Rocket Propulsion Questions and Answers Part-1

1. Which of the following is not an air-breathing engine?
a) Turbojet
b) Turbofan
c) Rocket
d) Scramjet

Answer: c
Explanation: Unlike turbojet, turbofan, scramjet, ramjet or turboprop engines, rockets are not air-breathing engines. It carries both fuel and oxidizer along with it, while the other engines suck in air to undergo combustion with the stored fuel.

2. Which of the following is not a valid classification of rocket engines based on their energy source?
a) Chemical
b) Geothermal
c) Solar
d) Nuclear

Answer: b
Explanation: Chemical, solar, nuclear, electric are all different types of rocket engines based on their energy sources. But geothermal energy source is limited locally on earth and can’t be harnessed for the whole course of the rocket’s journey.

3. What is the range of temperature of reaction product gases in a rocket engine?
a) 500 to 1700 °C
b) 1700 to 2500 °C
c) 2500 to 4100 °C
d) 4100 to 6900 °C

Answer: c
Explanation: The temperature at which product gases of the chemical reaction taking place within the combustion chamber is too high. It is even higher than the melting point of some of the more commonly used materials in aircraft industry like aluminum (660 °C). Appropriate cooling mechanisms are set up to ensure that this high temperature doesn’t transfer over to the rocket components and that it doesn’t lead to the development of thermal stresses.

4. Which of the following pressure feed systems is mostly used in attitude control of liquid propellant rocket engine with more than one thrust chamber per engine?
a) The Gas pressure feed system
b) Pump-fed system
c) Gravity assisted feed system
d) No feed system is used

Answer: a
Explanation: For attitude control of such vehicles, the thrust, as well as the total energy of the propulsion system should be low. Gas pressure feed system is suitable in such circumstances.

5. What is the typical liquid propellant feed system used in space launch vehicles?
a) The Gas pressure feed system
b) Pump-fed system
c) Gravity assisted feed system
d) No feed system is used

Answer: b
Explanation: In vehicles used for space launch, the thrust required and the overall energy of propulsion required is too high. In such cases, the rate of supply of propellants to the combustion chamber will be high. A pump fed system is capable of generating a large mass flow rate of the propellants. Hence it is used here.

6. For a constant thrust of 6000 N, find the total impulse for the burning time of 3s.
a) 36000 Ns
b) 18000 Ns
c) 9000 Ns
d) 0 Ns

Answer: b
Explanation: Here we assume that the start and stop transients are negligible during the operation of the system. Then, for a constant thrust (F), and for the total burning time of t seconds, the total impulse It = Ft.
It = 6000 x 3 = 18000 Ns

7. Suppose the thrust force of a rocket varies with time t as F = 250(3t2 + 2t). Then find out the total impulse for a burn duration of 4s.
a) 10000 Ns
b) 20000 Ns
c) 40000 Ns
d) 80000 Ns

Answer: b
Explanation: The total impulse can be obtained by integrating the force F with respect to time t from 0 to 3. The total impulse for a varying force F is obtained by It = ∫Fdt.
It = 250 x (tb3 + tb2) = 250 x (64 + 16) = 20,000 Ns.

8. For a rocket, the specific impulse was found to be 250s. Given that the thrust force is constant and assuming gravity-free and drag-free conditions, what will the thrust force be for a mass flow rate of the form (4t2-3t) kg/s and a burn duration of 3s?
a) 18375 N
b) 12426 N
c) 25234 N
d) 15595 N

Answer: a
Explanation: Specific impulse Isp = ∫Fdt/(go∫mdt) where m is mass flow rate, go is standard acceleration due to gravity at sea level.
Then Isp = Ftb/go(4tb3/3 – 3tb2/2).
Using tb = 3s, Isp = 250s, go = 9.8 m/s2,
F = 250 x 9.8 x 22.5/3 = 18375 N.

9. Given that the total impulse is 25000 Ns and specific impulse is 300s, find out the effective propellant mass for a burn duration of 8s.
a) 17 kg
b) 8.5 kg
c) 34 kg
d) 68 kg

Answer: b
Explanation: Given Isp = 300s and It = 25000 Ns.
Using Isp = It/mpgo, with go = 9.8 m/s2.
mp = 25000 / (9.8 x 300) = 8.5 kg.

10. Consider two rocket engines A and B with A having a specific impulse of 242s and B having a specific impulse of 150s. What can be said about their efficiency?
a) Both A and B are equally efficient
b) A is more efficient than B
c) B is more efficient than A
d) Efficiency doesn’t depend on specific impulse

Answer: b
Explanation: Higher specific impulse will result in better efficiency. Rocket A has a higher specific impulse than rocket B. Hence it is more efficient.