From Blue Skies to Green Skies

8/10/2019 From Blue Skies to Green Skies

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From blue skies to green skies: engine

technology to reduce the climate-

change impacts of aviationRic Parker a

aDirector of Research and Technology , Rolls-Royce , Derby,

United Kingdom

Published online: 08 Mar 2011.

To cite this article:Ric Parker (2009) From blue skies to green skies: engine technology to reduce

the climate-change impacts of aviation, Technology Analysis & Strategic Management, 21:1, 61-78,

DOI: 10.1080/09537320802557301

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Technology Analysis & Strategic Management

Vol. 21, No. 1, January 2009, 6178

From blue skies to green skies: enginetechnology to reduce the climate-changeimpacts of aviation

Ric Parker

Director of Research and Technology, Rolls-Royce, Derby, United Kingdom

The environmental challenge facing the aerospace industry is a significant one. Despite con-sistent, continued efforts by industry to reduce the fuel burn, emissions and noise produced byaircraft, concern over climate change means that further advances in technology are required tomitigate the climate impacts of aviation. This paper outlines the current status of the industry,the current understanding of aviations climate impacts and technologys progress to envi-ronmental targets, before investigating opportunities for further emissions reductions. Theseinclude technologies to improve the engines propulsive and thermal efficiencies, technolo-gies enabling reduced thrust requirements and alternative fuels. This paper discusses howRolls-Royces Technology Strategy, Rolls-Royce Technology Visions, enables these innova-

tive technologies to be brought to market, the timescales and challenges associated with theirdevelopment, and the partnership required between industry and government.

Keywords: aviation; climate change; emissions; engines; alternative fuels

1. Introduction

In 1903, getting a man airborne using a flying machine was regarded as nothing short of a

miraculous feat. Priorities and expectations have, however, changed dramatically since the Wright

brothers first flight. These days, flying an aircraft laden with passengers, safely, half-way around

the world is considered a standard requirement for aircraft manufacturers and equipment suppliers,who are now focused on new challenges: making sure the environmental footprint of this aircraft

is as small as possible.

The environmental challenge facing the aerospace industry is a significant one. The industry

has successfully made consistent, continued efforts to reduce the fuel burn, emissions and noise

produced by aircraft. However, growing concern over climate change means that both the general

public and political focus on air transport and its environmental impacts, has greatly intensi-

fied. Although noise and local air quality impacts remain extremely important considerations for

manufacturers, reducing the climate change impact of aviation is the primary focus of this paper.

Email: [email protected]

ISSN 0953-7325 print/ISSN 1465-3990 online 2009 Rolls-Royce PlcDOI: 10.1080/09537320802557301http://www.informaworld.com


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62 R. Parker

Emissions from aviation currently account for only a small proportion of global greenhouse

gas emissions, around 2% of global man-made CO2 emissions, however, if the current growth

rate of air travel continues, this contribution is forecast to grow to around 3% by 2050. 1 Aviation

also produces other emissions with climate and local air quality impacts.

Technology has a vital role to play in mitigating the environmental impacts of air transport.However, it in itself is not the whole solution. To ensure that new technology produces a net

reduction in aviations climate impact, improved understanding of the effectsof different emissions

is required. To enable this new technology to be developed and implemented in an appropriate

timeframe, policy that encourages the development and use of more efficient technology and

operating practices is needed. Finally, to ensure the most efficient use of this technology, improved

air traffic management (ATM) and operating procedures are fundamental. Hence, new, innovative

technology, robust scientific understanding, effective policy and improved ATM and operations

will together help to minimise aviations future climate impact.

2. Emissions and their impacts

In the combustion process in an aircraft engine, air and fuel are mixed, ignited and burned to

produce heat, and, as a by-product, emissions.

The emissions produced in the ideal combustion process are those directly dependent on the

chemical composition of the fuel. As shown in Figure 1, these are carbon dioxide (CO 2), water

(H2O) and sulphur dioxide (SO2). The quantity of these emissions depends directly on the amount

of fuel burnt in the engine and in the case of SO 2, on the sulphur content of the fuel.

However, real combustion is a multi step process involving hundreds of intermediate reactions

that produce a number of additional emissions to ideal combustion. The additional emissions are

oxides of nitrogen (NO and NO2which together are called NOx), unburnt hydrocarbons (UHC),carbon monoxide (CO), soot, sulphur trioxide (SO3) and sulphuric acid (H2SO4), which together

with sulphur dioxide (SO2) are called SOx. The quantity of these emissions depends on both the

amount of fuel burnt and on the conditions in the combustor (such as temperature and pressure,

combustion duration at different air/fuel ratios, etc.).

CO2 and H2O are produced in the largest quantities in all conditions. NOx and CO emissions

can also be produced in measurable quantities; however, the amount of these emissions is highly

dependent on the operating conditions.

