Sustainable and Green Engines (SAGE)

Open rotor technologies offer the potential for significant reductions in fuel burn and CO2 emissions relative to turbofan engines of equivalent thrust.

Higher propulsive efficiencies are achieved for turbofans by increasing the bypass ratio through increases in fan diameter but there is a diminishing return to this improvement as nacelle diameters and consequently weight and drag increase. Open rotor engines remove this limitation by operating the propeller blades without a surrounding nacelle, thus enabling ultrahigh bypass ratios to be achieved.

Further improvements in propulsive efficiency can be gained for open rotor engines by using a second row of propeller blades rotating in opposition to the front row to remove the spin from the column of air, thereby producing a more direct thrust. The technical challenges of counter rotating open rotor engines are many, but are principally:

• Reduction of the noise created by the propeller blades to counter the loss of attenuation provided by a turbofan nacelle.
• Definition of the propeller system to reduce the noise created by the counter rotating blades (counter rotating slotted discs in close proximity is the basis siren design and blade interaction noise was the principal issue noted when the open rotor engines were demonstrated in the 1980s).
• Complexity of communications and blade pitch control through the counter rotating power transmissions system.
• Installation of the open rotor engine on the airframe: In conventional engine-to-wing configurations with turbofans, the engines are isolated from the airframe by the nacelle. However, with open rotors, the airflow through the propellers interacts with the supporting airframe structure in a different manner, hence the installation impacts on the engine system noise and efficiency.

Rolls-Royce has developed an open rotor propeller design to minimise noise and has demonstrated the effectiveness of these designs through scaled rig testing in the FP7 DREAM programme.

The SAGE1 project plans to acquire technology for the propulsor system, increasing the Technology Readiness Level (TRL) to TRL 5.

Collaborating in the SAGE1 project under Rolls-Royce leadership are GKN Aerospace and ITP.

Examples of technology maturation studies are the successful Rig 145 “installed and uninstalled” high-speed testing of the Rolls-Royce second generation CROR Blades with Airbus in 2011, and the Z08 wind tunnel test successfully performed pusher/puller configurations with Airbus (Q2 2012).

After the strategic decision of Rolls-Royce to focus on Lean Burn Combustion in SAGE 6, the activities in SAGE 1 were re-scoped in 2012 and then focused on completing the work in 4 main areas:

• Validation of a fast Computational Fluid Dynamics (CFD) solver tool for the Open Rotor case. This work was completed in 2015 and has focused on validating extreme blade angles of attack against rig Z08 blade surface measurements.
• Component integrity and evaluation of blade design and material options, including a sample test programme under high strain rate, during shear extension experiments. This project was completed in 2015.
• Validation of methods for stability, flutter and forced response against available rig data. This programme was completed in 2015.
• Validation of far-field and near-field noise methods for open rotor designs using available rig data. This programme was completed in 2015.

The programme of work was focused on the R&T activities in place at universities to ensure continuation and completion of their research.

All current advanced engine concepts that are candidates for integration into large transport aircraft are based on the principle of High, Ultra-high Bypass Ratio (UHBR) or Geared Turbine Fan (GTF) turbofans.

After a short, intense feasibility phase conducted with two types of engines in the early 1980s, the approach to developing a Contra-Rotating Open Rotor (CROR) engine concept for the short/medium range class of aircraft is absolutely unique.

For large short- and medium-range aircraft, the first GTF engines will be the most advanced and fuel-efficient state-of-the art solution entering service shortly. However, compared to the GTF, the CROR concept promises a potential 15% further fuel burn reduction, a 30% reduction versus the well-known CFM56®. To realise this potential, Clean Sky 2 has to deliver convincing proof that the CROR engine concept can be manufactured, installed and operated in an industrial environment comparable to other latest state-of-the-art concepts.

Clean Sky will deliver ground testing of a geared pusher open-rotor engine within the SAGE 2 project, led by SAFRAN/Snecma (Figure 1), followed by a flight test in Clean Sky 2 under the Large Passenger Aircraft platform (LPA WP 1.1.) in cooperation with Airbus.

The SAGE 2 project focuses on demonstrating the following technologies:

• composite propeller blades research activities including aero-acoustic optimisation
• pitch control system
• lightweight front and rear rotating frames including certification issues of rotating casing, controlling leakage at interfaces and reducing weight
• the contra-rotating reduction gearbox
• the power turbine to reduce module weight and increase performance characteristics
• lubrication and cooling systems
• nacelle components and particularly rotating parts
• control, protection and monitoring system and equipment
• propeller blades
• electric de-icing system and equipment

The gas generator used in the SAGE 2 open rotor demonstrator is derived from a Snecma M-88 engine. The delivery of the SAGE 2 demonstrator to the ground test facility is planned by mid-2016 and the ground test will start in Q3 of 2016.

In Clean Sky 2, SAFRAN/Snecma will lead engine related activities required to perform flight tests of a 2nd version of a geared open-rotor demonstrator, carrying on the Clean Sky SAGE 2 achievements. The current assumption is to use the Airbus A340-300 MSN001 test aircraft as a flight test vehicle, with one full size CROR pusher engine attached to a representative pylon and engine mount installed at the port board side of the rear fuselage. 

It must be noted that the overall roadmap for the future propulsion system of short/medium range aircraft will be subject to a number of reviews, both by the airframer and the engine manufacturer

The trend to Very High Bypass Ratio (VHBR) engines requires technology developments across a broad range of complex gas turbine systems, from fan inlet through the complete compression, combustion and turbine to exhaust.

