GAS DYNAMICS AND JET PROPULSION
Short Questions and Answers
Unit – I: COMPRESSIBLE FLOW FUNDAMENTALS
1. Distinguish between compressible fluid and incompressible fluid flow. (Nov/ Dec2006)
Ans: Density is constant for incompressible fluid flow and it is variable for
incompressible fluid flow. The compressibility factor is one for incompressible fluid and
is less than one for compressible fluid. Examples are incompressible fluid - water, oil and
compressible fluid – air, hydrogen.
2. Define velocity of sound. What is the velocity of sound in air at 27oC? (May 2009)
Ans: The velocity of sound is defined as the velocity at which pressure waves are
transmitted through the medium. The velocity of sound in air is calculated by
a  RT .
The velocity of sound in air at 27oc,
a  1.4  287  300  347.2 m/s
3. Define Mach number. (Apr/May- 2008)
Ans: Mach number is defined as the ratio of local velocity of fluid to local sonic velocity
at the same point.
M 
Velocity of fluid
C

Velocity of sound a
4. What are the different regions of compressible flow? (April/May-2008).
Ans: When the fluid velocity and velocity of sound is plotted graphically on the c-c
coordinates, a steady flow ellipse is obtained. The various regions of flow are;
M<<1: Incompressible region
M<1: Subsonic region
M=1: Sonic region
M>1: Supersonic region
M>5: Hypersonic region
5. What is the difference between nozzle and diffuser?
Ans: Nozzle is a device in which there is an increase in the velocity and decrease in
the pressure. Diffuser is a device which there is a decrease in the velocity and increase in the
pressure.
6. Define stagnation state and stagnation enthalpy. (Nov/Dec-2009)
Ans: The stagnation state of a fluid is defined as the state of the fluid attained, when the
flow is isentropically decelerated to zero velocity at zero elevation. Stagnation enthalpy of a
gas is its enthalpy when it is isentropically decelerated to zero velocity at zero elevation.
7. Define stagnation pressure and stagnation density. (Nov/Dec-2007)
Ans: Stagnation pressure of a gas is defined as the pressure attained, when the gas is
isentropically decelerated to zero velocity at zero elevation. The stagnation density of a gas is
defined as the density corresponding to stagnation pressure and stagnation temperature.
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8. Define stagnation velocity of sound. (Nov/Dec-2009)
Ans: The stagnation velocity of sound is defined as the sonic velocity corresponding to
stagnation conditions of a gas. The stagnation velocity of sound a0  RT0 substituting
,
R=Cp (γ-1)/γ and simplifying a0    1h0
9. What are Mach cone and Mach angle? (April/May-2008)
Ans: In the supersonic flow, any disturbance will be propagated to the surroundings
with a velocity greater the acoustic velocity and hence the source point is always ahead of
wave front. Tangents from the source point to the spherical waves will form a cone and
this is called as Mach cone. The semi-angle of Mach cone is known as Mach angle and
given by
 1 

M 
  sin 1 
10. Write brief note on reference velocities. (April/May-2008)
Ans: Local velocity of sound a  RT
Stagnation velocity of sound ao  RTo
Maximum velocity of sound C max  ao
2
 1
Critical velocity of sound/fluid a *  C *  RT *
11. Define impulse function and write the impulse function in terms of Mach number. .
(May/June-2013)
Ans: The sum of pressure force (PA) and inertia force (ρAC2) is known as impulse
function (F) which is an important parameter in calculating the thrust in jet engine and
rocket engine.
F  PA  AC 2

