# Spaceflight Mechanics Questions and Answers – Rocket Propulsion – Optimal Rockets

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

1. What is a feature of an optimal rocket?
b) Highest structural ratio efficiency
c) Highest propellant mass ratio efficiency
d) Highest thrust

Explanation: Optimal rockets are those rockets which can carry maximum payload with least non-payload mass upto the burnout velocity. The payload mass is maximum for a particular total velocity impulse applied.

2. For an optimal rocket, the specific impulse and structural ratio can have any values.
a) True
b) False

Explanation: In case of optimal rockets, there is a unique set of structural ratio and specific impulse for each stage. Unless these specifications met, the total payload ratio is not maximum.

3. For optimal rockets, what should be the value of specific impulse of initial stages compared to later stages?
a) Zero
b) High vaue
c) Low value
d) Infinite

Explanation: In order to achieve optimal staging it is important to have lower values of specific impulse for initial stages compared to the later stages. This is because the lower stages are designed to operate in Earth’s atmosphere thus requiring higher efficiency. We trade-off between the efficiency and specific impulse in order to get higher thrust for initial stages. For later stages, it operates in vaccum thus it has a higher specific impulse. This would result in a higher delta-v.

4. Similar stages with same size and properties yield different echaust velocities.
a) True
b) False

Explanation: For achieving optimal staging, it is important for the similar stages with similar payload mass fraction, structural mass fracting to result in similar delta-v. Although in real life, this is not the case. It is imperative to split the velocities unevenly according to the trajectory.

5. In which type of staging, the specific impulse of all the individual stages are same?
a) Restricted staging
b) Optimal staging
c) Parallel staging
d) Serial staging

Explanation: In case of restricted staging, unlike optimal staging, all the the stages have the same value of specific impulse, structural ratio and payload ratio. One important aspect of restrcited staging is that total mass of initial stages is larger than the following upper stages.

6. How many stages of a rocket in considered to be an optimum choice?
a) 1
b) 2-3
c) 5-6
d) 8-10

Explanation: Intuitively it seems that more the the number of stages, more is the efffective exhaust velocity. But that is not true as when the number of stages go beyond three, the overall weight including the structural and propellant mass increase. This also results in increased complexity in design making it uneconomical.

7. In which case does the optimum velocity increment occur provided exhaust velocities and propellant are same for all the stages?
a) Payload ratios of all stages are equal
b) Structural ratios of all stages are unequal
c) Mass of each stage is unequal
d) Structural coefficient of all stages are unequal

Explanation: In case of multi-stage rocket, if it is assumed that the exhaust velocity and th propellant is same for all the stages, there’s an optimum velcoty increment achieved when the payload of all the stages are equal.

8. Which of these ratios is a measure of usefulness of the rocket?
a) Structural ratio
c) Efficiency ratio
d) Mass ratio

Explanation: The rocket’s measure of usefulness is found out using the payload ratio. It is the ratio of mass of payload to the mass of the rest of the rocket.
L = $$\frac{M_P}{M_S+M_F}$$
Where, L is the payload ratio (for a single stage)
MP is mass of the payload
MS is the structural mass
MF is the mass of fuel/propellant.

9. Which of these ratio is the measure of the degree of optimization for the enginerring design?
a) Structural ratio
c) Efficiency ratio
d) Mass ratio

Explanation: The rocket’s measure of the degree of optimization for the enginerring design is found out using the structural ratio. It is the ratio of structural mass to the combined mass of both the propellant and the structure. It is given by the formula:
σ = $$\frac{M_S}{M_S+M_F}$$
Where, σ is the structural ratio (for a single stage)
MS is the structural mass
MF is the mass of fuel/propellant.

Sanfoundry Global Education & Learning Series – Spaceflight Mechanics.

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