Aviation will suffocate in its own pollution early in the 21st century unless something radical is done, warns NASA. "The growth of aviation will be increasingly constrained by environmental issues," says Glenn Research Center director Dr Carol Russo, who maintains that meeting the US agency's drastic emissions and noise reduction targets are vital for the industry and the global economy.
"There are some models that show up to 5% growth in gross domestic product is fuelled by a 5% growth of aviation. So this is not just about keeping the General Electrics, Pratt & Whitneys and Boeings in business. It permeates the entire economy," says Russo.
In Europe and the USA, the propulsion community is working towards a common vision of aero-engines that are dramatically cleaner, quieter and smarter than today's turbofans. The new powerplants that emerge must also be substantially cheaper to produce and operate than current engines if aviation is to achieve its growth projections.
NASA is taking the first steps towards this smart, green and affordable engine vision with technology initiatives under way at Glenn, home of the agency's Aerospace Propulsion Systems programme, recently reorganised after the early-1999 cancellation of NASA's High Speed Research (HSR) and Advanced Subsonic Technology (AST) projects. The emerging successor is the Ultra-Efficient Engine Technology (UEET) programme.
Like the supersonic and subsonic engine efforts before it, UEET will take a host of embryonic developments from Glenn's basic research programmes and bring them up to component-level technology readiness. UEET is much broader in scope than AST and HSR. The programme's draft vision statement calls for it to "develop and transfer revolutionary propulsion technologies that will enable future generation vehicles over a wide range of flight speeds". Like the proposed Versatile Affordable Turbine Engine successor to the US Government's successful Integrated High Performance Turbine Engine Technology programme, UEET aims to do more with less. It shifts focus away from point design targets to more generic technology development, maximising the investment from research and development.
NASA is keenly aware of the potential drawbacks, as UEET programme manager Joe Shaw admits. "The fear is that you can have a programme a mile wide and a molecule deep, and we are sensitive to that. But we have to make sure we have a solid foundation of technologies, and that we can bundle those technologies together and produce a demonstrator. It was one of the lessons learned from the HSR. It was aimed at a Mach 2.4, 5,400nm [10,000km], 300-passenger target, and we were constantly bouncing technology decisions off that system requirement. The downside was that this was viewed as a one-vehicle programme and when Boeing said 'we can't do it', we lost the programme."
The most promising HSR and AST technology efforts have survived to become part of UEET, which was firmed up at the end of November 1999. Programme goals have a strong focus on the environment, with a de facto emphasis on efficiency. They will "address long-term aviation growth potential, without impact on climate, by providing technology for dramatic increases in efficiency to enable reductions in CO2 based on an overall fuel savings goal of up to 15%". Local air quality concerns, as well as ozone depletion worries, will be targeted with the development of technology to reduce NOx emissions by 70% at take-off and landing, and to allow aircraft to cruise without affecting the ozone layer.
The broad scope of UEET means that there will be not one, but possibly six reference airframes at which to aim the technologies. These include a 50-seat regional jet, 300-passenger subsonic and supersonic airliners, a supersonic business jet, an advanced combat aircraft and a reusable launch vehicle. "We are still struggling with the fighter and the access-to-space airframe requirements," admits Shaw.
Raw material for the programme will be mined from five main technology seams. They include combustion, turbomachinery, materials and structures, propulsion/airframe integration and systems integration and assessment. Each is fed from below by promising developments that emerge from the basic research and technology programmes.
Emission battle
Starting in the heart of the engine, the battle to drive down CO2 and NOx, particulates, aerosols and other emissions is being waged at source by the Turbomachinery and Combustion Technology (TCT) project. "Our primary goal is to reduce emissions through improved combustors and lower fuel burn," says project manager Bob Corrigan. "To do that encompasses making turbomachinery more efficient, or extracting more energy out of the combustion exhaust. So we do more work for the same fuel, or use less fuel for the same amount of work."
TCT's NOx emissions reduction target goes beyond the 70% specified by UEET, and is focused on developing lean burning combustors rather than traditional rich burning designs. "One of the problems when you go to leaner combustors is that instabilities are introduced. You get fluctuations in temperature and pressure which have a significant impact on the turbines downstream," says Corrigan.