F  PA 1  M 2

12. Define stagnation state of a system. (Nov/ Dec-2006)
Ans: Refer Q No. 6.
13. What is meant by steady flow ellipse and explain various regions of flow.
(May2013)
Ans: Incompressible flow (I): If the Mach number is very low (M<<1), the velocity of fluid
is much lower than velocity of sound. Here the effect of compressibility is negligible and the
flow is termed as incompressible flow.
Subsonic flow (II): If the Mach number is less than unity (M<1), the velocity of fluid is
lower than velocity of sound. The flow is termed as sonic flow.
Transonic flow (III): For Mach number slightly higher than and lower than sonic condition
(M=1) is known as transonic flow. For transonic flow usually: 0.9<M<1.1.
Supersonic flow (IV): The region in which Mach number is greater than unity, the flow is
termed as supersonic flow.
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Hypersonic flow (V): The region with higher Mach number (M>5) is known as hypersonic
flow. In this region of hypersonic flow, the velocity of fluid is far greater than velocity of
sound.
14. An air jet (γ = 1.4 & R = 287J/kg K) at 400 K is at sonic state. Determine its velocity (Nov/
Dec-2006)
Soln: M = c/a at sonic velocity M = 1
c = M x a = 1 x √ γ RT = 1x√ 1.4 x 287 x 400 =
15. Write the energy equation in differential form. (May/June-2013)
Ans: In adiabatic flow q = 0. Therefore, the energy equation becomes
dh + cdc + dz + Ws
=0
UNIT II: FLOW THROUGH DUCTS
1. Define Fanno flow (Nov/Dec-2012)
Ans: The one dimensional steady adiabatic flow in a constant area duct with frictional
effects is known as Fanno flow.
2. What are the assumptions for Fanno flow? ( Nov/Dec-2007,08,10)
Ans: The assumptions for Fanno flow are
i. The flow occurs in constant area duct
ii. The flow is adiabatic
iii. No work transfer across the boundary of the system
iv. The fluid behaves as perfect gas throughout the flow
v. The specific heat and molecular weight of the fluid remain constant
vi. The flow is steady and one dimensional
vii. Wall friction is the sole driving potential in the flow and the effect of body forces
are neglected.
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2. Sketch the Fanno line in T-S plane and explain the significance of it. (Nov/Dec-2006)
Ans: Figure 3. From the figure, it should be noted that F is the point where the Mach
number is unity and the flow is chocking. The specific entropy at this point is maximum.
The upper branch (AC) of the curve is for subsonic flow (M<1) and the lower branch (BC) is
for supersonic flow (M>1). The successive state points cannot lie along the branches of the
curve; because it violates the second law of thermodynamics (i.e) decrease in entropy. The
successive state points may lie only in the direction of arrow shown in the figure, (i.e) either
A to C for subsonic flow or B to C for supersonic flow. The limiting state for both the cases
is C, where the specific entropy is maximum.
4. Give few examples of Fanno flow (Nov/Dec-2012)
Ans: Fanno flow occurs in many engineering systems including fluid flow in aerospace
propulsion systems, thermal and nuclear power plants, chemical process plants, gas
industries and air conditioning systems.
5. Explain the phenomenon chocking in Fanno flow (Nov/Dec-2012)
Ans: In the Fanno flow, the chocking occurs when the Mach number is unity. For
subsonic entry condition, the Mach number increases and for supersonic entry condition,
the Mach number decreases along the direction of flow and reaches the chocking point.
Hence the mass flow rate is maximum at the critical state and the flow is said to be
chocked flow.
6. Discuss the effect of friction in constant area duct if the flow initially
subsonic (ii) supersonic
Variation of flow parameters in Fanno flow
Properties
Initially Subsonic
Initially Supersonic
M
Increases
Decreases
C
Increases
Decreases
Decreases
Increases

T
Decreases
Increases
To
Constant
Constant
P
Decreases
Increases
Po
Decreases
Decreases
F
Decreases
Decreases
4fLmax/D
Decreases
Decreases
4
(i)
7. Define Rayleigh flow (Nov/Dec-2012)
Ans: One dimensional steady frictionless flow through a constant area duct with heat
transfer is known as Rayleigh flow.
8. Outline the assumptions made in Rayleigh flow (Nov/Dec-2007,08,10)
Ans: The various assumptions for Rayleigh flow analysis are
(i) The flow takes place in constant area duct
(ii) The frictional effects are negligible
(iii) The gas behaves as perfect gas and properties are constant
(iv) The flow is steady and one dimensional
(v) The external shaft work and body forces are absent
9. State the two governing equations used in plotting Rayleigh line (Nov/Dec-2005
Ans: Continuity equation

