Todd Spierling, principal technical fellow at Collins Aerospace discusses with Ed Hill the latest developments in developing hybrid-electric powered flight, how it will affect the aviation landscape and the challenge of meeting net zero compliance.

Within both divisions of the RTX Corporation, Pratt & Whitney is one of the world’s leading engine manufacturers, while Collins Aerospace has a long history of providing electric power generation and distribution systems for commercial and military aircraft. Recently, these companies have been leveraging the synergies across RTX to explore how these technologies can fit together in future hybrid-electric propulsion systems. Something both businesses have been exploring to improve fuel efficiency and capability.

Q) What are the major challenges of developing hybrid electric power systems for flight?

The major challenge for hybrid electric systems is balancing the trade-off between improvement in fuel efficiency and optimising the weight and thermal management needed to enable the system. Configurations that rely on high power and energy use, typically provide the most upfront value due to gains in efficiency but typically trade out due to the weight added because of large battery requirement or increased drag due to increased heat rejection requirements. This can also be seen when looking at trades for electric motors and motor controllers, where the challenge is trying to optimise for size, weight, and operating environment limitations given the power required by the engine.

Another challenge is the hydraulic and pneumatic systems, the same fluid that provides power also carries away heat, while in electrified configurations two different systems are required, adding to the complexity of the system and further weight challenges.

The increased voltages associated with electrified aircraft, particularly electrified propulsion have also placed increased focus on phenomena such as corona, partial discharge and arcing. Through the RTX Solid-State Circuit Breaker programme with NASA, we are working to provide protection elements critical to the high-voltage electrified propulsion system, while through the Clean Aviation HECATE project we’re working with consortium partners to address how to interface that propulsion system with the aircraft’s secondary electrical system.

Q) Can you update us on where RTX is with its development of the demonstrator programme for hybrid-electric?

Testing has progressed on both the electric motor as well as the overall powertrain. Pratt & Whitney Canada has mated Collins’ 1MW electric motor with the thermal engine and are progressively operating the two in tandem to achieve the required combined system power levels and performance. Collins will also perform the detailed development and safety-of-flight tests to validate that our electrical equipment is ready for aircraft installation and safe flight. Our other major partners, such as H55 (batteries) and GKN (electrical wiring), are also supporting the development and integration of their components.

Q) These systems will be employed on different types of aircraft. How will that disrupt the familiar landscape of today’s commercial aviation?

Propulsion electrification brings many opportunities to significantly change the aviation landscape. The emergence of Advanced Air Mobility (AAM) has been enabled by quiet, reliable, low-cost electric powertrains that allow for safe, efficient short-range flight. For commuter/small regional aircraft, the switch to electrified propulsion with its lower cost and quieter operation will re-invigorate service at underserved airports due to its improved operating economics. For larger aircraft, electrification will allow fuel-burning gas turbines to become more efficient and will enable the creation of aircraft concepts that are not practical today. Overall, the mix of aircraft and route structures will need to change to best incorporate these new types of aircraft with their operating economics.

However, in the near term the overall landscape of commercial aviation will likely be unchanged. The hybrid-electric technologies that are currently being developed by RTX are focused on making the current fleet more fuel efficient but with the current trajectory of battery and powertrain technologies, the capabilities will be less disruptive and more evolutionary.

Two noteworthy dynamics that hybrid-electric technologies bring are (1) the modular nature of the technologies introduces possibilities for adaptability for variants using the same base engine to suit different mission profiles and (2) the platforms can improve over time through the evolution of battery technologies allowing for improvements in base performance.

Q) RTX is signed up to the SWITCH project to develop hybrid electric (combined with a water enhanced turbofan engine). Can you explain more about this technology?

SWITCH is focused on developing a novel propulsion concept built from two revolutionary and synergetic technologies: Water-Enhanced Turbofan (WET) and hybrid-electric propulsion. By combining these technologies with Pratt & Whitney’s GTFTM engine architecture, the SWITCH concept aims to enhance efficiency and reduce emissions across the full operating envelope of an aircraft.

Technologies developed as part of SWITCH will be compatible with cleaner alternative fuels – such as Sustainable Aviation Fuel (SAF) – and will be evaluated for future use with hydrogen. The hybrid-electric GTF powertrain will enable even greater efficiency across all phases of flight by leveraging highly efficient megawatt class electric motor generators, power electronics, and batteries to optimise the performance of the fuel-burning gas turbine.

Q) How important is it that companies collaborate on developing these systems in terms of the timeframe of 2050 and net zero?

Hybrid-electric propulsion is a key pillar of the aviation industry’s drive to net zero, with the potential to significantly reduce carbon emissions. What’s more, hybrid-electric architectures can also be adapted to support hydrogen propulsion applications for much larger efficiency gains towards net zero and beyond. With the emergence of new technologies, it will be imperative that both existing aircraft companies and emerging startups work together to invent new solutions and bring them to certification and industrialisation.

Q) What role do you think digitalisation in manufacturing, additive manufacturing and advanced composites will have in speeding up development of these type of systems?

Digitisation is transforming manufacturing and design by increasing the speed of the development cycle. It achieves this by reducing the time required for design, prototyping and virtual testing of systems. The combination of additive manufacturing and generative design facilitates efficient exploration of the design space, resulting in rapid prototypes for intricate lightweight structures that were previously challenging to manufacture. Additionally, digital twins – virtual replicas of physical assets – enable real-time monitoring and a better understanding of product behaviour, leading to better designs. When integrated with advanced simulation tools, they minimise the need for physical testing and enhance overall system performance.

Q) How will airlines adapt their operations to remain profitable with these new power systems and aircraft architecture?

Large commercial air carriers have a long history of optimising their operations with traditional fuel-powered aircraft and will now need to understand how to best consider new variables such as electric cost/carbon footprint and battery maintenance/replacement and continue to adapt their operational models to take advantage of new technologies.

We’ve seen this throughout the evolution of traditional gas turbine technologies and will likely see this with hybrid-electric technologies. How this will happen will depend on the platform economics and the operating costs, as well as the increasing influence of environmental policy measures, such as carbon taxes which further affect airline operating models.

Propulsion electrification will also create entirely new industries such as Urban Air Mobility. The business models of these industries will be fundamentally different from the traditional large commercial air carriers, including elements such as leasing and infrastructure in new private/public partnerships.

RTX will be at Chalet C301-305 and C631 at the Farnborough Airshow.

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