# Rocket Propulsion Questions and Answers – Ideal Rocket

This set of Rocket Propulsion Multiple Choice Questions & Answers (MCQs) focuses on “Ideal Rocket”.

1. For an ideal rocket, what should be the nature of the working substance?
a) Homogeneous
b) Heterogeneous
c) Anisotropic
d) Amorphous

Explanation: The working substance and chemical products should be homogeneous. When we say a substance is homogeneous, we intend to say that its composition and character is uniform throughout the substance.

2. What is the allowable amount of heat (in % of total available energy) that can be transferred across the walls of an ideal rocket?
a) 0%
b) 10%
c) 15%
d) 5%

Explanation: An ideal rocket doesn’t allow any heat transfer to take place across its walls. The flow, in this case, would be adiabatic. So, 0% of heat transfer takes place across the walls of an ideal rocket.

3. An ideal rocket has a chamber pressure of 1 MPa and a temperature of 1600 K. Determine the density of the combustion product if its molecular mass is 24 kg/kmol.
a) 1.2 kg/m3
b) 2.4 kg/m3
c) 1.8 kg/m3
d) 3.6 kg/m3

Explanation: Ideal rockets have working substances that follow perfect gas law. Hence using P=ρRT, we can determine the density of the combustion product.
R = Ru / Mmol = 8314 / 24 = 346.42 J kg-1K-1
ρ = P/RT = 1 x 106 / (346.42 x 1600)
= 1.8 kg/m3.

4. Which of the following statements is correct for an ideal rocket?
a) All kinds of frictional effects are not present, but the boundary layer cannot be neglected
b) All discontinuities are not present, but shock waves are allowable
c) Propellant flow is steady but constant
d) The expansion of the working fluid is steady, but need not be uniform

Explanation: In an ideal rocket, propellant flow is considered to be steady and constant. In such a vehicle, all kinds of frictional effects and boundary layers are neglected along with the possible discontinuities in flow such as shock waves. The expansion of the working fluid is both steady as well as uniform.

5. Select the correct option for an ideal rocket engine.
a) The exhaust gases leaving the rocket need not be axially directed
b) Across any cross-section normal to the nozzle axis, gas velocity need not be uniform
c) The gas composition can vary across the nozzle
d) Cryogenic propellants need to be stored at their boiling points

Explanation: Exhaust gases leaving the ideal rocket nozzle is axially directed. Gas pressure, temperature, and velocity needs to be uniform across the nozzle axis and the gas composition remains the same along with the nozzle.
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6. Which of the following relations won’t hold in an ideal rocket engine’s nozzle?
a) Isentropic expansion relations
b) Conductive heat transfer relations
c) Homogeneous flow relations
d) Ideal gas law relations

Explanation: The ideal rocket engine has adiabatic walls. Hence heat transfer cannot happen through the walls. At the same time, the nozzle flow obeys isentropic (reversible and adiabatic), homogeneous expansion relations and ideal gas laws.

7. For typical real rocket engines, what is the approximate amount of energy lost (from the available energy pool) due to heat transfer to the walls of the rocket?
a) 1%
b) 10%
c) 5%
d) 15%

Explanation: The energy loss by heat transfer to the walls is usually less than 1% and occasionally may go up to 2% of the total energy. Because of that, the heat transfer effects can be neglected in real rockets.

8. Which of the following is a reasonable depiction of the mass distribution in practical chemical rockets?
a) 50% propellants, 50% inert mass
b) 10% propellants 90% inert mass
c) 90% propellants 10% inert mass
d) 40% propellants 60% inert mass

Explanation: Much of the rocket mass is constituted by the mass of the propellants. Rest of the mass includes the rocket structure, its engines, and its payload.

9. Find the propellant mass fraction for a rocket having a structural mass of 150 kg, a payload mass of 20 kg and propellant mass of 1000 kg.
a) 0.65
b) 0.85
c) 0.95
d) 0.45

Explanation: The propellant mass fraction is the ratio of propellant mass to the initial mass of the vehicle.
Minert = Mstruct + Mpayload = 150 + 20 = 170 kg
Minitial = Minert + Mprop = 170 + 1000 = 1170 kg
Propellant mass fraction η = Mprop / Minitial = 1000/1170 ≈ 0.85.

10. Which of the following is not a feature of the combustion chamber in an ideal chemical rocket engine?
a) High pressure
b) High temperature
c) Low flow velocity
d) Higher transient time

Explanation: An ideal chemical rocket engine has high pressure, high temperature and low flow velocity. The transient time is very low to ensure that the flow and associated processes are steady and remains constant at all times.

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