Same gas generator core, three completely different ways of turning that core's power into thrust — and the split all comes down to one number: bypass ratio.
All three engine types share the same fundamental core: a compressor raises intake air pressure, fuel is burned in the combustor, and a turbine extracts just enough energy from the hot gas to keep driving that compressor. What each design does with the leftover energy is what makes it a turbojet, a turbofan, or a turboprop. A turbojet sends essentially all of it out the back through a single exhaust nozzle as high-velocity jet thrust. A turbofan uses an extra turbine stage to drive a large ducted fan, splitting the airflow into a fast "core" stream and a much larger, slower "bypass" stream around the outside. A turboprop takes that idea further, using nearly all of the core's spare power to spin an external propeller through a reduction gearbox, with only a small residual jet contribution. The single number engineers use to describe where an engine sits on this spectrum is the bypass ratio — the mass of air that goes around the core divided by the mass that goes through it.
Bypass ratio (BPR) is defined as the mass flow of bypass air divided by the mass flow through the core: BPR = ṁ_bypass / ṁ_core. A turbojet has BPR ≈ 0 — no bypass duct at all, every kilogram of air goes through the hot section. Modern high-bypass turbofans on airliners run BPR from roughly 5 up to 12 or more on the newest geared designs, meaning five to twelve times as much air flows around the core as through it. Pushed to its logical extreme — replace the ducted fan with an open, unshrouded propeller and drive it through a reduction gearbox — you effectively get a turboprop, whose "bypass ratio" in the same sense is enormous, often cited above 25 or 50. The underlying physics that explains every point on this spectrum is propulsive efficiency: it's highest when the exhaust (or propwash) leaves only slightly faster than the aircraft is already moving. A pure turbojet accelerates a small amount of air to a very high velocity — most of that kinetic energy squirts out the back unused. A high-bypass turbofan or a turboprop instead accelerates a much larger mass of air only a little, which is thermodynamically far more efficient — provided the aircraft is flying slowly enough that a fan or propeller can do that job without its blade tips going transonic and losing efficiency to compressibility drag.
It's a natural-sounding description, but it collapses a real engineering distinction. A turbofan's fan sits inside a shrouded duct (the nacelle), which lets its blade tips run supersonic locally without the efficiency collapse an open propeller would suffer at the same condition — that duct is precisely what lets turbofans cruise efficiently well above the speeds where turboprops become impractical, typically up to roughly Mach 0.85–0.9 on modern high-bypass designs. It is also not simply "a jet engine plus a fan" bolted on afterward — the fan is driven by its own dedicated low-pressure turbine stage, and the whole engine's core is sized specifically to produce exactly the shaft power that fan needs, not sized the way a standalone turbojet core would be. Meanwhile a turboprop's open, unducted propeller is a completely different and further extreme along the same bypass-ratio spectrum — mechanically similar to a turbofan in concept (a gas-generator core driving a downstream device that moves bulk air), but built and optimized for the very different, much lower speed regime where a duct isn't needed and an open propeller is simply more efficient and lighter.
Explains how turbojets, turbofans, and turboprops differ — not as three unrelated engine families, but as points on a single bypass-ratio spectrum — and why that ratio trades top speed for propulsive efficiency.
All three engine types share the identical core arrangement — compressor, combustor, turbine — so it is easy to assume the differences are cosmetic (a propeller here, a bigger fan there). In reality the differences follow directly from one design variable, bypass ratio, and understanding that one variable explains speed range, fuel efficiency, noise, and why each engine type dominates a different part of commercial and military aviation.
Bypass ratio (BPR) = mass flow of air bypassing the core ÷ mass flow through the core. A turbojet has BPR = 0: all intake air passes through the compressor, combustor, and turbine, and is expelled through a single nozzle at very high velocity. A turbofan adds a large fan at the front, driven by an additional low-pressure turbine stage, splitting flow into a bypass stream (moving around the core, mostly unheated) and the core stream. Low-bypass turbofans (BPR roughly 0.2–2, common on fighter aircraft) favor speed and afterburning capability; high-bypass turbofans (BPR roughly 5–12+ on modern airliners) favor fuel efficiency and quietness. A turboprop takes the bypass idea to its extreme: nearly all core power drives an external, unducted propeller through a reduction gearbox, with only a small fraction of thrust (commonly cited near 10%, engine-dependent) coming from the residual exhaust jet.
Propulsive efficiency describes how much of the energy imparted to the air actually becomes useful thrust power versus wasted kinetic energy left in the exhaust wake. It rises as the exhaust (or propwash) velocity approaches the aircraft's flight velocity — moving a large mass of air a little is more efficient than moving a small mass of air a lot, for the same thrust. This is why turboprops are the most fuel-efficient option at low subsonic speeds (large propeller, low exducted velocity), turbofans dominate at the ~Mach 0.75–0.85 airliner cruise regime (moderate bypass, moderate exhaust velocity), and turbojets — despite being the least fuel-efficient at low speed — remain viable at high supersonic speeds where fan or propeller blade tips would otherwise go transonic and lose efficiency to compressibility effects and shock formation.
Engine selection during conceptual aircraft design is driven directly by this spectrum: regional turboprops (BPR effectively very high) for short, low-altitude, low-speed routes where fuel burn per seat-mile matters most; high-bypass turbofans for the vast majority of commercial jet transports balancing speed, range, and efficiency; low-bypass (often afterburning) turbofans for fighter aircraft needing both efficient subsonic cruise and a supersonic dash capability; and pure turbojets, now largely superseded, historically used or still used in applications needing high-speed capability, particularly at high altitude, with less regard for fuel efficiency (early supersonic transports, some high-altitude reconnaissance aircraft, and many cruise missiles and target drones).
Not by conventional terminology, though the underlying principle (a gas-generator core driving a downstream device that accelerates a large mass of bypass air) is the same. The practical distinction engineers use is whether the fan/propeller is shrouded in a duct (turbofan) or open and unshrouded (turboprop) — the duct changes both the aerodynamics and the achievable flight-speed range substantially.
A larger fan needed for higher bypass ratio adds weight, frontal drag, and — critically — its blade tips reach the local speed of sound (become transonic) at a lower flight speed than a smaller fan would. Beyond that point, efficiency drops sharply due to shock losses at the blade tips, which is exactly why open propellers (the highest-bypass-ratio option) are limited to roughly Mach 0.6–0.7 and even ducted high-bypass fans are optimized for below about Mach 0.85–0.9.
An afterburner injects and burns additional fuel in the exhaust duct downstream of the turbine, where there is still leftover oxygen in the exhaust gas, producing a large but fuel-inefficient thrust boost. It is used almost exclusively on turbojets and low-bypass turbofans on military fighter aircraft, where the fuel penalty is acceptable for short bursts of supersonic dash or combat maneuvering performance.
Yes — even though nearly all useful thrust comes from the propeller, the core still exhausts through a nozzle, and that residual exhaust jet typically contributes a modest fraction of total thrust (often cited around 10%, though it varies by engine design). This is one reason turboprops are not purely "propeller engines" in the way piston engines are.
Turbofans, especially high-bypass ones, are markedly quieter than turbojets, because reducing the exhaust velocity mismatch with the surrounding air (mixing a large slower bypass flow with a smaller, faster core flow) directly reduces jet noise, which scales strongly with exhaust velocity. This noise reduction was one of the primary drivers behind the industry-wide shift from low-bypass turbojets to high-bypass turbofans starting in the 1960s–70s.
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