# Spaceflight Mechanics Questions and Answers – Rocket Propulsion – Multi-Stage Rocket Engine

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

1. In which of these multi-stage rockets are the stages arranged serially?
a) Tandem stage
b) Parallel stage
c) Ordered stage
d) Sequential stage

Explanation: A multi-stage rocket is required to achieve the delta-v to enter the escape trajectory. These rockets can have each individual stage aligned in two ways. Tandem stage is where each stage is arranged serially and parallel stage is where the stages are arranged alongside one another.

2. In which of these multi-stage rockets are the stages arranged alongside one another?
a) Tandem stage
b) Parallel stage
c) Ordered stage
d) Sequential stage

Explanation: Staging in a rocket is essential to obtain the delta-v required to escape the Earth’s atmosphere. When the stages are arranged alongside each other it is known as parallel staging. The propellants are burned at the same time and has an advantage of getting rid of propellant mass quickly thus resulting in higher acceleration and delta-v.

3. Why is single stage rocket not used often?
a) Low exhaust velocity
b) Lesser fuel
c) High mass
d) Low delta-v

Explanation: The maximum delta-v of the rocket for a given fuel to dry mass ratio is too less for the single stage rockets. The delta- v which is required to reach low earth orbit requires a large wet to dry mass ratio which is not present in single stage rockets. This is overcome by making use of multi- stage rockets by jettisoning the stages when they run out of propellant. This staging allows the thrust of the remaining stages to accelerate easily and reach the final speed.

4. What is the delta-v of the rocket that has an initial mass of 250,000 kg with an exhaust velocity of 28.3 km/s if the mass of the rocket without the propellant is 150,000 kg?
a) 9.23 km/s
b) 15.85 km/s
c) 14.46 km/s
d) 11.17 km/s

Explanation: The rocket equation to calculate the delta-v is:
Δv = ve ln$$\Big(\frac{m_0}{m_f}\Big)$$
Where, ve is the exhaust velocity = 28.3 km/s, m0 is the dry mass (without propellant) = 150,000kg and mf is the wet mass (dry mass propellant mass) = 250,000 kg
Δv = 28.3 ln$$\Big(\frac{250,000}{150,000}\Big)$$ = 14.46$$\frac{km}{s}$$.

5. The lower stages of the rocket has higher specific impulse.
a) True
b) False

Explanation: The relation between thrust and specific impulse is given by:
T = Isp g0 $$\frac{dm}{dt}$$
According to this formula, higher is the specific impulse, more is the thrust produced. For a multi-stage rocket, the initial stages have lower specific impulse because there’s a trade-off between the thrust and efficiency. Subsequent stages have higher specific impulse as it needs lesser exhaust velocity due to low atmospheric pressure since it is above the atmospheric layer.

6. Which of these is not a dimensionless performance quantity to assess the performance of the rocket?
a) Initial to final mass ratio η
b) Structural ratio ∈
c) Aerodynamic ratio α

Explanation: There are three dimensionless performance parameters to compare the performance of the rockets. The first one is the ratio between initial and final mass which gives us the ratio between the rocket’s stage’s full initial mass to rocket’s final stage’s mass. The second parameter is the structural ratio which is the ratio between the empty mass of stage to the combined empty and propellant mass of the stage. The final parameter is the payload ratio which is the ratio between payload mass to the combined mass of empty and propellant mass.

7. Parallel staging rockets are more efficient than the serial staging rockets.
a) True
b) False

Explanation: Parallel staging rocket offers the advantage of getting rid of the propellant mass more quickly, thereby increasing the total efficiency when compared to serial staging. This is why most of the rockets in the recent times employ only parallel staging to improve the efficiency.

8. Space-shuttles employ which of the staging?
a) Tandem stage
b) Parallel stage
c) Ordered stage
d) Sequential stage

Explanation: Space-shuttles have 2 solid rocket boosters attached parallel. Both of these burn simultaneously at the time of the launch. This is combined with cryogenic engine. The boosters after having all of the propellants exhausted are jettisoned.

9. A rocket engine produces a thrust of 1,500 kN at the sea level with a propellant flow rate of 250 kg/s. What is the approximate value of specific impulse?
a) 612 sec
b) 552 sec
c) 200 sec
d) 255 sec

Explanation: Given, F = 1,500,000 N, $$\dot{m}$$ = 250kg/s
The specific impulse is calculated using the formula:
Isp = $$\frac{F}{\dot{m} g}$$ = $$\frac{1,500,000}{250×9.8}$$ = 612.24 sec.

10. Which of these propellants is used for upper stages in multi-stage rockets?
a) Gas propellants
b) Solid propellant
c) Liquid propellant
d) Cryogenic propellant

Explanation: The upper stages of the multi-stage rockets are lighter and require lesser thrust this is because there is absence of atmospheric drag at higher altitudes, because of increase in mass ratio and efficiency, liquid propellants such as the liquid oxygen and liquid hydrogen is used.

Sanfoundry Global Education & Learning Series – Spaceflight Mechanics.

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