# Rocket Propulsion Questions and Answers – Flight Performance – Propulsion System Effect on Vehicle

This set of Rocket Propulsion Questions and Answers for Experienced people focuses on “Flight Performance – Propulsion System Effect on Vehicle”.

1. Vehicle’s final velocity increment can be increased by using a _________ propellant.
a) more energetic
b) less energetic
c) highly unstable
d) highly stable

Explanation: Using a more energetic propellant increases the specific impulse of the vehicle. Higher specific impulse results in higher final velocity increment.

2. The mass ratio has a/an ____________ effect on the final velocity increment of a rocket vehicle.
a) logarithmic
b) exponential
c) linear
d) parabolic

Explanation: R = m0/mf is the mass ratio, where m0 is the initial mass and mf is the burnout mass.
&#8710u; is proportional to c ln(R) where c is the exhaust velocity.

3. Which of the following is not a way of reducing final mass mf of a rocket?
a) Using a lesser portion of propellant for flight
b) Using lighter structures
d) Using lighter guidance/ control devices

Explanation: If a lesser portion of propellant is used for the flight of the vehicle, then the residual propellant mass will be more. This means that the final mass mf will be more than usual.

4. Which of the following is minimum for increasing the initial mass of the rocket?
a) Increasing the thrust
c) Increase in structure
d) Increase in specific impulse

Explanation: The increase in structure or propulsion system masses is kept minimum for increasing the initial mass of the rocket. The effective mass ratio will be more if the structural mass is not kept minimum.

5. Which of the following can the gravitational losses?
I) Reducing the burning time.
II) Increasing the angle between the normal to the local horizon and the flight path angle.
a) Only I
b) Only II
c) Both I and II
d) Neither I nor II

Explanation: Reducing the burning time will only reduce the gravitational losses. Keeping the flight path normal to the local horizon will also help in reducing gravitational losses
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6. The form drag depends on ___________
a) aeroelastic stresses
b) solid structures far downstream
c) solid structures far upstream
d) aerodynamic shape of the body

Explanation: Form drag depends on the aerodynamic shape of the body in the flow field. A streamlined body like a slender body with a pointed nose has lesser drag compared to a blunt object.

7. The rocket frontal area can be reduced by choosing a propellant with __________
a) higher propellant density
b) lower propellant density
c) higher propellant flammability
d) lower propellant flammability

Explanation: Higher propellant density will allow lesser tank volume for the storage of the same mass of the propellant. Hence the frontal area requirement will be less. Flammability has no role in controlling rocket frontal area.

8. Which drag is specifically caused by the friction of air flowing over the vehicle’s outer surface?
a) Form drag
b) Skin drag
c) Shock drag
d) Base drag

Explanation: Skin friction drag is the drag caused by the friction of flowing air over the vehicle outer surface. A polished and smooth outer surface will help in its reduction.

9. A shorter rocket length will help in _________
a) more vehicle structural mass
b) lighter vehicle structure
c) greater vehicle mass ratio
d) greater complexity of the vehicle design

Explanation: A shorter rocket length means that the overall vehicle’s structural mass will be lesser. This is because of the reduction in the amount of material required in the construction of the vehicle. It will also mean more mass ratio compared to longer rockets because of lesser inert mass in shorter vehicles.

10. If flight velocity is u and the vehicle exhaust velocity is c, when is the propulsive efficiency maximum?
a) u ≅ c
b) u ≫ c
c) u ≪ c
d) c → ∞

Explanation: Propulsive efficiency is maximum when the flight velocity and the exhaust velocity of the vehicle are closer to each other. This can be seen from the formula ηprop = 2u/(u+c).

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