The Next Great Engine War Was Not What You Thought It Was
Since the BCA results were not made public, and the decisions were made at the most senior levels of the DoD, interviews of multiple senior DoD officials involved in the decision-making process or directly involved in the programs were conducted by the author. Many of the officials still serve in the same roles; therefore, identities will be anonymous for this article to ensure transparent responses. A few themes emerged when asked what the primary factors were in the decision not to field AETP to the F-35. Nearly all interviews revealed that the dominant factors were cost and tri-variant commonality. From a cost perspective, the development/integration costs of $6.7 billion were considered manageable; however, the procurement costs weighed heavily in the decision. Although the AETP engine costs were eventually projected to be only slightly more expensive than an F135, the F135 had the benefit of being 1,000+ engines down the learning curve (anonymous personal communication, January 17, 2024). This phenomenon, paired with the question of what to do with the in-service F135 engines, drove DoD to estimate AETP cost between $20 billion-$38 billion (depending on whether fuel savings were included) when compared to upgrading the F135 (anonymous personal communication, February 7, 2024). Tri-variant commonality was the second primary factor that weighed in the decision. All U.S. and international users of the F-35 maintain a configuration that maximizes commonality across all three variants, including the F135 engine. The F135 engine comes in two variants: a conventional take-off and landing (CTOL) version for the F-35A and F-35C, and a STOVL-compatible version, which includes a drive shaft to power the lift-fan, bleed-air ports to feed the roll posts, and a vectoring nozzle, though both versions share significant part numbers. Incorporation of AETP into the fleet threatened further division of conventional configurations, stranding the STOVL engine and diluting the benefits of international partnership and the global sustainment strategy (anonymous personal communication, January 17, 2024). Although cost and tri-variant commonality were the primary factors weighed in the decision, senior DoD decision-makers also considered the value of the operational benefits of increased range, thrust, power, and thermal capacity, and the value of re-inserting the threat of competition back into the F-35 engine program.
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Defense ARJ , Summer 2025, Vol. 32 No. 2: 104—130
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