Keeping options open

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The projected price of aviation fuel will have a pivotal impact on the selection of architectures for the engines that will power the next generation of Airbus and Boeing narrowbodies. The more radical proposed solutions, such as open rotor powerplants, are likely to see the light of day only if fuel cost becomes the overriding factor in the equation, as their use would require a fundamental rethink of the design of the airframes that will carry them.

But fuel cost aside, these decisions will also have a massive impact on the aviation industry’s carbon footprint. Assuming an open rotor-powered airliner could be 30% more fuel efficient that the best 150-seater now in service, the reduction in emitted CO2 per aircraft would be equivalent to planting 250,000 trees.

“In terms of technology and the environment, you can argue [open rotor] is the biggest single technology potential in the world,” says Rolls-Royce vice-president strategic marketing Robert Nuttall.

But rather than being able at this early stage to back a specific powerplant configuration, R-R is having to maintain a dialogue with the airframers, airlines and regulators as it builds a toolkit of technologies that it hopes will enable it to offer the most attractive engine solution when the time comes. But meeting the next-generation narrowbody requirement will demand more than just technology, as the engine solution will need to be optimised for a complete air transport system, which may in turn demand that aircraft be operated in a different way to today.

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 © IAE
The prospective requirement to power future single-aisle families from Airbus and Boeing represents a massive opportunity for engine manufacturers

“When the requirement is defined, or definable even, we will then jump and say ‘this is the solution’,” says Nuttall. “I doubt that it will be quite as much of a ‘champagne-popping’ decision as that. I suspect it will be a creeping decision as things become more evident.

“It’s not our job to tell the industry what the answer is,” he adds. “Our job is to present the industry with options and solutions and help them understand these things, and then the industry can make the decision. Everybody’s getting quite seriously interested in the open rotor and would like to understand what potential it’s really got.”

UPHILL TASK

However, the engine manufacturers face an uphill task in trying to crystalise their approaches to meeting a requirement for which there are only two certainties – that a new generation of narrowbodies will be launched at some point, and that when they are, the powerplant makers will be presented with potentially the most lucrative opportunity in the history of aerospace propulsion.

Airbus and Boeing have repeatedly pushed back their projected service entry dates for their respective A320 and 737-family replacements, with the European manufacturer’s chief salesman John Leahy having recently suggested that a new design will not be available until at least 2024.

Nuttall, speaking before Leahy in late September publicly punted the likely service entry well into the middle of the decade after next, said the UK engine maker was working towards a supposed date of “2019, plus or minus one”.

To understand the loosely defined requirement, the engine manufacturers are having to stray outside their normal areas of expertise, questioning the basics of the air transport system as a whole. Should future airliners be designed for shorter ranges and make additional fuel stops? To maximise efficiency, should they fly more slowly than current aircraft?

R-R is determined to engage operators as well as airframers in its market research to ensure that the end product meets market needs. Nuttall says this approach follows lessons learned on the A380 programme, where R-R’s Trent 900 turbofan “did perfectly what the airframer wanted”, but ended up undergoing a late design change to add a larger fan to accommodate airline requests for even greater take-off and landing noise margins.

R-R is studying advanced two- and three-shaft turbofans as well as open rotor engines, as it works on “lego blocks” of technology.
“We’re developing technology pieces that we can then assemble into different architectures,” says Nuttall.

There are two factors that should combine to deliver substantial improvements in fuel consumption when the new engines are required. The first is the “iterative” development of engines, where the progressive introduction of small incremental enhancements historically tends to yield efficiency improvements at the rate of around 1% a year. The second is fundamental advances in the overall architecture of the engine, into which new technologies can be embedded.

“We have to be very flexible, with lots of options, and be prepared to actually commit to a product when it’s required,” says Nuttall. “At the point at which it becomes clear what that requirement is, both the date and the product requirement, that’s when we choose the architecture of the solution,” he adds.

“If you think of technology as a conveyer belt coming towards us all the time at a rate of 1% [efficiency improvement] a year, and the conveyer belt is 20 years long, out there at 20 years it’s not in R-R probably, but in a university. When you get to 10 years away there are bits that are in rigs or possibly development engines of some kind, and five years away they’re on a development engine.”

Option 15 would deliver an advanced turbo­fan offering a 15% improvement in fuel efficiency by around 2015 (the “15” in Option 15 refers to the efficiency gain rather than the projected service entry date). This would be largely be based on the technology in development for the Trent XWB, which is already under full-scale development for the Airbus A350 XWB family of widebody twinjets and due to enter service in 2013.

The Option 15 engine would be around 30dB quieter than Stage 3 standards. If developed, it seems increasingly likely its airframe application would be some sort of interim “refresh” of the A320 and 737 families, given recent pronouncements from the aircraft manufacturers on the timing of their all-new aircraft.

“Our position is that if you do [an interim refresh], it will delay the new aircraft by some period of time,” says Nuttall. “So if you are an airline that gets the first one off the line, it makes sense to do it. If you get one of the last ones and then a new aircraft appears, you are just as badly off as if you’d never had it.

