U.S. military planners have now broadly accepted that the only way to meet the advanced performance needs of “sixth-generation” combat aircraft, barring changes to the laws of physics, will be the adoption of variable-cycle, or adaptive engine, technology.

The future fighter will be required to not only fly farther than today’s aircraft, but will also need more speed and power when engaging the enemy. But from a propulsion perspective, up until now these objectives have been mutually exclusive. Longer range and subsonic loiter require lower fuel burn and good cruise efficiency, while higher thrust for supersonic dash demands larger cores and much higher operating temperatures, neither of which is good for fuel burn or stealth.

To solve this conundrum and combine both capabilities in one propulsion system, engine makers are working under the U.S. Air Force Research Laboratory’s (AFRL) Adaptive Engine Technology Development (AETD) program to test technology for a new generation of engines that can be reconfigured in flight. Although AETD is set to end with a flight-weight core demonstration in 2016, the Air Force is planning a follow-on initiative called the Adaptive Engine Transfer Program (AETP). This will pave the way for an adaptive, 45,000-lb.-thrust-class combat powerplant for sixth-generation combat aircraft as well as the possible reengining of the Lockheed Martin F-35 in the 2020s.

The timing, budget and content of AETP remains in flux, but the program is still expected to begin sometime in 2016. The roughly three-year effort will mature adaptive engine technologies and reduce risk in readiness for a competitive engineering and manufacturing development (EMD) program.

Adaptable engines use an array of variable geometry devices to dynamically alter the fan pressure ratio and overall bypass ratio—the two key factors influencing specific fuel consumption and thrust. Fan pressure ratio is changed by using an adaptive, multistage fan. This increases the fan pressure ratio to fighter-engine performance levels during takeoff and acceleration, and in cruise lowers it to airliner-like levels for improved fuel efficiency. 

To alter bypass ratio, variable-cycle engines add a third airflow stream outside of both the standard bypass duct and core. The third stream provides an extra source of airflow that, depending on the phase of the mission, can be adapted to provide either additional mass flow for increased propulsive efficiency and lower fuel burn, or to provide additional core flow for higher thrust and cooling air for the hot section of the engine, as well as to cool fuel, which provides a heat sink for aircraft systems. During cruise, the third stream can also swallow excess air damming up around the inlet, improving flow holding and reducing spillage drag.

The move to AETP is the latest of a series of major Air Force-backed initiatives to steer the variable-cycle concept toward a generational change in fighter-engine performance. Starting in 2006, with the launch of AFRL’s five-year Advent (Adaptive Versatile Engine Technology) program, GE and Rolls-Royce North America each developed high-pressure-ratio cores and adaptive-fan, variable-bypass, low-pressure system technology. Aimed at cutting combat-engine specific fuel consumption (SFC) by 25% compared with early 2000 baseline fighter engines, Advent tackled key technology challenges including maintaining constant engine flow with variable-fan-pressure ratios and dealing with higher-than-ever hot-section temperatures. Advent also saw development of methods for modulating cooling air that was itself cooled, and newer, simpler exhaust system designs. GE expects to complete a detailed assessment of its Advent engine with AFRL sometime this month.

Rolls-Royce began Advent core tests in late 2012 but has not discussed further milestones. GE, which ran its Advent core test in 2013, exceeded AFRL’s temperature goal by more than 130F, achieving an Air Force-validated record for the highest combined compressor and turbine temperature operation “in the history of jet engine propulsion,” says the engine maker. “We found the core could efficiently generate the power that would be needed for a three-stream architecture,” says Daniel McCormick, GE Military Systems Operation, Advanced Combat Engine Programs general manager. “It validated the 25% fuel-efficiency goals set by AFRL which translates into a 30%-plus-range improvement for the platform,” he adds. The core, which notched up 60 hr. of testing, was followed by the first full three-stream demonstrator engine, which ran from November 2013 to July 2014.

The Advent turbofan test was “extremely successful,” but also revealed some unexpected and highly relevant information. “One of the things we found with adaptive engines is that modeling techniques for understanding the performance of adaptive cycles is well-tuned, but modeling adaptive three-stream engines is a little different. Some of it we got right and some of it [we have to reexamine]. The turbofan engine has been an extremely valuable tool for us,” notes McCormick.

Advent broke ground for the subsequent AETD effort, for which GE and Pratt were selected over Rolls in 2012.    The program is aimed at technology for a new combat-aircraft engine with 25% lower thrust-specific fuel consumption, but 5% more military power and 10% higher maximum thrust than Pratt’s F135. AETD therefore goes beyond Advent in terms of efficiency and power, and unlike the smaller cores used in the initial effort, which were aimed at B-2 bomber-type power ranges, is based around a larger core.

Although GE initially aimed to size its AETD engine to suit the future U.S. Navy F/A-XX and Air Force F-X sixth-gen fighters, respectively, the precise thrust requirements for these remain “very rubbery,” says McCormick. With major questions unresolved—whether these will be one- or two-engine aircraft, for example—GE “defaulted to the F-35,” as the basis for its engine plan. “It is a known entity and it is a challenging installation. From a company perspective, if this technology was to go forward, at least one of these opportunities could potentially be a fifth-generation aircraft and the F-35 will still be in production for a while,” McCormick says. “So the engine will be larger than the existing propulsion system in the F-35 but will have the same inlet. We have to figure out how to create more thrust in a much more fuel-efficient way throughout the envelope without a major architecture change.”

