structure of the blade forms a single crystal. Most metallic objects have a crystalline structure. For example, you can sometimes see the crystal boundaries on the zinc coating of new galvanized steel cans, or on old brass doorknobs etched by years of wear. When metal objects are cast, the crystals in the metal form randomly due to uneven cooling. Metal objects usually break or fracture along the boundaries of crystal structures. To melt a crystalline object, the heat energy must break down the bonds that hold the crystals together. The bigger the crystals the more energy it takes. If these crystalline boundaries can be eliminated entirely, a cast metal object can have very high strength and heat resistance, qualities highly desirable in a turbine blade.
The first step in forming a single crystal structure is to precisely control the cooling process. In turbine blade manufacturing, this is done by very slowly withdrawing the mold from an induction furnace. This works like your microwave oven at home, only a lot hotter. Controlled cooling by itself, however, will not produce a single crystalline structure. For that you also need a 'structural filter.'
So the molten nickel alloy is poured into the turbine blade mold, which is mounted on a cold plate in an induction furnace. When the mold is filled, the mold/cold plate package is slowly retracted from the furnace. Immediately, multiple crystal structures begin to form in a crystal 'starter block' at the bottom of the mold. But because the cold plate is withdrawn vertically, the crystals can only grow toward the top of the starter block. At the top of the block is a very narrow passage that is shaped like a pig's curly tail. This pigtail coil is the structural filter, and it is only wide enough for one crystal structure to travel through. When the single crystal structure reaches the root of the turbine blade, it spreads out and solidifies as the blade mold is slowly withdrawn from the furnace. Once it is completely cooled, the turbine blade will be a single crystal of metal with no structural boundaries to weaken it. It now only requires final machining and polishing to make it ready for use.
While single-crystal turbine blades are very strong and heat resistant, they would still melt if directly exposed to the hot gases from the combustion of a turbofan engine. To keep molten turbine wheels from dribbling out the back end of the engine, a blanket of cool air from the compressor is spread over the turbine blades. This is possible because complex air passages and air bleed holes can be cast directly into the turbine blades. These bleed holes form a protective film of air, which keeps the turbine blades from coming into direct contact with the exhaust gases, while simultaneously allowing the turbine blades to extract work from those gases. Earlier non-single-crystal turbine blade designs had very simple cooling passages and bleed holes that were machined out by lasers or electron beams, and didn't provide as much thermal protection.
Thanks to single-crystal casting technologies, the turbine sections of turbofans not only operate at higher pressures and temperatures than turbojets, but are smaller, lighter, and more reliable. For example, a quick comparison between the J79 and the F100 shows that the turbine section that drives the compressor has shrunk from three large stages to two smaller ones.
The remaining problems resulting from a turbofan's higher pressure ratio include preventing the compressor blades from stalling at higher rotational speeds, and reducing the compressor section's weight. Weight is particularly critical, since every extra pound/kilogram has to be compensated for by the aircraft's designers. Fortunately, the solution to compressor stalling also reduces the compressor's overall weight.
Consider the problem: As the rotational speed of the compressor increases, so too does the speed of the airflow. At some point the airspeed becomes so high that a shock wave forms and the compressor 'stalls.' This is very similar to what happened to many early straight wing jet and rocket-powered aircraft when they went supersonic. As the aircraft exceeded the speed of sound, a shock wave (a virtual 'wall' of air) formed which caused the wing to undergo 'shock stall' and lose all lift. In an engine, excessive shock-induced drag stalls the airflow and the compressor is unable to push the air any further. In aircraft design, the remedy for shock stall was to sweep the wings back. The same solution works for turbofan engine compressor blades. Sweeping back the compressor blades not only avoids shock stalling, but allows the blades to do more work on the air because they are moving faster. This raises the pressure ratio. Since these higher-speed, swept-back compressor blades are much more efficient in compacting air, a smaller number of compressor stages are required to achieve a desired pressure ratio. A smaller number of stages means a reduction in the overall weight of the compressor and the engine itself. Again, comparing the J79 and the F100, we can see an overall reduction in the number of compressor stages from seventeen in the J79 to thirteen for the F100 (or really only ten if we exclude the fan section). Compressor weight has also been reduced through the use of titanium alloys in about half of the stages towards the front of the engine. Although titanium is lighter than nickel alloys, it cannot be used further aft than the midsection of the compressor (due to heat-resistance limits of titanium alloys), so heavier steel alloys are used in the remaining stages. Still, there is a significant weight saving from the use of titanium where it is applicable, and the current generation of fighting turbofan engines has greatly benefited as a result.
Once the problems with higher rotation speed compressors were solved, turbofan engines generally replaced the turbojet as the propulsion plant of choice for high-performance military aircraft. Their superior thrust made them a natural choice for the new generation of high-performance aircraft like the F-15 and F-16 that came on-line in the mid-1970s.
The latest version of the Pratt & Whitney F100 family, the F100-PW-229, is generally considered to be the best fighter engine in the world today. It is capable of delivering over 29,000 lb./13,181.8 kg. of thrust in afterburner, as well as providing improved fuel economy in dry-thrust ranges. Although it's not the first turbofan engine used in a fighter design (the F-111A was fitted with the Pratt & Whitney TF30), the F100 engine was the first true 'fighting' turbofan, and is the propulsion plant for all of the F-15-series aircraft and the majority of the F-16 fleet as well. The F100 engine first flew in July 1972 in the first prototype F-15; and by February 1975, the Eagle had established eight world records for rapid climbing, streaking past the records held by the turbojet- powered F-4 Phantom and the Soviet MiG-25 Foxbat.
The improvement in fuel economy at subsonic speeds came about because the smaller quantity of higher- pressure air entering the combustion chamber mixed better with the fuel and burned more completely. Since the fuel burns more efficiently, turbofans have about 20 % lower specific fuel consumption at subsonic speeds; and as an added bonus they do not produce as much smoke as a turbojet. This was a major tactical improvement. In Vietnam, the F-4 Phantom II usually announced its presence by the plumes of smoke belching from its twin J79 turbojets.
Another significant improvement in fuel economy and overall engine performance came with the development of an advanced electronic-control system called Full-Authority Digital Engine Control or FADEC. FADEC replaced the old hydromechanical control system found on turbojets, responding faster and more precisely to changes that the engine experiences in flight. Factors that FADEC monitors include aircraft angle of attack, air pressure, air temperature, and airspeed. Since FADEC can monitor considerably more parameters than a hydromechanical system, it is constantly fine-tuning the engine to maximize its performance.
Not everything about a fighting turbofan engine is an improvement over a turbojet. For instance, the afterburner of a turbofan actually consumes far more fuel (about 25 % more) than its counterpart on a turbojet. Because so much of the air entering a turbofan goes through the bypass duct, the afterburner is supplied with a larger supply of oxygen-rich air. With the greater amount of oxygen available for combustion, more fuel can be sprayed into the afterburner to produce even more thrust. For turbofan engines, the afterburner provides about a 65 % increase in thrust (compared with 50 % for a turbojet). The good news is that aircraft equipped with fighting turbofans don't need to use afterburners as often. The latest version of the F100 produces as much thrust without afterburner as the J79 does with it. Now, an F-15C still needs the afterburner to sustain supersonic flight, but it can cruise at high subsonic speeds, loaded with external fuel tanks and missiles, without using this fuel-guzzling feature.
Presently, all high-performance fighters are subsonic aircraft, with the ability to make short supersonic dashes through the use of afterburners. But the USAF's next-generation Advanced Tactical Fighter (ATF) will be required to sustain cruise speeds above Mach 1.5 (at altitude)