Figure 1. Schematic showing the ideal and real combustion processes for aero engines.


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From Blue Skies to Green Skies 63

Table 1. Local and global impacts of engine emissions

Emission Local air quality Global atmosphere

CO2

H2O NOx UHCs CO C (soot/smoke) SOx

As shown in Table 1, some of these emissions impact on local air quality; others affect the

global atmosphere with an attendant climate change impact.

There is considerable uncertainty about the exact environmental effects of many of these emis-

sions. However, it is widely acknowledged that the main contributors from aviation to climate

change are CO2, NOxand H2O emissions. In certain atmospheric conditions H2O emissions and

particulates can trigger contrails and aviation-induced cirrus clouds.

Of the three main aviation emissions contributing to climate change, CO2is believed to be the

most significant when considering effects over a number of years. CO2is a long-lived greenhouse

gas (50200 years) so gets mixed throughout the atmosphere and has the same effect whether it

is emitted by an aircraft engine at altitude or by another source at ground level. Its behaviour in

the atmosphere is simple and relatively well understood.

The chemical reactions following the release of NOxare complex and scientific understanding

is still developing. However, it is agreed that NOx emissions have two effects; they increase

the amount of ozone (O3) and reduce the amount of methane (CH4) in the troposphere. Thewarming/cooling effects of these two gases oppose each other and the net result is broadly

neutral with a slight net warming effect.

NOx formation is a function of temperature, pressure and time at high temperatures. As a

result, NOx emissions vary significantly across the flight cycle, with the highest levels (g/kg

of fuel) generated at take-off when the air entering the combustor has the highest pressure and

temperature, and the combustion temperatures are highest. However, as more time is spent at

cruise than at take-off, the total NOx emissions produced at both cruise and take-off conditions

can be significant.

In certain conditions, the H2O emitted from an aircraft engine, together with the H2O already

present in the atmosphere, can form condensation trails (contrails) behind the aircraft. Contrailformationdepends on properties of theexhaust(temperature,humidity andthe presence of soot and

SOxparticles) and the atmospheric conditions (temperature, humidity level). Although contrails

typically last for only a few hours, if the surrounding air is sufficiently cool and moist, contrails

may persist and spread to form cirrus clouds. Current scientific understanding of the formation

mechanisms and environmental impacts of contrails and cirrus clouds is relatively poor. More

work must be undertaken before decisive conclusions regarding their contribution to climate

change can be made.

The impacts of different emissions can be compared using radiative forcing (RF), the metric

that assesses the impact that an external agent has on the radiative energy budget of the earths

climate system. The Intergovernmental Panel on Climate Changes (IPCC) best estimate for theoverall climate impact of aviation is about 3.5% of the total RF due to anthropogenic activities in


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64 R. Parker

Figure 2. Estimates of globally averaged surface temperature changes for different aviation effects fora 100-year scenario (Marais et al. 2006). Figure in Rolls-Royce Environmental Advisory Board AnnualReport for 2006.

1992, growing to 5% (mid range scenario) by 2050.2 There is, however, significant uncertainty

regarding these figures. As shown in Figure 2, estimates of surface temperature changes due to

different aviation emissions can also be made.

Some policy makers advocate using a multiplier (i.e. using the ratio between the total aviationinduced RF and the RF due to CO2only), to compare the impacts of non-CO2aviation emissions

with CO2so that policy regulating emissions can be seen to be addressing aviations total climate

impact. However, many scientists believe that the use of a multiplier is inappropriate for aviation

on the grounds that it does not take into account the significantly different timescales of the

different emissions. As a result, applying such a multiplier to aviation, an area where measures

to reduce one emission can increase another, may mean that policy designed to reduce climate

impact actually has the perverse effect of increasing it. Given that CO 2is produced in the greatest

quantities and has the biggest long-term impacts, it is clear that reducing this emission must

remain a priority.

3. The current state of technology

As shown in Figure 3, efforts by engine and aircraft manufacturers since the late 1960s have

resulted in a 70% reduction in fuel burn and thus CO2emissions per passenger-km. NOxemissions

have also been reduced by approximately 50% relative to international regulations. The latter is

a significant accomplishment given that improvements in fuel efficiency have driven combustion

temperatures up, making NOx reduction a more challenging task.

Manufacturers have also reduced the aircraft noise by around 20 decibel (dB) since the late

1970s. This corresponds to a reduction in perceived noise of around 75%.

Traditionally, emissions reduction was driven mostly by operatorsdesireto reduce fuelburn andhence operating costs, and by the International Civil Aviation Organization (ICAO) regulations


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Figure 3. Jet engine-powered aircraft fuel burn vs certification date (ASD 2007).

governing emissions with local air quality impacts (smoke, UHCs, CO and NO x emissions).

