Military Aviation Engines
The Wright Brothers, Wilbur and Orville, were bicycle makers from Dayton, Ohio who achieved the first controlled, manned, powered flight on December 17, 1903, in an airplane which they designed, built, and took turns piloting. Prior to their success, they mastered an understanding of the aerodynamics of lift and drag, lightweight structures, stability and control, and of course, propulsion. They studied the work of others, verified and extended it, kept their own counsel when their experiences ran counter to the experts, invented, adapted, and perfected their work at every turn.
Since they could find no suitable engine for their airplane, Orville designed one based on the simple unit they had constructed to power the machine tools in their cycle shop. Their mechanic, Charlie Taylor, built it using aluminum, a new lightweight material, for the block. Their engine produced 12 horsepower and weighed about 200 pounds.
Finding little useful information on the theory of propellers, they conducted their own experiments and built a successful propeller which, when combined with their revolutionary engine, was enough to change the world.
In the ensuing decades, airplane piston engines were built in numerous sizes, shapes, and designs: air cooled and liquid cooled, radial, Vee, and in-line cylinder configurations, with fuel injection or carburetion, poppet valves and sleeve valves, normally aspirated and supercharged. Even within each of these designs, there were many different approaches.
Supercharging is a good example. Early on, it was realized that the limiting factor to achieving high altitude flight was the loss of power that a normally aspirated engine experiences as it ascends to thinner air. To concentrate more air into the combustion chamber, superchargers were introduced and configured with the compressor directly driven from the crankshaft, driven by exhaust gas, or combinations of the two. Intercoolers were sometimes included to cool the supercharged air, producing more power and decreasing engine knock. By the mid-1920s, variable pitch propellers were invented to optimize prop pitch with flight speed.
In WWII, American industry built hundreds of thousands of aircraft engines that delivered victory into the hands of the Allies. Some of the more notable engines include the Wright R-1820 (B-17), Pratt & Whitney R-1830 (B-24 and C-47), Allison V-1710 (P-38), Pratt & Whitney R-2800 (P-47, F-8U and C-46), and the Rolls-Royce developed, Packard built Merlin (P-51). The V-1710 powered America’s two top aces of the war, Major Richard Bong and Major Thomas McGuire, to a total of 78 victories. While development of the piston engine was curtailed dramatically with the advent of the jet engine, many types of piston engines powered America’s aircraft through the Korean and Vietnam Wars.
Progress in piston powered aircraft can be appreciated by considering that the propulsion system of the Wright Brothers’ 1903 airplane had a power-to-weight ratio of 0.04 Hp/lb and an overall efficiency (the product of thermal and propulsive efficiencies) of about 5%. By the end of WWII, the power-to-weight ratio of piston powered aircraft had improved by more than an order of magnitude and the overall efficiency by a factor of five.
The jet engine was concurrently and independently invented and developed by both Frank Whittle in Great Britain and Hans von Ohain in Germany prior to WWII. While the Whittle and von Ohain engines both ran in 1937, the Germans were first to fly a jet airplane and build a jet fighter, the twin engined Me 262, which saw action in the last months of the war. To power it, they built thousands of Junkers Jumo 004 jet engines. The Me 262 was 100 knots faster than the Merlin powered P-51, the fastest fighter America had in the skies. While the Me 262 was too little too late for the Germans, it clearly showed the superiority of jets.
In Britain, jet engine development also progressed rapidly and soon attracted the attention of USAAF General “Hap” Arnold. In 1941, after witnessing a flight of the Whittle powered Gloster E28/39 jet prototype, he negotiated with the Air Ministry to produce the Whittle/GEI-A in the United States. From this British “seed” engine, much of the US jet aircraft engine industry took root.
Significant increases in thrust may be obtained by increasing the operating temperatures of turbojet engines. Since the operating temperatures of turbojet engines are limited by high-temperature material properties, probably the most direct approach to advancing engine temperatures is to develop improved heat-resisting materials.
The introduction of jet propulsion systems gave rise to other design considerations. For example, the mass distribution was generally quite different from that for reciprocal engine designs. The typical reciprocal engine-propeller arrangement was located forward and resulted in a forward location of the center of gravity (c.g.). Jet engines, however, were placed near the mid-body or aft portion of the airframe and resulted in a more aft c.g. location. Thus, those force and moment factors that are related to the c.g. location would be influenced. The forebody lift will produce a destabilizing pitching moment with increasing angle of attack and the forebody sideforce will produce a destabilizing yawing moment with increasing angle of sideslip.
