Wide-ranging developments are taking place that will determine the future of aircraft propulsion systems – including sustainable aviation fuel (SAF), electric, hybrid and hydrogen technologies. Ian Harbison reviews their progress to date.
This article was published in the August/September edition of LARA’s magazine. To access more articles like these, apply for your complimentary subscription to LARA.
Geared Turbofan engine – Cutting emissions and boosting fuel efficiency
Dr Michael Winter, the newly appointed Chief Science Officer at RTX, parent company of Pratt & Whitney, says: “Eighty per cent of aviation emissions are produced on flights of greater than 1,500 kilometres in length.”
While there may be opportunities in the future to look at aircraft with lower range and lower payload, the current focus has to be on those types that are the main cause of CO2 emissions, with non-CO2 emissions likely to attract more attention as well.
Winter says the company is ideally placed to bring new technologies together and integrate them into propulsion systems. The main focus is continuous improvement of the gas turbine engine.
Since jets were first introduced about 85 years ago, that has been at a rate of about one per cent per year in terms of average fuel efficiency across the fleet.
There are two key factors affecting engine design:
- Thermal efficiency, which comes from the core of the engine, where the energy in the fuel is released and turned into mechanical energy to turn the shaft.
- Propulsive efficiency, which comes from the fan and the bypass ratio and the nozzle.
Pratt & Whitney made a significant breakthrough with the Geared Turbofan (GTF), which enabled fuel savings of up to 20 per cent over previous engine types.
By June 2023, having entered service in 2016, GTF-powered aircraft saved airlines more than 1.7 billion gallons of fuel and over 17 million tonnes of carbon emissions.
Pratt & Whitney is now working on the GTF Advantage, which should reduce fuel consumption and CO2 emissions by up to a further one per cent while offering a four to eight per cent higher take-off thrust and higher thrust at altitude.
New challenges – Thermal efficiency and environmental challenges in modern engine design
Winter says thermal efficiency has been pursued across the board by all the engine companies, but now the world is getting hotter and dirtier, more attention is needed on improving the resilience of the products.
Higher ambient air temperatures means that wings generate less lift, so more power is needed to maintain performance.
Reduced air quality means that non-volatile particulates in some parts of the world are continuing to increase.
While some of these, like sand, can erode blades or melt and form a glass that clogs cooling holes, the main problem is chemical changes to these pollutants caused by the extreme temperatures in the engine.
The role of sustainable aviation fuel in reducing CO2 emissions
Sustainable aviation fuel (SAF) is also important, says Winter.
“Today, the Pratt & Whitney modern products are capable of 100 per cent SAF. We are working with other members of the industry to bring SAF forward, as there is clearly not enough today, and are an active partner with the industry and the approval boards, such as ASTM International.”
He points out that jet fuel is “a really good molecule for powering a plane”. It releases about 45 MJ/kg and is relatively stable.
Even the best batteries available, or under development in the lab, have significantly less energy per unit mass and per unit volume. In addition, they have to be contained in a box and need a thermal management system and fire protection, making them heavier. In fact, jet fuel has 40 times more energy to release for every kilo.
However, there is continuous development, and the current technology is mature up to about one megawatt, including the batteries, motors, generators, drives and distribution systems.
The company is in partnership with Collins Aerospace, another RTX company, and is working on several hybrid-electric demonstrator programmes addressing all range of future applications.
For the entry level STEP-Tech project, the companies are developing a scalable system in the 0.1 to 1 MW range, intended for distributed propulsion concepts. This integrates a new, very efficient gas generator engine from Pratt & Whitney Canada (P&WC) with batteries, control logic and propulsors.
With funding from the Canadian and Quebec governments, a Dash 8-based hybrid-electric flight demonstrator is being developed that will integrate the 1 MW gas generator engine integrated with the 1 MW electric motor and use a gearbox to drive a propeller, broadly contained within the existing nacelle.
The fuel burn can be optimised depending on the mission, perhaps using both for climb and just the gas generator engine for cruise.
Making the switch – Hybrid-electric GTF and water enhanced turbofan
A derivative of that 1 MW motor will be used on the hybrid-electric GTF demo as part of the Sustainable Water-Injecting Turbofan Comprising Hybrid-Electrics (SWITCH) programme. This is supported by the European Union Clean Aviation Joint Undertaking with a consortium of Airbus, Aristotle University of Thessaloniki (Greece), Chalmers University of Technology (Sweden), Collins Aerospace, the German Aerospace Center (DLR), GKN Aerospace, MTU Aero Engines and the University of Stuttgart.
