A jet engine's job is to accelerate air and turn that into thrust: mass flow rate times the difference between exhaust velocity and flight velocity. How well it does that job is overall efficiency, which splits into two multiplied factors — thermal efficiency (the Brayton cycle, set by the overall pressure ratio) turning fuel's chemical power into kinetic power in the jet, and propulsive efficiency (the Froude efficiency) turning that kinetic power into useful thrust power on the vehicle rather than wasted motion left in the exhaust. At a standstill thrust is huge but propulsive efficiency is exactly zero — the engine's oldest paradox. Raising bypass ratio trades specific thrust for a lower TSFC, the fuel-per-thrust tradeoff behind every high-bypass turbofan. Set the airflow, bypass ratio, pressure ratio, fuel flow, flight speed, and component efficiency, then watch it all play out live. It all runs on your device.
Fuel power in, thermal and propulsive losses out — and how much of it survives as thrust.
Flight speed—
Exhaust velocity—
Propulsive eff.—
Thermal eff.—
TSFC—
Specific thrust—
Phase—
Efficiency delivered
Loss split — fuel power vs thrust power
Engine schematic
Thrust & efficiency vs flight speed
Efficiency & specific thrust vs bypass ratio
Overall efficiency is a product of two factors, and one of them can be zero.Thermal efficiency, set by the Brayton cycle from the overall pressure ratio, is how much of the fuel's chemical power becomes kinetic power in the jet. Propulsive efficiency, the Froude efficiency, is how much of that kinetic power becomes useful thrust power on the vehicle instead of being left behind as wasted motion in the exhaust — it equals twice the flight speed divided by the sum of exhaust and flight speed, so at a standstill it is exactly zero even though thrust — mass flow times exhaust velocity minus flight velocity — is at its highest. Raising bypass ratio spreads the same core power over more total airflow, lowering exhaust velocity toward flight speed to cut TSFC and raise propulsive efficiency, at the expense of specific thrust per unit of airflow.
Reading the simulation
What the engine schematic, the loss-split bar and the two plots are telling you — then what the model says about bypass ratio.
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The schematic and the efficiency-delivered gauge
The schematic shows core airflow burning through the compressor, combustor and turbine while bypass airflow slips around it, both streams joining the exhaust. The gauge beside it tracks overall efficiency — how much of the fuel's chemical power actually survives as thrust power once thermal, component and propulsive losses are subtracted (the same split the loss bar shows).
2
The loss-split bar and the two efficiency factors
The loss-split bar breaks the fuel's chemical power into three losses that never show up as a single number — thermal loss, component loss, and propulsive loss — plus what's left as delivered thrust power. Widen any slice and overall efficiency falls, since it's the product of thermal, component, and propulsive efficiency together.
3
The speed sweep and the bypass-ratio curve
The first plot ramps flight speed from a standing start up to cruise speed, tracking thrust falling and propulsive efficiency rising together — the standstill paradox made visible. The second plot draws overall efficiency and specific thrust against bypass ratio and marks where the current design sits, showing why high-bypass turbofans trade thrust density for fuel economy.
How It Works
Two efficiency factors multiplied together, three losses that eat into them, and a bypass dial that trades one for the other.
Fuel's chemical power enters the core & nozzle, where three losses are subtracted before what's left reaches the vehicle as thrust power: thermal loss from the Brayton cycle, component loss in the compressor and turbine, and propulsive loss from kinetic energy the jet leaves behind in the exhaust. Raising bypass ratio spreads the same core power over more total airflow, trading specific thrust for a lower fuel burn per unit of thrust.
1
Thrust: mass flow times velocity change
Thrust is the total exhaust mass flow rate — core airflow plus bypass airflow plus the fuel burned into the stream — multiplied by the difference between exhaust velocity and flight velocity. Either pushing more air or pushing the same air faster relative to the vehicle raises thrust; a jet engine's whole job is accelerating the air passing through it.
2
Thermal efficiency: the Brayton cycle
Thermal efficiency is set by the overall pressure ratio through the Brayton-cycle relation: one minus the pressure ratio raised to the negative of gamma-minus-one over gamma. A higher pressure ratio squeezes more useful kinetic power out of the same fuel, which is why modern engines keep pushing OPR higher — component efficiency in the compressor and turbine then takes its own cut of whatever the cycle delivers.
