1. Classification of power plants based on
methods of aircraft propulsion
Air breathing propulsion
• Reciprocating Engines
Piston Engines
Propeller engines
• Gas turbine engines
Ramjet
Pulse jet
Turbo jet
Turbo fan
Rocket propulsion
• Chemical rockets
Solid rockets
Liquid rockets
Hybrid rockets
• Nuclear Rockets
Fission
Fusion
• Electro dynamic
Ion rocket
Plasma rocket
Photon rocket
2. Jet propulsion and Rocket propulsion
Air breathing
• Turbo fan, Turbo jet ,Turbo
prop
• Uses atmospheric Oxygen
for combustion
• Thrust and rate of climb
decreases with altitude
• Flight speed less than jet
velocity
• Reasonable efficiency &
longer flight
Non Air breathing
• Rockets & Missiles
• Carries its own
Oxidizer
• Thrust and rate of
climb increases with
altitude
• Flight speed generally
more than jet
velocity
• Lower efficiency &
lesser flight duration
3. Jet and rocket propulsion
Rocket engines are more suitable for space
applications
In space the availability of oxygen is either limited or
not available. Rocket engines carry their own oxygen
in the form of oxidizer
Rocket engines have very less components and hence
high mass ratio can be achieved
Space applications require very high velocity. Geo
synchronous satellites require the speed of earth (7.8
Km/s) to be positioned at a constant location
High speed requires higher mass ratio & higher ISP.
Both can be achieved only by rockets
6. • Advantages:
• Less maintenance cost.
• It runs smoothly because continuous thrust is
produced by continuous combustion of fuel.
• Reheat is possible to increase the thrust.
• Disadvantage:
• It has low take off thrust
• Fuel consumption is high.
• Propulsive efficiency and thrust are lower at
lower speeds.
7. Components
• Diffuser converts the kinetic energy of the incoming air into
static pressure rise which is achieved by Ram effect
• Compressor increase the pressure of incoming air by doing
work on it
• Centrifugal compressor
• Pressure ratio 4:1 or 5:1 in a single stage
• Usually double sided rotor to reduce engine diameter
• Short & sturdy appearance
• High durability, ease of manufacture & low cost
• Good operation under adverse conditions such as icing and
when sand & small foreign particles inhaled at inlet
• Have 20% weight advantage over axial compressor
• Thrust per unit weight is more
8. Centrifugal Compressor
Working principle
• Inlet casing with convergent nozzle accelerates the air
• Impellor builds up high air velocity due to its rotation.
Energy transfer takes place resulting in a rise of kinetic
energy and static pressure.
• Diffuser to transform high Kinetic Energy at the impellor
outlet to static pressure.
• Outlet casing/volute/scroll is a fluid collector
• Energy transformation takes place in two parts
• Rotating impellor imparts high velocity to air and also
increase the static pressure
• Number of fixed diverging passages/diffuser decelerates the
air increasing the static pressure
9. Centrifugal Compressor
• Air is sucked into the impellor through an accelerating nozzle
and whirled round at high speed by the vanes on the
impellor disc.
• In the impellor the flow experiences centripetal acceleration
due to pressure head.
• Hence the static pressure increases from eye to the tip of the
impellor.
• Reminder of the static pressure takes place in diffuser.
• Generally 50% in impellor and 50% in diffuser since there
will be friction loss and clearance loss in impellor.
• Impellors are highly stressed and hence straight vanes are
preferred.
10. AXIAL COMPRESSOR
• Consists of Inlet guide vanes and alternate sets of rotor and
stator blades.
• Inlet guide vanes is at the entry to guide correctly the air into
the first rotor blade.
• Rotor consists of moving sets of blades fixed to the spindle. It
imparts kinetic energy to air which is then converted into
pressure.
• Stator consists of fixed sets of blades fixed to the outer casing.
It serves to recover part of kinetic energy imparted to working
fluid. Main purpose is to change the direction of the air flow
as it leaves each stage of compressor rotor and to give proper
direction for entry into next stage. Eliminates turbulence.
Fitted with shrouds to prevent loss of air.
11. COMBUSTOR
Factors affecting combustion chamber design
• Combustion temperature within the level suitable to blade material
• Temperature distribution to be reasonably uniform to avoid local
heating. Temperature and velocity distribution at turbine inlet must be
controlled.
• Combustion must be maintained for
Different load conditions- Full load/Idling
Different air fuel ratio -60:1 to 120:1 for simple gas turbine to 100:1 to
200:1 for gas generator
Different air velocity 30-60m/sec
wide range of chamber pressure with altitude and forward speed
• Total pressure loss should be kept minimum
Cold loss due to friction because of turbulence
Hot losses due to accelerations accompanying heat addition
PLF=Pressure loss factor
K1+K2((T02/T01)-1)
K1-constant for friction loss K2-constant for hot loss
12. Combustion Process
• Mixing of fine spray of fuel droplets with air
• Vaporisation of droplets
• Breakdown of heavy H2 molecules into lighter fractions
• Intimate mixing of these hydro carbons with these O2 molecules
• Chemical reactions
• A high temperature is essential if all the above processes are to
occur sufficiently rapidly for combustion in a moving air stream to
be completed in a small space by the combustion of
approximately stoichiometric mixture.