The SAGE 3 project, led by Rolls-Royce, has demonstrated technologies applicable to large 3-shaft turbofan engines in the 60-95,000lbs thrust class, with particular focus on an Advanced Low Pressure System (ALPS), externals and compressor structures technologies: 

• lightweight composite fan system, including composite fan blades, composite fan case and all supporting hardware
• lightweight integrated engine externals for fan case and core mounting dressings
• intake developed in conjunction with the composite fan case and optimised for the composite fan as an aerodynamic and structural system including noise reduction technologies
• lightweight new materials and construction techniques for low pressure turbines
• technologies for higher efficiency low pressure turbines, through aerodynamic developments such as blade clocking and more advanced secondary cooling flow control
• materials and construction techniques for compressor inter-cases, focussing on topology optimisation manufacturing techniques to enable thin sections and materials with higher temperature capability

The ALPS engine has undergone a series of ground tests on based on a Rolls-Royce Trent 1000 platform, looking at the functional and structural capability of advanced dressing for

• flutter and aerodynamic performance
• composite fan system flutter behaviour under cross-wind conditions
• noise performance
• low pressure turbine performance
•  thermal behaviour
• structural behaviour

Flight tests of the engine were successfully performed in October 2014 to demonstrate composite fan blade in-flight operability.

Additional ground testing is planned before end of 2016 to assess blade integrity and bird impact capability for the composite fan system, as well as icing tests to determine ice shedding behaviour of blades and impact damage tolerances of new liners.

This technology development should allow a -3 to 6% of CO₂ reduction due to the introduction of those new technologies, including a -3 to -6 EPNdB noise reduction from the composite fan system.

Read more: ALPS successful first flight 

A first generation of geared fan engine technology successfully demonstrated its performance and operating characteristics during flight trials on development aircrafts. This first generation of GTF engines have been selected to power newly designed aircraft that are entering the regional market within the next few years. Significant reduction in fuel consumption (~-15% versus reference year 2000) and noise emission compared to conventional turbo fan engines are the convincing benefits of the GTF engine design.

The SAGE 4 demonstration is led by MTU Aero Engines and is envisioned to assess further advancements of the current geared fan technology to provide significant contributions towards the ambitious targets of ACARE in 2020. Components and modules with new technologies have been developed and validated through rig testing as required before being implemented into a GTF donor engine: high efficient HP compressor technology building upon the NEWAC programme (Active Core work package) and national research programmes, high speed low weight LP turbine technologies derived from CLEAN and VITAL programmes as well as from national research programmes, light weight exhaust frame.

 These technologies have recently been demonstrated with a successful campaign of engine tests in November 2015. Technologies for high efficient and reliable advanced power gearbox are underway at Avio Aero and will be tested on a dedicated new rig by mid-2016.

The SAGE 5 project under Clean Sky will provide TURBOMECA with the necessary technologies for the development of a new engine family, suitable for helicopters with a take-off weight from 3 tons (single-engine) to 6 tons (twin-engine).

The SAGE 5 project focuses on demonstrating the following technologies:

• high efficiency compressor stages
• small size high efficiency cooled HP turbine in order to improve performance and reduce dimensions of turboshaft engines and prepare a power growth capability
• high efficiency single stage LP Turbine
• materials and coating development and manufacturing techniques to enable static parts to cope with higher temperature,
• technologies for low noise devices for quiet exhaust,
• technology for inter shaft architecture
• technologies for control systems through development of equipment aiming to reduce cost, weight/size or to enable higher temperature operation

Additionally, technologies have been studied to develop a reliable and compact combustion chamber through development of size and weight decreases by studying the following:

• a small volume combustion chamber
• reliability improvement and life limit increase
• optimisation of the combustion process and fuel injection system
• technologies for Low NOx Combustion chamber and integration on turboshaft engine
• study of the associated injectors and fuel system

The first engine tests of the TECH 800 demonstrator were performed in February 2013, and subsequent runs were carried out in 2015, demonstrating an effective 15% SFC reduction.

Most of these technologies have been incorporated in Turbomeca’s new engine in the 1,100 to 1,300 shp power class: the ARRANO, which was selected in early 2015 as the exclusive engine for Airbus Helicopters’ new twin-engine H160.

Combustor partial rig test

1. Instrumented Reduction gear box ready to be assembled with the air intake module.
2. Turbine casing is a main part of the gas generator module

Besides the ongoing work on the engine architecture itself in SAGE3, the SAGE6 project has been created to house the Lean Burn programme and will deliver a Lean Burn Combustion System demonstrator engine for ground testing.

Lean burn combustion is a vital technology acquisition for the European aerospace industry if it is to comply with future CAEP & ACARE emissions legislation, and for its products to remain competitive in the world marketplace.

The aim of the SAGE 6 lean burn project is to demonstrate a lean burn whole engine system to TRL6, suitable for incorporation into civil aerospace applications in the 30,000 lbs to 100,000 lbs + thrust classes. In doing so, it is expected that the resulting technology will achieve compliance (with some margin) with all planned future legislation.

Work to develop the technology for civil aerospace applications has, to date, used demonstrator programmes such as ANTLE/POA, E3E and EFE. All of these experiments have identified significant challenges beyond combustor that require development of complementary systems to achieve an operable and certifiable product. The integrated system, which consists of technologies across the engine architecture (e.g. control systems, sensing and noise technologies) must be able to address many often conflicting requirements to provide a high accuracy fuel supply to separate pilot and mains feed within the combustor.

Significant technologies being developed within this programme consist of, but are not limited to:

• combustion
• hydro-mechanical fuel control
• control laws and associated sensing devices
• whole engine thermal management
• acoustic attenuation
• turbomachinery thermo-mechanical integration
• system health monitoring and maintenance functions