For example, the ICAO NOx limit, which has become progressively more stringent since its

introduction in 1986, has greatly assisted in driving advances in low NOx technologies. This

clearly has benefits for both local air quality and the global atmosphere.

However, even before the growing concern over climate change, the focus on reducing emis-

sions from aviation contributing to global warming was of utmost importance to the industry.In 2000 a European industry Group of Personalities formulated its Vision 2020 targets for

aviation. In 2001, the Advisory Council for Aeronautical Research in Europe (ACARE) captured

the challenging targets for aviation in its Strategic Research Agenda: a 50% reduction in CO 2emissions (and perceived noise) and an 80% reduction in NOx for aircraft entering into service

after 2020, relative to their year-2000 counterparts. Although non-mandatory, these targets have

been embraced by major European aerospace companies and have helped to both drive and focus

research and development effort in the field.

The industry has committed to achieving the ACARE target of a 50% reduction in CO2through

the following contributions:

2025% reduction due to airframe improvements

1520% reduction due to engine improvements

510% reduction due to increased Air Traffic Management (ATM) efficiency

As part of its commitment to the 2020 vision, Rolls-Royce monitors its progress towards the

ACARE targets and periodically reports on its achievements. The following charts show the

progress towards the CO2, NOxand noise targets for new Rolls-Royce Trent engines entering the

civil market since 2000.

As seen in Figure 4, Rolls-Royce has made substantial progress towards meeting the ACARE

2020 targets. However, this progress has not been easy; it represents a multi-billion pound invest-ment by the company with some government support. Not only are the ACARE goals demanding,


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66 R. Parker

Figure 4. Rolls-Royce progress to ACARE 2020 targets (Peacock 2008).

Figure 5. Schematic showing tradeoffs between different aviation emissions and noise.

representing a doubling of the rate of improvement over the past 30 years, but the technology

requirements for reduced CO2, NOxand noise are often conflicting, hence a reduction in one area

can result in an increase in another (see Figure 5).

Balancing tradeoffs between different emissions and noise is an extremely difficult task. Despite

the intense media focus on climate change, public opinion, and emphasis remains on the more

immediate concerns of noise and air quality in and around airports. Rolls-Royce recognises theneed to balance these requirements against the need to reduce CO 2 without sacrificing the relia-

bility, durability or any safety critical performance characteristics of its products. This represents

a significant challenge going forward.

4. Bringing technology to market

Engine technology must not only fulfil difficult technical requirements and have a long working

life (2030 years), but the safety criticality of any hardware or software intended for use on an

aircraft means that it is subject to extremely stringent certification requirements. As a result, the

time frames associated with developing and implementing new engine technology are long (inthe order of 1020 years) compared to other industries.


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To ensure that new and innovative products are brought to market, Rolls-Royce invests heavily

in R&D, around 800 million per year. Around two thirds of this spend is on technology with direct

environmental benefit. To guide its investment decisions and technology choices, the company

employs a consistent, committed, research and development strategy, Rolls-Royce Technology

Visions. Three strategic visions, Vision 5, Vision 10 and Vision 20, make up the Rolls-RoyceTechnology Visions and allow the company to drive near, medium and long-term technology

developments simultaneously.

Vision 5 describes technologies Rolls-Royce currently has available off-the-shelf that can be

implemented in new products or used to improve the performance of existing products. Vision 5

helps to ensure that Rolls-Royce products are market leaders in performance, cost and reliabil-

ity. Technologies brought to market using this approach include the swept, hollow titanium fan

technology with reduced noise, improved damage resistance and improved efficiency introduced

on the Trent 900 engine for the Airbus A380, and the host of electric technologies (including

electric start-up and electrically driven Environmental Control and anti-ice systems) put into the

Trent 1000 engine for the Boeing 787 Dreamliner. Such electric technologies offer many benefits

including improved efficiency, reduced fuel burn and lower emissions.

Vision 10 encompasses technologies that are currently at the validation stage, due to go into

Rolls-Royce products within the next 10 years. Technologies developed and validated through

collaborative programmes such as Environmentally Friendly Engine (EFE), enVIronmenTALly

friendly aero engine (VITAL), DREAM and NEWAero engine core Concept (NEWAC) areVision

10 technologies. These innovations are responsible for ensuring that the next generation of aircraft

engines have a significantly smaller environmental impact than those currently in service. The key

to Vision 10 is large and very expensive demonstrator programmes that validate the technology

in the appropriate and demanding operating environment.

