# Rocket Propulsion Questions and Answers – Flight Performance – Basic Relations of Motion

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This set of Rocket Propulsion Multiple Choice Questions & Answers (MCQs) focuses on “Flight Performance – Basic Relations of Motion”.

1. Which of the following can be non-zero for an airborne vehicle moving in rectilinear equilibrium flight?
a) Control forces
b) Lateral forces
c) Moments
d) Thrust

Explanation: Control forces, Lateral forces and moments are taken to be zero to ensure that the vehicle doesn’t turn or slip in flight. However, the vehicle may need thrust to propel forward.

2. What is the trajectory of an airborne vehicle in rectilinear equilibrium flight?
a) One-dimensional
b) Two-dimensional
c) Three-dimensional
d) Four-dimensional

Explanation: The trajectory for such a vehicle is considered to be two-dimensional and contained within a plane. In real cases, the flight is much more complex and three-dimensional and requires computational usage for the trajectory analysis.

3. The angle at which the wings of the vehicle are inclined to the flight path is called as __________
a) angle of attack
b) flight path angle
c) pitch angle
d) roll angle

Explanation: The vehicle wings are at some angle with the flight path. This angle is called the angle of attack. It plays a significant role in determining the lift generated under atmospheric conditions in the vehicle.

4. Lift is generated __________ to the flight path for an airborne vehicle in rectilinear equilibrium flight.
a) parallel
b) perpendicular
c) at an acute angle
d) at an obtuse angle

Explanation: Lift generation is normal to the direction of the flight path for such a vehicle. Lift is normal to the relative wind, while drag acts parallel to it and in the opposite direction.

5. In the direction of the flight path, the product of mass and acceleration of the vehicle is not the sum of which of the following forces?
a) Propulsive
b) Aerodynamic
c) Gravitational
d) Electromagnetic

Explanation: Propulsive, aerodynamic and gravitational forces are the major forces that contribute to the flight of an airborne vehicle. Propulsive forces include the thrust produced by the vehicle’s engine, while the aerodynamic forces include lift and drag forces.

6. Which of the following is acceleration perpendicular to the flight path?
a) udθ/dt
b) θdu/dt
c) d(uθ)/dt
d) d(u/θ)/dt

Explanation: udθ/dt is the correct representation of the acceleration perpendicular to the flight path. du/dt is the acceleration in the direction of the flight path. It has a contribution from the lift, propulsive forces, and the gravitational forces.

7. Determine the acceleration perpendicular to flight path for constant value of flight speed u = 100 m/s and instantaneous radius R = 200 m.
a) 100 m/s2
b) 50 m/s2
c) 200 m/s2
d) 125 m/s2

Explanation: Required acceleration perpendicular to flight path is atan = u2/R.
atan = 1002/200 = 50 m/s2.

8. Determine the component of gravitational force in the direction of the flight path if the angle of the flight path with the horizontal is 30°, angle of direction of thrust with the horizontal is 20° and mass of the vehicle is 300 kg. Assume the acceleration due to gravity to be 9.8 m/s2.
a) 2250 N
b) 2980 N
c) 1470 N
d) 3343 N

Explanation: Required component of gravitational force = mgsinθ.
Fgrav comp = 300 x 9.8 x sin(30) = 1470 N.

9. For a propulsive force of 1500 N, determine the magnitude of its component in a direction normal to the flight velocity if the angle of flight path with the horizontal is 30° and the angle of direction of thrust with the horizontal is 10°. Assume g=9.8 m/s2.
a) 513 N
b) 1026 N
c) 279 N
d) 813 N

Explanation: The required propulsive force component is Tprop = F x sin(ψ-θ), where F is the total propulsive force, ψ is the angle of direction of thrust with the horizontal and θ is the angle of the flight path with the horizontal.
Tprop = 1500 x sin(10-30) = 513 N in magnitude.

10. If the flight is not following a vertical path, then the gravity losses are a function of the angle between the flight direction and the local horizontal.
a) True
b) False 