Beneath steel scaffolding and a thicket of instrumented cables and bolted-on accessories, the world’s most advanced jet engine—GE Aerospace’s XA100—made a rare appearance to a group of journalists and think tank analysts within the normally classified space of Altitude Test Cell 44, offering a glimpse of a future propulsion system that could still be canceled this year.

The U.S. Air Force has proposed canceling the program in fiscal 2024, leaving the fate of the world’s first adaptive turbofan with Congress. Although the Defense Department has two competing 45,000-lb.-thrust-class engines by GE and Pratt & Whitney to reengine the Lockheed Martin F-35A, it still could walk away from the project on which it has spent more than $4 billion on adaptive engine development since 2007.

• Engine-maker starts third phase of tests on second XA100
• The company’s new goal is to complete the detailed design review for the production version

Pratt supports the Air Force’s decision to upgrade the F135 power module and thermal management system rather than face the prospect of losing future F-35A engine sales from a reengining competition between production versions of GE’s XA100 and Pratt’s XA101. But GE is lobbying Congress to extend the Adaptive Engine Transition Program (AETP) at least one year, David Tweedie, vice president and general manager for GE Advanced Products, told journalists on May 2 at the company’s facilities in Evendale, Ohio. “Let’s not close off long-term options,” Tweedie said, describing GE’s message to lawmakers. “Let’s try to take [the AETP] to the next logical milestone.”

For GE, the goal now is to complete a detailed design review in fiscal 2024 for a production version of an A100 engine, which would remove the experimental “X” prefix from the designation. Although GE has long described the XA100 as a “production-representative engine,” several changes are required for an operational version. These include lighter hardware throughout the engine and a new full-authority digital engine control (FADEC) system, replacing the computerized ignition and control system borrowed from the canceled F136 program for the first two experimental XA100s.

Adaptive engines promise the next breakthrough in turbofan engine performance: a 4,000-lb. increase in thrust or, depending on mission needs, 360 nm of increased range for the F-35A, along with more than four times the cooling capacity for onboard electronics. While the F135 meets the original requirements with 14 kW of cooling capacity, new avionics and sensor upgrades in Block 4 F-35As demand 47 kW. The first version of the XA100 can provide up to 62 kW of cooling power, with a growth path up to 80 kW.

In an effort to prove these capabilities, GE obtained approval to allow access to the classified test chamber.

On the floor inside the test cell, a disassembled piece of the first XA100 engine was available for inspection. This section of the engine fan frame reveals the revolutionary geometry of three-stream, adaptive turbofan technology. The frame is carved by a series of circular passages. First, a roughly 8-in.-dia. inner duct leads air ingested by the fan directly into the engine core. Surrounding the inner duct, a roughly 2-in.-dia. channel guides the airflow that bypasses the core. Finally—and most important—a second, similarly sized duct wraps around the first bypass stream. By opening a valve to release air into this duct during cruise flight, the XA100 can increase bypass air flow around the engine core significantly.

Next to the fan frame, GE had laid out two versions of blades used in the second stage of a low-pressure turbine, one made of metal and the other composed of ceramic matrix composites (CMC). The edges of the metal blade, which is exposed to temperatures up to 2,400F in a normal fighter engine, are perforated by dozens of tiny holes, which allow cooling air to flow through the blade and help reduce blade surface temperature to 1,900F. By contrast, the CMC blade is 30% lighter despite being solid, with its material properties capable of enduring the heat of the exhaust gases without active cooling.

Roughly 10% of the XA100’s weight comes from additively manufactured parts, including intricately fashioned precoolers and heat exchangers that would be impossible to build using conventional fabrication processes.

Not visible inside the test chamber is the software inside the FADEC that controls the complex adaptive bypass flows. Over a 16-year period of simulations, the algorithms have matured. The software, which once required hours to manage the transition between cruise and dash modes in the engine, now can make instantaneous and seamless changes based on throttle inputs, GE says.