To get around this, NASA is investigating active combustion control, which counteracts potential instability by modulating fuel injection in the combustor. By pulsing the fuel in a "smart" way, the engine could teach itself to control "spikes" and instabilities. Using the same smart system, NASA is also looking at a combustor that would alter its own burn pattern to avoid thermally overstressing turbine blades in which sensors have detected signs of weakness and possible failure. Microelectromechanical systems (MEMS) technology is the key to this approach, says Corrigan.
Another technique offering potential for reducing NOx is lean direct injection. This involves building a complex series of fuel mixers in a laminated construction. Each layer is etched with channels and drilled with holes to allow the fuel to mix with air, and then stacked on top of the next layer. "We are testing it in a flame tube and it is showing around an 80% cut in NOx emissions," says Corrigan, who adds that the relatively simple construction produces a large mixing area at low cost.
"We are working on the analytical codes to look at exactly what is going on in combustors," he adds. The result, if all goes well, will be the National Combustor Code - an engineering Rosetta stone for future combustor design.
In the turbomachinery arena, advanced fans, highly loaded compressors, high/low-pressure (HP/LP) turbines and coupled HP/LP turbines form a major part of both the UEET and TCT efforts. Here NASA has set the goal of demonstrating "the turbomachinery technologies required for lightweight, reduced-stage cores, LP spools, and propulsors for high-performance, high-efficiency and environmentally compatible propulsion systems". Targets set by UEET include reducing component weight by 20% and total engine weight by 5%. Efficiency will be increased by 1- 2%, average stage loading increased by more than 50%, cooling flow reduced by 25% and turbine inlet temperature increased by more than 200°C.
Supporting these goals is TCT work on improving fan and compressor stability. "We are looking at techniques to move the stall line to the right, so that the engine can operate at high efficiency levels and still have adequate margins even if it encounters some sort of perturbation like a bird strike," says Corrigan. A promising area of research is active control of compressor-blade tip stall using carefully injected bursts of air. The air would be injected into "bubbles" that form around the blade tips at the onset of a stall. Researchers have discovered that a tiny amount of injected air is enough to burst the bubble, and destroy the conditions needed for a stall. "You don't need to have air being fed all the time. It could be pulsed only when you need it, or when the engine detects [a stall]," says Corrigan, who adds that the air would be injected by jets in the casing, rather than in the blades. NASA is also investigating a passive form of stall margin improvement involving slots in the casing which change the local pressure gradient around blade tips.
Tests have been carried out on a two-stage compressor section and plans are in hand for tests on a six-stage LP compressor. Initial tests were made using 12 injectors, but later ones using just four proved to be as effective "through good timing. It reduces the amount of bleed-air energy needed and still moves the stall line far enough back to give the benefit", says Corrigan. "Overall, it produces a more efficient engine because you can operate the compressor close to highest efficiency where you'd normally be close to the stall line. With tip injection, it moves the stall line away from that high efficiency regime, so for the same fuel it will reduce emissions".
Another promising area is "aspirated" turbomachinery, in which boundary layer air is bled off at discrete locations on the blade surface. The result is a much higher pressure ratio for a given tip speed. "There is limited proof-of-concept lab work going on in that in the base programme which we'll take into UEET to see how it works for turbofans," says Shaw, who adds that the goal is to build a compressor with just four stages. NASA projects potential fuel savings of more than 10% from aspirated technology, which could lead to reduced rotational speeds and a 40% cut in component weight.
A big focus of the turbomachinery research is on developing accurate computational analysis tools. "We are working on enhanced codes for understanding what's going on in the compressors and for designing bigger stall margins," adds Corrigan.
Modelling codes
An emerging follow-on to TCT, called Smart Efficient Components, will use the new modelling codes and MEMS technology to take engine efficiency to new heights. NASA hopes the ultimate result will be a thinking, adaptable engine that can make changes in flight to optimise performance and minimise emissions throughout the envelope. "It could allow morphing of fluidic and structural components so we can have carefully controlled flow and active combustion control," says Corrigan. One near-term study is a synthetic jet which can be used to produce a variable inlet. The fluctuating jet is created by an oscillating piezoelectric membrane, which forces the jet into the flow by squeezing the air in an adjacent cavity.
Other technologies on the horizon include seals and bearings which use fewer parts, are more efficient and considerably more reliable. They also offer the first realistic chance for engine makers to achieve another dream - the oil-free, electric-actuated engine. Up to half of United Airlines' maintenance costs, for example, are directly tied to oil-related systems. Acoustic seals, for example, would perform the same function as traditional mechanical seals but would be formed by actively generated high-pressure waves which would be "guided" to act as seals in the appropriate area.
