The F135 Jet Engine’s History
The Pratt & Whitney F135 Turbofan Jet Engine entered low-rate initial production (LRIP) in 2009 and was selected as the preferred jet engine for the F-35 Joint Strike Fighter Program by the U.S. Air Force in 2001. The F135 was a variation of the F-22 Raptor’s P&W F119 engine.
To start the process of choosing a prime military contractor for the future F-35 Lightning II, the Pentagon’s Joint Strike Fighter Program (JSF) published a Request for Information (RFI) in 1994.
RFIs were submitted in 1995 by British Aerospace, McDonnell-Douglas, Northrop-Grumman, Boeing, and Lockheed-Martin. Each of the five contractors was requested to specify which prototype powerplant from a jet engine manufacturer best fit their JSF vision.
Only one prime contractor chose General Electric’s suggested powerplant, while four chose the Pratt & Whitney F135 engine. Boeing and Lockheed-Martin were down-selected by the JSF Program Office in 1997 after both companies had selected the P&W powerplant, the F135 predecessor.
In actuality, businesses other than Pratt & Whitney and Lockheed-Martin faced a formidable obstacle. The Defense Advanced Research Project Agency, or DARPA, had awarded Lockheed-Martin’s Skunk Works in Palmdale, California, a classified research and development contract a number of years prior.
In order to replace the Marine Corps’ outdated fleet of AV-8B Harrier II jump jets, DARPA commissioned the Skunk Works to provide a roadmap for a new STOVL fighter. P&W was selected by the Skunk Works to collaborate with them on the DARPA project.
The scope of DARPA’s technologies is astounding. The Manta Ray sub-surface UAV Program from Northrop Grumman serves as one illustration.
When the JSF Program took off a year or two later, they were searching for a fighter jet that would satisfy the needs of the USAF, be able to fly off a Navy aircraft carrier, and serve as the new STOVL fighter for the Marine Corps. It goes without saying that the JSF piqued the interest of both Pratt & Whitney and Lockheed-Martin.
Pratt & Whitney and Lockheed Martin, two seasoned defense firms, had an advantage thanks to the DARPA initiative.
Referring back to JSF’s down-selection ruling from 1997, each contractor was required to construct two concept demonstrator prototypes, or CDPs. Boeing’s CDP was called the X-32 (shown below), while Lockheed Martin’s was called the X-35.
The contract was awarded to Lockheed-Martin and Pratt & Whitney in 2001 following a multi-year flight test program on both aircraft concurrently. The aircraft and jet engines were contracted separately in accordance with Pentagon protocol. As government-supplied equipment (GSE), P&W would manufacture the F135 turbofan engine, sell it to the JSF, and then ship it to Lockheed-Martin.
The Pentagon’s dilemma with two producers of jet engines
Other companies in the globe produce jet engines, but they are not used in American jet fighters, which are vital national security assets. Fighter jets would be heavily loaded if the United States entered a multidomain shooting conflict similar to the one in Vietnam.
There would be a huge supply chain breakdown if the Pentagon relied on a single American producer of high-performance jet engines to support its fleet of fighter jets, including a significant ramp-up in manufacturing.
Lack of production capacity that could be rapidly ramped up would be a major national security issue, even if the Pentagon only had enough work for one fighter jet engine manufacturer during peacetime. Both Pratt & Whitney and General Electric must be able to produce fighter aircraft engines in America.
To maintain both engine makers’ jobs and profitability, the Pentagon fighter jet program community must perform a careful balancing act.
Over the past 60 years, Pentagon program managers have realized that attempting to purchase jet engines from both G.E. and P&W at the same time for a new fighter plane that is still in development is a recipe for problems.
Getting one engine OEM completely integrated into a new aircraft, much alone two, is a challenging task. When it comes to new aircraft programs, the Pentagon attempts to let each business be the primary engine OEM.
Pentagon program managers will work with the non-prime engine manufacturer to certify their product for use if a particular fighter jet, such as the F-16, remains in production for a sufficient amount of time. There is no plug-and-play interchangeability between the two engines.