Vision 20 looks at emerging, yetto be proven technologies that could potentially go into products

in the twenty-year timeframe. This vision ensures that Rolls-Royces long term research prioritiesare aligned with predicted market developments and other important changes affecting future

generations of products, such as increased focus on the environment and climate change. Much

of the work that contributes to Vision 20 is undertaken through Rolls-Royces global network of

University Technology Centres (UTCs). These 29 UTCs provide Rolls-Royce with world-leading

Figure 6. Wide-cord, swept, fan blade, 3D aerodynamic design of compressor, tiled low-NOxcombustor andturbine profiled end-walls (Parker 2007).


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68 R. Parker

research capabilities in core areas such as materials, aerodynamics and turbomachinery as shown

in Figure 6.

The long timescales associated with Rolls-Royce Technology Visions means that to be suc-

cessful they must be supported by a similar, long-term strategic government view. This should

combine a consistent, committed R&D investment strategy with policy and regulation that allowsthe industry to develop responsibly and sustainably.

5. Opportunities for engine development

To reduce the amount of CO2emitted, the specific fuel consumption (SFC), which is the amount

of fuel that must be burned per unit of thrust, must be reduced. This is achieved by increasing the

efficiency of the engine.

The overall efficiency of an engine depends on its propulsive and thermal efficiencies. The

thermal efficiency is the effectiveness of converting the heat energy in the fuel to mechanical

energy in the engine. It can be improved by increasing the pressures and temperatures insidethe engine (cycle efficiency) and by reducing the losses of individual components (component

efficiencies). The propulsive efficiency is the effectiveness with which the mechanical energy

generated by the engine is used to propel the aircraft forward.

Thus far, enginedesigners have worked to reduce SFCby improving both propulsiveandthermal

efficiencies simultaneously. Although they have made significant gains since the early jet engines

of the 1950s and 1960s, there is still further opportunity to improve these efficiencies. Figure 7,

which shows the SFC of an uninstalled engine as a function of propulsive and thermal efficiencies,

indicates that current engine designs could be improved by up to 30% before theoretical limits

for propulsive and thermal efficiencies are reached. However, this limit is reduced to 20% if

acceptable levels of NOxare to be achieved.

5.1. Technologies to improve propulsive efficiency

The best way to improve the propulsive efficiency is to pass a large volume of air through the

engine relatively slowly. As shown in Figure 8, this can be achieved using a turbofan engine with

a high bypass ratio as this passes a large amount of air and has a low mean jet velocity.

Figure 7. Uninstalled SFC as a function of thermal and propulsive efficiencies (Parker 2007).


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Figure 8. Turbojet, high bypass ratio turbofan and turboprop engines and their jet velocities (Rolls-Royce2005).

Engine manufacturers have exploited this characteristic by progressively increasing the bypass

ratios of modern turbofan engines. However there is a limit to how much the bypass ratio of an

engine can be increased, as eventually the fuel burn benefit associated with the high bypass will

be offset by the weight of the fan system, the weight and drag of the large nacelle (engine casing)

and the associated fuel penalty. At this point, a better approach can be to utilise an open rotor

engine, an engine where the propeller/fan is not contained within the nacelle, thereby offering

a significant further increase in bypass ratio without the fuel burn penalty associated with the

nacelle.

Rolls-Royce is exploring open rotor engines as one potential technology for the so-called New

Short Range(NSR) opportunity. This opportunity, which includes replacing a significant numberof retiring aircraft (AirbusA320s, Boeing 737s, etc.) and producing new aircraft to satisfy growing

global demand, is forecast to be in excess of 15,000 aircraft. This represents a market opportunity

of around US$1 trillion (Airbus 2006). However, this significant demand for next generation

aircraft offers more than just economic benefit. It provides the biggest opportunity for new engine

and aircraft designs with radically improved environmental performance to be implemented on a

grand scale. This opportunity, if missed, will not come around again for over 30 years.

Rolls-Royce predicts that installing open rotor engines on the next generation of single aisle

aircraft could result in a reduction in SFC and CO2 of 2530% relative to current single aisle

turbofans. It is currently investigating possible open rotor designs (e.g. pusher or tractor config-

uration as shown in Figure 9, geared or direct drive, etc.) and working to address a number ofchallenges associated with this new engine type. These include noise, optimum speed, integration

with airframe, possible maintenance and certification issues. While a turboprop engine offers

greater fuel efficiency at the expense of significantly reduced aircraft speed, by using a second

set of contra-rotating propellers, an open rotor engine can offer low fuel burn and CO 2emissions

without a significant speed penalty. The small increase in flight time (estimated to be under 5

minutes on a 1-hour journey and 510 minutes on a 2-hour journey) will not affect the number of

sectors an operator can fly per day and will be acceptable to passengers once they are aware of

the environmental benefit offered by open rotors.

Rolls-Royce is part of a number of collaborative, technology development programmes being

undertaken to advance the open rotor engine from concept design to a full-scale demonstrator.These include VITAL (enVIronmenTALly friendly aero engine), DREAM (Validation of Radical


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70 R. Parker

Figure 9. Possible contra-rotating, open rotor configurations.