Work on oil-free bearings for high-speed and high-temperature turbomachinery has accelerated thanks to technology breakthroughs in foil bearings, coatings and analytical modelling. Although air-cycle machines have used air bearings for almost two decades, the latest advances promise maintenance-free small gas turbines, and savings in larger turbofans.
NASA Glenn has combined advanced foil bearings with solid lubricant coatings and analytical modelling to design and test an oil-free heavy duty diesel engine turbocharger. Current oil-lubricated turbochargers suffer from high power loss, coking, leaking seals, fires and low overall efficiencies of around 60%.
A foil bearing cushions a layer of air between a compliant (bump foil) support structure, which lines the bearing housing, and a rotating top foil which lines the shaft. The self-acting hydrodynamic bearing has no theoretical speed limit, requires no oil, tanks, coolers, filters or plumbing and needs no maintenance. The experimental oil-free unit includes two journal foil bearings, two thrust foil bearings, a NASA-developed solid coating and a rigid rotor. "So far we've run the bearing at red hot temperatures up to and beyond 1,200°F [648°C], which opens up the possibility of high-speed flight," says foil bearing research scientist Chris Della Corte.
According to Corte, the successful development of oil-free bearings for the stressful turbofan environment could "start a turbomachinery revolution. Bearings are the backbone of the engine and this is the way to start major advances". NASA plans to test a full-scale demonstrator of a small gas turbine in four years, and to have a mid-range engine demonstrator running around 2007. The current schedule calls for an oil-free turbomachinery demonstration in a large engine by 2011. Overall, NASA hopes to achieve a 15% reduction in weight and 15% lower maintenance costs through the introduction of the new bearing technology.
New materials
New materials will be needed to make more efficient engines. Ceramic matrix composite (CMC) liners and vanes, advanced materials for disks and aerofoils, lightweight nozzle structures and high-temperature polymer matrix composite (PMC) materials form the core of NASA's ambitious drive for an 8-15% reduction in fuel burn and CO2, and a 70% cut in NOx.
Objectives include the development of materials able to guarantee long disk and airfoil lives at turbine-rotor inlet temperatures of 1,700°C and pressure ratios in excess of 55:1. They include the development of a CMC vane able to meet the same turbine temperature, and overall weight reductions through new uses for lighter, higher-temperature PMCs. The tougher CMC material is also intended to line new low-NOx combustors. Tests have seen CMCs run in combustors at temperatures up to 1,200°C. "We want to ratchet that up to 2,400°F [1,314°C] next, which is a pretty sporty challenge," says Shaw. A relatively high temperature capability of 290°C is targeted for PMC engine components assembled using low-cost and environmentally friendly resin transfer moulding.
Ways of reducing emissions through better engine-airframe integration are being studied, including smart nacelle technology and unconventional installations that reduce aircraft CO2 emissions by cutting the drag of propulsion systems on everything from future general aviation aircraft to reusable launch vehicles.
Main objectives include the computational design of integrated, low-drag nacelles, developing reduced-length inlets, and eliminating boundary layer diverters. The design challenge requires the development of advanced computational fluid dynamics grids and optimised algorithms, while the main hurdle to shorter "S" inlets will be preventing flow separation. The elimination of boundary layer diverters means facing the problem of minimising inlet distortion caused by ingestion of the boundary layer. These latter two challenges may be met by active flow control developments, says NASA.
Ostensibly the least glamorous star in NASA's aerospace propulsion pantheon, systems integration and assessment is quickly assuming a vital role in the future of powerplant technology. Not only will it provide essential guidance to the UEET programme, but it will be used to track the progress and model the effects of engine exhaust on the atmosphere and humans. Potentially most important, it also involves the development of high-fidelity system simulations with the ultimate goal of reducing engine development time by almost half.
Glenn Research Center plays a significant part in NASA's High Performance Computing and Communications programme, which aims to accelerate the use of supercomputing to solve problems of national interest. "Our unique role is propulsion and our vision is to computer-model the entire engine from inlet to exhaust at very high levels of fidelity - both in terms of three- dimensional aerodynamics and structurally. This has never been done before," says John Lytle, chief of the computing and interdisciplinary office at Glenn.
