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December 2, 2022
F-35 Joint Strike Fighter (JSF) Engine Options
The Department of Defense (DOD) is considering whether
for bleed air (compressed air taken from within the engine)
to upgrade the F-35 Joint Strike Fighter’s (JSF’s) existing
during early development. However, engine capability
F135 engine—the Engine Enhancement Package (EEP)—or
design modifications and new requirements emerged during
to develop and procure a new engine for the aircraft—the
the F-35 Continuous Capability Development Delivery
Adaptive Engine Technology Program (AETP). Congress
(C2D2) program. The F-35 C2D2 program provides
has long expressed interest in issues relating to the F-35’s
“incremental ... improvements to maintain joint air
engine. Section 242 of the FY2022 National Defense
dominance against evolving threats.” He stated that “[t]o
Authorization Act (P.L. 117-81) required DOD to develop
provide the necessary bleed air, the engine was required to
an acquisition strategy for transitioning the engine of the
run hotter, and early engineering assessments suggest that
Air Force version of the JSF (the F-35A) to the AETP.
this increase in operating temperature could decrease
Section 243 required DOD to develop a separate acquisition
engine life, driving earlier depot inductions and an increase
strategy for transitioning the Marine Corps and Navy
in life cycle cost.”
versions (the F-35B and F-35C, respectively) to some form
of advanced propulsion.
A July 2021 Government Accountability Office (GAO)
report (GAO-21-39) stated that “[a]ccording to multiple
History of F135 Engine
service and program officials, challenges related to F-35
The F135 (see Figure 1)—designed and built by Pratt &
engine sustainment are currently affecting the program and
Whitney (P&W) of Middletown, CT—is the only engine
may pose its greatest sustainment risk over the next 10
that currently powers the F-35. (For more on the F-35
years.” The report described two issues affecting the
program, see CRS Report RL30563, F-35 Joint Strike
engine:
Fighter (JSF) Program, by John R. Hoehn.) DOD awarded
The need to “[remove] engines for unscheduled
P&W the F135 contract in 2001. P&W decided to derive
maintenance more often than expected, primarily to
the F135 from the F119 engine, which powers the Air
Force’s
repair the power module—a key component of the
F-22 fighter, to speed up the F135’s development.
engine that generates thrust for the aircraft to fly”; and
Figure 1. Diagram of F135 Engine
DOD’s ability “to repair only 43 percent of removed
power modules in 2020, thereby resulting in a backlog
of power modules needing repair.”
The report stated that these issues resulted in not meeting
goals for engine repair turnaround times, and that “DOD
recognizes that it lacks the capacity to make both
unscheduled and scheduled engine power module repairs at
the levels needed to support the F-35 program.”
An updated July 2022 report (GAO-22-104678) stated that
the “number of power modules needing repair was largely
Source: CRS adapted graphic from GAO report GAO-22-104678,
due to coating distress of the high-pressure turbine blades.
July 2022, p. 6, at https://www.gao.gov/assets/730/721771.pdf.
F-35 aircraft operations in dusty or sandy environments, as
well as the higher running temperatures, have caused
General Electric (GE) and Rolls Royce (RR), alternatively,
accelerated coating distress on the blades.” The report
collaborated to develop an engine for the JSF called the
further stated:
F136, and the F-35 program initially planned to use both
engines. The Navy ended its participation in the F136
Annual engine sustainment costs, a portion of total
program. Following DOD’s F135 contract award in 2001,
sustainment costs, have increased from $79 million
GE and RR continued to develop the F136. In FY2011,
in fiscal year 2016 to $315 million in fiscal year
Congress ended development funding for the F136, and GE
2020.... By fiscal year 2028, maintenance costs for
and RR announced in December 2011 that they would no
the F-35 aircraft engine are projected to be over $1
longer continue developing the F136.
billion annually. According to Pratt & Whitney
officials, scheduled maintenance has the potential
P&W has experienced design challenges with the F135
to be over 70 percent of total engine maintenance
engine, such as bleed air requirements and sustainability
issues. Air Force Lieutenant General Eric Fick, the Program
costs by 2030.
Executive Officer for the JSF program, testified in April
2022 that DOD originally defined the F135’s requirements
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F-35 Joint Strike Fighter (JSF) Engine Options
The report added that to address these F135 sustainment
GE contends that the XA100 can support both the F-35 and
issues, DOD is assessing two potential options: the EEP and
NGAD programs.
the AETP, reviewed below.
