As fuel prices force the world’s airlines to put planes into cold storage, John Dwyer examines the new urgency behind the development of clean flying machines
A year ago easyJet unveiled a ‘concept plane’ – its idea for the next generation of ‘clean’ aircraft. It would emit half the CO2 and a quarter of the nitrogen oxides (NOx) of today’s Boeing 737 and Airbus A320 families and be 25 per cent quieter. All this, says easyJet, by 2015.
It’s do-able, says the Society of British Aerospace Companies’ (SBAC’s) aviation and environment policy adviser, Carrie Lambert. It has to be. Other industry insiders have said that if, as predicted, air travel quadruples over the next 30 years, technology and operational changes will struggle to compensate for the extra noise and emissions the new traffic generates.
But the green lobby is the least of aviation’s worries. Rising fuel costs – airlines’ second biggest burden after salaries – now threaten the industry’s very survival. The commercial incentive to cut consumption is pushing manufacturers and operators towards the ‘green’ corner at a dizzying lick.
Their first imperative is to meet the targets set by Europe’s Advisory Council for Aeronautics Research (ACARE): a 50 per cent cut in noise, the same in CO2 and an 80 per cent cut in nitrogen dioxide (NO2) by 2020.
Where will these improvements come from? EasyJet says a rear-mounted ‘open-rotor’ (OR) engine will contribute a quarter of its concept plane’s CO2 saving. The jet’s lightweight airframe will chip in another 15 per cent, and the rest will come from changes to air traffic management technology and design.
Engine maker Rolls-Royce’s estimate is that new airframe design can cut emissions by up to a quarter, engine changes will deliver up to another fifth, and the remainder of the 50 per cent Acare target can be met by changing the way the aircraft are operated.
There’s no guarantee. In a series of SBAC policy papers Lambert lists a string of emissions-busting projects involving the industry’s biggest – and some of its smallest – players. In most cases, says Lambert, “these targets are very aspirational. It’s new stuff, and it can’t be guaranteed that the participants are going to achieve expectations.”
The technologies deployed all have different levels of ‘maturity’. In the all-important materials sphere, for example, while composites are well understood and already widely used, ‘smart’ materials – which Rolls-Royce, for example, hopes might be used to alter exhaust-nozzle shapes in different conditions – are more cutting edge. And though plenty of work is being done on alternative ‘drop-in’ replacements for kerosene fuel, there is little prospect of a return within 30 years.
Later this year the Sustainable Aviation group, set up by the UK’s airlines, manufacturers and airports in June 2005, will publish a climate change road map of the ‘more promising’ sustainable technologies that the market should consider for urgent adoption. They include aerodynamics (laminar flow technology and the use of different fuselage and wing shapes) and new materials (not just composites, now regarded as mature, but super-alloys and smart metals like those Rolls-Royce exhaust nozzles).
In another part of the forest, the UK’s Technology Strategy Board (TSB) – fast shaping up as a major spur to UK innovation – is supporting 17 companies, led by Airbus, in a £103 million Next Generation Composite Wing programme to ‘revolutionise’ UK aircraft wing development.
Some mature technologies have already delivered most of their benefits. For years now wings and other aerodynamic components have been designed using Catia and other computer-aided design programs. To win significant further benefits will mean going to radically new plane designs like the ‘flying wing’. These are being explored in the New Aircraft Concepts Research (NACRE) programme.
Further off are extreme wings that change shape at different stages of a f light, and selfhealing wings. In May aerospace engineers at Bristol University described a technique which would repair minor wing damage by bleeding epoxy resin into the ‘cuts’ in fibre-reinforced polymer (FRP). The Engineering and Physical Sciences Research Council (EPSRC) funded Bristol’s £171,000 three-year joint project with Hexcel Composites.
But the industry has particularly high hopes for new engine developments. The Environmentally Friendly Engine (EFE) consortium, led by Rolls-Royce (Bristol), is investigating lean burn combustors, high-temperature turbine blades and vanes, reduced cooling air f lows, higher efficiency high-pressure turbines and reducing engine drag, all for conventional turbofan engines.
In parallel, the Snecma-led Vital programme seeks to achieve high engine bypass ratios with less drag and lower weight by using lightweight structures and components. Vital’s 18 per cent target would satisfy the engine part of the ACARE target 11 years before 2020, says Lambert.
For Lambert some of the most promising projects are in the Clean Sky programme. The 1.6 billion euro seven-year initiative, launched in Brussels in February, is made up of six demonstrators – five for new engine developments – to show reductions in noise, fuel consumption, emissions, maintenance, system complexity and cost.
Most of these programmes rely on new materials, and innovative structural design and manufacturing techniques. One of Vital’s challenges, says Lambert, is to transmit higher torque on reduced-weight shafts – up to a 50 per cent increase in torque density.
To meet a 20 per cent weight reduction goal, the cold end of the turbine – the front fans – will make greater use of titanium castings and forgings and non-structural polymer matrix composites, and the ‘hot’ end, further back, may use cast or fabricated super-al loy (nickel-based) structures.
All this implies the need for competitive automated processes for titanium fabrication and the modelling of composites. One radical change in the Vital programme, for example, is to make the booster rotor bleed fan from composites. The programme will also evaluate two designs for the low-pressure shaft: one using metal matrix composites, the other multi-metallic (welded) materials.
