1. Which of the following is not a reason why the wall surface throughout the nozzle is smooth?
a) Minimize friction
b) Minimize radiation absorption
c) Minimize convective heat transfer
d) Minimize flow velocity
Explanation: Smooth wall surface allows smooth passage of flow through the nozzle. It won’t minimize the flow velocity. Since friction is dependent on surface roughness, it will be less for a smoother nozzle. Smoother surfaces will also increase the reflectivity of the material. Since reflectivity + transmissivity + absorptivity = 1, increasing reflectivity reduces the absorptivity of the material.
2. Which of the following is permissible in a rocket nozzle?
a) Gaps
b) Smooth edges
c) Holes
d) Protrusions
Explanation: Gaps, holes, and protrusions obstruct the flow passage through the nozzle. An ideal rocket requires the flow to be uniform at any cross-section throughout the length of the nozzle.
3. Which of the following is not an objective of a good nozzle configuration?
a) Decrease nozzle inert mass
b) Increase nozzle length
c) Obtain the highest practical specific impulse
d) Decrease nozzle diameter
Explanation: Increase in nozzle length would lead to more material consumption, which means an increase in inert mass. It will also mean a larger exposed surface area of the rocket and while operating under atmospheric conditions will lead to higher drag.
4. What is the nozzle correction factor for a conical nozzle angle of 200?
a) 0.992
b) 0.969
c) 1.939
d) 0.939
Explanation: λ = 1/2 (1 + cosα), where α is the semi-divergence angle for a conical nozzle.
Here, α = 20/2 = 10°
∴ λ = 0.5 x ( 1 + 0.985)
= 0.992.
5. To which of the following terms can we multiply the nozzle correction factor with?
a) Total thrust
b) Momentum thrust
c) Pressure thrust
d) Pressure ratio
Explanation: Nozzle correction factor needs to be multiplied with momentum thrust term only. The flow momentum is affected by the change in divergence angle of a conical nozzle.
6. Which of the following is a typical optimum divergence angle for a conical nozzle?
a) 10°
b) 30°
c) 40°
d) 60°
Explanation: Typical half-angle of a conical nozzle lies in between 12° to 18°. So the conical nozzle divergence angle should be between 24° to 36°, so 30° angle is a plausible option.
7. How does the divergence angle vary along a bell-shaped nozzle?
a) Sharply increases, then decrease
b) Sharply decreases, then increase
c) Remains constant for a while and then increases
d) Remains constant for a while and then decreases
Explanation: For a bell-shaped nozzle, immediately after the throat section, the nozzle divergence angle increases rapidly and then it gradually decreases. It allows the flow to smoothly follow the contour without separation and leave the nozzle with more axially directed flow momentum (for eg. relative to conical nozzles).
8. What is the difference between the angle at the nozzle inflection point and the angle at the exit called?
a) Contour angle
b) Divergence angle
c) Semi-divergence angle
d) Turn-back angle
Explanation: Turn-back angle is the difference between the nozzle angle at the exit and the angle at the inflection point. Between inflection point and the exit, when the gas flow is turned in the opposite direction, it leads to the formation of oblique compression waves.
9. If the divergence angle for a conical nozzle is 30°, nozzle exit diameter is 30 cm, exit pressure is 10% more than the ambient pressure, mass flow rate is 14 kg/s and the jet exhaust velocity is 700 m/s, determine the total thrust of the engine under standard sea level operating conditions.
a) 11.2 kN
b) 12.9 kN
c) 10.3 kN
d) 9.8 kN
Explanation: Total thrust = Corrected momentum thrust + Pressure thrust
Ttot = λTmom + Tpres
λ = 1/2 (1 + cosα)
α = 1/2 x 30° = 15°
So, λ = 1/2 (1 + 0.966)
= 0.983
Tpres = (Pe – Pa)Ae
Since the operation is under standard sea level conditions, Pa = 101325 Pa.
Pe = 110% of Pa.
So Pe – Pa = 10% of Pa = 10132.5 Pa
Ae = π de2/4 = 0.071
∴ Tpres = 10132.5 x 0.071 = 719.075 N
Tmom = μe
= 14 x 700 = 9800 N
So Ttot = (0.983 x 9800) + 719.075 ≅ 10.3 kN.
10. If for a bell-shaped nozzle, the divergence angle immediately after the throat is 40°, contour angle at the inflection point is 32° and the nozzle full-angle at its exit is 10°, determine the turn-back angle.
a) 8°
b) 35°
c) 27°
d) 22°
Explanation: Divergence angle is the difference between the angle at the inflection point and the angle at the nozzle exit.
θi = contour angle at the inflection point = 32°
θe = 1/2 x full-angle at the exit = 5°
θturn-back = θi – θe
= 32° – 5° = 27°.