Rocket Propulsion Questions and Answers – Nozzle Theory – Variable Thrust

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This set of Rocket Propulsion Multiple Choice Questions & Answers (MCQs) focuses on “Nozzle Theory – Variable Thrust”.

1. Thrust is a weak function of ______________
a) throat area
b) chamber pressure
c) mass flow rate
d) thrust coefficient
View Answer

Answer: d
Explanation: Thrust is a weak function of thrust coefficient CF. It is in turn dependent on specific heat ratio, pressure ratio, expansion ratio, and altitude.
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2. A two-to-one thrust decrease can be achieved using _________ in a liquid propellant rocket engine.
a) throttle valves
b) gas pressure feed system
c) spark ignitor
d) using BATES grain
View Answer

Answer: a
Explanation: Throttle valve helps in controlling the mass flow rate thereby reducing the thrust. It usually consists of a flow controlling unit and an electro-mechanical-actuator.

3. Thrust can be varied by changing the throat area along with throttling the flow.
a) True
b) False
View Answer

Answer: a
Explanation: Some methods of changing the area involves using a tapered plug or movable contoured pintle into the nozzle. Pintle has a pin or bolt shaped structure with a rod inside the nozzle and a tip outside of it.

4. Attitude and maneuvering corrections require impulses with _________ pulse width.
a) extremely short
b) extremely large
c) moderately short
d) moderately large
View Answer

Answer: a
Explanation: The engine might be operating at very high thrust levels. In order to bring changes to the vehicle orientation and for maneuvering purposes, time constraints might be tight and required impulses should be delivered with an extremely short pulse width.

5. Which of the following is not a way for thrust modulation?
a) Controlling the propellant flow rate to the injector
b) Controlling injection orifices
c) Varying chamber pressure
d) Deploying control surfaces like fins
View Answer

Answer: d
Explanation: Thrust modulation can be achieved by varying mass flow rate or axial component of gas flow velocity or both. Deploying control surfaces may help in controlling the attitude of the vehicle in flight within the atmosphere, but it doesn’t help in varying thrust manually.
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6. Assertion: Oxygen addition in the control flow gases of solid rocket engine will result in a higher characteristic velocity of the exit jet.
Reason: Because of secondary combustion, stagnation temperature and pressure increase. Higher stagnation temperature results in exhaust jet characteristic velocity.
a) Both assertion and reason are correct
b) The assertion is correct but the reason is wrong
c) The assertion is wrong but the reason is correct
d) Both assertion and reason are wrong
View Answer

Answer: a
Explanation: Usually solid propellant combustion gases are fuel rich and addition of oxygen into this control flow results in a secondary combustion. It increases both stagnation temperature and pressure of the flow. Because of pressure sensitivity on burning rate, higher stagnation pressure results in a higher mass flow rate, while the increase in stagnation temperature results in an increase in the characteristic velocity of the exit jet.

7. Which of the following thrust modulation technique makes use of symmetric injection for jet area control?
a) Throat Shifting Thrust Modulation
b) Shock Thrust Modulation
c) Shock Vectoring Control
d) Scarfed Nozzle Thrust Modulation
View Answer

Answer: a
Explanation: In Throat Shifting Thrust Modulation (TSTM), by shifting the throat of the nozzle, thrust is controlled. For modulation of thrust, the jet area is controlled using symmetric injection near the throat.

8. Which of the following thrust modulation techniques uses a normal shock to reduce total thrust?
a) Throat Shifting Thrust Modulation
b) Scarfed Nozzle Thrust Modulation
c) Shock Thrust Modulation
d) Fluidic Thrust modulation
View Answer

Answer: c
Explanation: Shock Thrust Modulation (STM) involves an axisymmetric injection of secondary flow into the diverging section of the supersonic nozzle and the creation of a normal shock by the interaction of oblique shocks. Across a normal shock, a supersonic flow becomes subsonic, thereby reducing exhaust velocity and total thrust. Fluidic Thrust Modulation is divided into STM and TSTM.

9. For a conical nozzle of throat diameter 0.007 m, exit cross-section diameter 0.02 m, and divergent section length of 0.038 m, determine the nozzle semi-divergence angle.
a) 10.5°
b) 12.6°
c) 7.3°
d) 9.7°
View Answer

Answer: d
Explanation: Required angle α = tan-1((re-rt)/L), where rt is the throat radius and re is the exit cross-section radius.
∴ α = tan-1(1/2(0.02-0.007)/0.038) = 9.7°.

10. Which of the following cannot be the nature of the actuation system for thrust vector control?
a) Mechanical
b) Fluidic
c) Either mechanical or fluidic
d) Neither mechanical and fluidic
View Answer

Answer: d
Explanation: The force applied for vectoring the thrust is modulated in its magnitude and direction using thrust vector control systems, which generally use heavy actuators. The actuation system can be either mechanical or fluidic.
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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He is Linux Kernel Developer & SAN Architect and is passionate about competency developments in these areas. He lives in Bangalore and delivers focused training sessions to IT professionals in Linux Kernel, Linux Debugging, Linux Device Drivers, Linux Networking, Linux Storage, Advanced C Programming, SAN Storage Technologies, SCSI Internals & Storage Protocols such as iSCSI & Fiber Channel. Stay connected with him @ LinkedIn