Figure 2. GE XA100 Engine
Overview of F135 EEP
P&W contends that the EEP would provide “an affordable,
low risk, and agile pathway to fielding meaningful
propulsion capability for all F-35 customers.” More
specifically, P&W states that the EEP would mitigate
current sustainment issues with the F135, meet the needs of
the Block 4 version of the F-35, and provide $40 billion in
lifecycle savings. GAO’s July 2022 report notes the EEP
“would result in an increase in capability, such as improved
range and thrust.” P&W states that if the EEP is required to
work with all three F-35 variants, “some degradation in
performance would be experienced to accommodate the lift
fan that is part of the engine for the [the Marine Corps’] F-
35B [variant].” DOD has not provided an estimate for how
Source: Photograph accompanying GE Aerospace, “Testing on GE’s
much the EEP would cost to develop. Some analysts
First XA100 Adaptive Cycle Engine Concludes, Proves Out
estimate the EEP’s development cost at about $2 billion.
Transformational Capabilities,” May 20, 2021, at
https://blog.geaerospace.com/technology/testing-on-ges-first-xa100-
Overview of AETP
adaptive-cycle-engine-concludes-proves-out-transformational-
AETP is the Air Force’s next-generation engine research
capabilities/.
and development program, intended for use with the
service’s envisioned Next Generation Air Dominance
P&W’s Version of AETP—the XA101
(NGAD) aircraft. As mentioned above, DOD is also
The XA101 reportedly is in testing as of September 2022.
assessing the potential for using AETP with the F-35.
P&W stated that “XA101 testing remains on track and
Typically, designers optimize jet engines for either fuel
aligned with the U.S. Air Force’s AETP development
economy (as in airliners and military cargo aircraft) or high
timeline.... P&W is committed to the continued maturation
thrust (as in fighters). By permitting changes to the bypass
of the technology suite in AETP, as it is foundational for
ratio (the amount of air that goes around the engine core
the sixth-gen [sixth-generation] capabilities needed for
compared with the amount that goes through the engine
NGAD family of systems in the 2030s.”
core), adaptive engine technology allows jet engines to
switch between fuel-efficient and high-thrust modes, as
Potential Issues for Congress
needed. Adaptive engines can also improve thermal
Congress may consider whether or not it should authorize
management, which can permit increased power generation.
and fund an upgrade to the F135 engine and consider the
Two companies are developing adaptive-engine
potential impact current F135 sustainment issues have on
technologies: GE (which is developing the XA100; see
F-35 readiness. In addition, Congress may also consider
Figure 2) and P&W (which is developing the XA101). A
whether
September 13, 2022, trade press report states that both the
it would be more cost-effective for the Air Force to
GE and P&W engines “are expected to increase the F-35’s
pursue EEP or AETP;
range by at least 25 percent, increase its thrust by 10
percent and double the power management compared to the
Congress has sufficient information on potential costs
F135.”
(including development, procurement, and life-cycle
operation and support costs), development risks, and
Secretary of the Air Force Frank Kendall testified in April
performance improvements of EEP and AETP to
2022 that he anticipated AETP offering a cost savings and
adequately compare and assess these two options for
an increase in capability (e.g., range and power generation)
upgrading the F135;
compared with the current F135. He testified that he
it might be more cost effective if the AETP were
anticipated that it would cost about $6 billion to transition
pursued to only apply it to the F-35A, or to also apply it
the AETP into production. Kendall has said that the Air
to the F-35B and/or the F-35C as well; and
Force could make a decision on whether to pursue AETP in
the FY2024 budget.
there are potential secondary impacts for other Air Force
aircraft if the AETP were pursued as an upgrade to the
GE’s Version of AETP—the XA100
F135. For example, AETP engines might reduce the
General Electric advertises the XA100 engine as increasing
requirements for aerial refueling capacity.
the F-35 aircraft’s range by 30%—achieving 25% from
improved fuel economy and 10% from increased thrust.
John R. Hoehn, Analyst in Military Capabilities and
According to defense press reporting, the XA100 engine
Programs
completed testing at Arnold Engineering Development
Patrick Parrish, National Defense Fellow
Complex in September 2022. In addition, testing for the
IF12262
XA100 engine reportedly demonstrated that the engine
could potentially be modified to power the F-35B aircraft.
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F-35 Joint Strike Fighter (JSF) Engine Options
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