Engine Architecture Systems) and the CLEAN SKY JTI (Joint Technology Initiative), a 1.6 bil-

lion, seven-year EU programme that aims to accelerate the delivery of technologies and operating

practices that could dramatically reduce the environmental impact of air transport.VITAL is a four-year, 90 million EU research programme that began in January 2005. It aims

to integrate the results and benefits of earlier research efforts to provide an 8 dB noise reduction

per aircraft operation and an 18% CO2 reduction (relative to engines in service prior to 2000)

by the end of the programme. VITAL is developing and testing three different light-weight fan

architectures for very high bypass ratio turbofan engines, in order to achieve the very high bypass

ratio required for low CO2 emissions with minimal weight and drag penalty. This is realised

through new materials, innovative structural design and manufacturing techniques. Technologies

developed in VITAL include Rolls-Royces novel light-weight fan.

DREAM is a Rolls-Royce lead, three-year research and development programme that aims to

demonstrate new open rotor engine concepts. The40 million EU programme, which commencedin 2008, involves both component (e.g. gearbox, low pressure turbine) and rig testing in an effort

to improve understanding of noise and other performance characteristics of open rotor technology.

Both geared and direct drive contra-rotating, open rotor engines are being considered in DREAM.

Technology successfully developed in DREAM will go into in the CLEAN SKYs Sustainable

and Green Engine Integration Technology Demonstrators, with the aim of running an open rotor

engine ground demonstrator around 20112012.

Despite exploring the open rotor engine and the environmental benefits it offers, Rolls-Royce

continues to develop other engine options for the NSR aircraft such as an advanced turbofan

with a novel three-shaft architecture. Rolls-Royce predicts these variants will have an SFC that

is around 15% less than current two- and three-shaft engines employed in this market.

5.2. Technologies to improve thermal efficiency

An increase in the engines thermal efficiency can be achieved by increasing the peak pressure

in the engine (air entering the combustor) or by increasing the temperature of the gas leaving

the combustor and entering the turbine. Therefore, to reduce fuel burn and CO 2 emissions, the

pressure and temperature of the air entering and exiting the combustor should be high. This can

be achieved by using highly efficient turbomachinery (compressors and turbines) and effective

combustor design.

However, there are two main challenges associated with raising the peak temperatures and pres-sures in the engine. First, as explained previously, increased combustor pressures and temperatures


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results in an undesirable increase in NOx emissions. Second, despite advanced cooling systems,

the temperature capability of turbine components limits peak engine temperatures (the temper-

ature of the exhaust leaving the combustor is currently limited to around 1700 C). In order to

address these challenges, Rolls-Royce undertakes research both individually and collaboratively,

into solutionssuch as low NOxcombustors, advanced thermodynamic cycles and high temperaturematerials.

The Environmentally Friendly Engine (EFE) programme is a five-year, 95 million Rolls-

Royce led collaborative programme that aims to address these challenges. It is supported by the

UKs Technology Strategy Board and the Regional Development Agencies (RDAs). In order to

limit NOx while reducing CO2, a lean burn, low NOx combustor is being developed and tested

as part of the EFE programme. Building on the successful results of the DTI and EU funded

Affordable Near Term Low Emissions (ANTLE) programme, the EFE combustor is designed to

limit combustor peak temperature and the time spent at this temperature, to reduce NOxformation,

while demonstrating stable operation throughout the flight envelope. Satisfying these requirements

is challenging and as a result the design of EFEs lean burn combustor is complex, featuring

sophisticated air and fuel injection systems.

EFE also aims to develop and validate advanced materials and coatings, and cooling techniques,

like the cooling air passages shown in Figure 10, that will allow the turbine components to survive

temperatures around 200 degrees above todays operating temperatures. Improved cooling air

distribution will permit an increase in thermal efficiency and as the cooling air needs to be

compressed before entering the turbine components, if less cooling air is used, fuel burn and

CO2 emissions will be reduced. Furthermore, as more air is now available for combustion, the

combustion process can be leaner resulting in reduced NOx formation.

Figure 10. Cooling passages and cooling holes in a turbine blade (Rolls-Royce 2005).


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72 R. Parker

The EFE programme is also developing many technologies to improve the efficiency of the

high pressure turbine to reduce weight and therefore fuel burn and CO2emissions. These include

improved aerodynamics, shroud-less turbine blades and improved tip clearance control. EFE is

well underway with components being manufactured in preparation for the first engine test due

in 2009.Another important collaborative programme is NEW Aero engine core Concept (NEWAC), a

71 million, four-year EU project that commenced in May 2006. NEWAC aims to develop and

validate novel core engine technologies to reduce CO2 and NOx emissions. The programme is

investigating four different engine concepts including an intercooled core and an intercooled,

recuperative core.

