GUY NORRIS / LOS ANGELES
The next century of propulsion promises ground-breaking advances with steps towards cleaner engines rivalled only by the search for greater speed
Ever since the Wright brothers' home-made, four-cylinder piston engine sputtered into life in 1903, aerospace has been fundamentally reliant on propulsion technology for every major advance.
The first century's passion for power brought air-breathing engines to the once unthinkable thrust level of 100,000lb (445kN) and beyond. While the main focus for the next century will be on developing greener, cheaper engines and alternative power sources such as fuel cells, the search for speed will continue to push the frontiers. Some of this effort will be aimed at military and civil supersonic aircraft, but the true cutting edge will be the search for new powerplants, such as pulse detonation engines and high Mach number air-breathing hypersonic projects linked to military strike vehicles and access to space.
Work on green and exotic projects alike is underpinned by the vital new emphasis on affordability. The fantastic rate of aerospace progress in the first 70 years slackened in the 1980s and 1990s as money began to run out. Pundits agree that engineering achievements such as Concorde, Boeing 747 and Space Shuttle would not happen under today's tight budgets.
Affordability goals
Nowhere is this "get real" phenomenon more acutely seen than in the US Department of Defense's (DoD) switch from the largely performance-driven targets of the Integrated High Performance Turbine Engine Technology programme to the affordability goals of its successor, the Versatile Advanced Affordable Turbine Engines (VAATE) programme. By about 2020, the US goal is a family of engines from a common pair of "versatile" cores, with the potential for 10 times lower cost (development through maintenance) than today's Pratt & Whitney F119.
Crucially, VAATE's versatile core concept includes some intended spin-off to future commercial engines. The DoD projects a 10:1 ratio between civil and military engine production rates into the first quarter of the 21st century. A close relationship between the two is therefore vital to maintaining industry investment.
At the heart of VAATE, and several other engine technology programmes around the world, is the aim of developing an "intelligent engine" that can "learn" as it runs. The engine will be flexible and able to adapt, actively or passively, to changing situations, either internal (engine health), or external (different missions). The learning engine will need a smart management system with the ability to auto-optimise, self-diagnose and self-prognose. The system will rely on models of engine performance, against which data on pressures, temperatures and speeds will be compared.
A key target for new engines will be the use of smart structures, materials and sensors combined with advanced control algorithms to mimic the responsiveness of a living organism. Active compressor stabilisation, for example, could control post-stall behaviour in the event of a sudden throttle change. Using smart algorithms, response settings and actuation, the operating point can be moved to the stall line and back. With this ability, designers will drive stage loading up significantly.
This should be good news all round. Because the trade between pressure ratio and stability margin will be simplified, actively controlled compressors will be easier to develop. Fewer design iterations will cut development costs, and reducing the compressor stages will trim manufacture and maintenance costs. Elimination of stall and surges will lead to the development of maintenance-free compressors. Similar "intelligent engine" concepts include active blade vibration control and active combustor control.
Engine versatility may also embrace variable-cycle systems, which could be the next major innovation to the jet engine. Variable-cycle engines tune bypass and pressure ratio to the unique mission requirements, while all propulsion systems are designed for the critical point in the mission. This could be take-off power, take-off noise, acceleration or cruise. The concept could provide more efficient operation than the current fixed cycles.
Along with versatility and intelligence, a third focus area is durability. A major part of the drive for affordability will be preventing component failure, increasing engine life and reliability, and making them easier to repair. To make this possible, engines of the future will move towards high-speed, high-temperature, oil-free turbomachinery using technology such as foil bearings and tribological coatings.
NASA is also examining the potential advantages of an oil-free rotor system as part of Glenn Research Center's aerospace propulsion and power programme. The aeropropulsion programme offers a blueprint for NASA's vision for the 21st century and, in common with many engine makers and global study groups, sees a gradual shift from today's reliance on chemical combustion energy, through a phase of hybrid systems to a largely electrochemically-based aerospace world.
The first steps toward these greener engines involve the experimental development of fuel cell-powered unmanned air vehicles and general aviation aircraft. Fuel cells are electrochemical devices that convert hydrogen directly into electricity and heat without combustion, making them more than twice as efficient as an internal combustion engine. They are also "green" because they are powered by hydrogen and exhaust only water as a by-product.
Working with NASA on the ultra-low-emissions air vehicle demonstrator (a converted Diamond Katana Xtreme motor-glider), are Advanced Technology Products, Aerlyper of Spain, Boeing and Sener, together with Intelligent Energy, a UK-based developer of the proton exchange membrane (PEM) fuel cell. Boeing aims to demonstrate a fuel cell-based auxiliary power unit (APU) on a 737 as part of possible plans to use the technology in future commercial aircraft from 2010, as well as paving the way for potential retrofits.
Unlike the hydrogen-based PEM technology used in the demonstrator, the parallel advanced APU development will be based on a solid oxide fuel cell (SOFC). Boeing hopes big gains can be found with a 45% efficiency improvement. This equates to savings of about 340,500kg (750,000lb) of fuel a year on a typical 777 cycle, but would save up to 1,360,000kg per aircraft a year for a typical 737 operation.
The technology will be sufficiently mature for incorporation from "around 2010", which means Boeing's planned 7E7 will, initially at least, not be offered with an SOFC APU. In the meantime, Boeing expects to test an experimental unit in a 737, possibly from 2005 to 2008, in which the APU will power the DC bus only. But there seems little doubt that hybrid fuel cell-based propulsion and eventually all-electric powered aircraft will appear.
