Pratt & Whitney’s near-term focus remains squarely on improving the architecture of the geared turbofan (GTF) engine, and possibly developing larger variants, while waiting for future low-emission technologies to mature.
But behind the scenes, a cadre of engineers and scientists with P&W and other companies within the Raytheon Technologies group are busy advancing next-generation technologies, like electric propulsion and the use of hydrogen fuel.
But do not expect hydrogen- or electric-powered airliners to be carrying passengers any time soon – the industry needs years to perfect those designs, executives say.
But the research happening today will ensure that Raytheon, P&W and other subsidiaries can both update existing engines and potentially offer a clean-sheet powerplant when Airbus, Boeing and other airframers come calling, they say.
Among Raytheon’s many facilities, its Research Center along the Connecticut River in East Hartford is where much of its aircraft powerplant research and development occurs.
Workers at that facility, which dates to 1929, evaluate technologies including “complex integrated systems”, advanced materials, model-based digital engineering tools, autonomy, machine learning and electrification, says Andreas Roelofs, Raytheon vice-president of research and development and director of the Research Center.
The site houses Raytheon’s “compressor test facility” – a room in which massive, looping ducts carry fast-moving air through a test chamber. The company uses the site to study low- and high-pressure compressors, including smaller, higher-pressure and faster-spinning cores. Such characteristics can boost efficiency but can require use of advanced high-temperature-resistant materials.
“It’s almost like a windtunnel for compressors,” Raytheon’s Scott Kearney, who helps oversee the facility, said during a tour of the site on 19 May. The equipment can simulate altitudes of 30,000ft, and will soon have ability to test components whirling up to 35,000rpm.
“This facility focuses on thermal efficiency, where maybe competitors have a step up,” Kearney says.
An engine’s total efficiency depends on both thermal efficiency (how well it converts a fuel’s chemical energy to mechanical power) and propulsive efficiency (how well it converts mechanical power to propulsive output).
In developing the GTF – which powers Airbus A220s, A320neo-family jets and Embraer E-Jet E2s – P&W optimised propulsive efficiency. It did so with a gear that decouples the fan and turbine, allowing for a larger, slower-turning fan that can send more air around the core. Because that bypass air generates most of a turbofan’s thrust, the design improves propulsive efficiency. The A320neo’s PW1100G has a 2.06m (81in)-diameter fan and 12:1 bypass ratio.
CFM International’s line of competing Leap turbofans – variants of which power A320neo-family jets and Boeing 737 Max – lack such a gear. But CFM, a joint venture of GE Aviation and Safran Aircraft Engines, instead opted to optimise thermal efficiency. It accomplished that through use of high-temperature materials like ceramic matrix composites (CMCs), which are made from silicon carbide, ceramic fibres and resin.
CMCs can withstand temperatures up to 1,320°C (2,400°F), meaning less of a turbofan’s air must be diverted for cooling, which boosts thermal efficiency.
Both companies say their engines are roughly 15% more efficienct than comparable previous-generation turbofans.
P&W president Christopher Calio confirmed during P&W’s 18 May investor day that the company is now focusing on improving thermal efficiency. “What’s next in the efficiency space? It’s thermal efficiency,” Calio says. “It’s developing materials to make engines even more capable.”
He cites use of “high-temp composites” like CMCs, noting P&W recently opened a site in Carlsbad, California to advance such technology.
“This will enable us to both retrofit the existing fleet when it’s ready, and then also, in the future, we can design around it for those applications where we need to drive more heat through the core,” Calio says.
The aviation industry faces enormous pressure to curb carbon output and has committed by 2050 to slash emissions to half of 2005 levels.
Also in East Hartford, Raytheon is advancing additive manufacturing, better known as 3D printing. The company’s additive manufacturing machines can “print” components made from metals including aluminium, titanium and “high-temperature nickel-based super alloys”, it says.
