greater than atmospheric, the valve closes, and when it is smaller than atmospheric, the valve opens. Some fuel is injected into the engine, and ignited. The explosion raises the inner pressure, closing the valve, so the expanding gas is forced to leave out the back, like smoke from a cannon. However, the inertia picked up by the backwards-leaving air means a slight vacuum forms in the engine, which opens the valve again, sucking in fresh air, so more fuel is injected and ignited and the cycle repeats. The vacuum also sucks in some of the air that was exiting out the back, and because this entering air has inertia, it compresses the fresh air prior to combustion. The valve is the crucial bit: it opens and closes hundreds of times per second, so it must be flexible but must not break, crack or leak. Most home-made pulsejets have flower-shaped valves, but many newer ones have rectangular valves, and other arrangements. Below, a “normal” round pulsejet inlet with a flower valve, and what that valve looks like after a few minutes of operation. Lower right, the alternative: a double-rectangular valve, kinda like the one shown in the cycle diagrams above and to the right. Valve damage causes pulsejets to be impractical, as no pulsejet engine can be run for more than half an hour before the valve requires replacement. Below; Some homemade pulsejets (made by Dewey King and Bruce Simpson, two of the masters of the art of homemade jets). Bottom: the Marquardt Whirlajet, a helicopter powered by pulsejets on the rotor tips