m
 G  C
A
Momentum equation
P
G2

 constant
10. Distinguish between Rayleigh flow and Fanno flows. (Apr/May-2005,13)
Ans:
Fanno Flow
Rayleigh Flow
One dimensional stead frictional flow
One dimensional steady frictionless
flow
Stagnation temperature is constant
Stagnation temperature is not
constant
There is no heat transfer
Heat transfer occurs
Friction is the driving force
No friction
11. How the flow properties changes due to heating in Rayleigh flow, when the flow is
initially subsonic? (Nov/Dec-2006)
Ans:
Property
Subsonic flow (M<1)
Heating
Cooling
M
Increases
Decreases
T0
Increases
Decreases
P0
Decreases
Increases
P
Decreases
Increases
c
Increases
Decreases
T
Increases for M<1/ Decreases for M<1/
Decreases for M>1/ Increases for M>1/
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12. How the flow properties changes due to heating in Rayleigh flow, when the flow is
initially supersonic? (Nov/Dec-2006)
Ans:
Property Supersonic flow (M>1)
Heating
Cooling
M
Decreases Increases
T0
Increases Decreases
P0
Decreases Increases
P
Increases Decreases
C
Decreases Increases
T
Increases Decreases
13. Define isothermal flow.
Ans: One dimensional steady flow through a long pipe line with heat transfer and friction
may be considered as isothermal flow. In a long pipe line, the heat transfer rate is
relatively low and the static temperature is maintained constant. The buried pipe line in
which gas is transported may be modeled as isothermal flow.
14. What are the assumption made for the isothermal flow?
Ans: The various assumptions made for the analysis of isothermal flow are
a. The static temperature remain constant
b. The driving potential are friction and heat transfer
c. The fluid viscosity remain constant
d. The friction factor remain constant along the duct
e. The flow is steady and one dimensional
f. The fluid properties remain constant and it behaves as perfect gas
14 .Explain the difference between Fanno flow and isothermal flow. (Nov/Dec-2009)
Ans:
Isothermal flow
Static temperature is constant
Heat transfer occurs
Flow occurs in a long ducts where
sufficient time is required for heat
transfer
Fanno flow
Static temperature is not constant
No heat transfer
Long ducts are not required
UNIT III: NORMAL AND OBLIQUE SHOCKS
1. Define normal shock. (Nov/Dec-2006)
Ans: When the shock waves are right angles to the direction of flow and the rise in
pressure is abrupt are called normal shock waves.
2. What is Prandtl-Meyer relation? (Nov/Dec-2006)
Ans: The fundamental relationship between gas velocities before and after the normal
shock wave and critical velocity of sound is known as Prandtl-Meyer relation.
a *2  C x C y ;
Cx C y
 1;
a * a*
M *x M *y  1
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Write the expression for Mach number downstream of the normal shock
Ans:
2
 M x2


1
M y2 
2
M x2  1
 1
3. Write the expressions for change in static pressure, static temperature, density, and
stagnation pressure across a normal shock in terms of upstream Mach number.
(May/June-2013)
Ans:
Static pressure ratio:
Py
Px

2
 1
M x2 
 1
 1

   1 2  2
M x 
M x2  1
1 
T
2

   1

Static temperature ratio: y 
2
Tx
  1 M 2
x
2  1
 1 2
Mx
y
2

Density ratio:
 x 1  1 M 2
x
2

   1 2   1
1
 1
 2 Mx 
 2



1
M x2 
Stagnation pressure ratio: 
 


1


1
  1
2

1 
Mx 
2


4. Give two useful applications of shock waves. (Nov/Dec-2008)
Jet engines, Shock tubes and Supersonic wind tunnel.
5. State Rankine - Hugoniot equation. (April/May-2008)
Ans:
  1 Py
 1  y
1
1
 y   1 Px
Py   1  x

;

x
Px
  1 Py
 1  y


  1 Px
 1  x
6. Explain why the shock cannot occur in subsonic flows. (Nov/Dec-2008)
Ans: For subsonic flow Mx<1, then the entropy change will take a negative value that
represents a decrease in the entropy, which is not possible as per second law of
thermodynamics. Therefore, it is impossible for shock wave to develop in subsonic flow.
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8. Define the term: Strength of shock wave. (Nov/Dec-03, May/June-2004)
Ans: Strength of the shock wave is defined as the ratio of the increase in static pressure
across the shock to the upstream static pressure.

