Much has been written about the coming of the more-electric aircraft (MEA), especially in the context of Boeing’s leap into the innovative, higher-risk era with the 787 Dreamliner. The Dreamliner product line broke with tradition and adopted systems that rely more on electrically generated power than pneumatic and hydraulic solutions that harness engine bleed-air. It was a bold step to take, and it brought initial problems when the first deliveries entered regular service. But, with a massive early back-log of orders to fill, it was clearly an issue that would have to be solved without delay (and it was with an interim fix).
Every major step forward in aerospace innovation has issues that arise before the product can fully mature in everyday usage. And, as new designs emerge, the trend toward using electrical power systems (EPSs) is going to gain momentum. EPSs offer improved performance and reliability while saving weight, which in turn has an impact on the customer’s bottom line. The key to unlocking significantly greater performance from onboard EPSs is at the heart of the aerospace initiatives currently being undertaken by Raytheon in the U.K.
Raytheon has invested considerable funds into developing innovative EPSs for a wide range of industrial applications, especially those demanding new standards of operation in harsh environments. Its family of EPSs are smaller, lighter, and more efficient than legacy systems, with products that include primary and secondary stage power conversion, solid state distribution and switching, electrical engine start, and motor drives.
“Everyone in the sector is keenly aware that this market is becoming deeply competitive with new and emerging providers taking share,” said Steve Clerkin, Head of Aerospace Power Systems at Raytheon Integrated Power Solutions. “The rapid increase of aircraft in service drives a focus on procurement cost, operating profitability, and legislative environmental boundaries.”
Raytheon—one of the first worldwide to achieve the BS11000 partnering and collaborative working standard—is performing fundamental materials research through software algorithm developments that are being applied to control actuation sub-systems, cyber and intelligence solutions, big data developments, and other programs. The company is aiming higher with a whole new generation of integrated power systems utilizing silicon carbide (SiC).
Processing capabilities and expertise has grown enormously over the last decade. Raytheon is partnering with many leading aerospace customers and research academia as new applications are added. An advanced SiC manufacturing technology has been developed—high-temperature SiC (HiSiC)—to produce complementary metal–oxide–semiconductor (CMOS) integrated circuits capable of operating above 250°C. This method has been developed for sensors, instrumentation circuits, and gate-drivers operating in extremely harsh environments where existing silicon-based semiconductors fail.
To meet the technological challenges associated with MEA, Raytheon has demonstrated a scalable Technology Readiness Level (TRL) 5 primary power converter. Demonstrated at 90 kW, the bi-directional non-isolated power supply can convert three-phase 115-VAC generator-supplied power into 540-VDC to meet the broad electrical network requirements of MEA. It can also convert DC into three-phase AC required for engine starting using novel high-frequency SiC power semiconductor inverters.
Raytheon’s philosophy is to move away from traditional one-way power distribution in favor of a more versatile and intelligent power architecture, making use of electricity as a common energy carrier. More efficient and higher density conversion technology is seen as the key to making the most of new opportunities for reducing weight.
These demonstration converters were developed during a series of U.K. research and development initiatives undertaken by the Aerospace Technology Institute and the government-supported Innovate U.K., which included projects led by Rolls-Royce (Siloet2) and Airbus (the Integrated Power and Propulsion Architectures project). These projects, which have also embraced partners from the U.K.'s leading experts in aircraft electrical systems, are aimed at providing aerospace engineers with the data needed to optimize future aircraft electrical architectures and systems for greater efficiency, while reducing emissions and operating costs.
The role that Raytheon has played in these projects has been to help identify how the future power architecture can best be modeled. By operating in environments where traditional silicon-based gate driver circuitry cannot cope with the heat, the new HiTSiC-module capability represents a major industry breakthrough. The end-result should be fit-for-purpose, reliable, and high-density power converters; other products will be ready for OEM decisions on the configuration and specification for the next generation MEA.
“The benefits of lighter, more fuel-efficient aircraft with lower emissions and greener systems will deliver these goals, but the drive toward more electric solutions has been relatively slow in reality. While investment in future technologies that push the boundaries has risen, there are obsolescence issues in today’s market that affect our customers in different ways,” said Clerkin.
It’s a question of balance for many in the industry: many current civil aircraft will remain in service for the next 20-30 years. Thus, there is a need to look at lower-risk technology insertions that might bring greater efficiency through having lighter systems.
