Flight fan-tastic

Boeing’s 7E7 is an aero-engine first: the whole focus during Rolls-Royce’s development of its Trent 1000 engine has been on reducing the plane’s life cycle costs. George Coupe reports.


In many ways the Trent 1000 is today’s most significant civil aerospace project – the first of a new breed of engine that arguably represents the future of air travel. Much has been spent on researching the aero-engines of the future — from hypersonics to hydrogen-powered electric motors, but little has actually changed in the way aircraft fly.


And such great leaps forward are unlikely to happen in today’s straitened climate: prices are still rising and legislation on emissions and noise is set to get tougher and tougher.


Aircraft manufacturers have come under pressure to reconcile the demands of viability and sustainability. Airbus has responded with the pack-’em-in-and-sell-it-cheap approach. The giant airframe of the Airbus A380 will no doubt be breathtaking in sheer bulk, if not in technological refinement. The alternative is the much smaller, and more flexible aircraft, the Boeing 7E7.


Instead of trying to squeeze in more passengers to make the economics work, this project has aimed its engineering brains at the root of the problem. A huge amount of effort has gone into improving efficiency, reducing emissions and, most significantly, cutting the life cycle costs of the aircraft as a whole. But the focus has been on the engine, the Trent 1000, designed by Rolls-Royce at Derby.


Although the design borrows heavily from the unique three-shaft architecture of the Trent family and in particular from the Trent 900, which will power the A380, the engine was conceived in accordance with an entirely new philosophy: that of reducing the life cycle cost of the aircraft. This is a new departure for the company and when the engine takes to the air in 2007, it will be a first for the aero-engine world in general.


Gary Cutts, assistant chief engineer on the Trent 1000 programme, explained: ‘There has been a gradual trend towards becoming more focused on life cycle costs, but in my mind there has been an absolute step-change on that with the 7E7.’


Boeing is also seeking a ‘step-change’ in the economics of its aircraft. The 7E7 is a replacement aircraft that will take others out of service, and it therefore has to show a clear economic advantage. ‘We feel a very strong market pull for this engine, supporting the aircraft so it has the minimum life cycle cost, much more so than we normally did. So the decision process on virtually everything was: what’s the total lowest life cycle cost?’


So far the Japanese airline ANA has ordered 50 7E7s, to be powered by the Trent 1000. Cutts claimed that the engine alone would reduce the amount of fuel burnt by around six per cent on every mission.


While some of the Trent 1000’s features represent a significant leap forward into the future, others are incremental changes or improvements on the three-shaft design. There is a balance that must be achieved, said Cutts, to make the engine reliable, affordable and capable of achieving the low life-cycle costs.


Over the years Rolls-Royce has been designing Trent engines with larger and larger fan diameters to reduce engine noise and, more importantly, improving propulsive efficiency by increasing the bypass ratio: the air flow bypassed through the fan duct divided by the airflow through the combustor.


Increasing fan size can give more thrust, but this also increases weight. The Trent 1000, however, has a larger diameter than the 900, but the weight has been kept down.


‘We’ve done something quite clever with the hub-to-tip ratio,’ explained Cutts. ‘We’ve pulled the inner annulus of the fan in a little bit for a fixed outer tip of the fan, so there’s actually more effective useful aerodynamic blade for a given outer diameter. That’s effectively given us a 112in fan with the airflow that a 113.5in fan used to give. So we are finding a way of passing more airflow down a given maximum outer diameter.


‘If you put the fan size up then several things happen, but principally you get a higher drag and a significant increase in weight. So we are striving to find ways of creating more airflow and therefore bypass ratio and propulsive efficiency from a physically smaller unit.’


But the most significant changes to the fan are the modifications to its aerodynamics. The blade is a conventional hollow-titanium blade and it is of swept aerodynamic design, similar to the Trent 900. However, the tip speed of the fan has been slowed down; it will be eight to 10 per cent slower than a Trent 900. Cutts explained that slowing the fan down changes the way the engine cycle behaves: ‘We get an efficiency improvement with the fan running slow, but also the way in which the fan maintains its efficiency with power is different. It tends to keep its efficiency over a larger portion of the running range, so it gives a good advantage for this aircraft which has to operate over a wide range of missions.’


After the fan is the intermediate pressure (IP) compressor. Again the design owes much to the Trent 900. However, the profile of the leading edge of the blades in the IP and high-pressure (HP) compressor has been ‘sharpened’ for better aerodynamic performance.


Next is the combustion system, which is where the emphasis on life cycle cost has an altogether more significant impact on design.


Through its various technology programmes, particularly the Affordable Near Term Low Emissions project (ANTLE), Rolls-Royce has developed a very low NOx combustor. This combustor is expected to be put into future Rolls-Royce engines and could have been used on the Trent 1000, but it wasn’t. Cutts said his team thought long and hard about which combustion technology to employ in the Trent 1000, but ultimately decided not to go for the ANTLE as there are significant life cycle cost implications involved in using low-NOx combustors.