Hybrid motor, part of the SWITCH programme.
The goal is to achieve up to a 25 per cent improvement in fuel efficiency and reduced CO2 emissions compared to current engines in short- and medium-range aircraft.
The consortium is co-ordinated by MTU. It combines hybrid-electric propulsion and the Water Enhanced Turbofan (WET) using a Geared Turbofan (GTF) engine.
The hybrid-electric powertrain will optimise the performance of the gas turbine while WET recovers water vapour from the engine exhaust and re-injects it into the combustion chamber to significantly improve fuel efficiency, reduce NOx emissions and lessen contrail-forming emissions.
For a typical narrowbody aircraft – which Winter describes as “the heart of the market” – an Airbus A320 has two 30,000 lb thrust engines, equivalent to a total of 36 MW.
About five per cent of that power could be augmented by electrical motors, by putting a 1 MW motor starter generator on the core of each engine and a 0.5 to 1 MW on the low spool of the engine that turns the fan. Optimising efficiency by moving power between the two provides opportunities to optimise efficiency throughout the flight.
Winter says that whenever the next generation of aircraft starts development – likely around the mid-2020s or later – they will still be flying in the 2060s, so entirely different battery technologies will be available, although it will still come back to how the engine and batteries are integrated and optimised throughout the flight.
Running cool – Historical insights and future potential of hydrogen propulsion
When it comes to hydrogen, Pratt & Whitney has a surprisingly long involvement with hydrogen propulsion systems.
In 1956, it was working on an engine for Project Suntan, a CIA long-range reconnaissance aircraft. This was the Lockheed CL-400, from the (in)famous Skunk Works. It was to be capable of flying at Mach 2.5 at altitudes of around 100,000 ft, where the air is so thin that hydrogen is the only possible fuel.
However, the programme was cancelled in 1958, turning into the SR-71.
Hydrogen, as Winter explains, releases 102 to 122 MJ/kg but takes up four times the volume of jet fuel even when stored as a liquid (at -253°C).
When burned, it could produce more NOx than jet fuel, as well as 2.6 times more water, meaning more contrails, both of which are undesirable.
In 2022, the company was selected by the US Department of Energy (DoE) to develop the Hydrogen Steam Injected, Inter-Cooled Turbine Engine (HySIITE) for commercial aviation, as part of the DoE’s Advanced Research Projects Agency-Energy (ARPA-E).
HySIITE has a thermodynamic engine cycle that incorporates steam injection, hydrogen combustion and water vapour recovery to achieve zero CO2 emissions, while reducing NOx emissions by up to 80 per cent and fuel consumption by up to 35 per cent for future generation single-aisle aircraft.
The water will be used for cooling and intercooling, reducing temperatures and increasing durability. It will also increase thrust levels, so the core of the engine can be smaller, and the water capture may assist with some of the non-CO2 emissions in the form of contrails.
Cutting emissions – the CFM RISE program
Arjan Hegeman, General Manager of Future of Flight Technologies at GE Aerospace, says the CFM RISE (Revolutionary Innovation for Sustainable Engines) programme is the future for the company and partner Safran Aircraft Engines.
CFM RISE Program Open Fan
The target is to achieve more than 20 per cent lower CO2 emissions than today’s engines by using an advanced open fan architecture.
Progress is through demonstrations starting at component level, then system level, and culminating in flight tests before the end of this decade.
This is the largest demonstration programme in the company’s history because there are four technology pillars that will eventually merge together.
The first is the open fan, which is about propulsive efficiency. The second is the core. By using next-generation compressor technologies and materials, and higher operating pressures, increased thermal efficiency means the core can be smaller.
Third is a MW class hybrid electric powertrain. This could be used to drive the fan, or to generate power for aircraft systems or to charge batteries.
Finally, the use of alternative fuels, from SAF to hydrogen, is possible.
Again, RISE is targeted at the A320/737 replacement market in the 2030s – but it is scalable, so could be used for regional aircraft as well.