The process can manufacture, as single units, complex components now made from multiple parts, thereby reducing weight and complexity. Raytheon is producing prototypes of components that could find their way onto engines, such as heat exchangers and fuel pump assemblies, it says.
Raytheon executives also view wider use of sustainable aviation fuel (SAF) – known as biofuel – as key to reducing carbon output. Burning biofuel releases carbon that had previously been absorbed from the atmosphere by plants, making the fuel’s CO2 impact “almost neutral”, according to IATA.
Regulations permit commercial jets to fly on fuel blends composed of 50% SAF, but Calio says P&W is assisting regulators to achieve approval for 100% biofuel.
“We think that can reduce emissions up to 80%,” he says. “We don’t think it will cause us to have to do tremendous modifications to the engine.”
Other companies have put weight behind SAF, including Boeing, which this year committed that all jets it produces by 2030 will be capable of burning 100% SAF. Additionally, US lawmakers recently introduced a bill that would subsidise SAF production.
IN THE PIPELINE
Raytheon’s Research Center has also been advancing high-power electric and hybrid-electric technologies, evaluating the use of 1MW electric motors.
Calio views hybrid-electric propulsion as “an opportunity in the short-to-medium range”, particularly for general aviation aircraft, turboprops and helicopters. Such systems could deliver 30% efficiency improvements, he says.
Applications for larger aircraft could potentially include a GTF engine paired with a 1.5MW “electric assist” system, according to a P&W diagram.
How soon such technology will be viable remains unknown. In 2020, Raytheon slowed Project 804, an effort to equip a De Havilland Canada Dash 8-100 with a supplemental 1MW electric system.
P&W is also evaluating potential future use of hydrogen fuel, which, when used to power fuel cells, emits water vapour.
“There’s absolutely potential there,” Calio says of hydrogen fuel. “But that’s a 2035 prospect.”
That timeline aligns with Airbus’s target for potential development of hydrogen commercial aircraft. The Toulouse airframer unveiled three hydrogen-powered aircraft concepts in 2020, saying such technology could be ready in 15 years.
“We are talking about re-engineering the entire airplane” to accommodate hydrogen, says Raytheon fellow of strategic technology Jeff Cohen, who helps oversee the “jet burner test stand” at the East Hartford site.
There, engineers evaluate low-emission fuels like SAF and hydrogen.
Hydrogen “burns easy” in turbofans but has “mass and infrastructure limitations”, Cohen says, citing hydrogen’s “volumetric energy-density problem”.
While 1kg (2.2lb) of hydrogen has better energy density than the same mass of jet fuel, that quantity of hydrogen takes up much more space – meaning larger and likely heavier fuel tanks will be required.
Challenges aside, use of hydrogen could open other technological possibilities, says Raytheon research and development head Roelofs.
Hydrogen is extremely cold as a liquid – the form envisioned for aviation applications, he notes. That characteristic could potentially be leveraged to cool electric components like superconductors, helping enable electric propulsion.
IMPROVING THE GTF
During Raytheon’s recent investor day, executives stressed that the group’s near-term priority is improving the existing GTF architecture, saying step-change technologies remain insufficiently mature for prime time.
“We think it’s the architecture of future,” says P&W’s Calio of the GTF. “We are working on packages to [improve] the fuel burn. Also working on a way to, maybe, just scale it up, to the extent that engine bypass ratios get larger, applications get larger [so] we are in a position to offer the GTF.”
He does not mention potential future applications for GTF derivatives, but P&W has previously said the design could possibly be scaled to power widebody aircraft.
“The outlook we provided today does not contemplate a new centreline engine,” Raytheon chief financial officer Neil Mitchill tells investors. “However, it does contemplate… enhancements to the gear, and the geared-turbofan engine performance improvements.”
Executives note that major airframers, likewise, seem unlikely to launch a new jet in the near term. “We don’t see a major new clean sheet on the immediate horizon,” says Stephen Timm, president of Raytheon subsidiary Collins Aerospace.