The AETP-based engine design “is more aggressive than today’s standard F-35 requirements but not to the level of [powering] directed energy weapons,” comments McCormick. Instead, the potential benefits of the third stream would be aimed at opening up the low-altitude/high-speed corner of the F-35’s flight envelope to enable extended operation at Mach 0.8/0.9 and 500 ft. “Today, the F-35 has flight restrictions at lower altitudes because of thermal management. You just can’t get heat off the airplane,” he adds. “The program we have laid out says you could be in the F-35 before mid-2020s. It depends on funding profiles and how big AETP is, but it’s early in the 2022-24.”

For true sixth-generation-fighter applications, however, the third-stream benefits will be channeled to supporting advanced weapons and systems, as well as performance. It is “almost a given that directed energy weapons will be in play for these future platforms,” says McCormick. He anticipates a power offtake requirement for at least 1 megawatt. “We are trying to define the design space. Under a portion of AETD, we are conducting next-gen trade studies. Money is flowing from AFRL to GE and we are funding the three aircraft manufacturers [Boeing, Lockheed Martin and Northrop Grumman] to work with us as part of these trade studies.”

“The vision of AFRL is that those studies will help weapon system contractors inform aircraft capability as an analysis of alternatives (AOA) progresses forward,” he says. “What’s in the art of the possible? That is evolving over time.” The AOA is in the planning stage and has not yet been launched by the Air Force or the Navy, he adds.

While GE hopes to employ a common-core strategy for these fighters, it already knows through discussions with the Air Force’s Life Cycle Management Center that there will be differences to the engine. It may have to be scaled. There is probably going to be a reasonable change to the low-pressure turbine architecture and, if the next-generation fighter becomes a twin, we won’t need a big airflow inlet,” he adds.

Despite the growth from a B-2- to an F-35-thrust-class engine, the AETD core size “hasn’t changed a lot,” says McCormick. The baseline core size is established for thrust in supersonic operation and for acceleration capability. “Both have continued to grow. When we go to a next-generation fighter type of airplane, that core size may be adjusted again if we go to a twin-engine design. ” The AEDT configuration outline outwardly resembles the architecture of the F136, the alternate engine developed with Rolls-Royce for the F-35 but axed in 2011. However “relatively speaking, it is a clean-sheet design,” says McCormick. The three-stage fan is significantly different as it includes variable geometry and adaptive features, while the external profile incorporates an annulus duct for the third stream. The high-pressure compressor is based on the 22:1 pressure ratio, 10-stage design used in the CFM Leap commercial engine, while the turbine and combustor are “dramatically improved” over the F136.

One of the greatest areas of advance, which GE believes to be a major trump card in the sixth-generation engine contest, is the extensive application of lightweight, heat-resistant ceramic matrix composite (CMC) materials. “The F136 had one part made of CMCs, the third-stage nozzle. Now CMCs [are] all the way back through the hot section from the combustor to the low-pressure turbine, including rotating parts,” says McCormick. The pioneering application of CMCs on a rotating stage was successfully tested on an F414 in late 2014 (see sidebar).

The third stream will also house two sets of heat exchangers. “One of the things we found with these high-compression-ratio fans and compressors is the air temperature is hot. So we have a cool-cooling air system before we pipe it back into the turbine section,” McCormick explains. Configuration details for the heat exchangers and other features of the scaled-up AETD will be defined in time for an engine system preliminary design review with AFRL in early March.

A detailed design review of the core engine was completed late in 2014, as were tests of a third-stream cold-flow and jet-effects rig at NASA Glenn Research Center, Ohio. Several component tests of lightweight stators made from polymer matrix composites (PMCs) were also completed. As with CMCs, GE intends to leverage extensive materials and aerodynamics advances made in its commercial powerplants, in its pursuit of the next generation of military engines.

“There is a company appetite to be ready,” says McCormick. “We have spent about $1 billion of GE’s money investing in technologies that specifically apply to AETD and what will follow on.” This comprises $600 million of corporate funds to support commercial-centric advances such as CMCs for the Leap, GE9X and Passport business jet engine; another $400 million of direct cost share were dedicated to R&D for military-specific technologies in Advent and AETD.

Following the completion of full annular combustor rig tests that began last November, GE is set for a busy 2015 with full-scale PMC component evaluations and runs of a high-pressure compressor rig at Wright-Patterson AFB, Ohio, to be conducted by year-end. Augmenter tests are scheduled to begin in December in the run-up to fan rig tests at Wright-Patterson in the first half of 2016. Although afterburner (augmenter and nozzle) technology development is not part of these adaptive-cycle technology demonstrator programs, McCormick says GE is “bringing today’s augmenter technology forward to see how we mix three streams of air back together at the back end of the engine.” GE’s AETD program will end following fan tests and runs of a core engine at its Evendale, Ohio, facility in 2016.

Pratt will meanwhile also conduct a preliminary design review of its AETD engine early this year. The manufacturer, which has released few details of its next-generation plan, expects to conduct demonstration testing of an advanced high-pressure-ratio core in early 2016, to be followed later in the year by full engine testing of a three-stream adaptive fan and compatible augmenter and exhaust system.

A version of this article appears in the February 2-15, 2015 issue of Aviation Week & Space Technology.