Thermal Engineering

TSadhu Singh, Sukumar Pati

Chapter 17

Jet Propulsion - all with Video Answers

Educators

Chapter Questions

Problem 1

In solid propellants rocket, ammonium picrate is usually added as:
(a) an additive
(b) an inhibitor
(c) a darkening agent
(d) a plasticiser

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Problem 2

A turbo prop is preferred to turbo-jet because
(a) it has high propulsive efficiency at high speed
(b) it can fly at supersonic speed
(c) it can fly at high elevations
(d) it has high power for take-off

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Problem 3

The thrust of a jet propulsion power unit can be increased by
(a) injecting water into compressor
(b) burning fuel after gas turbine
(c) injecting ammonia into the combustion chamber
(d) All of the above

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Problem 4

Figure $17.16$ shows the propulsive efficiencies of three different engines. Based on the figure, match List I with List II and select the correct
answer using the codes given below the List:

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Problem 5

If $V$ is the jet velocity $V$ is the vehicle velocity, the propulsive efficiency of a rocket is given by
(a) $\frac{2\left(v_{0} / v_{1}\right)}{1+\left(\frac{v_{0}}{v_{1}}\right)}$
(b) $\frac{\left(v_{0} / v_{1}\right)}{1+\left(\frac{v_{0}}{v_{1}}\right)^{2}}$
(c) $\frac{v_{0}}{v_{0}+v_{1}}$
(d) $\frac{v_{1}}{v_{0}+v_{1}}$

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Problem 6

Consider the following statements relating to rocket engines:
I. The combustion chamber in a rocket engine is directly analogous to the reservoir of a supersonic wind tunnel.
II. Stagnation conditions exist at the combustion chamber.
III. The exit velocities of exhaust gases are much higher than those in jet engines.
IV. Efficiency of socket engines is higher than that of jet engines. Of these statements:
(a) I, III, and IV are correct
(b) II, III, and IV are correct
(c) I, II, and III are correct
(d) I, II, and IV are correct

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Numerade Educator

Problem 7

Consider the following statements about a rocket engine:
I. It is a very simple in construction and operation.
II. It can attain very high vehicle velocity.
III. It can operate or very long duration. Of these statements:
(a) I and II are correct
(b) I and III are correct
(c) II and III are correct
(d) I, II, and III are correct

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Numerade Educator

Problem 8

Only rocket engines can be propelled to space because
(a) they can generate very high thrust.
(b) they have high propulsion efficiency.
(c) these engines can work on several fuels.
(d) they not air-breathing engines.

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Problem 9

Consider the following statements:
As compared to turboprop, a turbojet
I. can operate at higher altitudes
II. can operate at higher flight velocities
III. is more fuel efficient at lower speeds Of these statements:
(a) I, II, and III are correct
(b) I and II are correct
(c) II and III are correct
(d) I and III are correct

Hunza Gilgit

Numerade Educator

Problem 10

Propulsion efficiency of a jet engine is given by (where $u$ is flight velocity and $\mathrm{V}$ is jet velocity relative to aircraft).
(a) $2 u[V-u]$
(b) $[V+u] 2 u$
(c) $2 u[V+u]$
(d) $[V-u] 2 u$

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Problem 11

Consider the following statements:
In open cycle turbo-jet engines used in military aircraft, reheating the exhaust gas from the turbine by burning more fuel is used to increase
I. thrust
II. the efficiency of engine
III. the range of aircraft Of these statements:
(a) I and III are correct
(b) I and II are correct
(c) II and III are correct
(d) I, II, and III are correct

Hunza Gilgit

Numerade Educator

Problem 12

In a turbojet engine, subsequent to heat addition to compressed air, to get the power output, the working substance is expanded in
(a) turbine blades, which is essentially an isentropic process
(b) turbine blades, which is polytropic process
(c) exit nozzle, which is essentially an isentropic process
(d) exit nozzle, which is a constant volume pro-
cess

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Problem 13

Which one of the following is the correct sequence of the position of the given components in a turboprop?
(a) Propeller, compressor, turbine, burner
(b) Compressor, propeller, burner, turbine
(c) Propeller, compressor, burner, turbine
(d) Compressor, propeller, turbine, burner

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Problem 14

The absolute jet exit velocity from a jet engine is $2800 \mathrm{~m} / \mathrm{s}$ and the forward flight velocity is 1400 $\mathrm{m} / \mathrm{s}$. The propulsive efficiency is
(a) $33.33 \%$
(b) $40 \%$
(c) $66.67 \%$
(d) $90 \%$

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Problem 15

The efficiency of jet engine is
(a) higher at high speed
(b) lower at low speed
(c) higher at high altitude
(d) same at all altitude