In the intercooled core concept, a heat exchanger is placed between the intermediate and high

pressure compressor stages to cool the air entering the high pressure compressor. This means that

less work and hence fuel burn is required to compress the air (as cooler air is easier to compress

than hotter air) and that the temperature of the air entering the combustor is lower, reducing NOx.

As this compressed air is also used for cooling the turbine components and as this air is now

cooler, less of it is required for cooling, leading to further reductions in CO 2emissions.

In the intercooled, recuperative core concept, an intercooler and a recuperator, a heat exchanger

that transfers the heat from the engine exhaust gas to the air entering the combustor, are used. Using

a recuperator allows for a reduced pressure rise over the compressor for a given thrust and there-

fore reduced compressor work, fuel burn and CO2emissions. It also enables the use of ultra-low

NOxcombustor technologies that cannot be used in engines with high compressor pressure rises.

Alongside the four novel core engine technologies, NEWAC is developing three novel lean

burn, low NOx combustor concepts. The NEWAC programme is progressing well with the first

rig tests of components scheduled to run in 2008.

In addition to contributing to collaborative programmes such as EFE and NEWAC, Rolls-

Royces undertakes individual research into areas such as advanced materials. As shown inFigure 11, over the years, Rolls-Royce has developed and refined many advanced materials with

Figure 11. Improvement in temperature capability of turbine blade materials (Parker 2005).


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improved properties such as increased specific strength (strength per unit mass) and increased

temperature capability. One example is its single crystal, nickel alloy turbine blades. The con-

tinued development of Rolls-Royces advanced alloying techniques and processing technologies

have culminated in the development of turbine blades made from single crystal alloys. These

materials are devoid of crystal boundaries, the points of weakness in a material, and thus havesignificantly improved temperature capability, allowing the turbine blades to withstand the high

forces, temperatures and pressures found in the high pressure turbine. However, despite the fact

that the latest high pressure turbine blades can withstand forces of around ten tonnes per blade at

temperatures around 1700 C, Rolls-Royce must continue to evolve these materials so they can

survive the ever more harsh conditions in the future. It is currently developing fourth and fifth

generation single crystal blades made with increasingly more exotic alloying additions.

5.3. Technologies to reduce thrust requirements

Improving SFC is one way to reduce CO2 emissions. However, a better way may be to reduce

the amount of thrust required. There are many options for reducing the thrust requirements of an

engine including reducing the weight and drag of the aircraft. Reducing the total distance flown

and hence the net fuel burn and CO2 emissions, through changes to operations such as using

optimal sector lengths and avoiding stacking, results in a smaller fuel load and therefore reduced

engine thrust.

As discussed in the previous section, materials with high specific strength can be used to

reduce the weight of the engine and other aircraft components. Composite materials have been

progressively developed and utilised by the aerospace industry. The high specific strength and

other desirable properties (fatigue strength, manufacturability, etc.) of composites have resulted

in this material choice becoming increasingly popular with airframe manufacturers. Compositeshave also been developed and used for low temperature engine applications such as the nacelle

(engine casing), bypass duct, propeller and fan blades. Other, less mature material technologies

being developed by Rolls-Royce and others in the industry include ceramic matrix composites,

smart materials (materials which have properties that can be dramatically altered by changing

the conditions they are subject to) and nano materials (materials with crystals approximately

1 109 meters in size that display very different properties to materials with ordinary-sized

crystals). Potential applications for smart materials include changing the form of the aircraft

wing to minimise drag and fuel burn, and altering the shape of the engine exhaust nozzle for

optimal performance and reduced CO2 in all operating conditions. Aerospace opportunities for

nano materials include enhanced structures, modified surfaces and coatings and new sensor andmanufacturing technologies.

Reducing the drag of the aircraft through improved aerodynamics can also assist greatly in

reducing thrust requirements and fuel burn. There are many opportunities for improving aircraft

aerodynamics, including improving the engines integration with the airframe. Rolls-Royce is

working closely with airframe manufacturers to improve the integration of engine designs with

airframe configurations, as well as develop ways to integrate novel engines such as the open rotor

engine with new airframe designs.

NACRE is a four-year 30.3 million EU programme that is developing generic design solutions

for major aircraft components that will then be further developed and assessed on some novel

aircraftconcepts. These include two Pro Green aircraftconceptsfocused on reduced environmentalimpact and a Flying Wing aircraft concept designed to maximize the efficiency of passenger


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74 R. Parker

Figure 12. The pro green concept (Airbus) (Frota 2006).

Figure 13. The flying wing concept (Airbus) (Frota 2006).

transport, thereby minimising fuel consumption. Two of these concept designs can be seen in

Figures 12 and 13 respectively.