3
Propulsive efficiency and the bypass tradeoff
Propulsive efficiency, the Froude efficiency, is twice the flight speed over the sum of exhaust and flight speed — it is zero at a standstill and climbs as exhaust velocity approaches flight velocity. Raising bypass ratio spreads the core's kinetic power over more total airflow, lowering exhaust velocity toward flight speed to cut TSFC and lift propulsive efficiency, but it also lowers specific thrust per unit of airflow, which is why a high-bypass turbofan needs a larger engine for the same thrust a low-bypass turbojet delivers.
Classroom activity
A ready-to-assign lab using the simulation above. Free to use — nothing to install or sign up for, and no student data leaves the browser.
High school, College10–20 minInteractive simulation
Learning objectives
Explain how a jet engine produces thrust.
Name the stages of the Brayton cycle.
Relate thrust to airflow and exhaust speed.
Variables to change
Airflow rate
Compression ratio
Fuel flow
Procedure
Run the engine and follow air through intake, compressor, combustor, turbine, and nozzle.
Increase fuel flow and observe thrust and exhaust speed.
Change the airflow and observe the effect on thrust.
Observations
Record thrust versus fuel and airflow, and note where air is compressed, heated, and expanded.
Questions
How does a jet engine create thrust?
Name the Brayton-cycle stages.
Write thrust in terms of airflow and speed.
What does the turbine do?
Why do high-bypass turbofans improve efficiency?
Explanation
A jet engine follows the Brayton cycle: it compresses incoming air, burns fuel to heat it, and expands it through a turbine and nozzle. Thrust ≈ ṁ·(v_exhaust − v_inlet) — accelerating air backward pushes the engine forward.
Answer key (for instructors)
1. By accelerating a mass of air rearward; the reaction force pushes the engine forward (Newton’s third law).
2. Intake, compression, combustion (heat addition), expansion through the turbine, and exhaust.
3. Thrust ≈ ṁ·(v_exhaust − v_inlet) — mass flow times the increase in speed.
4. Extracts energy from the hot gas to drive the compressor.
5. They move more air at lower speed, which is more efficient for subsonic flight.
Educators: link or embed this simulation freely in your LMS or course guide.
How does a jet engine produce thrust?
Thrust equals the exhaust mass flow rate — core airflow plus bypass airflow plus the fuel that's burned into the stream — multiplied by the difference between exhaust velocity and flight velocity. The engine's job is to accelerate the air passing through it; the faster that air leaves relative to how fast the engine itself is already moving, the more thrust the same mass flow delivers. This is Newton's third law in action: the engine pushes the air backward and the air pushes the engine forward with equal force, which is why thrust falls as flight velocity rises even at constant engine settings — the exhaust velocity stays fixed while the exhaust-minus-flight speed difference driving thrust shrinks.
What is overall engine efficiency, and why does it split into two factors?
Overall efficiency is thermal efficiency times propulsive efficiency (component efficiency also enters as a third factor). Thermal efficiency, given by the Brayton cycle from the overall pressure ratio, is how much of the fuel's chemical power becomes kinetic power in the exhaust jet. Propulsive efficiency, the Froude efficiency, is how much of that jet's kinetic power actually turns into useful thrust power on the vehicle rather than being left behind as wasted motion in the exhaust. Losing energy at either stage lowers the product.
Why is static thrust the highest an engine produces, yet the least efficient?
At zero flight speed, thrust is exhaust mass flow times the full exhaust velocity — as large as the engine's design allows. But propulsive efficiency is twice the flight speed divided by the sum of exhaust and flight speed, so at zero flight speed it is exactly zero: none of the jet's kinetic power does useful work on a stationary vehicle, all of it stays behind in the exhaust stream. An idling engine on the ramp makes its biggest thrust number and its worst efficiency number at the same time.
What is bypass ratio, and why do most airliners use high-bypass turbofans?
Bypass ratio is the airflow that skips the combustor and passes around the core, divided by the core airflow that actually gets burned. Raising it spreads the same kinetic power from the core over a larger total mass flow, which lowers exhaust velocity toward flight velocity — raising propulsive efficiency and cutting fuel burned per unit of thrust — but it also lowers specific thrust, the thrust delivered per unit of total airflow, so a high-bypass engine needs a larger, heavier nacelle for the same thrust. Airliners favor low fuel burn at cruise; fighter jets favor the thrust density of a low-bypass turbojet or afterburning turbofan instead.
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