Will spiralling fuel prices reverse the decline in engine technology development and herald a new wave of environmentally friendly sustaining research?
When the top US propulsion scientists and engineers met in the intense heat of an Arizona summer last month, they delivered an icy warning that research in engine technology is in serious decline. More amazingly, industry and government speakers blamed the downturn on the growing perception that jet engine technology has virtually peaked and is a “sunset technology”.
Yet hope for gas turbine research may be coming from an unexpected source. Hydrocarbon fuel, the finite availability of which has spurred the search for alternative hydrogen and electrical propulsion technology, is the key to a possible short and mid-term renaissance. The astronomic rise in oil prices, so long the nemesis of the gas turbine, is forcing a hurried re-examination of research and development priorities.
“The phone has been ringing off the hook with people asking us what we’re planning to do on that – failing to realise that everything we’re doing today is the result of investments made five years ago,” says William Koop, chief of the turbine engine division of the US Air Force Research Laboratory’s Propulsion Directorate.
The US military operates around 24,350 aircraft powered by 47,630 turbine engines, which burn a lot of fuel. With the price of oil surging past $60 a barrel, even the deeply buffered US military stocks have started to feel the pinch. “Every year we burn around 5 billion gallons [19 billion litres] – and 15 months ago that cost around $4.7 billion. But now it costs us $8-9 billion – so that’s almost double.”
Wake-up call
The fuel rise “woke up some people in the department who are calling for more efficiency and asking why we didn’t have it today”, says Koop. The fuel price rise is, and always has been, a double-edged sword for jet engine researchers in prompting both sustaining technology breakthroughs as well as the hunt for non-fossil fuel propulsion options. Although the emphasis has been on increased reliability and lower fuel consumption over most of the six-decade lifespan of the gas turbine, the combined pressure of environmental emissions regulations and rising oil prices haveforced the search for non-hydrogen-based alternatives.
With most of the “low hanging fruit” long ago snatched from the fuel-consumption tree, the focus has moved to lowering emissions, maintenance costs and making gas turbines more affordable until someone can come up with something better. In the meantime, researchers have suffered because their dollars are from the same pot that pays the fuel and maintenance costs.
“We’re starting to eat our own here,” says Koop. “We’ve had a long history of science and technology [S&T] investment until recently, but there’s been a dramatic reduction from $134 million last year to below $90 million. That’s having a big impact on what we’re able to do, and how fast we can do it.”
Key efforts such as VAATE – the US Department of Defense-industry Versatile Affordable Advanced Turbine Engines programme – continue to be funded, but the outlook for many other US Air Force efforts looks bleak, with “lots of uncertainty beyond 2007”, says Koop, who was speaking at an American Institute of Aeronautics and Astronautics meeting in Tucson, Arizona.
It is even worse for NASA and its UEET (ultra-efficient engine technology) programme, now effectively moribund after the dramatic changes in the agency’s priorities towards space and away from aeronautics. Beginning in 2000, the UEET was sketched out as a five-year effort aimed at developing and “handing off” revolutionary turbine engine propulsion technologies that “will enable future-generation vehicles over a wide range of flight speeds”. In particular, UEET was aimed at carbon dioxide reductions based on fuel savings of up to 15%, as well as technologies for “70% NOx [nitrous oxides] emissions reduction at take-off and landing conditions, and also technology to enable aircraft to not impact the ozone layer during cruise operation”.
Several times it looked as if UEET would be extended rather than chopped. “Towards the middle of the programme the plan was to extend it a few years to do engine demonstrations, so we were planning to keep alive until 2007,” says UEET acting project manager Mary Jo Long-Davis. “But when it became part of Vehicle Systems it formed part of a new project plan that took us into 2008.” However, the seismic shift in NASA priorities with President George Bush’s new 2004 space initiative put an end to all this and “UEET will no longer formally exist in 2006”, she adds.