Turbine blade design is another focus, says Lambert. EFE is exploring the development of blades that don’t have a ‘shroud’ at the tip – a weighty chunk of metal that links with adjacent shrouds to form a ring round the turbine and seal the turbine ends from air leaks. Eliminating the shrouds would reduce weight, allowing the turbine to work faster and more efficiently. Smart materials might offer better clearance control of the gap between tip and casing to stem air leakage round the tip.
Another innovation is the use of ceramics for the low-pressure nozzle guide vanes. “Ceramics have not been used in engine parts before, just for thermal barrier coatings.” These ‘miracle’ coatings have been a main reason why combustor turbine blades have been able to operate at up to 200oC above their melting point. Their use as vane material “will be a first,” says Lambert.
Clean Sky, indicates Lambert, is a high-maturity programme that undertakes final steps of development required for market. And Clean Sky’s OR (open rotor) ground demonstrator shows how close the easyJet design, for one, is to reality. ORs are not new – the turbo-prop is an OR design. But new ORs won’t merely blow the dust off these old configurations, says the SBAC. They might use advanced turbine and compressor systems using new materials and coatings, improved aerodynamics and less cooling air. They could use better control electronics, sensors, actuators and software to optimise engine performance and emissions, and leanburn combustors to reduce NOx formation. New propeller designs could incorporate advanced aerodynamics and pitch-control systems.
Clean Sky’s OR ground demonstrator, to run by 2012, is the last step before the design is mounted on a plane. Products will be on the market between 2015 and 2020. “The easyJet timescales are achievable,” Lambert says.
Some have doubts. Engine maker Pratt & Whitney is sceptical that the OR’s fuel burn benefit will offset its extra weight and noise. OR planes also travel slower than conventional jets. P&W prefers the geared turbo fan (GTF) design, which it is building for Bombardier’s sub-150 seat C series, to enter service in 2013. Vital also supports GTF development. Rolls- Royce, though involved in EFE and the DREAM 40 million euro OR programme, is also investigating a new three-shaft turbofan engine.
Operators won’t worry about extra OR travel time, says Lambert. But higher maintenance costs for both OR and GTF designs are of more concern. Turbofan engines are easily mounted and maintained under aircraft wings. ORs are not.
The challenge posed by GTFs in operation is that they are new technology, says Lambert. Putting a gearbox into the engine increases its weight and makes it difficult to maintain.
She isn’t concerned: “There are potential issues with new hardware [and] maintenance burdens for airlines could potentially put operators off. So the manufacturers will look for solutions to the problems before they arise.”
In any case, says Lambert, many of the engine makers are moving to, or already operating, ‘power by the hour’ maintenance agreements that mean the problems will fall back on the manufacturers themselves.
All this effort is riven with contradictions and trade-offs. The SBAC has plotted power settings (percentage of take-off thrust) against emissions of CO (carbon monoxide) and UHC (unburnt hydrocarbons), smoke and NOx emissions. All the graphs show completely different characteristics, showing starkly the impossibility of reducing any of them without raising at least one of the others.
Too little is known either about how emissions are formed at altitude or what their effects are: “CO2 is understood, NOx is a bit understood, but the rest aren’t,” says Lambert. Developers have had to make a judgement that CO2 and NOx are emitted in greater quantities and have greater impacts than the others. So lean burn engines are being designed with just their reduction in mind.
A lean burn combustor can reduce NOx by limiting both peak combustor temperature and its duration. The downsides are poor f lame stability, poor ‘altitude relight’, and the likelihood of incomplete combustion – which increases CO and UHC emissions.
Increasing lift – by increasing wing span, for example – reduces fuel burn. But wider wings may increase weight – and the airports don’t have the space for them.
Once the trade-offs are overcome, how quickly the market reacts to these developments depends on the rate of the fleet’s renewal – largely a matter of economic conditions. The big prize the industry now eyes is replacement of the world’s ageing fleet of single aisle aircraft (SAAs) – typically Boeing 737s and Airbus A320s. The SBAC estimates that more than 15,000 new craft will be needed from now to 2025, a market that European plane maker Airbus puts at $1,000 billion, or $67 million per aircraft.
The SAA replacement programme will be a challenge, says Lambert: “The capability requirements for manufacturing are going to be immense. We are demanding new, very complex, innovative technology with these programmes.” That means new materials, new processes, new equipment and, crucially, new skills. Many new to the industry won’t have experience of older technologies, and older workers won’t have experience of composites and other leading-edge production processes.
“It gets more and more difficult,” says Lambert. “The more complex the technology, the more capability is required.” More of the industry supply chain will have to adopt joining technologies like electron-beam and laser-beam welding and make greater use of automation.
The investment will be there, says Lambert: “Fuel prices are not going to get cheaper, and that will generate the investment.” But some of the aviation-supply SMEs will need prime customers’ help in developing their expertise and technology.
Much of this groundwork has already been done in the SBAC’s supply chain 21 (SC21) programme. And that it’s working is clearly evident, says Lambert, from the fact that 20 of the 54 companies in the Clean Sky programme are SMEs.
The replacement of the world’s SAA fleet raises the issue of how much extra CO2 would be produced in manufacturing the replacements. There has been a fear that some processes, like superplastic forming (SPF) and composites manufacture, are so energy intensive that they could wipe out much of the environmental advantage in using them.
The concern is exaggerated, says Rolls- Royce, which calculates that 99.9 per cent of GHG emissions occur during the in-service, postmanufacture phase.