# Rocket Propulsion Questions and Answers – Flight Performance – Forces Acting on a Vehicle in the Atmosphere

This set of Rocket Propulsion Interview Questions and Answers for freshers focuses on “Flight Performance – Forces Acting on a Vehicle in the Atmosphere”.

1. Which of the following is not an external force acting on a vehicle flying in the earth’s atmosphere?
a) Thrust
b) Aerodynamic forces
c) Gravitational forces
d) Forces due to thermal stress

Explanation: For a vehicle flying in the earth’s atmosphere, thrust, aerodynamic (lift and drag) and gravitational forces are the major external forces. Thermal stress inside an engine leads to internal forces only.

2. Which of the following is a reasonable factor of increase for effective exhaust velocity (c) as altitude is increased?
a) 1.5
b) 2.1
c) 3.2
d) 4.5

Explanation: The value of c can increase by a small factor between 1.2 to 1.6 as altitude is increased. Rocket motors produce greater thrust at higher altitudes due to the lower atmospheric pressure.

3. If L denotes the lift and ∅ denotes flight path angle, then what is the aerodynamic force acting normal to the flight path?
a) Lcos ∅
b) Lsin ∅
c) Lsec ∅
d) L

Explanation: Lift (L) is the aerodynamic force acting normal to the flight path. Flight path angle is the angle between the local horizon (perpendicular to r) and the flight path.

4. In writing the expressions for CL and CD for missiles and rockets, what does area A denote?
a) Sum of all the fin areas
b) The total surface area of the missile/rocket including the fins
c) Maximum cross-sectional area normal to the missile axis
d) Minimum cross-sectional area normal to the missile axis

Explanation: Area A denotes the maximum cross-sectional area perpendicular to the longitudinal axis of the missile. So for a cylindrical missile body having a radius r, A = πr2.

5. What is the angle of attack for a missile?
a) The angle between the local horizon and the missile axis
b) The angle between the local horizon and the missile flight path
c) The angle between the missile axis and its flight path
d) The angle between the relative wind and the missile longitudinal axis

Explanation: Angle of attack for a missile is the angle between its flight path and the missile axis. For the pitching movement of a cruise missile, a range of angle of attack of 15° to 24° is found to be effective.
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6. The lift and drag coefficients are not primarily a function of ____________
a) vehicle configuration
b) flight Mach number
c) angle of attack
d) vehicle mass

Explanation: lift and drag coefficients are primarily a function of vehicle configuration, flight Mach number, and angle of attack. CL depends on lift force and not vehicle mass.

7. What is the loss of velocity increment experienced by a rocket as a result of drag in the atmosphere?
a) 5-10%
b) 15-20%
c) 25-30%
d) 35-40%

Explanation: For space launch vehicles and ballistic missiles, the drag loss is about 5 to 10% of the final vehicle velocity increment. It will be lesser for space vehicles than atmospheric vehicles like fighter aircraft because the time spent in the atmosphere is lesser for the space vehicles.

8. Which of the following is the correct expression for gravitational acceleration (g) at a height h, given go refers to gravitational acceleration on the earth’s surface and Ro refers to effective radius of earth?
a) g=go[Ro/(Ro+h)]2
b) g=go[Ro/(Ro-h)]2
c) g=go[(Ro+h)/2Ro]2
d) g=go[(Ro-h)/2Ro]2

Explanation: g=go[Ro/(Ro+h)]2 is the required formula.
go = GM/Ro2
g = GM/(Ro + h)2
⇒ g/go = Ro/(Ro+h)2.

9. Dynamic pressure experienced by the rocket in the lower atmosphere is ______ compared to that experienced at higher altitudes.
a) more
b) less
c) equal
d) any random value

Explanation: At lower atmospheric altitudes, the dynamic pressure experienced will be low because of the lower velocity of the rocket even though the air density is high. Dynamic pressure Pdyn is proportional to ρV2.

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