1. What will happen when the thrust line doesn’t intersect the center of mass of a flying vehicle?
a) The vehicle experiences a centripetal acceleration
b) The vehicle drops in altitude
c) The vehicle trajectory becomes zigzag
d) The vehicle tries to rotate in flight
Explanation: When the thrust line doesn’t intersect the flying vehicle’s center of mass, turning moments are generated about the COM and it tends to rotate the vehicle in flight. Moment M = r x F, where F is the thrust force and r is the position vector measure from center of mass to any point on the force vector.
2. Which among the following is the most necessary for controlled turning and attitude control?
a) Turning moments
b) High exhaust velocity
c) Vortex generators
d) Dorsal fins
Explanation: By varying the direction of the thrust line, turning moments can be generated. Controlling the turning moments will help in attaining the desired vehicle attitude.
3. What will not help in generating a turning moment in a flying vehicle?
a) Thrust vector deflection
b) Aerodynamic fins
c) Using separate engines for attitude control
d) Altering the nozzle exit area by using nozzle extensions
Explanation: Altering the nozzle exit area will not produce a thrust component not passing through the center of mass of the rocket. In that case, turning moment will not be generated.
4. In general, the rocket nozzle exits surface’s geometric axis is taken to be the __________
a) gimbal axis
b) roll axis
c) thrust axis
d) lift axis
Explanation: Both the thrust axis and the geometric axis of the rocket nozzle exit surface are taken to be the same. For the high-thrust booster system, even a small misalignment can lead to the rocket deviating from its path during the vehicle operation period.
5. Which of the following doesn’t belong to misalignment due to irregularities in nozzle geometry?
a) Out of round
b) Unsymmetrical roughness
c) Protuberances
d) Nozzle extensions
Explanation: Nozzle extensions are a means of improving the efficiency of rocket engines by increasing nozzle expansion ratio. It is not an irregularity in nozzle geometry. Protuberances refer to obstructions or protrusions from the nozzle wall. Out-of-round means having an unbalanced spherical or circular form or density.
6. If the nozzle exit is flush with the missile skin, then the nozzle scarf angle and cant angle is the same.
a) True
b) False
Explanation: In surface mounting, the device is fixed to the outer surface while in flush mounting, the attached device won’t extend out of the outer surface. Cant angle is the angle between the longitudinal axis of the rocket motor grain and the nozzle center-line.
7. Conventional axisymmetric conical nozzle with a cylindrical extension is called as ______
a) cant nozzle
b) bell-shaped nozzle
c) scarfed nozzle
d) aerospike nozzle
Explanation: Scarfed nozzle is a passive way of thrust vector adjustment. Even though the flow Mach number of scarfed nozzles is lower compared to conical nozzles, it is more preferable because unwanted turning moments due to vehicle configuration can be avoided using it.
8. Motor axial specific impulse _____ with the scarf angle.
a) increases
b) decreases
c) increases and then decreases
d) decreases and then increases
Explanation: With the increase in scarf angle, the motor axial specific impulse will decrease. This is because the component of exhaust velocity directed along the vehicle’s longitudinal axis decreases with the scarf angle increase.
9. For a scarfed nozzle, thrust coefficient, thrust and specific impulse are a function of ambient pressure.
a) True
b) False
Explanation: Thrust coefficient, specific impulse, and thrust are independent of ambient pressure for a scarfed nozzle. Scarfed nozzle generally lies on the missile skin. The pressure forces acting on the scarfed nozzle are therefore perpendicular to the vehicle’s longitudinal axis and hence doesn’t contribute to thrust forces. Similar argumentation can be made for thrust coefficient and specific impulse as well.
10. If the motor axial specific impulse of a given nozzle is 340s for propellant ‘A’ and its characteristic velocity is 1400 m/s, determine the specific impulse for the same nozzle using a propellant ‘B’ with a characteristic velocity of 980 m/s. Assume both propellants ‘A’ and ‘B’ have the same specific heat ratio.
a) 259 s
b) 286 s
c) 277 s
d) 238 s
Explanation: Isp2 = Isp1 x C2*/C1*
= 340 x 980 / 1400 = 238 s.