By its very nature, fuel cell technology also allows designers to play with unconventional powerplant and airframe configurations. Distributed vectored propulsion, distributed exhaust and distributed engine concepts are being studied in which mini or micro engines can be clustered or spaced around the airframe. Another concept involves several smaller fans being driven by two or three main power units. In some concepts, namely highly efficient shapes such as the blended wing body, large sections of the wing become, in effect, giant fuel cells.
In the nearer term, engine makers are developing technologies to keep the hydrocarbon-fuelled jet engine environmentally acceptable. Targets for emissions established by the Advisory Council of Aeronautical Research in Europe (ACARE) suggest fuel consumption and carbon dioxide should be cut by up to 10% by 2007-2010 and by 50% (including contributions from the airframe) by around 2020, compared with emissions from a current turbofan. Notional targets set by the US Aerospace Technology Enterprise (ATE) are a 25% cut in fuel burn and CO2 by 2010 and a 50% cut by 2025. ACARE also targets noise for a 40dB cutback by 2020, and the ATE says reductions of up to 75dB are obtainable by 2025. Emissions of nitrous oxides, now around 30% below International Civil Aviation Organisation standards in many current engines, are targeted for an 80% cut by ACARE and ATE.
One option being studied is a derivative of the geared turbofan concept championed for so long by Avio, MTU, Pratt & Whitney and P&W Canada. The inter-cooled recuperated aero engine (IRA) is being developed in a pan-European research programme rivalling NASA's Ultra Efficient Engine Technology initiative. Dubbed the Efficient and Environmentally Friendly Aeroengine project, it is split into two main thrusts. The first is the short- to medium-term Affordable Near Term Low Emissions (ANTLE) validation vehicle led by Rolls-Royce, and the longer-term Component vaLidator for Environmentally-friendly Aero Engine (CLEAN) programme.
ANTLE will provide the technology basis for future 50,000-110,000lb-thrust engines, while CLEAN is aimed at new narrowbody engines. R-R, which provides a Trent 500 as the baseline validation platform, has outlined a "Vision" plan for the next 20 years that embraces several ANTLE features. Electric starter-generator technology, for example, will feature new offerings for more-electric aircraft such as the 7E7. Gone will be the conventional gearbox, replaced by internal starter motors, active magnetic bearings, intelligent sensors and shaft-mounted generators. Further off, R-R's 20-year study is examining more exotic advanced cycles, tip-driven fan and contra-rotating aft fan concepts.
Nestled within CLEAN, the IRA concept has potential to maximise thermal efficiencies and reduce nitrous oxides by modifying the thermodynamic cycle through a heat exchanger or recuperator. The recuperator will sit in the hot gas exhaust while a system of collectors, splitters and tubes will deliver compressed air to the heat exchangers and then return it to the burner inlet. MTU, leading the project, believes the result will be a 200¡C rise in compressor air temperature. Combined with a geared fan, the IRA concept, compared with current generation turbofans, is expected to generate up to 80% fewer nitrous oxides, 35dB less noise and reduce fuel consumption and CO2 by up to 18%.
While further improvements to turbofans are well under way, the big breakthroughs in future air-breathing propulsion appear to be much further off. One concept gaining ground is the pulse detonation engine (PDE), a simple propulsion system in theory, yet in practice one that is proving hard to demonstrate and develop.
PDEs operate on the principle of the Humphrey cycle, a constant volume combustion process that far exceeds the efficiency of the Brayton constant-pressure cycle-based jet engine. The PDE has few moving parts and begins a cycle by filling a tube with a fuel/air mixture. A fast-acting valve closes one end and a detonation is initiated, sending a propagation wave down the tube at supersonic speed. On leaving the tube, a series of rarefaction waves travel back into the chamber and cause the burned gases to exhaust. The valve then reopens to restart the cycle.
Hybrid applications
PDEs could be 25% cheaper to produce, and have the potential to improve fuel consumption by 25% as well as increase specific power by 45%. P&W appears to be closer than most to developing initial PDEs for missile applications but, like General Electric, is studying hybrid applications, including its use as an afterburner device or as a core replacement for a turbofan.
PDEs could also be a key component of future access to space projects, working in combination with ramjets and supersonic-combustion ramjets (scramjets) and/or the Revolutionary Turbine Accelerator (RTA). High Mach cruise configurations aimed at the sustained supersonic M2 to M4-plus range are seen as ripe for PDE, advanced variable-cycle turbofans or turboramjets.
Beyond Mach 4-plus pushes the need for turbine-based combined cycles (TBCC), where the gas turbine provides acceleration to a takeover Mach number at which a dual-mode ram/scramjet powers the vehicle beyond M6. Reusable, air-breathing access to space concepts rely on the TBCC as a first stage of a two-stage to orbit system.
P&W is developing a scramjet for flight tests under the US Air Force's Hypersonic Technology programme, and GE and NASA Glenn are working on a ground demonstration RTA/TBCC for validation tests in 2006. The RTA is a turbofan ramjet, designed to evolve from an augmented turbofan to a ramjet that produces enough thrust to accelerate to Mach 4-plus. The first mid-scale engine, RTA-1, will use many GE YF120 components and lead to a system-level validation engine called RTA-2. This will have a projected thrust-to-weight ratio of 15 and a maximum speed of M5-plus. A follow-on flight test depends on whether NASA chooses the TBCC over a rocket-based alternative around 2009.
The programme faces a phenomenal challenge to demonstrate take-off and climb in single-bypass mode to M1.5, transition to double-bypass mode for the M2-plus transition to ramjet mode, and transonic acceleration to M4-plus. While in ramjet mode, it will also have to show that the turbomachinery can successfully windmill at high Mach speeds.
Given the emergence of advanced, "intelligent" engines, fuel-cell technology, PDEs, scramjets and RTAs, to name just a few, it seems the next century of propulsion will be even more exciting than the first.
Source: Flight International
