AETP awards set to put sixth-gen U.S. combat engines in motion

Originally published in AviationWeek by Guy Norris on June 22nd 2016

The U.S. Air Force is poised to award General Electric and Pratt & Whitney contracts for adaptive cycle technology development that will pave the way toward an active procurement program for a sixth-generation fighter engine as well as the potential reengining of the F-35 Joint Strike Fighter.

Contracts for the Air Force Research Laboratory’s (AFRL) Adaptive Engine Transition Program (AETP) are expected to be valued at up to $1 billion apiece for the two engine-makers, setting the stage for a 21st-century version of the “great fighter engine war” between GE and Pratt over dual-sourced engines for the F-15 and F-16. Although Pratt now runs both key U.S. military development programs with the F135 for the F-35 and the engine for Northrop Grumman’s B-21 Long-Range Strike Bomber, AETP opens up potential competition for both the reengining of F-35s as well as proposed sixth-generation fighters for the U.S. Navy and Air Force.

AETP is specifically aimed at maturing three-stream engine technology now considered vital to achieving the high-speed, long-endurance performance requirements of the Navy’s future F/A-XX and the Air Force’s F-X sixth-generation fighters. Although it remains unknown whether the F/A-XX will emerge as a twin-engine design, the three-stream concept is designed to be scalable across a wide thrust range. The AETP is, however, targeted initially at a 45,000-lb.-thrust-class engine baselined to fit within the existing confines of the F-35A engine bay. This makes it a contender to replace the F135 from the mid-2020s onward.


AETP will test full engine versions of concepts, such as this GE adaptive engine, developed in the current AETD program. Credit: GE Aviation

AETP will test full engine versions of concepts, such as this GE adaptive engine, developed in the current AETD program. Credit: GE Aviation

AETP is scheduled to run through 2019 with several tests of full engines; it follows the Adaptive Engine Technology Development (AETD) program that is helping prove the basic viability of the adaptive cycle. AETD, which is set to wrap up between now and early 2017 with a series of demonstrations by GE and Pratt, itself builds on Advent (Adaptive Versatile Engine Technology), the Air Force’s pioneering research effort into variable cycle architectures, three-stream flowpaths and adaptive fans, conducted from about 2007 on.

The third stream provides an extra source of air flow that, depending on the phase of the mission, is designed to provide either additional mass flow for increased propulsive efficiency and lower fuel burn, or additional core flow for higher thrust and cooling air. It also can be used to cool fuel that provides a heat sink for aircraft systems. The third stream can also swallow excess air damming up around the inlet, improving flow holding and reducing spillage drag.

At the heart of adaptive engines are variable-geometry devices that 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 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. The third stream, which is external to both the core and standard bypass duct, is used to alter the bypass ratio.

“We have seen a huge amount of technology progress and a huge amount of risk reduction,” says AFRL Adaptive Engines program manager Matt Meininger. “The objective here is to burn as much risk down as you can before you walk into an acquisition program for a potential product in the mid-2020s. It is an exceptional accomplishment to not only demo the technology but to transition it as well. But we bridged that ‘valley of death’ [the gap between early prototype technology and readiness for further development] with AETD and the follow-on transition program, because adaptive engine technology was viewed as a significant capability and a cost saving for the future of the nation.”

AFRL paved the way for an early transition to procurement by partnering with the Air Force’s Life Cycle Management Center (LCMC) on AETD. “They are the development guys, if you will. If something is going to get out into the field, they will be the agents for that,” says Meininger. “As we move into the AETP program, which is an active procurement program, this transitions it out of the lab and into LCMC as the primary agency. They will mature this into a full design and deliver an engine to test.” To feed more technology into AETP, AFRL is also pursuing a follow-on effort called the Air Dominance Adaptive Propulsion Technology (ADAPT) program (see related story).

Pratt & Whitney's adaptive engine will include a three-steam fan design due to be tested soon on an F135. Credit: Pratt & Whitney

Pratt & Whitney’s adaptive engine will include a three-steam fan design due to be tested soon on an F135. Credit: Pratt & Whitney

The involvement of LCMC is key to transitioning the adaptive engine from research to full-scale development because currently the technology, or an engine incorporating the technology, “is not really a program of record. This is still all technology maturation and risk reduction,” he says. “The beautiful part is we are transitioning it to LCMC. Normally we would do a demo program, and when we are done it either sits on the shelf or works its way into a program of record.”

Meininger also notes, “We started this program together by developing a set of requirements that were robust and comprehensive. But we could not have done that without the partnership of the LCMC guys because they were worried about life cycle, and we worry about making it work. It has been a real education for our team.” The cooperation affected the trade studies for the life, cost and weight of the engine. “They are all as highly prioritized as meeting the performance requirements,” he adds.

GE, which concludes its AETD work in the coming months with a fan rig test and a core engine test in the third quarter at Wright-Patterson AFB, Ohio, says it “remains very engaged” with the Air Force on the AETP program and its timing. GE believes the program “would certainly include more design work, maturing the design to a Detailed Design Review (which is the next formal design milestone), and we envision several full-up engines being tested in the AETP program.”

USAF Details Sixth-gen Combat Engine Research Plan

The company enters the AETP phase with confidence, having successfully passed through a gauntlet of sophisticated tests for AETD. These included completion of two combustor sector development tests, third-stream cold flow and jet effects testing at NASA Glenn Research Center, advanced lightweight fan stator hardware evaluation and runs of an F414 with flaps and seals made from an oxide-oxide (ox-ox) ceramic matrix composite (CMC) material. Other components tested included an advanced heat exchanger and an F414 fitted with second-stage low-pressure turbine blades made from CMCs. Various rigs have also been run to test an advanced augmentor assembly, bearings, mechanical systems and a high-pressure compressor.

“Next year is a big one for AETD,” says Jimmy Kenyon, senior director of Advanced Programs and Technology at Pratt & Whitney, which is also undertaking AETD testing. The company demonstrated a three-stream fan in a rig in 2013. In early 2017, “We want to take the next step and demonstrate that in an engine environment, so that we get interaction between the fan and engine,” he says. “So we are taking the fan off an F135 engine, [then] putting the three-stream fan in its place and simulating three-stream flow back to the back.” Pratt also plans to demonstrate a “very high-efficiency core” on a test stand early next year, he adds.

“AETP will mature that design and go to a series of tests where we will ring out the engine in a true prototyping sense of the word,” says Kenyon. “We will mature it to put some time on it and that will be a huge risk reduction for any sort of follow-on EMD type program.”

AFRL also notes the different approaches taken by GE and Pratt, particularly on materials. “GE has made a big bet on CMCs for low-density, high-temperature materials as well as other areas, and Pratt is leveraging all their combat experience. They are leveraging what they have done in the past and evolving that through the AETD program. So we have two unique approaches to the core and material set and also two unique adaptive fans,” says Meininger.