Py  Px
Px
9. What do you understand by strong and weak waves? Which one is preferred? (Nov/Dec2008)
Ans: Based on the pressure rise across the shock wave, it is classified as strong and weak
waves. The pressure rise is higher for strong waves and low for weak waves. The strong
shock waves are preferred in the air propulsion application
10. What do you understand by oblique shock? (May/June-2004)
Ans: The shock wave which is inclined at an angle to the direction of flow is called
oblique shock. When the flow is supersonic, the oblique shock occurs at the corner due to
the turning of supersonic flow.
11. What are the differences between oblique and normal shocks? (Nov/Dec-2006)
Ans:
Normal Shock
Oblique shock
The shock wave are right angles to the
The shock waves are inclined at an
direction of flow
angle to the direction of flow
May be treated as one dimensional
Oblique shock is normally not one
analysis
dimensional analysis
12. How is the shock formed? (Nov/Dec-2007)
Ans: Shocks are introduced to increase the pressure and hence it is a deceleration process.
Therefore shocks are possible only when the fluid velocity is supersonic.
13. What is meant by mach reflection? (Nov/Dec-2006)
14. Shock wave cannot develop subsonic flow? Why? (Nov/Dec-2007)
Ans: Refer Q No. 7.
15. Differentiate between impulse and specific impulse. (Nov/Dec-2006)
Ans: Impulse - This is an important parameter in calculating the thrust in jet engine and
rocket engine.
Specific impulse - It is defined as the thrust produced per unit weight flow rate through the
propulsive system.
UNIT-IV: JET PROPULSION
1. What is jet propulsion?
Ans: Jet propulsion is an act of pushing forward a body in a fluid medium by the reaction
of jet produced by the combustion of fuel and air. The principle of jet propulsion is
obtained from Newton’s second and third law of motion.
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2. How is jet propulsion classified? (Apr/May-08)
Ans: Jet propulsion is classified into two types
a. Air breathing propulsion engine
b. Rocket propulsion engine
3. List different types of jet engine?( Apr/May-2008)
Ans:
i. Turbo propeller engine
ii. Turbo jet engine
iii. Turbofan engine
iv. Ram jet engine
v. Pulse jet engine
4. List the components of turbo jet engine?(Nov/Dec-2006)
Ans: The components of turbo jet engine are Diffuser, Compressor, Combustion chamber,
Turbine, and Exhaust nozzle.
5. Give the difference between turbo propeller engine and turbo jet engine
Ans:
Turbo Propeller Engine
Turbo Jet Engine
TSFC is low
TSFC is high
Propulsive efficiency within the range
Propulsive efficiency is low
of operation is higher
Take of roll is sort and require shorter
Take of roll is longer and require longer
runway
runway
Higher frontal area
Lower frontal area
Higher weight per unit thrust
Lower weight per unit thrust
6. Give the difference between turbo jet and ram jet engines. .(Apr/May-2006)
Ans:
Turbo Jet Engine
Ram Jet Engine
Compressor and turbine are used
Compressor and turbine are not used
but diffuser and nozzle are used
Lower thrust per unit weight
Higher thrust per unit weight
Design and Construction is complicated Design and construction is easy
No launching machine is required
Launching machine is required
7. Give the difference between ram jet and pulse jet engines. .(Apr/May-2004)
Ans:
Pulse Jet Engine
Ram Jet Engine
Mechanical valve arrangements are
Works without the aid of any
used in combustion chamber
mechanical device and needs no moving
parts
The stagnation temperature at diffuser
Ram jet engine works at supersonic
exit is comparatively less
speed, the stagnation temperature is
very high
Vibration occurs due to valve
No vibration occurs
arrangement
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8. What is ram effect? ?( Apr/May-2005)
Ans: When an aircraft flies with high velocity, the incoming air is compressed to high
pressure without external work at the expense of velocity energy is known as ram effect.
9. Briefly discuss about thrust and its types. ?( Apr/May-2005)
Ans: The force which propels the aircraft forward at a given speed is called thrust. There
are two types of thrust, namely, propeller thrust and jet thrust. Thrust is calculated from
the Newton’s second law of motion; the thrust produced is equal to the rate of change of
momentum. The thrust is calculated by
F=ma(Cj – u)
where ma = Mass of air
Cj = Jet speed
u = Speed of aircraft
10. Define propulsion efficiency and thermal efficiency.(Apr/May-2004)
Ans:
Propulsive power (or) thrust power
2
Propulsion efficiency:  P 