Jet engines were a paradigm shift in technology and immediately new US aircraft developments were based on them. Jet engines powered all new US fighters beginning with the P-80, and all bombers beginning with the B-47. While jets offered terrific speed, they were notoriously fuel hungry and short lived. Through cycle analysis, it was known that higher thermal efficiency required a higher cycle pressure ratio, and a corresponding increase in turbine inlet temperature. In the late forties, the generally acknowledged pressure ratio limit for a multistage axial flow compressor with fixed stators was about 6:1. Above that value, the compressor simply was not operable; it could not be started at low rpm nor accelerated to high rpm without stalling. Since the rotor and stator angles were set to produce high pressure ratio at high rpm, the angles were far from optimum at low rpm.
In the US, Pratt & Whitney led the way with a successful high pressure ratio (12:1) design consisting of two separate compressors in series (one with nine stages and one with seven stages). The two compressors ran at different rotational speeds and were only aerodynamically coupled. Pratt & Whitney’s two spool compressor development was the foundation of the J52, J57, and J75 series of military engines and the JT3 and JT4 series of commercial engines.
General Electric’s approach was to design a single spool 17 stage axial flow compressor with variable stators to prevent rotor blades from stalling. These controllable stators, or the variable geometry compressor as it came to be known, were the foundation for the J79 series of military engines and the CJ805 series of commercial engines.
Both the dual (and even triple) spool and variable stator configuration compressors would come together a decade later in the high bypass turbofan engines powering wide body aircraft.
Because of its high power-to-weight ratio, the jet engine in the form of a turboshaft also became the engine of choice for low subsonic speed, fixed-wing aircraft and helicopters. Engines such as the Lycoming (now Honeywell) T53 and T55, the Allison T56, and the General Electric T64 have been in production and operational use for nearly 50 years.
Turbofan engines supplanted the pure jet in both military and commercial applications beginning in the early 1960s. While the earlier doubling of the compressor pressure ratio had improved the thermal efficiency of the jet engine, the propulsive efficiency was improved with the turbofan cycle. The Pratt & Whitney JT3D/TF33 family of turbofans dominated early US commercial (Boeing 707 and Douglas DC-8) and military (C-135, B-52H, and C-141) applications for turbofan engines.
The first afterburning turbofan, the TF30, powered the F-111 multirole fighter. Afterburning turbofans, with bypass ratios of one or less, provide both good subsonic cruise fuel efficiency and high augmented thrust for supersonic flight. Even today, the afterburning turbofan remains the dominant cycle for all fighters.
High bypass turbofans, meaning bypass ratios in the range of 5 to 9, power virtually all transports designed to cruise at high subsonic speeds. High bypass ratio engines provide increased takeoff thrust, low environmental noise, and low specific fuel consumption. The development of the first high bypass ratio turbofans, the TF39 for the C-5A and the JT9D for the Boeing 747, required nearly doubling the cycle pressure ratio from the 12:1 of the JT3/J79 series of jets, and increasing the turbine inlet temperature.
The newest high bypass turbofans have cycle pressure ratios greater than 40:1 and have been made possible by advancements in high temperature materials and cooling technology. In a general sense, increases in hot section materials capability and turbine cooling techniques have paced the development of high pressure ratio engines. Today, turbofans range in size from small missile engines by Teledyne and Williams International, to behemoths in the 100,000 pound thrust class for large transports.
Since the first United States-built aircraft gas turbine engine was flown in 1942, engine control technology has evolved from a simple hydro-mechanical fuel metering valve to a full-authority digital electronic control system (FADEC) that is common to all modern aircraft propulsion systems. At the same time, control systems have provided engine diagnostic functions. Engine diagnostic capabilities have also evolved from pilot observation of engine gauges to the automated on-board diagnostic system that uses mathematical models to assess engine health and assist in post-flight troubleshooting and maintenance.
Using system complexity and capability as a measure, it is possible to break the historical development of control systems down to four phases: (1) the start-up phase (1942 to 1949), (2) the growth phase (1950 to 1969), (3) the electronic phase (1970 to 1989), and (4) the integration phase (1990 to 2002).
As engine capabilities advanced in the 1950s, engine control technologies also accelerated to deliver the new capabilities. By 1969, a number of well-known and long-service engines had been tested, such as GE's J79 and F101, and PW's TF30 and F100. As the engine technology had matured to high compression ratio, by-pass flow turbofans during this period, the control technology had also matured to variable geometry controls, e.g., the compressor stator control, intake and nozzle controls. This period exemplified several successful transfers of military engine technologies into commercial engines. The most notable examples are the PW JT3 (originated from the J57) for the Boeing B-707 and the Douglas DC-8, PW JT8D (from the J52) for the B-727 and the DC-9, PW JT9D (from the TF30) for the B-747, GE CF6 (from the TF39) for the DC-10, and CFM56 (from the F101) for the B-737-300.