Instead, some elements of UEET work will continue within the reformulated Vehicle Systems Programme that makes up one of the four main “Barrier Breaking Technology Systems”, which superseded many of the aeronautics research efforts within NASA. These include variable-cycle engine technology, such as a supersonic inlet, as part of planned sonic boom mitigation demonstrations and lightweight gearbox studies for a subsonic noise reduction demonstration project.
Research areas
But other areas of UEET research, such as intelligent propulsion “adaptive control” technology, will also live on as part of foundational technology within the Aeronautical Research Directorate. “This is like the seedcorn for the future,” says Long-Davis, who adds that NASA, General Electric and several state universities are under contract to continue to develop intelligent propulsion technologies for low CO2 and NOx emissions. “They have been kept under UEET funding until 2006,” and are effectively the last vestige of the former programme, she adds.
The UEET technology portfolio was extensive (see box P32), and produced significant results during its short timespan. “We achieved a 10% reduction in CO2 emissions on a large subsonic design and with Pratt & Whitney in a full annular rig test [of the Talon X combustor UEET01] saw a 67% NOx reduction,” says Long-Davis, who adds: “And they know what they have to do to get the rest.” Further work is planned using a sector rig to gain the extra 3%, while at the same time achieving adequate smoke and cruise efficiency margins. The tests will also look for additional lean blow-out margin, and improved altitude relight performance.
“This all contributes to the argument that this is a not a ‘sunset’ industry. We have a lot more to do in terms of smart control capability, and making vast improvements to the engine.
“We have wrung a lot out of them in terms of efficiency, now it is time to bring advances in terms of computerised control and to be ‘smart’ about the way we can lower emissions,” she says, underlining the elemental fact that attacking emissions is also a de facto attack on fuel consumption.
Viable future
To keep the future of gas turbines viable and sustainable in the face of all these threats Long-Davis says “we need to do something different, and we think through bringing more adaptive controls is one of those ways. It’s a different way of handling engines if you will. It uses more sensors and lighter weight technology – such as shaped memory alloys. What we need to do is to take advantage of controlling the engine cycle by making the engine smarter and getting rid of heavy actuators. We can even make them self-healing. If a turbine blade gets partially rubbed away we’d like them to be able to automatically replace that or compensate.”
Achievable emissions targets, relative to 1996 ICAO limits are 20-25% CO2 and 85% NOx says Long-Davis. “If we stretch temperatures, increase the overall pressure ratio of engines and work on control we can achieve those sorts of numbers.”
Military threats
Bennett Croswell, vice-president for military development programmes at P&W, adds that the industry even faces a threat from its own success. “The propulsion S&T research budget is in decline, so why is propulsion taking a disproportionate share of cuts? The health of engines today is sort of hurting us – incredibly too, there’s a belief that gas turbine propulsion is a sunset industry, and that the major innovations have already been found.”
Croswell says the downturn poses a particularly serious threat to the USA. “Aerospace workers represent 4% of all US manufacturing jobs, and its worth $31 billion in trade. I think we should be concerned we’re losing that leadership position, and we should be investing more, not reducing the budget. NASA Aeronautics and VAATE funding have been reduced, that worries me the most. It’s through funding of efforts like that we get the best minds. We’re losing our ability to do that, and to attract top people.”
GE’s advanced technology and preliminary design special projects manager Clay Haubert adds: “We are in dire need of restocking the technology bank, and with VAATE we need a new centreline engine with new pressure ratios and cycles. Right now we don’t have the funding for it.”
VAATE follows on from the successful IHPTET (Integrated High Performance Turbine Engine Technology) programme, a US government-industry S&T effort that since 1987 has chased the aggressive target of doubling propulsion capability. Now drawing to a close, IHPTET is seen as a more traditional “flange to flange” or fan to exhaust research effort, while VAATE is aimed at a more holistic propulsion systems approach. IHPTET is credited with developing the technologies that enable super cruise for the Lockheed Martin/Boeing F/A-22, the short take-off and vertical landing (STOVL) ability for the Lockheed F-35 and the enhanced range and increased useful load of helicopters.