"The current state of the art is to model components, subcomponents and blades row by row using CFD. But over the last few years we've been working to model the full component such as an entire high-pressure compressor with all 21 blade rows. The key is to compute that in a timeframe which is of use to companies - overnight or in a 15h period. The next step is to model the full engine, and by the end of 1999 we expect to have done that with a full turbofan," says Lytle. Although this will initially be limited to structural analysis at the part level (a blade row or blade), it will eventually be extended to cover the full engine, he adds.
Massive computer power
Current work is performed on "massively parallel" computers with "tens to hundreds of processors needed", says Lytle. Eventually, NASA hopes to migrate the work from the present engineering workstations down to PC cluster level. "As cost comes down and performance goes up on PC processors we expect that in the next couple of years this will be possible," he adds.
Lytle believes the holistic design capability offered by this computing technology is invoking a major shift in engine development. "We are taking engine design to the next step. By putting everything together in a simulated environment that allows you to easily assemble these complex interactions, you can speed up the design of an engine that will also turn out to be more efficient. Say you get a high-frequency vibration at the LP turbine because of a shockwave in the transition duct between the HP and LP turbines that was not predicted because you didn't see the interaction. Simulation would discover those early.
"There is a major shift going on in how subsystems are designed as a coupled subsystem. There are significant changes being made as a result for HP and LP turbines in high bypass-ratio turbofans and in transition ducts which would otherwise have distorted the flow and affected the efficiency of the LP turbine. So there is a lot to be learned by coupling subcomponents together. The next step is to bring the combustor together to establish its life and that of the HP turbine," he says.
NASA is developing a "plug and play" programme which will allow engine companies to assemble a full engine simulation at the initial engine cycle level. "At that level we've brought airframe people in because they often share the same data, and we have developed some standards for how the data are shared among companies so the engine makers can plug their model into the airframer's requirement. It really enhances collaboration between the two," Lytle says. The result is expected to bring engine and airframe designs together much earlier in the process.
While much of the propulsion research is shaped by emissions concerns, the older battle to cut noise goes on. At Germany's DLR aerospace research agency, for example, active noise control (ANC) is emerging as one way to suppress engine fan noise.
The agency is testing a system which generates an "anti-sound" wave to counteract the primary sound wave generated by the fan rotor. The sound field in the intake duct is sensed continuously by an array of microphones in the inner duct lining. These signals are fed to an electronic controller which then computes input signals for the secondary noise sources so as to achieve maximum sound attenuation. According to the DLR, ANC promises larger attenuation rates in short-duct engines than produced by conventional passive absorbers.
The bulk of engine noise on approach is generated by acoustic interaction between the fan rotor and downstream stator vanes. Reducing this noise source plays an important role in current engine design and the DLR believes that widespread adoption of ANC would enable "aircraft engines to be designed entirely for optimum aerodynamic performance with subsequent savings in engine length and weight".
NASA, on the other hand, leans more towards a modelling-based holistic approach to treating fan/nacelle noise. Predictive codes are being developed that will allow precise design for noise reduction covering everything from the positioning and shape of stators, to alignment of splices in the inlet duct. William Willshire, AST noise reduction programme manager, says: "There is a paradigm shift going on today. Engines have been designed for the 'cut-off' condition, in which the blade pass frequency of the fan does not pass out of the nacelle. If we are successful, they will no longer have to do that. Liners are designed to absorb discrete tones and, if we are able to eliminate that at source, the engine can be designed for cut-on, rather than cut-off."
The two main noise targets are discrete tones produced by blade movement, fan wake and hot exhaust gases and broadband tones produced by movement of air over blades and by combustion. "Liners can have a new purpose. They no longer need to be optimised for one, two or maybe three fan tones. Now they need to be broadband devices," Willshire says.
Novel low-noise exhaust designs continue to be evaluated. Tailored to achieve a 20dB jet noise suppression target with less than 10% thrust penalty, they range from multiple mini-nozzles to multi-hole designs. Manufacturers are also researching designs, including a chevron nozzle which GE is developing for early application on its CF34 engine, and possibly the CFM56 produced with Snecma. This reduces the energy of the high-velocity jet exhaust up to 50% by aggressively mixing it with the ambient airflow, and has produced 3.5dB reductions in lab tests.
Source: Flight International