Final answer:
Correct answer is (D) First increases and then decreases. The specific thrust of a turbojet engine typically first increases and then decreases with an increase in flight altitude due to changes in air density. The engine is most efficient at an optimal altitude where the air density allows for the best thrust generation, beyond which the thrust decreases due to thinner air.
Step-by-step explanation:
The specific thrust of a turbojet engine changes with an increase in flight altitude for a given flight speed. It is generally observed that specific thrust first increases and then decreases as altitude increases. At lower altitudes, the engine is more efficient because the air density is higher, which helps the engine to develop more thrust. However, as altitude increases, the air becomes thinner, and the engine cannot produce as much thrust since it depends on the mass of the air it intakes. For turbojet engines, there is an optimal altitude where the engine operates most efficiently. Beyond this altitude, the specific thrust decreases due to reduced air density.
According to Bernoulli's equation, if the pressure in a fluid is constant and the kinetic energy per unit volume increases, the potential energy per unit volume of the fluid must decreases. As for the rocket's acceleration, it is affected by the exhaust velocity of the gases (Ve), the rate at which mass is ejected from the rocket (Am/At), and the rocket's changing mass (m). The rocket's acceleration will tend to increase as the fuel burns and its mass decreases, hence optimizing the thrust.