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Problem 16

The propulsive efficiency of a turbojet aircraft approaches $100 \%$ when the thrust approaches
(a) Maximum
(b) $50 \%$ of the maximum
(c) $25 \%$ of the maximum
(d) Zero

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Problem 17

Which one of the following is the correct expression for the propulsive efficiency of a jet plane (neglecting the mass of fuel)?
(a) $\eta_{p}=\frac{2}{\left(\frac{v_{a}}{v_{j}}\right)+1}$
(b) $\eta_{p}=\frac{2}{\left(\frac{v_{j}}{v_{a}}\right)+1}$
(c) $\eta_{p}=\frac{2}{\left(\frac{v_{a}}{v_{j}}\right)-1}$
(d) $\eta_{p}=\frac{2}{\left(\frac{v_{j}}{v_{a}}\right)-1}$

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Problem 18

The relative jet exit velocity from a rocket is 2700 $\mathrm{m} / \mathrm{s} .$ The forward flight velocity is $1350 \mathrm{~m} / \mathrm{s} .$ What is the propulsive efficiency of the unit?
(a) $90 \%$
(b) $66.66 \%$
(c) $50 \%$
(d) $33.33 \%$

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Numerade Educator

Problem 19

Consider the following statements regarding performance of turbojet engines:
I. The thrust decreases at higher altitudes due to reduced density of air and consequently lower mass flow of air.
II. Any subsonic speed, the effect of increased velocity is to increase the air flow and the thrust increases.
III. The relative velocity of jet with respect to the medium decreases at higher speeds which tends to reduce the thrust.
IV. For turbojet engine, the thrust of jet at subsonic speeds remains relatively constant. Which of the statements given above are correct?
(a) I, II, III, and IV
(b) I and III
(c) I, II, and IV
(d) II, III, and IV

Hunza Gilgit

Numerade Educator

Problem 20

Consider the following statements:
The thrust of rocket engine depends on
I. Effective jet velocity
II. Weight of the rocket
III. Rate of propellant consumption Of these statements:
(a) I and II are correct
(b) I and III are correct
(c) II and III are correct
(d) I, II, and III are correct

Hunza Gilgit

Numerade Educator

Problem 21

Consider the following statements:
In a turbojet engine, thrust may be increased by
I. Increasing the jet velocity
II. Increasing the mass flow rate of air
III. After burning of the fuel
Of these statements:
(a) I and II are correct
(b) II and III are correct
(c) I and III are correct
(d) I, II, and III are correct

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Numerade Educator

Problem 22

The propulsive efficiency of a turbojet aircraft approaches $100 \%$ when the thrust approaches
(a) maximum
(b) $50 \%$ of the maximum
(c) $25 \%$ of the maximum
(d) zero

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Numerade Educator

Problem 23

An aeroplane travels at $400 \mathrm{~km} / \mathrm{h}$ at sea level where the temperature is $15^{\circ} \mathrm{C}$. The velocity of the aeroplane at the same Mach number at an altitude where a temperature of $-25^{\circ} \mathrm{C}$ is prevailing, would be
(a) $126.78 \mathrm{~km} / \mathrm{h}$
(b) $130.6 \mathrm{~km} / \mathrm{h}$
(c) $371.2 \mathrm{~km} / \mathrm{h}$
(d) $400.10 \mathrm{~km} / \mathrm{h}$

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Numerade Educator

Problem 24

In turbo prop, the expansion of gases takes place approximately
(a) $100 \%$ in the turbine
(b) $80 \%$ in the turbine and $20 \%$ in the nozzle
(c) $50 \%$ in the turbine and $50 \%$ in the nozzle
(d) $100 \%$ in the nozzle

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Numerade Educator

Problem 25

With reference to turbojet and rocket engines, consider the following statements:
I. Efficiency of rocket engines is higher than that of jet engines.
II. Exit velocities of exhaust gases in rocket engines are much higher than those in jet engines.
III. Stagnation conditions exist at the combustion chamber in rocket engines.
IV. Rocket engines are air-breathing engines. Which of these statements are correct?
(a) I and II
(b) I, III, and IV
(c) II, III, and IV
(d) I, II, and III

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Problem 26

Consider the following statements indicating a comparison between rocket and jet propulsion systems:
I. Both rocket and jet engines carry the fuel and oxidant
II. Rockets do not employ compressor or propeller
III. Rockers can operate in the vacuum too
IV. Rockers can use solid fuels and oxidants
(a) I, II, III, and IV
(b) Only I and II
(c) Only II, III, and IV
(d) Only I, III, and IV

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Problem 27

Match List I with List II in respect of a chemical rocket engine and select the correct answer using the codes given below the Lists:

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Numerade Educator