‘Predominantly we were looking at the total life cycle cost, and those systems tend to have an impact on the reliability and maintainability of the combustion system. In our judgment we are getting very low levels of NOx anyway, well within all the foreseeable future legislation. Therefore going to an even lower level but giving the operator a burden of an extra maintenance cost was not the right balance.’


In the final stages of the engine, the turbine suit, other modifications have been made to cope with the higher temperatures needed for improved thermal efficiency. A complex internal cooling passage will be included in the HP turbine blades to improve cooling efficiency and, for the first time on a Trent, it will also be necessary to cool the IP turbine blades. ‘In the past because we have the three-shaft architecture our second-stage blade has run more slowly than the HP, so the stresses are lower, and we have been able to get away without cooling for longer than on a two-shaft architecture,’ explained Cutts.


While such changes are likely to increase the price of the engine, they will also improve efficiency and running costs. Another striking example of this is in the choice of materials. Rolls-Royce will use its own nickel-base superalloy, RR1000, for the first time in a civil engine. This relatively exotic material has been specified for parts of the HP compressor and turbine, to improve their durability and life span.


Cutts said that a clear decision was taken to pay substantially more for the unit cost of the engine to obtain a better cyclic life from the turbines. ‘You’ll get several more shop visits for the engine before you throw away those components — and these are very expensive components,’ he said.


One of the most significant features of the new engine, in terms of efficiency and lower fuel burn, is the means by which it will generate electrical power to run the other systems on the aircraft. With the exception of the hydraulics, all the other systems on the aircraft are electrically powered, in particular anti-icing of the wings and the operation of the compressors that provide the cabin air.


The 7E7, however, has much higher electrical demands than other aircraft. While most require power within a range of 120 to 150kVA, the 7E7 will need more than 500kVA. All engines today, two shaft and three shaft, take their power from the HP spool. Engineers realised the 7E7, even at very low engine power, would still need full electrical power. The ability to supply power in the order of 500kVA when the engine is idling is very difficult. In view of this, Rolls-Royce has elected to take the power from the IP spool rather than HP. The IP spool is a high-power unit and will easily satisfy the demand. But this configuration offers other benefits that Rolls-Royce has only recently discovered.


At present on an HP compressor the more power you take from it, the more unstable it becomes. On the IP compressor the opposite happens: its ‘operating line’ drops away from the point at which the compressor would become unstable. ‘On a three-shaft or even a two-shaft engine the more electrical demand on the high spool, the worse it gets, and the more you have to work to make it stable again,’ said Cutts. ‘If you take the power from the IP spool it does the opposite: it gets better and better.’


On existing Trent engines handling bleeds or vents are used at lower power to stabilise the HP compressor, but this results in up to 20 per cent of air flow being lost, which in turn has an impact on the engine’s overall efficiency. The Trent 1000, however, is further optimised by the fact that air bleed from the engine is no longer required, said Cutts.


‘What we find with the electric demand is that we reduce the working line of the IP compressor to such an extent that we can run through the entire operating range without needing to use compressor bleed.


‘This is a unique benefit of the three-shaft engine which has become obvious in the past few years. There’s less of a fuel burn penalty as you come to low power conditions.’This configuration also allows the engine to reduce its idle speed because it is determined by the operability and stability of the compressor. By taking electrical power from the IP spool, both it and the HP become more stable, allowing much lower idle speeds. ‘We have probably reduced the idle thrust by about 30 per cent. So that has big advantages,’ claimed Cutts. ‘If you look at Japanese operations, 400 to 500 nautical mile missions, the combination of this power off-take is worth about six per cent of fuel burn improvement, which is just huge. That is worth the last five years of engine development. That all comes from the point at which you pull back to idle, so it’s the idle descent and the on-the-ground fuel burn. Even in longer missions the saving will be in the region of two per cent.’


The life costs will be further reduced by a new generation of engine-management tools. Traditionally engine monitoring has consisted of taking ‘snapshots’ of the engine’s performance, one per flight, to look for long-term changes to the operating trends. The Trent 900 has continuous monitoring to pick up transient changes and Rolls-Royce has now gone a step further with the Trent 1000.


This includes more vibration transducers to pick up mechanical problems before they result in a failure. There are also more performance sensors, which will enable engineers to isolate changes in individual components. ‘The philosophy is to minimise look-and-see type maintenance inspections and only carry out inspections when the system tells you that an event has taken place,’ said Cutts.


The engine’s concept design was ‘frozen’ last October, and the conceptual design for the components is due to be completed next month. The first engine will run in February 2006 before certification in the middle of 2007.


The Trent 1000 will look, at first glance, like most other engines, yet it is anything but. Low running costs, extended durability and reliability are most likely to be the next big thing in air travel.



On the web