For decades, aerospace manufacturers have been incorporating composite components into their aircraft to cut back on weight, save on fuel and reduce the environmental impact of commercial, business and military flights. With the development of new technologies, composite materials will play an even bigger role in the aircraft of the future.
The work on composites for aerospace is continuing on many fronts, three of which are highlighted in this article. At a new technology hub in England, aerospace engine manufacturer Rolls-Royce is showcasing automated production of composite fan components. In Wichita, Kan., Spirit AeroSystems is finding ways to improve the properties and production of composite materials designed for hypersonic flight. And in Florida, university researchers have significantly increased the strength of 3D-printed composite materials, making them better candidates for structural components of aircraft.
New Methods of Production
More sustainable, more automated production of composite materials could lead to their increased use in the aerospace industry. Rolls-Royce’s new composite technology hub in Bristol, England, provides a possible model for future manufacturing. Opened in January 2020, the facility is manufacturing CFRP fan blades and fan cases for demonstrator models of Rolls-Royce’s innovative UltraFan® engine. The UltraFan represents the first big architectural change in Rolls-Royce’s aircraft engines in several decades.
“UltraFan brings together a whole series of new technologies that combine the propulsive efficiency benefit of a large, low-speed fan driven through a power gearbox with a high-technology, high-speed and very power-dense, gas generator core,” says Andy Geer, chief engineer, UltraFan product development and technology at Rolls-Royce. This enables both fuel burn reduction and emissions reduction. “From a cost point of view and an environmental footprint point of view that is very much where the future of aerospace is pointed,” says Geer.
The company estimates that the incorporation of composite fan blades and fan cases will reduce the weight of the jet engine by more than 1,500 pounds, the equivalent of seven passengers. Overall, UltraFan will deliver a 25% reduction in fuel burn and CO2 emissions compared to Rolls-Royce’s first Trent engine family member.
Rolls-Royce partnered with the United Kingdom’s National Composites Centre on the development of UltraFan’s composite fan components. The initial work was done at the company’s academic-style research facility on the Isle of Wight. The next stage will happen at the composite technology hub, a pre-production facility that features state-of-the-art, automated manufacturing methods. The building is powered primarily by solar power and designed for sustainability with low-energy, very-low-emissions manufacturing processes.
“We paid a lot of attention to ensure that the composite lay down and consolidation are relatively low-temperature processes and that the outgassing, or the products that are released during the assembly process, are in no way harmful,” says Geer. Sustainability concerns also guided Rolls-Royce’s choice of materials for the CFRP components and its design of manufacturing processes that result in minimal waste material.
To manufacture the fan blades, Rolls-Royce builds up hundreds of layers of carbon fiber materials, pre-filled with a resin that increases its toughness. Toray manufactured the carbon fibers, which Hexcel Composites incorporates into the prepreg materials that it supplies. Rolls-Royce partnered with Delaware-based Accudyne to develop the specialized automated robotic systems that perform the material lay-up. The composite components are cured through the application of heat and pressure, and the blades are finished with a thin titanium leading edge to provide protection against erosion, foreign objects and bird strikes. Inspection systems are integrated throughout the process to ensure the quality of each part produced.
The factory’s pre-production technology can be easily scaled up to meet actual production demands, according to Geer. That’s especially important in the aerospace industry, where new aircraft components must go through rigorous qualification testing.
“Once you’ve achieved that certification, you have to be able to prove that the parts that you make subsequently are equivalent to those that were certified. So precision, accuracy and repeatability are a key part of that process,” says Geer. By controlling the manufacturing process, the company is able to demonstrate that the 100th part it builds will be made the same way and to the same quality standards as the first part it built. That’s harder to do when there’s a human factor involved, as in manual lay-up.
The fan blades and cases are not the only composite components on the UltraFan engine. Geer says that organic polymer matrix composites are used extensively throughout the fan system, and other composite materials are incorporated into the acoustic panels, into the infill for aerodynamic fairings and for the annulus (part of the engine combustion system). Ceramic matrix composites are being used in the hot sections of the engine.
Rolls-Royce plans to have the demonstrator model of its UltraFan engine completed by the end of 2021. While the company is in discussions with aerospace manufacturers about including the engines in new aircraft designs, COVID-19-related slowdowns in the industry make it difficult to predict when such projects will move forward.
“Our intent is to put a number of demonstrator engines through a pretty rigorous characterization and test program, prove that we understand the technology and that we’re completely ready to productionize it,” Geer says.
Standing up to Super Speeds
Hypersonic aircraft, used primarily by the defense and weapons industry, are designed to travel high in the earth’s atmosphere at speeds ranging from Mach 5 (about 3,800 mph) to Mach 20 (about 15,000 mph) or more. One of the biggest challenges in designing aircraft for these speeds is developing materials that can handle the effects of atmospheric drag.
A hypersonic vehicle must withstand temperatures ranging from around 1,200 degrees F to more than 4,000 degrees F in certain areas, depending on how fast the aircraft is traveling. “The ability of the materials and structure to react to rapid changes in temperature and to work properly when the vehicle experiences large temperature differences over various parts of the structure is absolutely essential,” says Chris Boshers, senior director and chief engineer of defense engineering at Spirit AeroSystems.
The structures that provide thermal protection for a hypersonic aircraft require high strength and stiffness because they also have to withstand the large dynamic pressures and g-loadings (forces of acceleration) of hypersonic flight. Composite materials can provide these properties, but those unidirectional fibers may suffer from delamination and premature failures.
“The strongest and most efficient temperature materials [for hypersonic aircraft] have fibers oriented in three directions to react to three-dimensional applied loads,” Boshers says.
To produce these hypersonic-ready composite materials, Spirit AeroSystems, which has expertise in material design and fabrication, recently acquired FMI, which has been developing and producing high-temperature composites for more than 50 years. The combined teams hope to improve the value proposition for hypersonic flight by making the required materials, and thus the aircraft, more affordable, more efficient and producible in quantity.
Spirit is working with two different types of composite materials for hypersonic applications – carbon/carbon and ceramic matrix. Carbon/carbon (C/C) composite materials can maintain high strength at temperatures above 4,000 degrees F while also retaining a predictable aerodynamic strength. In hypersonic aircraft, C/C composites are used for nose tips, control surfaces, thermal protection systems (TPS), aeroshell applications, and the nozzles and throats of rocket motor exhaust systems. Other potential applications include TPS for space re-entry vehicles, heat shields and high-temperature engine components, as well as oven walls and high-temperature probes for industrial applications.