The concept described in this question closely mirrors what aerospace engineers have been developing for decades under the category of Variable Cycle Engines (VCEs) and, more recently, Adaptive Cycle Engines (ACEs). A conventional turbofan operates with a fixed bypass ratio — the ratio of air flowing around the core to air flowing through it — which is optimized for a narrow band of flight conditions. A turbojet, by contrast, has effectively zero bypass ratio, pushing all ingested air through the core, which makes it thermodynamically better suited for high-speed, high-altitude flight but severely inefficient at subsonic cruise. The hybrid concept posed in this question — an engine capable of shifting between these two configurations — is not theoretical. It is an active area of military and commercial propulsion development, and prototypes already exist.
The most advanced realization of this concept is GE Aerospace's XA100 Adaptive Cycle Engine, developed under the U.S. Air Force's Adaptive Engine Transition Program (AETP) as a candidate replacement for the F135 powering the F-35. The XA100 uses a three-stream architecture: a traditional core stream, a first bypass stream, and an outer third stream that can be opened or closed to dramatically shift effective bypass ratio in flight. In high-efficiency mode, the engine behaves like a moderate-bypass turbofan, reducing fuel burn during subsonic transit. In high-thrust combat mode, the third stream is largely closed and the engine behaves more like a low-bypass turbofan approaching turbojet behavior, delivering higher specific thrust. Pratt & Whitney developed a competing design, the XA101, based on similar principles. Variable-geometry fan stages, adaptive nozzles, and active cooling stream management are the engineering mechanisms that enable these shifts. The concept is mechanically complex but aerodynamically coherent.
For pilots and operators in business aviation and supersonic transport contexts, this technology has direct near-term relevance. Boom Supersonic's Overture airliner — aimed at Part 135 charter operators and scheduled carriers — is designed to cruise at Mach 1.7 using modified medium-bypass turbofan engines without afterburners, a deliberate compromise between subsonic efficiency and supersonic performance. The absence of a true variable-cycle capability in Overture's current engine selection is one reason analysts have questioned its economics on transatlantic routes, where significant subsonic transit time at climb and descent must be flown on engines optimized for a different regime. If adaptive cycle technology matures into commercial certification, it would allow an aircraft like Overture — or its successors — to fly efficient turbofan profiles during subsonic segments and shift toward low-bypass turbojet behavior during Mach cruise, fundamentally improving block fuel economics. The Concorde, by contrast, used pure Rolls-Royce/Snecma Olympus 593 turbojets with afterburners and consumed fuel at a rate that made it commercially viable only in a narrow niche, partly because no such adaptive capability existed.
The broader implication for the aviation industry is that the turbofan-turbojet binary is dissolving. Bypass ratio is increasingly treated as a variable rather than a fixed design parameter, and the same core engine architecture can serve subsonic efficiency and supersonic performance goals within a single airframe. This shift parallels other convergence trends in aviation propulsion — hybrid-electric architectures for regional aircraft, open-rotor designs for short-haul fuel efficiency — where the industry is moving away from engines optimized for a single point design condition toward systems that adapt across a flight envelope. For professional pilots operating or evaluating supersonic business jets such as the Aerion-era concepts or forthcoming platforms from Spike Aerospace and Exosonic, understanding bypass ratio variability and its effect on fuel planning, thrust management, and engine certification basis will become increasingly relevant as these technologies approach entry into service within the next decade.