GATE 2021 AE – Question 16

Correct answer: (A) Increases monotonically

Explanation

For a turbojet at a **fixed flight speed**, the specific thrust (thrust per unit mass flow of air) depends on the cycle temperature ratio.

• As the altitude increases (in the troposphere), the ambient temperature $T_0$ **falls**.
• If the maximum cycle temperature (turbine inlet temperature) is held fixed, the ratio $T_{04}/T_0$ **rises**.
• A higher temperature ratio gives more energy per kilogram of air to the exhaust, so a higher exhaust speed and a higher specific thrust.

So the specific thrust **increases** with altitude (option A). (The absolute thrust still falls because the air density falls.)

Official GATE 2021 answer key: https://gate2026.iitg.ac.in/doc/download/Answer_keys2021/ae_2021.pdf