# Rocket Propulsion Questions and Answers – Flight Performance – Aerodynamic Effect of Exhaust Plumes

This set of Rocket Propulsion Question Paper focuses on “Flight Performance – Aerodynamic Effect of Exhaust Plumes”.

1. In which of these following altitudes does a rocket exhibit narrow pencil shaped exhaust plume?
a) Low altitude
b) Above atmosphere
c) Outer space
d) During launch

Explanation: In general, rocket exhaust plumes are pencil shaped when they are at low altitudes. This is because the exhaust gases are restricted from expanding against ambient air pressure.

2. What are the primary sources of radiation in a chemical rocket engine?
a) Excitation of molecules in the combustion chamber
b) De-excitation of internal modes of gas molecules
c) Rapid combustion of injector fuel
d) Shock layer at the nozzle exit

Explanation: The molecules are excited in the combustion chamber. But these excited molecules de-excite in their internal modes causing radiation to be emitted.

3. Determine the chamber pressure for a rocket engine if the thrust coefficient is 1.8, the throat area is 0.2 m2 and the total thrust is 20000 N.
a) 61.33 kPa
b) 67.98 kPa
c) 45.67 kPa
d) 55.56 kPa

Explanation: Thrust force F = Pc At Cf, where Pc is the chamber pressure, At is the throat area and Cf is the thrust coefficient.
Then, Pc = F/(At Cf) = 20000/(0.2×1.8) = 55.56 kPa.

4. Determine the mass ratio (Burnout mass/ Initial mass) of a vehicle in a gravitation less space if its velocity is boosted from 800 m/s to 1600 m/s and the effective rocket exhaust velocity is 2000 m/s.
a) 0.85
b) 0.33
c) 0.67
d) 0.25

Explanation: δu = ueq ln (Mo/Mb)
where δu = uf – ui = 1600 – 800 = 800 m/s.
Then Mb/Mo = 1/exp(800/2000) = 0.67.

5. Which of the following correspond to the spectral bands of CO2 and H2O respectively in the IR signature of rocket exhaust plumes?
a) 2.7μm and 4.3μm
b) 7.2 μm and 3.4μm
c) 4.3 μm and 2.7 μm
d) 3.4 μm and 7.2μm

Explanation: Spectral band of H2O is in the 2.7 μm region, while for CO2, it is 4.3 μm. In missile launches, these are the two principal bands observed in the atmospheric absorption.
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6. For strong emission signature of exhaust plumes, afterburning of fuel-rich gases is optimum for invisibility.
a) True
b) False

Explanation: Exhaust plumes with strong emission signatures easily detectable. Afterburning will enhance such an emission. For stealth aircraft, this isn’t an ideal mode of operation, unless the emission signature can be well suppressed.

7. If A is the pre-exponential factor, n is the temperature exponent, Ea is the activation energy, then the expression for forward rate constant according to Arrhenius law is ______
a) kf = ATn exp(-Ea/RT)
b) kf = T exp(-EaA/RTn)
c) kf = ATn exp(-Ea/RTn)
d) kf = AT exp(-Ea/nRT)

Explanation: The expression for the forward reaction is given by kf = ATn exp(-Ea/RT). This is given by the Arrhenius law, which plays an important role in the calculation of the rate of chemical reactions and activation energy.

8. Which of the following is the mixing layer of a nozzle core flow?
a) Inviscid inner core flow
b) Viscous outer core flow
c) Inviscid outer core flow
d) Viscous inner core flow

Explanation: Nozzle core flow can be divided into inviscid inner core flow and viscous outer core flow. The viscous outer core flow is also called the mixing layer. No chemical reactions are assumed to take place in the nozzle inner core flow.

9. At which of the following altitudes does the infrared emissions from the mixing zone of the rocket exhaust higher?
a) near sea level
b) Stratospheric region
c) Just outside the atmosphere
d) Outer space

Explanation: Afterburning in mixing zones lead to higher temperatures and higher infrared emissions. Afterburning is dependent on altitude as it depends upon the amount of oxygen readily available in the atmosphere. Oxygen concentration decreases at higher altitudes. So higher infrared emissions are observed near sea level.

10. Afterburning can lead to ozone layer depletion.
a) True
b) False

Explanation: It is true because studies have found that afterburning significantly plays a role in HCl conversion. It can lead to the formation of Cl and other active species containing chlorine which can act as a catalytic agent in the ozone depletion process.

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