VAATE builds on the IHPTET lead by expanding the focus beyond the basic turbine engine to include the integrated inlet, exhaust, power generation and thermal management systems. In terms of specific fuel consumption, VAATE “advances IHPTET’s sfc with a 15% improvement, thereby benefiting a broad range of air platform capabilities. Additionally, when integrated with the installed propulsion system components, VAATE technologies will further improve sfc by 25%,” says the US AFRL which is aiming for a 200% thrust-to-weight improvement in turbofan/turbojets compared to a 2000 state-of-the-art technology engine.
VAATE is made up of three main “focus” areas: the versatile core, the intelligent engine and durability. The versatile core focus looks at cost, performance, materials and structures and is the target of teams put together to study combustors, compressors, turbines, fuels and mechanical systems. Key technologies in this area include areas such as low-density high-temperature rotors made up of low ductility materials like gamma titanium aluminides and high-pressure (HP) turbine cooling concepts which need 10% less cooling flow, hence saving fuel.
Strutted dome
Others include an integrated lightweight combustor with novel “strutted dome” aerodynamics which are 35% shorter than conventional combustors, enhanced carbon-carbon composite bearing cages and lubricants, and fuel stabilisation work which, to date, has shown deoxygenated fuel can be used successfully in an engine at 315°C (600°F). They also include an integrated lightweight combustor mixing vane, a metal foam/air heat exchanger, which reduces cooling flow temperatures up to 200°C, and demonstrator tests with a new exchanger design that have shown with JP-8+100 fuel could increase high Mach system heat sink capability to 315°C.
The intelligent engine focus brings together components, active control, subsystem integration, engine health management and inlet-exhaust integration. Specific research areas within the “intelligent” grouping include integrated thermal management systems that combined advances in fuel heat load capability with high-performance components to increase life, cut maintenance and increase safety.
Mechanical- and fluidic-controlled exhaust systems for vectored thrust and low-observable performance and associated high-response air valves are also a key focus, as are advanced hot-section health sensors, pulsed vortex generator jets and passive dimple designs for flow control in the inlet, compressor and turbine.
Durability focus
The VAATE durability focus is aimed at doubling component life with increased “hot time” capability, and brings together studies into areas such as advanced HP turbine blades and a host of life-prediction, manufacturing and repair technologies. An advanced probabilistic life prediction system, for example, is being developed under VAATE, which is based on physics-derived models that consider material properties, flight-mission loading and effects, “design scatter” (or variations introduced as a result of minor irregularities) and other life-reducing factors.
However, under threat of funding cuts, VAATE is slowly morphing to survive. Calls are being made to find ways to demonstrate the technology more affordably, possibly by developing a series of common demonstrators involving more than just one engine company. Other potential moves include increasing the relevance of VAATE to improving safety, and reducing engine operations and support costs.
Koop says VAATE, like IHPTET is vital to the future of aerospace in general and not just propulsion. “Had we not continued to advanced the state of the art, then I submit the JSF would not have been able to go to the vertical mode, and if the JSF did not have the vertical component derivative, then we would not have had the UK involved, and it would not have the strong international component that makes it the JSF as we know it.”
The future, say the researchers, should include air-breathing jet engines and the hope is that the growing fuel crisis will spark renewed urgency in studies of more advanced and efficient designs, rather than condemn them to the junkyard.
UEET achievments
- Rich burn, low-NOx combustor full-annular rig test
- Single-stage, 5.5 pressure ratio HP turbine for rig tests
- A 1,480°C (2,700°F) ceramic matrix composite (CMC) system with thermal barrier coating (TBC) in a vane geometry in an HP burner rig test
- Single crystal blade alloy with low- conductivity TBC in rig test
- Functional, HP turbine microwave tip-clearance sensor in lab test
- Integrated active flow control in boundary layer ingesting, offset inlet demonstration in wind tunnel
- Low leakage, aspirated seal demonstrated on GE90
- 1,200°C CMC combustor liner demonstrated in CFM56
- 70% NOx and 10% CO2 cuts in emissions
Guy Norris / Los Angeles
Source: Flight International