Power output of engine
1 
Thermal efficiency:
2
 th 

Cj
Power output of the engine

1 2
Power input to the engine through fuel 2 f  CV

where f=Fuel-air ratio, CV=Calorific value,
σ = Effective speed ratio=Cj/u
11. Define Thrust specific fuel consumption , specific thrust and specific impulse
May/June-2013
Ans:
Thrust specific fuel consumption is defined as the amount of fuel consumed to produce
unit thrust.
Specific thrust is defined as the thrust produced per unit flow rate through the propulsion
system.
Specific impulse is defined as the thrust produced per unit weight flow rate through the
propulsive system.
12. What is thrust augmentation? (Nov/Dec-03)
Ans: To achieve better take-off performance, higher rates of climb and increased
performance at altitude during combat maneuvers, higher thrust output is required for a
short interval of time. This is achieved by addition of fuel in the tail pipe between the
turbine exhaust and entrance section of the exhaust nozzle. This method of thrust
increase is called thrust augmentation.
13. Why afterburners are used in turbojet engine? ?( Apr/May-2008)
Ans: Exhaust gases from the turbine have large quantity of oxygen, which can support the
combustion of additional fuel. Thus, if suitable burner is installed between the turbine
and exhaust nozzle, a considerable amount of fuel can be burned in this section to
produce temperatures entering the nozzle as high as 1900oC. This increased temperature
greatly augments the exhaust gas velocity and the thrust produced.
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14. Distinguish between air breathing engine and rocket engine?(May/June-09,Nov/Dec08
Air breathing engine
Altitude limitation
Thrust decreases with altitude
Rate of climb decreases with
altitude
Flight speed is less than the jet
velocity
Engine drag increases with flight
speed
Rocket engine
No altitude limitation, it can work in
space
Thrust improves slightly with altitude
Rate of climb increases with altitude
Flight speed is not limited, can be
greater than jet velocity
Engine has no ram drag; thrust is
constant with speed
15.How are rockets classified? (Nov/Dec-2006)
Ans:
i. Based on source of energy: chemical rocket, solar rocket, nuclear rocket, and
electrical rocket.
ii. Based on propellant used: liquid propellant rocket and solid propellant rocket
iii. Based on application: space rocket, military rocket, weather rocket, booster rocket
and retrorockets.
iv. Based on number of stages: single stage rocket and multistage rocket
v. Based on size: short range small rocket, long range small rocket and long range
large rocket
UNIT-V: SPACE PROPULSION
1. What are the types of liquid propellants? Give examples.
Ans:
i.
Storable propellants (e.g.) monomethyl hydrazine and unsymmetrical dimeylhydrazine
ii. Cryogenic propellants (e.g.) liquid oxygen, liquid hydrogen, liquid fluorine and liquid
ammonia.
2.What is monopropellant? Give examples. .(May/June-2004,13)
Ans: Monopropellant contains both fuel and oxidizer in single chemical. (e.g.) hydrogen
peroxide, hydrazine, nitroglycerine and nitro methane
.
3. List the desirable properties of liquid propellants.
Ans:
i.
The heat released during combustion per kg of the propellant should be high.
ii. The propellant should have higher density, so that they can be stored in smaller tanks
iii. The propellant should have lower freezing point.
iv.
The propellant should be non-corrosive
v. The propellant should be chemically stable
vi.
Propellant should not be poisonous and hazardous.
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4. What are the types of solid propellants? Give examples. .(May/June-2010)
Ans: Solid propellants are two types, heterogeneous or composite propellants and homogeneous
mixture of organic substances.
Heterogeneous propellants: Ammonium nitrate, potassium nitrate, ammonium percolate, and
potassium percolate as oxidizer and polymers, plastics and PVC as fuel.
Homogeneous propellants: Nitroglycerine and nitro cellulose
5. List the desirable properties of solid propellants
Ans:
i.