“From a net industry point of view, let’s say CO2 [emissions], if you delay the new aircraft because you’ve brought in an interim step, you’ve actually lost as an industry, so we are not really in favour of this. However, should an airframer wish to do it we will support them to the hilt,” says Nuttall.

EFFICIENCY IMPROVEMENT

Later in the next decade, potentially by 2018, Option 20 would incorporate iterative technology advances to deliver an efficiency improvement of roughly 20% versus today’s turbofans, and a slight additional noise reduction compared with Option 15, for a relatively low technical risk. “In the same sort of timescale, we can do a quantum step and go up by 10%-plus and get to the open rotor,” say Nuttall. The resulting “Option 30” open rotor would probably be based on the same two- or three-shaft core as the Option 20 advanced turbofan.

R-R’s studies of Option 15/20 two- and three-shaft advanced turbofans in the 20,000-30,000lb thrust range (89-134kN) are known internally as the RB282 and RB285, respectively, while the Option 30 open rotor is referred to as the RB3011.

“Beyond there we can get to Option 50, but the 50% is talking about an enterprise level – aircraft, engine, air traffic management, operator, the airport,” says Nuttall. “You can postulate it could be 50% better, but it may not give a solution that people want to have.”

Moving to Option 50 would require the industry to confront difficult issues in terms of the operation of airliners, such as the desirability of cruising more slowly, and possibly lower, at altitudes more susceptible to inclement weather, as well as flying over shorter ranges to conserve the fuel expended by today’s long-range aircraft as they “tanker” the fuel needed for the latter part of a flight.

“We are quite confident that for the next 10-20 years, the engine will be the biggest single contributor to reducing the environmental footprint [of commercial aviation]. We understand that, but the other guys have got stuff to contribute as well,” says Nuttall.

The open-rotor, as a “first-of-type”, would be an inherently riskier technical proposition than an advanced turbofan. “There are no rules for this, it doesn’t exist in the certification world as an engine,” says Nuttall.

R-R has held exploratory discussions with the certification authorities in an effort to envisage how certification rules for an open rotor engine might look. A conservative approach is to assume adoption of a combination of existing “worst-case” rules for turboprops and turbofans, with the incorporation of some additional margin.

The diameter of the blades – about 4.3m (170in) – on an open rotor engine means that a conventional arrangement with podded engines on a low wing is not an option.

The airframe therefore needs to be designed with a high wing, or provide for the powerplants to be mounted on the rear fuselage.

Each option introduces specific challenges. High wing mounting means locating rotating blades adjacent to the cabin and exposing a relatively large section of wing to prop-wash.

Fuselage mounting introduces centre-of-gravity issues (although it has the advantage of allowing the wing to be left “clean”), and requires a pylon that separates the centre of the engine from the side of the fuselage by around 2.54m. “That’s a short walk,” says Nuttall.

PULLER CONFIGURATION

For aerodynamic reasons, wing-mounted open rotor engines would require a “puller configuration” with the external blades mounted at the front, while a “pusher” design would be needed for the fuselage-mounted configuration. From an engine architecture perspective, the pusher configuration is harder to design because it requires a more complex gas path.

R-R is unique among the big three engine makers in offering three-shaft turbofans, and a significant spike in fuel prices would be likely to prompt it to base the advanced turbofan or open rotor on a three-shaft architecture.

“If fuel became really expensive, you’d go down the three-shaft route without really any great doubt,” says Nuttall. “If you are in a world where fuel isn’t massively expensive and therefore maintenance cost becomes more important, then you might go for two-shaft,” he adds. “The three shafts just add complexity.”

In support of its open rotor studies, R-R has examined the data and designer’s notes from the Pratt & Whitney/Allison Engine unducted-fan flight-test programme carried out in the late 1980s. It has also stripped down the engine, which used an industrial core and had the same number of blades on the front and rear rotors, which would not be done on a modern design to avoid passing-frequency and tip-vortex interference. The blades themselves were of a rudimentary design.

Nuttall says the engine was an “unsophisticated concept demonstrator” that was “quite noisy, but the physics showed promise”.

Earlier this year R-R finished rig tests of a one-sixth-scale, titanium-bladed open rotor configuration with the aim of demonstrating that noise could be controlled sufficiently for the concept to be viable. Trials involved varying the ratio of the diameters of the forward and rear rotors, blade numbers and speed.

“It’s changed from being a physics problem to an engineering challenge,” says Nuttall. “We have mitigated the noise risk down.

“We’re fairly confident [open rotor will be] 25-30% better than current turbofans, which means it’s 10-15% better than an advanced turbofan [such as] an RB285, a [CFM International] Leap X or a [P&W] geared turbofan. They’re all claiming 15-16% by about 2015 or 2016,” he says.

“We’re in a position where we haven’t reduced anything like all the risks. There’s the gearbox risk, a pitch-change risk, the blade, and there is the noise, although we think we’ve mitigated that down from the go/no-go position it was in, and then there’s the installation.”