Rocket Engines - Troy University

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Rocket Engines
• Liquid Propellant
– Mono propellant
• Catalysts
– Bi-propellant
• Solid Propellant
– Grain Patterns
• Hybrid
• Nuclear
• Electric
Performance
Energy
Safety
Simplicity
Expanding Gases
Thrust Termination
Restart
Rocket Propulsion
Liquid Rocket Engine
Oxidizer
Propellants
Fuel
Combustion Chamber
Throat
Nozzle
Newton’s Laws
The force required to accelerate a body is
proportional to the product of the mass of
the body and the acceleration desired.
F = ma
F
m=
a
F
a=
m
Rocket Thrust
• Thrust is produced by the expanding propellants.
• There is thrust from the difference between the ambient
pressure and that of the exhaust gases at the nozzle exit
(Pressure Thrust) and from the momentum of the
propellants (Momentum Thrust).
Total Thrust = Momentum Thrust + Pressure Thrust
Propellant Mass Flow
times Velocity
F=
.W
g
Nozzle Area times
pressure differential
Ve + Ae ( Pe - Pa)
Exhaust Plumes and Nozzles
Pexhaust < Pambient
Pexhaust = Pambient
Pexhaust > Pambient
Under Expanded
Ideal Expansion
Over Expanded
Expansion Ratio
• Ratio of the nozzle exit area divided by the
area at the nozzle throat.
x=
Ae
Throat
At
Exit
Specific Impulse
• A measure of the energy in the propellants and of
the efficiency of the rocket engine design
• Specific Impulse is the ratio of the Thrust (Force)
produced divided by the weight rate flow of
propellants
Isp =
F
.W
Mass Ratio of a Vehicle
Mass Ratio is the ratio between the booster mass before
the rocket engine burn divided by the booster mass
after rocket engine burn.
MR =
mi
mf
The Mass Ratio for a multistage rocket is the product
of the Mass Ratios of all the stages, i.e.
MROver All = MR1 x MR2 x MR3 x …x MRn
Thrust-to-Weight Ratio
• Measure of booster or stage design and
manufacturing technology.
Thrust
Y=
Vehicle Weight
=
F
W
• The higher the thrust-to-weight ratio the
faster the vehicle will accelerate
• The initial acceleration of a vehicle in “g’s”
equals
a=(Y-1)
Ideal Rocket Equation
• The ideal velocity change ( DV ) for each
stage of a rocket is a function of the mass ratio
( MR) of the stage and the specific impulse
( Isp ) of the rocket
DVi = Isp x g x ln MR
• Ideal means you do not consider gravity
changes, drag, or rotating Earth

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