GE Aviation has provided a glimpse into design challenges that the company is working through as it progresses on the twin-shaft, medium-bypass Affinity engine selected to power the Aerion AS2 supersonic business jet. Aerion Supersonic and GE announced plans in October 2018 to move forward with the Affinity for the AS2, saying it would be designed using a “proven engine core adapted from GE’s commercial airline portfolio” with the latest technology full authority digital engine control, an advanced twin-fan, durable combustor, and advanced acoustic technology that would meet or exceed regulatory requirements.
While GE has not specified beyond saying the engine pulls from its commercial engine core expertise, it is believed to borrow from the CFM56. But GE does say the Affinity adopts features from its new Passport business jet engine such as the front fan blisks and slimline composite core ducting.
Anticipated to be part of a family in the 16,000- to 20,000-pound-thrust range, the Affinity will be certified to operate at altitudes up to 60,000 feet and capable of providing efficient super and subsonic transport.
Speaking during the American Institute of Aeronautics and Astronautics Aviation Forum in June, Joel Kirk, GE Aviation’s executive leader of advanced systems design and technology, said the engine maker is partnering “across the board” on technology development for the next wave of supersonic flight and the AS2 is a “major driver for our strategy.”
The initial design review with Aerion was completed in the fall of 2018 and GE says it is remaining on track with the AS2 schedule, now targeting a first flight in 2025 and entry-into-service in 2027. Further, some component testing has been completed this year with more lined up over the next year, GE said, adding that trials have included “a first-of-its-kind engine acoustics test to give us sound references for the multi-stage fan design.”
Kirk outlined some of the differentiating factors required for Affinity, much of which surrounds supporting the mission of making the AS2 and other supersonic aircraft as environmentally friendly as possible. This is critical because the main obstacle to the return of supersonic flight is environmental regulations. Kirk called this an “imperative” for the engine.
The Affinity requirements include the goal of being able to run 100 percent on sustainable fuels and meet or exceed international levels for noise and emissions. To honor those promises—including meeting Stage 5 noise levels—Kirk said it has been “real important for us to have this good partnership earlier on in the development of the aircraft with so we can work through interfaces and integration with Aerion together. That has been an enabler to open up the design spaces solutions to help make this airplane friendlier relative to noise and friendlier relative to emissions and performance.”
He added that GE’s objective “is to burn as little fuel as possible and burn as clean as fuel as possible. But we have other considerations around how to get to supersonic speeds and also how to do that in a noise-friendly environment.”
These goals are requiring a “series of compromises” the company has to make around the installation of the engine on the aircraft, he said, adding, “integrating that has been a key part.” GE Aviation introduced new technologies around the fan and the “special, non-augmented supersonic exhaust system” to mitigate emissions and fan noise.
“We’re working on reduced fan size,” and eying technologies around acoustics for fan noise, he said. In addition to completing baseline testing for fan acoustics, GE also has completed initial testing around the exhaust. “Those technologies are being developed and matured,” Kirk said.
The ability to run on 100 percent biofuel has further involved several considerations, including with the fuel system, combustion, and the turbine, he said. “The biggest challenge is the fuel system,” he said. GE Aviation has taken a close look at how biofuels would impact the system in areas such as sealing. Biofuels typically lack key aromatics that are important for the seals.
“We’ve done a lot of work around that and we think we have solutions around that now,” Kirk said, adding that he believes the system now could “entertain both Jet-A and sustainable fuels. That technology has really been matured and developed to the point where we think it’s in a good position for introduction.”
Beyond combustion, GE Aviation had to examine capabilities and material systems involved with the turbine to minimize reaction with combustion byproducts.
He said by engineering a new technology engine, GE Aviation is able to incorporate materials and designs that would not necessarily fit with a legacy engine but that enables them to solve some of these issues. “We think we’re in a good position for that.”
Heat is also one of the key differentiators required for the supersonic-capable engine, Kirk said. This includes running the core hot for maximum efficiency. In subsonic designs, he explains, hottest temperatures would be set at a takeoff or “severe climb point” and then at cruise “it would fall out.”
But Kirk added that with temperature “in supersonic super-cruise condition, the cruise point is nearly at [that of the] takeoff point. So…GE is designing an engine that can sustain that environment throughout the entire flight or at least the entire super-cruise portion of the flight.”
This has required technologies drawn on through both its military and commercial programs, he said. “We’re leveraging over and inserting new technologies…so we can run the engines hotter, using best-in-class materials.” Central to all of that, he added, is “our combustor and how we are maximizing the heat extraction.”
At the same time, GE Aviation must ensure efficiency. To maximize the airflow velocity, subsonic commercial engines have continued to pursue higher-bypass ratios with larger fans on smaller cores. “However, Aerion doesn’t really want us to strap 100-inch fans onto the airplanes when they are trying to do super-cruise. So there are compromises that have to be made to fly supersonics,” he said.
Even so, GE Aviation promises the engine will have the highest bypass ratio ever for a supersonic engine.