In 1987, the Integrated High Performance Turbine Engine Technology (IHPTET) program was established to double aircraft propulsion capability. Supercruise (supersonic flight without afterburner) and advanced STOVL (Short Takeoff Vertical Landing) are made possible by investments in IHPTET technologies that are transitioning today to the F-22 and F-35.
Military and commercial gas turbine engines used for aircraft, ships, and utility power generation require more durable and more reliable hot-section components in order to achieve their "designed for" life. Less durable parts lead to increases in unscheduled (and costly) inspections, engine repairs, and major engine overhauls. For aircraft applications, this equates to less time "on-wing," a severe reduction in flight safety, and a significant reduction in operational readiness. Typical in-service Naval aircraft that are affected include the AV-8B with the F402 engine and the E-2/P-3/C-2/C-130 aircraft with T56 engines and Global Hawk Unmanned Aerial Vehicle (UAV).
Advanced turbine engines, such as the F414 (F-18E/F) and the F135/F136 (Joint Strike Fighter (JSF)), require much higher levels of turbine inlet temperatures, and hence much hotter hot-section components, to achieve robust engine performance. In the case of JSF and AV-8B, short takeoff/vertical landing (STOVL) operations are even more severe and critically limit the life of high temperature turbine engine components. A high temperature protective coating on components is key to enable longer life durability on the engine during these adverse conditions. In addition, T56-derivative engines (501K) are used for power generation on many Naval ships, and are experiencing thermal corrosion issues on turbine airfoils and have similar turbine durability issues.
A 40 degree F reduction in a turbine airfoil metal temperature can result in a doubling of stress rupture life and LCF life of aircraft gas turbine engine hot section components. The proposed program will address advanced TBCs with specific thermal resistance that are up to double that of conventional EB-PVD zirconia based thermal barrier coatings (TBCs). Many of these advanced TBC coatings are not very durable and prematurely spall-off the vane and blade turbine airfoils. At least doubling the Durability that of today's baseline of platinum aluminide, zarconia electron beam-physical deposition (EB-PVD) coating systems is one of the projects goals. In addition, while the insulation qualities provided by TBCs are highly desirable, TBCs add non-load bearing weight and thickness to the rotating components.
Too much coating will significantly decrease blade creep life and detrimentally impact airfoil aerodynamics and result in significant loss of turbine efficiency and operability. A very durable low thickness (below 125 mils) high-temperature protective coating system; comprised of a robust bond coat and an advanced TBCs with half the low thermal conductivity of today's platinum aluminide, zarconia EV-PVD coatings is the goal of the project.
For the future, the DoD, DOE, NASA, and industry program known as Versatile, Affordable, Advanced Turbine Engines (VAATE) will assure further dramatic improvements in turbine engine affordability, not only for military applications such as aircraft, rotorcraft, missiles, and Unmanned Air Vehicles (UAVs), but also for America’s domestic applications. VAATE will develop technologies that enable affordable growth to legacy systems and provide propulsion and power for future air, land, and sea applications.
Future engine designs will yield continuing improvements in performance with additional emphasis being placed on operational suitability, durability, and life cycle costs. Characteristics which promise highly survivable, reliable, easily maintained engines will be in demand and will be influenced by the performance trends described herein. Specific power and specific fuel consumption are, in general, improved by increasing pressure ratios and turbine inlet temperatures. As stronger, lighter-weight materials become available and more precise temperature measurement and control become possible through developing pyrometry, electrical controls and turbine cooling technology), increased pressures and temperatures are forecast.
Standard engine nomenclature used: R for radial aircooled (generally followed by a number indicating displacement); J for jet; T for turboprop; TF or F for turbofan, O for horizontally opposed; all others were in line or Veetype, generally liquid cooled. Standard power terminology is horsepower for propeller drives and pounds thrust for jet units. The practice on turboprops has varied. Sometimes the horsepower absorbed by the propeller and the residual thrust in pounds are both given; at others, the two are combined in equivalent shaft horsepower "eshp."
Identifying nomenclature for engine manufacturers was adapted from standard practice as follows: AL, Allison; ACM, Aircooled Motors; AIR, AiResearch; AM, Aeromarine; BO, Boeing; CO, Continental; FR, Franklin; GE, General Electric; LA, Lawrance; LI, Liberty; LY, Lycoming; PK, Packard; PW, Pratt & Whitney Aircraft; RA, Ranger; WAC, Wright Aeronautical Corporation; WE Westinghouse; WR, Warner.
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