# Spaceflight Mechanics Questions and Answers – Rocket Propulsion – Rocket Engine

This set of Spaceflight Mechanics Multiple Choice Questions & Answers (MCQs) focuses on “Rocket Propulsion – Rocket Engine”.

1. What does the propellant comprise of?
a) Fuel
b) Oxidizer
c) Fuel and oxidizer
d) Exhaust gases

Explanation: The rocket motor is a heat engine which converts the heat produced by burning the propellants composed of both fuel and oxidizer in the combustion chamber. This converts it into the kinetic energy from the exhaust gases. This results in thrust as produced by the engine.

2. What is the role of an expansion nozzle in the rocket engine?
a) Exhaust high velocity gas
b) Pumping fuel and oxidants
c) Mixing the propellants
d) Storing the propellants

Explanation: The role of the expansion nozzle in an engine is to convert the high-pressure hot gas that is generated in the combustion chamber into a high velocity exhaust stream. This results in providing thrust to the rocket.

3. What is the formula for the thrust produced by the rocket engine?
a) F = m $$\frac{dv}{dt}$$
b) F = mve
c) F = v $$\frac{dm}{dt}$$
d) F = dm dve

Explanation: Thrust produced by the rocket engine is the effective exhaust velocity multiplied by the actual mass floe in the exhaust stream produced by the engine. m is the mass flow rate and ve is the effective exhaust velocity.

4. What should be the pressure of gas at the end of the nozzle compared to the ambient pressure for its optimum performance?
a) Should be equal
b) Should be lower
c) Should be greater
d) Can be lower or greater

Explanation: For an optimum performance of the rocket engine, the pressure at the end of the nozzle should be equal to the ambient pressure. If it was greater than the ambient pressure, then the energy is wasted and if it was lower then rocket would be slowed down due to difference in pressure.

5. Which propellant is used by the thermal rocket engine?
a) Propellants in solid phase
b) Propellants in liquid state
c) Inert propellant
d) Propellant decomposed by catalyst

Explanation: Thermal rocket engines make use of inert propellants which are heated by electricity which is known as electrothermal propulsion or a nuclear reactor. Propellants in solid and liquid phase are used in solid-fuel rocket engine and liquid- fuel rocket engine respectively.

6. When the exit pressure of the nozzle is greater than the ambient pressure, then what is it called?
a) Grossly over-expanded
b) Perfectly expanded
c) Over-expanded
d) Under-expanded

Explanation: When the exit pressure is greater than the ambient pressure, the nozzle is said to be under-expanded. In this scenario flow expands outward after exiting the nozzle. This reduces the efficiency of the nozzle as external expansion does not exert any force on the nozzle wall. The nozzle with fixed area becomes under-expanded on reaching higher altitudes.

7. Which of these cases do not help in achieving a higher propellant efficiency in an engine?
a) Using high energy fuel
b) Using propellants which decompose to molecules with few DOF
c) Using low specific density fuel
d) Decreasing chamber pressure

Explanation: High propellant efficiency is achieved by heating propellant to a high temperature or using a high energy fuel, using a low specific density gas and using propellants which can be broken down o simple molecules with less DOF. The exhaust speed is unaffected by the chamber pressure.

8. Higher specific impulse implies lower efficiency of the rocket engine.
a) True
b) False

Explanation: Specific impulse is impulse per unit propellant which determines the efficiency of the rocket engine. Higher is the value of specific impulse, lesser the propellant is required to produce the desired impulse.

9. Which of these materials can sustain the high temperatures of the rocket’s exhaust?
a) Aluminum
b) Nickel
c) Tungsten
d) Steel

Explanation: There are two materials which can sustain the high exhaust temperatures which are graphite and tungsten. Aluminum, Nickel, Steel are common construction materials for the rocket where high temperatures are not achieved.

10. What is an advantage of a solid rocket engine?
a) Throttleable
b) No moving parts
c) Low temperature in combustion chamber
d) Solid fuel is inert without oxidizer

Explanation: One of the main advantages of a solid rocket engine is that it has a simple design with no moving parts. Hybrid rocket’s advantage is that it can be easily throttleable and for monopropellant rocket there’s low temperature in combustion chamber.

Sanfoundry Global Education & Learning Series – Spaceflight Mechanics.

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