Propellant should be abundantly available at low cost and safe to handle
ii. Physical and chemical properties should not change during processing
iii. The heat energy released during combustion should be high
iv.
The propellant should not chemically react during processing
6. What is bipropellant? Give examples. (Nov/Dec-2005)
Ans: The fuel and oxidizer is separate chemical in a bipropellant rocket.
(e.g.) monomethyl hydrazine and unsymmetrical dimeylhydrazine, liquid oxygen,
liquid hydrogen, liquid fluorine and liquid ammonia.
7.Differentiate between monopropellant and bi-propellant. (Nov/Dec-2007)
a. Ans: Monopropellant: A liquid propellant which contains both a fuel and oxidizer in single
chemical is known a monopropellant. E.g. Nitroglycerine, Nitro methane.
b. Bi-propellant: If the fuel and oxidizer are different from each other in its chemical nature then the
propellant is called bi-propellant. Example: Liquid, oxygen, gasoline
7. What are the advantages of solid propellant rockets over liquid propellant rockets? List any
two.(Nov/dec-2005)
a. Ans: I) Liquid propellant can be reused or recharged. Hence it is economical
b. ii) Increase or decrease of speed is possible when it is in operation.
c. iii) Specific impulse is very high.
d. iv) Storing and transportation is easy as the fuel and oxidizer are kept separately.
8. Compare the propulsive efficiency of jet engine with that of rocket engine.( Nov/Dec-2006)
Ans: Propulsive efficiency of jet engine: The ratio of propulsive power or thrust power to the
power output of the engine for same jet engine and rocket engine.
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9.What is meant by restricted burning in rockets?(Nov/Dec-2006)
Ans: In this case, the inhibition material (or) restrictions prevent the propellant grain from burning
in all directions. The propellant grain burns only at some surfaces while other surfaces are prevented
from burning.
9. What is a Bypass engine and define bypass ratio?(Nov/Dec-2007)
Ans: Bypass engine
The atmospheric air enters the turbofan engine is divided into two streams namely primary air and bye
pass air. The primary air flows through the components of turbojet engine. The bye pass air flows
through the ducted passage around the turbojet engine.
Bypass ratio:
The ratio of mass flow rates of bypass air and primary air
10. Give an example for hypergolic propellant.(Nov/Dec-2010)
Ans: Hypergols do not require the ignition and they start chemical reaction as soon as the two liquids
come in contact with each other
11. List out any two uses of solid propellant rocket systems. (Nov/Dec-2009)
Ans: i) Solid fuels and oxidizers are used in rocket engines
a. ii) Burning in the combustion chamber is uncontrolled rate
b. iii) Generally stored in combustion chamber.
12. Give any two advantages and disadvantages of rockets compared to air breathing engines.
(Nov/Dec-2009)
a. Ans: Altitude limitation
b. Flight speed is less than the jet velocity
c. Engine drag increases with flight speed
d. Thrust decreases with altitude
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13. Why ram jet engine does not require a compressor and a turbine? .(Nov/Dec-2011)
Ans: In general, the speed of a ram jet engine is supersonic is very high. At this flight speed the
contribution of the compressor to the static pressure rise is insignificant. Hence, ram jet engine does not
require compressor and turbine.
14. What are inhibitors? .(Nov/Dec-2010)
Ans: Inhibitors are used to regulate (or prevent) the burning of propellant at some sections.
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1. What are the types of liquid propellants? Give examples.
Ans:
i. Storable propellants (e.g.) monomethyl hydrazine and unsymmetrical
dimeylhydrazine
ii. Cryogenic propellants (e.g.) liquid oxygen, liquid hydrogen, liquid fluorine and
liquid ammonia.
2.What is monopropellant? Give examples. .(May/June-2004,13)
Ans: Monopropellant contains both fuel and oxidizer in single chemical. (e.g.) hydrogen
peroxide, hydrazine, nitroglycerine and nitro methane
.
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