Both a P&W and a G.E. version of the F-16 exist. Lockheed-Martin is required by the Air Force to construct the F-16 in “production blocks,” which allow them to differentiate between batches of aircraft with engines from one OEM and another.
Here’s an illustration: The aircraft underwent a major upgrade in the mid-1980s, which resulted in the F-16C and F-16D replacing the F-16A single-seaters and F-16B two-seaters. These were the first F-16s to use the P&W F100 engine in addition to the original G.E. F110 engine.
Because the two engines are not interchangeable either electronically or physically, the F-16C/Ds have Block 52 engines with P&W and Block 50 engines with G.E.
For more than 20 years, General Electric and the JSF program office have been considering this situation. Lockheed Martin and the program office were unable to validate G.E.’s alternative engine for the F-35 due to an excessive number of open action items.
A completely different kind of jet engine is proposed by General Electric.
To their credit, G.E. suggested a turbofan engine type for the F-35 that went beyond a “me too” in an attempt to take market share away from P&W’s F135 engines.
Using “Adaptive Versatile Engine Technology” (AVET), they developed the XA100 engine. Although AVET has been thoroughly researched over the last 20 years, no fighter plane currently in production has given it any significant attention.
The turbojet served as the foundation for jet engine technology from World War II until the middle of the 1960s. Turbojets were underpowered, inefficient gas-guzzlers. For this reason, big airplanes like the Boeing B-52 had eight engines in the past.
The McDonnell F-4 Phantom II was the primary fighter plane in the 1960s. The plane’s two G.E. J79 afterburning turbojets could reach Mach 2.2. The gasoline tanks would be completely dry in eight minutes if the pilot continued to fly at that speed!
In active air combat, that type of flying profile is acceptable, but not in other situations. The flying envelope of large aircraft, including bombers, cargo planes, and airliners, does not require that kind of “instant speed.”
The turbofan jet engine was created as a substitute for the turbojet in subsonic aircraft. All of the air that the engine intake has collected is directed through the combustion chamber, or engine core, in a turbojet. Fuel and combusts are combined with any air passing through the jet’s core.
Because so little air travels through the core, a turbojet is regarded as a low-bypass engine. In contrast, large, civilian jets with turbofans have a disproportionately huge fan in front of the intake. 80% to 95% of the air surrounding the core is bypassed by the fan.
Compared to its turbojet ancestors, turbofans are renowned for their superior power, silent operation, and fuel efficiency. They are classified as high bypass engines.
It’s incredible that Boeing needed four turbojet engines to achieve performance standards and provide a sufficient safety margin in case one of the engines failed when it first flew the 747 jumbo jet in 1969.
Twenty-five years later, Boeing unveiled the 777 widebody plane, which could fly safely on just one turbofan engine despite having just two!
With one significant drawback, turbofans appear to be the solution for all jet aircraft: the majority of their intake air bypasses the primary combustion chamber.
More fuel is delivered into the combustion chamber, increasing thrust, if the aircraft has to accelerate quickly utilizing the throttle. Although the airspeed will rise, it won’t accelerate quickly unless more air is delivered into the combustion chamber.
For an airliner, slow acceleration would be acceptable, but not for a fighter plane. Fighter planes kept their low bypass engines to achieve optimal performance, but they are now turbofans to benefit from features other than acceleration.
Despite being a powerful and advanced motor, the P&W F135 engine of the F-35 is still a low-bypass engine. Despite being a turbofan, the F135’s engine is still not very efficient.
However, the Pentagon will not compromise performance for increased fuel efficiency when it comes to the Joint Strike Fighter’s intended function!
The origins of the experimental XA100 engine from General Electric
In short, the fighter jet community in the Air Force and Navy required a hybrid jet engine that combined the advantages of high-bypass turbofans and low-bypass turbojets. Consequently,
In 2007, G.E. and Rolls-Royce were awarded research and development contracts by the Air Force for the Adaptive Versatile Engine Technology (ADVENT) program.
For the following ten years, ADVENT carried on with a number of sub-level projects. Pratt and Whitney would eventually become part of the ADVENT program as well. The Air Force oversaw the program for the first few years without focusing on any particular aircraft.