Hegeman says the aim is always to increase the bypass ratio (CFM56: 6, LEAP: 11) but there is a limit in size with a ducted fan, the duct adding weight and drag.
In the late 1980s, there was the GE36 Unducted Fan, which was flown on a McDonnell Douglas MD-81. While it demonstrated excellent fuel efficiency, it was extremely complex and noisy, being cancelled in 1989.
With improvements in Computational Fluid Dynamics, it has now been possible to optimise the airfoil shapes. Instead of two contra-rotating fans, as on the GE36, there is now a single fan with a static second stage.
As for noise, the airfoil shapes have also been tuned to produce noise levels below LEAP engines. The fan diameter is slightly larger than the current LEAP nacelle, so there should be no configuration problems with next-generation narrowbody designs.
Research and development – Propulsion technologies
Meanwhile, GE Aerospace is working with NASA on a number of propulsion systems research programmes.
Under the Electrified Powertrain Flight Demonstration (EPFD) project, it has partnered with Boeing and its subsidiary Aurora Flight Sciences to fly a megawatt-class hybrid electric powertrain in the middle of this decade using a modified Saab 340B aircraft with CT7 engines.
It is also developing a hybrid electric demonstrator engine with NASA that will embed electric motor/generators in a high-bypass commercial turbofan to supplement power during different phases of operation (also applicable to RISE).
This includes modifying a Passport engine with hybrid electric components for testing through NASA’s Hybrid Thermally Efficient Core (HyTEC) project.
In addition, it is working with Airbus on a hydrogen demonstration programme that will take flight around the middle of this decade.
CFM will modify the combustor, fuel system, and control system of a GE Passport turbofan to run on hydrogen, which will be fitted to an A380 test bed equipped with liquid hydrogen tanks.
Airbus will also define the hydrogen propulsion system requirements and oversee flight testing.
Maturing technologies – Deutsche Aircraft
Deutsche Aircraft has a unique perspective on the development of alternative technology powerplants.
Currently under development, for certification in 2027, is the D328eco. This is based on the original Dornier 328 but the extra weight of a 2.2-metre fuselage stretch to increase capacity from 32 to 40 seats, a larger tail assembly to retain stability and a requirement for good field performance means the original Pratt & Whitney Canada PW119A engines have been replaced by more powerful PW127XT-S turboprops.
Technology improvements in the new engine means it can offer a three per cent lower specific fuel consumption compared to earlier PW127 models.
While capable of using normal aviation fuel, the aircraft is designed to use Sustainable Aviation Fuel (SAF) produced by Power-to-Liquid (PtL) technology in regular operation. PtL uses renewably generated electricity, water and CO2 from the atmosphere to create a syngas from which SAF can be produced.
In June 2023, the company signed a Letter of Intent (LOI) with Sasol ecoFT, which will deliver fuel with an increasing amount of SAF to D328eco customers until 100 per cent SAF can be achieved. Sierra Nevada Corporation (SNC), which owns Deutsche Aircraft, participated in a programme to install hybridised GE Aerospace CT7 engines on a 328.
Four years ago, with financial support from the German government, the programme was shifted to Germany, linking up with local partners and engineering expertise. In the event, CT7 integration proved too difficult and battery weight cut payload/ range too much.
However, Deutsche Aircraft is also working with the German Aerospace Center (DLR) on the UpLift program. The DLR acquired a Dornier 328 and is converting it into an open innovation platform for decarbonising aviation. In May, it completed a successful test flight using 100 per cent synthetic fuel with zero aromatics.
As this is an open innovation platform, with the DLR collecting and testing the best ideas from industry, its involvement means the company can keep a watching brief on new technology that might make sense.
For hydrogen, the D328ALPHA programme is funded by the Bundesministerium für Wirtschaft und
Klimaschutz (BMWK, German Federal Ministry for Economic Affairs and Climate Protection) through its LuFo Klima Civil Aviation Research Programme.
Building on the lessons learned from the hybrid engine programme, D328ALPHA includes extensive ground verification and testing of systems with the ultimate aim of flight demonstration.
The technologies being matured are megawatt-class electric engines, power distribution systems, high-power fuel cells, thermal management systems and liquid hydrogen tanks.
Rather than replacing the PW119engines, the new electric engines and propellers will be mounted outboard of the existing engines – a configuration that Deutsche Aircraft says is well suited for
technology development and maturation.
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