# Rocket Propulsion Questions and Answers – Definitions and Fundamentals – Thrust

This set of Irrigation Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Definitions and Fundamentals – Thrust”.

1. If the exit pressure is thrice the atmospheric pressure for a rocket A and if it is twice for rocket B, what will be the ratio of pressure thrust of rocket A to that of B?
a) 1
b) 2
c) 3
d) 4

Explanation: Pressure thrust is the difference between nozzle exit pressure and ambient pressure multiplied by nozzle exit area.
Pe = 3Pa for rocket A
Pe = 2Pa for rocket B
Pressure thrust Tp = (Pe-Pa)Ae
So TpA/TpB = 2Pa/Pa
= 2.

2. In the expression of pressure thrust, what does the area (A) represent?
a) The cross-sectional area of the throat
b) The total surface area of the rocket casing
c) The cross-sectional area of the nozzle exit
d) The total surface area of the combustion chamber

Explanation: Area (A) represents the nozzle exit cross-sectional area. The expression for pressure thrust is (Pe-Pa)A, where Pe is the exit pressure and Pa is the ambient pressure.

3. If Pe represents exit pressure and Pa represents the atmospheric pressure, what is the condition for a nozzle to be overexpanded?
a) Pe < Pa
b) Pe > Pa
c) Pe = Pa
d) Pe = 0.5*Pa

Explanation: For the overexpanded nozzle, Pe < Pa is the correct condition. In this case, the exhaust jet pressure is lower than the atmospheric pressure and the flow will be suppressed in its size because of compression resulting from outside pressure. When that happens, our nozzle (which was designed for the optimal condition or correctly expanded flows, where Pe = Pa) will look as if it was bigger than what was required.

4. What will happen to thrust and specific impulse as the rocket is propelled to higher altitudes?
a) Thrust decreases, specific impulse increases
b) Thrust increases, specific impulse increases
c) Thrust decreases, specific impulse decreases
d) Thrust increases, specific impulse decreases

Explanation: Atmospheric pressure decreases with increasing altitude. So the pressure thrust part of the total thrust increases and hence the total thrust increases. Specific impulse is impulse per unit weight of the propellant and proportional to thrust, so it too will increase.

5. The rocket nozzle is usually so designed such that the exhaust pressure is ____ the ambient fluid pressure.
a) greater than
b) less than
c) equal to
d) equal to or slightly higher than

Explanation: Exhaust pressure should be such that it is equal to or slightly greater than the atmospheric pressure. Else the pressure thrust will be negative and lead to low overall thrust.

6. How does the total thrust vary with an extendable nozzle?
a) First increases, then decrease
b) First decreases, then increase
c) Keeps on increasing
d) Keeps on decreasing

Explanation: Initially as the nozzle is extended, thrust keeps increasing. This is because of the rate of increase in exhaust velocity being high compared to the decrease in exit pressure. This increase happens until the point where exit pressure equalizes atmospheric pressure. Afterward, the decrease in pressure thrust is more significant and as a result of this, the total thrust also decreases.

7. A rocket has a throat area of 10 mm2 and nozzle exit area of 25 mm2. If the mass flow rate of its propellants is 2 kg/s, exit velocity is 30 m/s, and if exit pressure is twice the standard atmospheric pressure at sea level, what will be the total thrust?
a) 60 N
b) 62.53 N
c) 61.26 N
d) 65.06 N

Explanation: Total thrust = momentum thrust + pressure thrust.
Momentum thrust = 2*30 = 60 N
Pressure thrust = (Pe-Pa)*Ae = 101325*25/1000000 = 2.53 N
Total thrust = 60 + 2.53 = 62.53 N.

8. Rocket A has twice the pressure thrust of rocket B while it has half the momentum thrust as that of B. If the momentum thrust of B is 5 times its pressure thrust, find the ratio of total thrust of A to B.
a) 0.5
b) 0.75
c) 1
d) 0.25

Explanation: Total thrust is the sum of momentum and pressure thrusts for a rocket engine. But for a gas turbine engine, it is the difference between the gross thrust and the ram drag.

9. Consider a rocket engine A and a jet engine B. Which of them will have higher specific impulse?
a) A
b) B
c) B doesn’t have a specific impulse
d) A doesn’t have a specific impulse

Explanation: B will have much higher specific impulse because it uses large amounts of outside air for combustion. Since specific impulse is impulse per unit amount of propellant and the stored propellant in jet engines is only the fuel (and not both fuel and oxidizer like in rocket engines), it will have a higher specific impulse.

10. If exhaust pressure is Pe and surrounding fluid pressure is Pa, what is the condition for the optimum expansion ratio of a rocket nozzle?
a) Pe=Pa
b) Pe<Pa
c) Pe>Pa
d) Pe=0.5*Pa

Explanation: For the optimum expansion of the nozzle, exhaust pressure should be equal to the ambient pressure. When that happens, the size of the rocket exhaust plume matches the size of the nozzle and losses are minimum.

Sanfoundry Global Education & Learning Series – Rocket Propulsion.

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