GATE 2025 AE – Question 61
An ideal turbofan with a bypass ratio of 5 has core mass flow rate, $\dot m_{a,c}$ = 100 kg/s. The core and the fan exhausts are separate and optimally expanded. The core exhaust speed is 600 m/s and the fan exhaust speed is 120 m/s. If the fuel mass flow rate is negligible in comparison to $\dot m_{a,c}$, the static specific thrust ($T/\dot m_{a,c}$) developed by the engine is ____ Ns/kg (rounded off to the nearest integer).
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Correct answer: 1200
Explanation
The bypass flow is $5 \times 100 = 500$ kg/s. At static conditions the flight speed is zero and both streams are optimally expanded, so $T = 100 \times 600 + 500 \times 120 = 120000$ N. The specific thrust is $\frac{120000}{100} = 1200$ N s/kg.