Rocket Propulsion Questions and Answers – Liquid Propellants – Combustion Instability

This set of Rocket Propulsion Multiple Choice Questions & Answers (MCQs) focuses on “Liquid Propellants – Combustion Instability”.

1. Which of the following terms best describes the combustion when the pressure fluctuation during steady operation is 4% of the mean chamber pressure?
a) Smooth combustion
b) Rough combustion
c) Combustion instability
d) Complete combustion

Explanation: When the pressure fluctuations do not exceed 5% of the mean chamber pressure, the combustion is called as smooth combustion. Combustion in a liquid rocket engine in reality is never smooth.

2. Which of the following terms best describe the combustion when the pressure fluctuations during steady operation is 12% of the mean chamber pressure?
a) Complete combustion
b) Incomplete combustion
c) Smooth combustion
d) Rough combustion

Explanation: If the pressure fluctuations exceed 5% of the mean chamber pressure, the combustion is called rough combustion. Normal liquid rocket engines in reality display rough combustion.

3. low-frequency combustion instability is called ________
a) chugging
b) buzzing
c) squealing
d) screaming

Explanation: Chugging is the term used to denote combustion instabilities with low-frequency of occurrence of fairly large concentrations of vibratory energy. The frequency range for this instability is 10-400 Hz.

4. Intermediate frequency combustion instability is called ________
a) squealing
b) chugging
c) screaming
d) buzzing

Explanation: Buzzing is the term used to denote combustion instabilities with an intermediate frequency of occurrence of fairly large concentrations of vibratory energy. The frequency range for this instability is 400-1000 Hz.

5. high-frequency combustion instability is called ________
a) squealing
b) buzzing
c) chugging
d) entropy waves

Explanation: Squealing is the term used to denote combustion instabilities with high-frequency of occurrence of fairly large concentrations of vibratory energy. The frequency range lies above 1000 Hz. It is also called screaming.

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6. Which of the following combustion instabilities occur due to propellant pump cavitation?
a) Buzzing
b) Chugging
c) Pogo
d) Screaming

Explanation: Chugging can occur in test facilities due to the propellant pump cavitation. It can also happen due to gas entrapment in propellant flow or due to tank pressurization control fluctuations.

7. Which of the following combustion instabilities happen due to propellant flow rate disturbances?
a) Screeching
b) Entropy waves
c) Screaming
d) Pogo

Explanation: Pogo instability is a longitudinal instability due to propellant flow rate disturbances. They are low-frequency waves, usually lying in the 10-50 Hz frequency zone.

8. Transverse mode of high-frequency instability propagation can be broken into ___________
b) longitudinal and tangential modes
c) radial, tangential and longitudinal modes

Explanation: High frequency (HF) instabilities are troublesome as they can lead to destructive results on the engine. Their destructive effect comes from the accelerated heat transfer as a result of these instabilities causing pressure fluctuations of the magnitude greater than about 20% of the mean combustion chamber pressure. Longitudinal and transverse modes are the two modes through which high-frequency instability occurs. The transverse modes can be further divided as radial and tangential modes.

9. Tangential mode of instability has the following modes ____________
a) standing and travelling waves
b) travelling and spinning waves
c) travelling and longitudinal waves

Explanation: Tangential mode can be considered as two waves: Standing wave and traveling wave. Traveling wave is also called as spinning wave. Standing wave remains fixed in position while the traveling tangential wave has a rotation of the whole vibratory system.

10. Acoustically stimulated variations in droplet mixing and/or vaporization can cause ___________
a) screeching
b) spinning wave
c) traveling wave
d) pogo

Explanation: Screeching (or squealing) can be caused by acoustically stimulated variations in droplet mixing and/or its vaporization. It can also be caused due to acoustic changes in combustion rates or due to local detonations.

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