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to allow for thrust growth (Murch & Bolkcom, 2008). The OSD CAIG’s prediction turns out surprisingly accurate during the future F135 and Adaptive Engine Transition Program decisions, which will be described later. Independent reports from the GAO and Institute for Defense Analyses similarly concluded that a financial break-even point was uncertain; there were numerous nonfinancial benefits, including fleet readiness, contractor responsiveness, industrial base resilience, and more robust international relations (Murch & Bolkcom, 2008). Despite these potential benefits, Congress relented, and the F136 program was canceled. From 1994 to 2009, P&W received $7.3 billion for F135 work, while the GE/Rolls Royce (RR) team received $2.4 billion for F136 work (Gertler, 2012). The Adaptive Engine War (GE’s XA100 vs. P&W’s XA101) In 2007, the DoD chose to eliminate funding for the F136 in the President’s Budget request and instead initiated the Adaptive Versatile Engine Technology (ADVENT) program through the Air Force Research Laboratory (AFRL). The program was intended to support technology development from initial design to a full-engine demonstrator. Unfortunately, the funding provided would require a down-select after design and allow for only one contractor to proceed from design to building the demonstrator. The program office described the vision for the new program as “multi-point design engines that automatically adjust fan & core airflow and pressure for optimized performance & fuel efficiency at all flight conditions.” In other words, unlike previous engines optimized at a single design condition (typically cruise), these “variable cycle” engines could adapt airflows to provide maximum efficiency across various flight conditions, significantly decreasing fuel consumption. A stated program goal was to reduce fuel consumption by up to 25% (Hunter, 2021, p. 57). An important concept to enable these fuel efficiencies is the introduction of variable bypass. An engine’s bypass ratio refers to the amount of air flowing through an engine fan compared with the portion going through the combustor. Higher bypass ratios are generally found on engines designed for long-distance cruise, such as transport or passenger aircraft. Lower bypass ratios are generally found on fighters that require more thrust. A variable bypass is designed to change the bypass ratio from low
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Defense ARJ , Summer 2025, Vol. 32 No. 2: 104—130
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