NACRE will use the ultra-high bypass engine concepts explored in VITAL in its Pro Green

aircraft concepts. This will help to improve understanding of the integration and noise issues

associated with tail-mounted engines and to develop more optimised engine integration solutions.

Another way to reduce thrust requirements is through the development of a more electric

aircraft. Aircraft systems are typically powered by a mixture of hydraulic, electric and air power.

However, electric systems are a better choice as they are lighter, more compact and efficient than

hydraulic and bleed air systems. Furthermore, as bleed air systems rely on compressed air from

the engine, which is produced at the expense of fuel consumption, replacing bleed air systemswith electric systems helps to further reduce fuel burn and emissions.

Rolls-Royce is working with airframe and aircraft systems manufacturers to develop the engine

and aircraft electrical technologies required for a more electric aircraft. As part of the Power

Optimised Aircraft (POA) programme, which was completed in 2008, Rolls-Royce tested a high

pressure starter generator and low pressure system generator, designed to produce power for both

existing and new electric technologies in the engine. The generation of 125 kW and 195 kW from

the low pressure system generator and the high pressure starter generator respectively, as well

as the successful demonstration of many new electric technologies in the programme (electric

engine controls and actuators, electric fuel and oil systems, etc.), showed that the goals of POA:

an integrated power system delivering electrical and motive power with reduced fuel burn, areachievable.

The results of POA are now being further developed in the More Open Electrical Technologies

(MOET) programme, a three-year 70 million EU effort that was launched in July 2006. MOET

aims to improve aircraft design and utilisation through power source rationalisation and electrical

power flexibility. This will provide multiple benefits (maintenance costs, unexpected operating

delays, production improvement, etc.) including a 2% reduction in fuel burn and CO2.

6. Opportunities for alternative fuels

In the long term, alternative fuels could assist in significantly reducing aviations environmen-tal impacts. However, finding an alternative fuel that could be used for aviation is extremely


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challenging. Rolls-Royce has three primary requirements against which it assesses any candidate

fuel: suitability, sustainability (both environmentally and socially) and production capability (i.e.

must be capable of economic production at industrial levels).

(1) Suitability. Any fuel proposed for aviation must have very specific characteristics includinga high energy content (to minimise fuel burn and CO2emissions), a low freezing point, good

flow properties (lubricity, low leakage, etc.) and compatibility with materials and components

in the fuel system. Thus far, the only fuel that has been able to fulfil all of these criteria, at

a reasonable cost, is kerosene. Consequently, kerosene has been utilised almost exclusively

by the aviation industry for many years, resulting in fuel supply and storage systems, engine

and aircraft systems being specifically designed for optimal performance with this fuel type.

Given the high capital cost and long lifetime of such equipment, it is highly desirable that

any alternative fuel is a drop-in replacement.

(2) Sustainability. An alternative fuel should offer a real environmental benefit. The feedstock for

the alternative should come from a renewable source reducing reliance on finite fossil fuels

and the alternative fuel should produce less greenhouse gas emissions over its life cycle than

conventional aviation fuel. One way to reduce total CO2 emissions is to use biomass as a

feedstock. However, the biomass must be able to be produced sustainably. It must not depend

on energyintensive farming methods, make excessive use of nitrogen based fertilisers (which

can result in increased nitrous oxide emissions, one of the most harmful greenhouse gases)

or compete for water or land with food production (nor result in further deforestation).

(3) Production capability. Any alternative fuel must be capable of industrial levels of production.

To replace 50% of todays aviation kerosene will require the production of 100 million gallons

per day of alternative fuel.

There are many alternative aviation fuels currently being considered by industry and researchgroups with the most viable alternatives generally considered to be XTL, FAMEs (Fatty Acid

MethylEsters) andhydrogenated plant or vegetable oils. Figure14 andTable 2 provide an overview

of the challenges associated with these alternatives.

XTL is the generic name given to synthetic kerosene produced from coal (coal to liquid

CTL), gas (gas to liquid GTL) or biomass (biomass to liquid BTL) using the FischerTropsch

process. The properties of the synthetic kerosene produced using this process are independent

of the feedstock and can be similar to, or better than, those of conventional kerosene. The life

cycle CO2emissions associated with XTL fuels varies significantly according to which feedstock

is used. However, both GTL and BTL could potentially result in life cycle emissions that are

comparable to or less than kerosene, depending on factors such as the type of power used for theproduction process, feedstock availability and location.

