Why a Jet Engine Is Useless Standing Still (and Loves a Big Fan)
By Lucian — builder & engineer, LK Forge
A jet engine's overall efficiency is the product of two very different numbers: thermal efficiency, set by how hard the core compresses air before burning fuel, and propulsive efficiency, set by how closely the exhaust speed matches the aircraft's own flight speed. We swept the Jet Engine Lab's model across bypass ratio, flight speed, and pressure ratio for a 60 kg/s core airflow engine to measure exactly how bypass trades specific thrust for efficiency, why standing still wastes every bit of exhaust energy, and why raising pressure ratio lifts efficiency independently of either.
· 6 min read · measured from the bypass/speed/OPR benchmark
A bigger fan trades specific thrust for overall efficiency
Overall efficiency is thermal efficiency times component efficiency times propulsive efficiency. Bypass ratio doesn't touch the first two — it only changes how the same core power is spread across the exhaust: a low-bypass engine concentrates it into a small, fast jet, while a high-bypass engine spreads it across a much larger, slower one. A slower jet closer to flight speed wastes less kinetic energy behind the aircraft, which is why propulsive — and therefore overall — efficiency rises with bypass, even though the same engine now produces less thrust per kilogram of airflow moved.
At this lab's 250 m/s cruise speed and 30:1 pressure ratio, moving from a 0:1 to a 12:1 bypass ratio takes overall efficiency from 22.32% to 45.16%, while specific thrust falls from 0.768 to 0.12 kN per kg/s of airflow.
Overall efficiency versus specific thrust, across bypass ratio
Overall efficiency (blue, left axis) rises steadily while specific thrust (orange, right axis) falls, for the 60 kg/s core airflow engine at 250 m/s cruise, swept from a 0:1 to a 12:1 bypass ratio.
● overall efficiency (left axis) ● specific thrust (right axis)
Standing still, none of that thrust does useful work
Propulsive efficiency measures how much of the exhaust jet's kinetic energy ends up moving the aircraft, versus how much is left behind in the wake at a speed the aircraft never reaches. At zero flight speed there is no "behind" to compare against — the aircraft never advances into the energy the engine spent, so propulsive efficiency is exactly zero even while the engine produces enormous static thrust. As flight speed rises toward the exhaust's own speed, the gap the engine has to make up shrinks, and propulsive — and therefore overall — efficiency both climb.
Thrust (blue, left axis) and propulsive efficiency (orange, right axis) versus flight speed, for the same engine held at a fixed 6:1 bypass ratio.
● thrust (left axis) ● propulsive efficiency (right axis)
The exact numbers
Every row below is a direct sample from the lab's model: a 60 kg/s core airflow engine at a 30:1 pressure ratio, for the bypass-ratio sweep at 250 m/s cruise.
| Bypass ratio | Overall efficiency | Specific thrust | TSFC |
|---|---|---|---|
| 0:1 | 22.32% | 0.768 kN/(kg/s) | 26.04 g/(kN·s) |
| 1:1 | 28.42% | 0.489 kN/(kg/s) | 20.45 g/(kN·s) |
| 2:1 | 32.25% | 0.37 kN/(kg/s) | 18.03 g/(kN·s) |
| 4:1 | 37.09% | 0.255 kN/(kg/s) | 15.68 g/(kN·s) |
| 6:1 | 40.15% | 0.197 kN/(kg/s) | 14.48 g/(kN·s) |
| 8:1 | 42.3% | 0.162 kN/(kg/s) | 13.75 g/(kN·s) |
| 10:1 | 43.91% | 0.137 kN/(kg/s) | 13.24 g/(kN·s) |
| 12:1 | 45.16% | 0.12 kN/(kg/s) | 12.87 g/(kN·s) |
And the flight-speed sweep, at a fixed 6:1 bypass ratio:
| Flight speed | Thrust | Propulsive eff. | Overall eff. |
|---|---|---|---|
| 0 m/s | 155.94 kN | 0% | 0% |
| 80 m/s | 125.84 kN | 34.88% | 19.51% |
| 150 m/s | 105.07 kN | 54.6% | 30.54% |
| 220 m/s | 88.73 kN | 67.62% | 37.83% |
| 280 m/s | 77.58 kN | 75.25% | 42.1% |
| 340 m/s | 68.51 kN | 80.7% | 45.14% |
And the pressure-ratio sweep, isolating thermal efficiency alone:
| Pressure ratio | Thermal efficiency |
|---|---|
| 15:1 | 53.87% |
| 25:1 | 60.13% |
| 35:1 | 63.79% |
| 45:1 | 66.3% |
Engine: 60 kg/s core airflow, 30:1 default pressure ratio, 250 m/s cruise speed. Generated 2026-09-11.
Try it yourself
Open the lab, set your own core airflow, bypass ratio, pressure ratio, fuel flow, and cruise speed, and watch thrust, thermal, propulsive, and overall efficiency move in real time.