“The OEMs are always looking at improvements including system upgrades and this extends into such areas as power electronics, galley equipment, in-flight entertainment, baggage handling systems, global connectivity, and new cabin lighting, and these needs can generate new business for us,” said Clerkin. “They are not all safety-critical items, but fitting and enabling more and more electronic devices on the aircraft may require a revision of onboard power requirements.”
“Large segments of existing aircraft do not have distributed electrical systems and need a solution that does not add more weight in delivering extra electrical power,” he said. “We believe that there is also a significant market for upgrading regional aircraft and here lower risk programs may be a practical solution. Solid state switching might be a vanguard for new applications and we are looking closely at the technologies best suited for these changes.”
Raytheon is working directly with R&D groups, with new funded programs, and looking at more electric architectures so that its engineers can better understand future customer requirements.
“These studies are examining what applications are needed and so we are doing the homework now for products needed in the mid-2020s,” said Clerkin. “Some of this work is evolutionary and there is genuine interest feeding back from customers who are starting to realize that they may be missing out on not leveraging the real weight savings that can be made. These solutions will be lighter and with a lower cost of entry, so despite a degree of inertia in the market at present, we are considering how further exploitation of even current technology might move things on.”
Additionally, because HiTSiC is all about working in extremes, there is growing recognition that devices mounted adjacent to very high temperatures, such as for taking measurements beside an engine, could bring big weight-saving advantages in the OEM market.
“The battery problems that received much publicity when the 787 entered service changed the dynamic on other programs and manufacturers stepped back from adopting such a radical dependency on electric systems and Li batteries,” said Clerkin. “The early adoption of these features caught the attention of a wide audience as boundaries on a very advanced design were being pushed and stretched. The good news is that the risk has been brought to the surface and development work is continuing on charging systems and more technical solutions beyond strengthened containment. We believe that silicon carbide products represent the way forward and we are not just setting out to compete with traditional solutions but will raise performance and make a difference in the size and weight challenge.”
In regard to HiTSiC weight reduction benefits, the higher the power density, the lighter the solutions could be, further reducing thermal stresses and the need for cooling systems and components. Density was a function of three important principles—quality, reliability, and efficiency. With a need to cover safety critical requirements, a power density ratio of 6-10 kW/kg was achievable, but while a 6 kW/kg ratio will do the job, beyond those numbers more sophisticated systems would be needed. Systems such as electrical starting are critical and can be improved, and the OEMs expect better performance solutions to become available through innovation.
Further applications include electric taxiing systems at airports. Electric options have been shown to work on 737s and A320s and the OEMs are looking at this closely. Although some products have entered the market, uptake is slow, and it is evolutionary.
“No doubt powered systems, such as the electric wheel, can do the job and some high-level innovation could make this more widely adopted, but I think that before that happens more of today’s aircraft will first have to be consigned to history,” he said. “Airlines don’t have to immediately change the way they taxi but in the future they may be subject to more pressure on this and then a suitable technological solution will emerge. It is probably a matter of when this is likely to arrive, rather than if.”
Asked if Raytheon was actively talking with OEMs about participating at the design stage of future aircraft, Clerkin remarked, “Yes we are and it is ongoing. The OEMs will be looking for reusable architectures, solid state solutions, good partners, and trying to keep things as simple as possible—not over-complex. Technical research will bring forward big improvements at the high end this might include aspects such as full system redundancy, modular and “smart” systems that reconfigure gracefully so as not to lose service. Self-healing architectures will also be made possible by system management software where the protocol tells you what to do, but corrective action can be made manually or automatically.”
Clerkin believes embedding power systems into the structure might become commonplace with sensors placed about the aircraft collecting and distributing data and then reacting directly with other systems, such as actuators.
“These electrical devices will be controlled and supported by multi-task systems that will deliver plenty of power where it is needed but within a reduced physical space,” he said. “Fault-finding aboard an aircraft will be made easier, and as the systems and resulting data will be integrated it will be available to pilots and also communicated to the ground for both planned and unscheduled maintenance. New tools will be needed in a commercial aircraft to counter the cyber threat that will grow with ever-expanding open connectivity, and this is being studied at system levels.”
There can be little doubt that the essential technological groundwork for the more-electric aircraft is being tested very comprehensively and is very much on-going, in this instance ahead of the development of the aircraft, which is as it should be to help de-risk one of the most crucial aspects of what will be the next game-changing step in the story of commercial aviation.