Under the Next Generation Air Dominance (NGAD) program, this non-specificity status ultimately came together in 2016 as a new engine for the sixth generation fighter. The NGAD program is the center of G.E.’s adaptive cycle engine, the XA100.
In 2018, the F-35 reengining endeavor was added to the ADVENT program. G.E. proceeded with the development of the XA100. In order to compete with the P&W F135 engine, they had also been striving to qualify their F136 low bypass ratio turbofan.
ADVENT’s R&D findings were promising. When the Air Force Life Cycle Management Center (AFLCMC) advised G.E. to take funds away from the NGAD program in order to qualify the XA100 for the F-35, it was very encouraging. In order to create their version of an adaptive flexible jet engine, P&W was given the XA101.
In 2021, G.E. concluded their XA100 testing program. G.E. was awarded a contract by the USAF life cycle engineers to begin production of the engine, which would be used on the F-35A and C variants beginning in 2027.
When the USAF decided to integrate the G.E. XA100 with the F-35 fighter in 2023, things took a different turn. Concerns were raised about the XA100’s engineering integration with the “B” and “C” F-35s, as well as the exorbitant cost of conversion. For the NGAD Program, the XA100 still has the inside track.
After the Air Force completed testing P&W’s Engine Core Upgrade (ECU) for its F-135 turbofan engine, they were more receptive to the USAF’s decision to postpone using the XA100. In 2029, the F-35 fleet will be introduced.
Initially, G.E. was interested in qualifying their F136 engine, which is comparable to P&W’s F135 engine, for the F-35. They now have no contracts that bring them money, and it’s unlikely that the XA100 ADVENT engine for the sixth generation NGAD fighter will turn a profit until around 2030.
Highlighting the primary distinctions between the F135 and XA100
The primary distinction is that the P&W F135 is exclusively a low-bypass turbofan engine. The XA100 is classified as a variable or adaptive cycle engine by G.E.
The XA100 is a hybrid engine that is intended to operate effectively in a variety of flight profiles, including above Mach 1, high sub-Mach 1 (transonic), and subsonic (0.3 Mach –.75 Mach), as opposed to being a high bypass turbofan or low bypass turbofan. The XA100 would have to function as a high bypass turbofan due to the subsonic flight
Engine modes of partial low bypass turbofan and partial high bypass turbofan are determined by transonic speeds between.7 and.95 Mach. The XA100 is a pure, low-bypass turbofan that would be used in an afterburner at supersonic speeds.
When utilizing a G.E. XA100 adaptive cycle turbofan engine in the same flying envelope as the P&W F135 low bypass turbofan, the plane’s range increased by 30%. Acceleration was 20% better with the XA100.
By switching from a high-performance, low-bypass turbofan to a high-bypass turbofan, the XA100 can increase fuel efficiency by 25%.
The F135 is an example of a low bypass jet engine, such as a turbojet or turbofan, that struggles to control its thermal profile.
Either the engine is operating too hot or not hot enough. The thermal profile of the XA100 is 200% more regulated. Component failures brought on by excessive heat cycling lower the MTBF.
The XA100 has undergone some flight testing, but not on an F-35. The F135 and XA100 differ greatly in their physical characteristics, which is the issue.
The XA100 weighs 6,400 pounds, whereas the F135 weighs roughly 4,000 pounds. The XA-100 is six feet wide and the F135 is not quite four feet in diameter. Finally, the F135 is two feet shorter than the XA100, which is 20 feet long.
I know from my engineering experience that it would be extremely challenging to convert the XA100 to the F-35 Lightning II.
A clean sheet design, such as NGAD, might be preferable than integrating the new engine. According to USAF cost estimates, integrating the XA100 into the F-35 would cost twice as much as integrating the F135 into the F-35.
Although P&W’s F135 Engine Core Upgrade (ECU) has been successful, I doubt that Elon Musk and Vivek Ramaswamy would approve spending a billion dollars on it given the white papers and reports that the Congressional Research Service, Government Accountability Office, and a number of nonprofit organizations, including Citizens Against Government Waste, have been publishing about the risk of doing so.