Question

How does the specific thrust of a turbojet engine change for a given flight speed with increase in flight altitude?

• A. Increases monotonically
• B. Decreases monotonically
• C. Remains constant
• D. First increases and then decreases

Hint:

At higher altitudes, colder ambient temperature increases jet velocity faster than the thrust decreases due to density drop—so specific thrust always rises.

Answer 1

The Correct Option is A

Specific thrust is defined as the thrust produced per unit mass flow rate of air entering the engine.
\[ \text{Specific Thrust} = \frac{F}{\dot{m}} \] When altitude increases, two important atmospheric changes occur: reduction in air density and reduction in ambient temperature.
1. Effect of decrease in air density:
Lower density means a lower mass flow rate into the engine for the same inlet area and same flight Mach number. Since mass flow rate decreases, if the net thrust does not decrease proportionally, the specific thrust increases. In turbojets, the loss in thrust due to lower density is much smaller compared to the drop in mass flow.
2. Effect of decrease in temperature:
At higher altitudes, the air is significantly colder. Lower ambient temperature increases the stagnation temperature ratio inside the engine, resulting in a larger difference between jet exhaust velocity and flight velocity. Jet velocity rises because colder inlet air requires less energy addition to reach the same turbine entry temperature. This directly increases net thrust.
3. Combined effect:
While mass flow rate reduces with altitude, the increase in jet velocity more than compensates for it. Therefore, the ratio \(\frac{F}{\dot{m}}\) increases continuously with altitude. The relation remains monotonic and does not show any turning points.
Hence, the specific thrust increases monotonically as the aircraft climbs to higher altitudes.