Vegetable/plant oils can be derived from oilseed crops such as such as rape, sunflowers and

soya beans or from plants such as palm and from algae, and processed to produce Fatty Acid

Methyl Esters (FAMEs) or hydrogenated oils. FAMEs are produced using a process known as

transesterification. The resultant biodiesel fuel has several properties that make it undesirable for

use in aviation including a high freezing point, low thermal stability and lower energy density

than kerosene. Although some of these undesirable properties can be reduced through additional

processing and the inclusion of additives, FAMEs suitability is still limited for use in relatively

small proportions (around 2030%) in FAME/kerosene blends. Hydrogenation consists of a

series of chemical reactions in which unsaturated compounds (e.g. plant oils) are converted intosaturated compounds (e.g. hydrocarbons) through the addition of hydrogen. Hydrogenated plant


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The big challenge associated with plant/vegetable oils is finding a feedstock that has the

appropriate chemical structure and can be produced in a sustainable and cost effective way.

Two potential feedstocks that may be able to be produced sustainably are algae and jatropha. The

development of algae-based fuels is still in its infancy; however, its high yield rates (could produce

150300 times more oil than a crop of soya beans (Daggett et al. 2007) and ability to grow onnon-cultivatable land (although still possibly requiring scarce water) means that it offers promise

as a future sustainable feedstock. In the nearer term, jatropha plants, which are resistant to drought

and pests and can be grown easily on non-cultivatable land, are a good potential feedstock.

In February 2008 the Airbus, Rolls-Royce and Shell International PetroleumA380 flight test of

a 40/60 GTL/kerosene blend took place.This and other demonstrations provided the first essential

steps in understanding the in-service performance of an alternative aviation fuel. Although Rolls-

Royce plans further flight tests of promising candidate fuels, including the testing of a jatropha-

based second generation biofuel on an Air New Zealand Boeing 747-400 aircraft shortly, flight

evaluations are only one part of its wider research programme into renewable fuels. Rolls-Royce

and British Airways are undertaking an in-depth scientific test programme to investigate the

viability of practical, sustainable alternatives. The companies have invited suppliers to provide

fuel samples to be tested on a Rolls-Royce RB211 engine in an indoor test bed facility in mid 2009.

This ground-based, back-to-backtesting of four fuels will enable accurate data to be acquired using

advanced test bed instrumentation. These tests will evaluate the engine performance, handling

and emissions for each fuel type.

Despite these promising steps forward, significant further development and validation effort

is required before an alternative aviation fuel can be implemented. FAMEs, GTL and BTL fuels

are under development and are likely to be available by 2015. Hydrogenated plant/vegetable oils

derived from traditional feed stocks may also be available within this timeframe. Fuels made

from algae still require significant work if they are to become viable alternatives, their practical,

wide-scale implementation is still around 510 years away.

7. Conclusions

The continued growth of aviation brings both social and economic benefits, but these will only

continue to be enjoyed if aviation can become more sustainable. The aerospace industry has

made significant progress towards reducing CO2 and NOx emissions and noise, and is commit-

ted to developing new, innovative technology and using alternative fuels to further reduce its

environmental impacts. However, technology will ultimately be only part of the solution and full

sustainability may require other effectors, such as carbon trading.

Making technological advances is challenging: technology must fulfil many (often conflicting)requirements. As a result manufacturers like Rolls-Royce must develop technologies to address

both long and short-term priorities, local and global effects, and advise the legislators on the art

of the possible.

A long-term strategic vision for both the industry and governments, and a solid understanding of

the climate impacts of aviation is still needed. These key ingredients, advanced technology, robust

science, effective and supportive policy, and improved Air Traffic Management and operations,

can together make this difficult challenge achievable.

Acknowledgements

I wish to acknowledge the very considerable contribution of Carrie Lambert at Rolls-Royce to the creation of this paper.


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78 R. Parker

Notes

1. http://www.grida.no/climate/ipcc/aviation, cited 31/01/08

2. http://www.grida.no/climate/ipcc/aviation, cited 31/01/08

Notes on contributors

Richard (Ric) Parker was appointed Director of Research & Technology, Rolls-Royce plc in January 2001, and is based in

Derby, United Kingdom. He is responsible for direction and co-ordination of Research & Technology programmes across

all the Rolls-Royce businesses. Ric is a Fellow of the Royal Academy of Engineering, a Fellow of the Royal Aeronautical

Society and a Fellow of the Institution of Mechanical Engineers. Ric joined Rolls-Royce in 1978, and has held various

posts including Chief of Composites and Ceramics, Chief of Compressor Engineering, Managing Director Compressor

Systems and Director of Engineering & Technology, Civil Aerospace. Ric gained a BSc in Physics at Imperial College,

London in 1975 and an MBA with distinction at Loughborough University in 1992. He is a visiting Professor inAerospace

and Automotive Technology at Loughborough University.

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WA: The Boeing Company.

Frota, J. 2006. New aircraft concepts research: NACRE presentation. Presentation made at the International Council of

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