FLUG REVUE June 2004: GE and RR engines for 7E7

BOEING SELECTS GE AND RR FOR 7E7

By Patrick Hoeveler

The dice have been cast: the chosen powerplants for the Boeing 7E7 are the GE GENX (Next Generation) and the Rolls-Royce Trent 1000. Pratt & Whitney has dropped out of the running, reportedly because its offer for the PW-EXX, whose core is based on the F119 engine of the F/A-22 Raptor, may have been the most expensive. According to industry insiders, Boeing, which is under a lot of pressure from its rival Airbus, decided in favour of the candidates that could deliver the fastest and with the least risk. Overnight the aircraft manufacturer has gained two competing engine types. The airlines will now be able to play the two companies off against each other for the purposes of obtaining discounts. “That is an advantage which Boeing can also actively market,” says the insider.

Moreover, apparently the Seattle-based company did not want to lose the traditional Rolls-Royce market, especially in Asia. However, according to 7E7 Senior Vice President Mike Bair, such considerations did not play any part in the decision. He would not comment on the suggestion that the engine manufacturers may have had to put up some of the funding for the airplane themselves in order to buy their way into this important programme. The aviation analysts of Forecast International estimate the value of engine orders for the Dreamliner over the next 25 years at $40 billion.

According to Bair, the new engines will be responsible for just under ten percent of the 7E7's efficiency improvements. For the first time the engine types will be interchangeable at relatively short notice, “in order to make the airplane more liquid to move in the marketplace.” However, the main focus of attention is on the more electric engines”, which will work without bleed air, thus enhancing efficiency. According to Professor Heinrich Weyer, who until recently headed the Institute of Propulsion Technology at the German aerospace research centre DLR, this makes the 7E7 a “not insignificant milestone, as it goes a good way down the path” which could lead in the distant future to electric aircraft propulsion.

Nevertheless, the engine specialists are still sticking with tried and tested technology. Thus, GE's two-shaft GENX engine, which will produce a thrust of between 244 and 311kN, will be the fifth engine to be based on the architecture of the GE90. As Tom Brisken, General Manager of the GENX, explained in an interview with FLUG REVUE, the engineers have scaled down the ten-stage high pressure compressor from the compressor of the GE90-94B.

“It is not a derivative of the GE90, it is a brand new engine,” he says. Therefore it is likely to be given a conventional designation, but this will not be GE90. The engine possesses four booster stages, a new seven-stage low pressure turbine and a two-stage high pressure turbine. The twin GENX

annular, premixing swirlers (TAPS) technology used in the combustor comes from CFM's TECH56 research programme. The fan is 2.82m in diameter and its blades are made from composite materials. “The bypass ratio will be the largest that we have ever done, over 9.5”, explains Brisken.

“Forty percent of the sfc reduction comes from the scaling of the engine. The rest comes from the components.” Here, for the first time GE is using counter-rotating shafts, as are already employed on the Rolls-Royce Trent 900. The low pressure system rotates in an anti-clockwise direction, when viewed from the front, so as to avoid any loss of efficiency as the air flows from the high pressure turbine to the low pressure turbine.

According to Brisken, the new design without engine bleed air is less complex and has advantages when it comes to reliability. “But it still has to be proven across the whole engine.” Maintainability and access to the heavy generators are also important considerations. A version with bleed air will depend on the next application. GE is open for any suggestions: “We want to make the most of the engine” says Brisken. This includes the proposed 747 Advanced, although not much activity seems to be going on in this area at present.

He does not envisage that there will be any problems in optimising the engine for the full 7E7 family: “The percentage difference between maximum thrust and lowest rating is similar to that on the CFM56-7. The GENX will be a cycle machine with high time-on-wing.” New materials should make the design more robust, although, Brisken was not prepared to go into any more detail at this time. Overall, the entire engine benefits from the latest generation of three-dimensionally designed aerodynamics and hence fewer blades.

Meanwhile, to minimise risk, Rolls-Royce is once again relying on the well established modular system of the Trent family and, in particular, on the Trent 900. The biggest challenge, according to Robert Nuttall, Vice-President Marketing, will be to optimise the three-shaft Trent 1000 for the entire planned thrust bracket of 236 to 311kN so as to reduce the life cycle costs. It

was these life cycle costs, rather than weight or simply fuel consumption, that were foremost in the requirements specified by Boeing. He was not prepared to divulge the exact number of years over which these costs have been calculated.

One of the design features which will help to achieve the required values is to reduce the fan speed by 5 percent by aerodynamic means. The fan itself is very similar to the Trent 900, but has a smaller diameter of 2.84 metres. Despite this, it achieves a bypass ratio of 11, the highest on any Rolls-Royce engine. To make this possible, for the first time the engineers are using a low hub-tip ratio fan originating form Vision 10.

At the present state of planning, the Trent 1000 will have a counter-rotating six-stage high pressure compressor which is about ten percent in physical size smaller than the Trent 900 as well as a single stage intermediate pressure compressor. The high and intermediate pressure turbines consists of a single stage each, the low pressure turbine has six stages.

The power offtake for the generator comes from the eight-stage medium pressure compressor instead of from the high pressure compressor. As a result, according to Nuttall, the engine is more stable and the fuel consumption with the engine at idle or at low thrust, for example, on the approach, is lower. On short-range missions with the 7E7-3, the total savings could come to six percent.

In Nuttall's view, the advantages offered by more electric engines mean that they are the way of the future. Hence, a Trent 1000 with engine bleed air would not make any sense for the A330. In his view, every aircraft requires a specially tailored engine.

Both companies are planning to use seven engines during the development programme. The first test runs are scheduled for 2006, with certification in 2007 and entry into service in 2008. Both firms are currently in discussion with possible partners, which are likely to come from the range of companies with which they have previously collaborated.

Meanwhile, following its defeat in the civil area, Pratt & Whitney has no large, independent product for the future. However, the engineers could develop the PW-EXX without time constraints for a possible Airbus platform. There could be opportunities for a completely new engine with high growth potential. At any rate, P&W CEO Louis Chenevert hinted at such a possibility in a letter to his staff.

From page 86 of FLUG REVUE 6/2004

Last updated 7 May 2004

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