• Approximate air fuel ratio is in the range of 100:1 while the
stoichiometric ratio is approximately 15:1, it is essential that air
should be introduced in stages
• Primary Zone
• Secondary zone
• Tertiary or dilution zone
13. Combustion Process
Primary Zone 15 to 20% of air is introduced to provide
necessary high temperature for rapid combustion.
• Flame stabilizers such as baffles establish a recirculation
zone.
• Vigorous mixing action is provided to mix air and fuel and
then to mix un burnt fuel with burnt gases.
• Stability parameter indicates it is better to have small
number of large baffles than large number of small baffles
Secondary zone 30% of air is introduced through holes in
the flame tube to complete the combustion.
• Necessity of high temperature for significant reaction rate
requires diluting air must be added only when the reaction
has gone to completion
14. Combustion Process
Tertiary or dilution zone Remaining air is mixed to
combustion products to cool them down and bring and
bring down the temperature acceptable to turbine blade
material
• Sufficient turbulence created so that hot and cold streams
are mixed thoroughly to give desired temperature
distribution with no heat streaks
Fig shows a typical combustion chamber.
• Annular space serves the purpose of
• separating the required air from total air
• providing cooling air stream which limits the temperature
of the liner, which contains a reaction zone where the
temperature can reach locally as high as 2000K
corresponding to the stoichiometric combustion
temperature
15. Combustion Chamber Types
• CAN Type -Individual combustion chamber
• Annular
• Can Annular or Cannular ( combination of 1&2)
• CAN Type
• Consists of an outer shell and a removable liner with openings to
permit compressor discharge air to enter from the outer chamber.
• 25% of air that passes through the combustion section is actually
used for combustion and the remaining air is used for cooling.
• Fuel nozzle is located at the front of the combustion chamber thro
which fuel is sprayed into the inner liner.
• Flame burns in the centre of the inner liner and is prevented from
burning the liner by a blanket of excess air which enters thro the
holes and surrounds the flame
• Advantage- Large degree of curvature and hence less warpage
• Disadvantage-i) Less utilisation of space ii) Large area of metal
required to enclose required volume
16. Annular Type
• High by pass turbo fan engines generally employs annular
type combustion chambers.
• This is a two piece assembly consisting of an inner and outer
liner.
• At the front are many fuel nozzle openings as high as 20 in
numbers with swirl vanes to vaporise the fuel.
• Two of the openings on the opposite sides are kept to keep
igniter plugs
• Advantages i) Efficient handling of air and gas ii) Efficient use
of available space iii)Requires only half the area for a
required flow.
• Disadvantages i) Lower curvature makes it susceptible to
warping
17. CAN Annular
• Can annular type has the characteristics of both
annular and can types.
• Composed of combustion chamber liners located
circumferentially within an annular combustion
chamber case.
• The large curvature of liner surface is retained
thereby maintaining a high degree of resistance to
warpage.
• Each liner has its own fuel nozzles.
• Space available is well utilised although not to the
same extent of annular type.
• Can annular combustion operates at a high pressure
level aiding efficient combustion at reduced power
and high altitude.
18. TURBINES
• Major components are Stator Nozzles and rotor blades
• Impulse and reaction turbines are the two different
types
• Impulse- Expansion of gases takes place in stator.
• Rotor blades acts as directional vanes to deflect the
flow and converts the kinetic energy of the gas into
work by changing the momentum of the gas nearly at
constant pressure.
• Reaction- Expansion takes place both in rotor and
stator. The function of stator is same as that in Impulse
turbine. But the rotor blade has two functions.
• Convert kinetic energy of the gas into Work
• Contributes a reaction force on the rotor blades
21. Turboprop engines:
• Similar to turbojet engine.
• Turbine drives the compressor and propeller. The angular velocity
of the shaft is very high and a reduction gear box is provided
before the power is transmitted to the propeller.
• Consists of an intake, compressor, combustor, turbine, and
a propelling nozzle.
• Air is drawn into the intake and compressed by the compressor.
• Fuel is then added to the compressed air in the combustor, where
the fuel-air mixture then combusts.
• The hot combustion gases expand through the turbine. Some of
the power generated by the turbine is used to drive the
compressor. The rest is transmitted through the reduction gearing
to the propeller.
• Further expansion of the gases occurs in the propelling nozzle,
where the gases exhaust to atmospheric pressure. The propelling
nozzle provides a relatively small proportion of the thrust
generated by a turboprop.
22. Turboprop Engine
• Advantages:
• High take off thrust
• Good propeller efficiency at a speed below 800km/hr
• Better fuel economy
• Sudden decrease of speed is possible by thrust
reversal
• Disadvantage:
• Propeller efficiency is rapidly decreases at high
speeds due to shocks and flow separation.
• More space is needed than turbojet engine.
• Engine construction is more complicated.
24. Turbofan Engine
• Combination of the turbo prop and the turbojet engines
combining the advantages of both.
• Similar to a turbojet uses the gas generator core (compressor,
combustor, turbine) to convert internal energy in fuel to kinetic
energy in the exhaust.
• Turbofans differ from turbojets in that they have an additional
component, a fan which is powered by the turbine section of the
engine.
• Unlike the turbojet, some of the flow accelerated by the
fan bypasses (primary air)the gas generator core of the engine and
is exhausted through a nozzle.
• The bypassed flow (secondary air)is at lower velocities, but a
higher mass, making thrust produced by the fan more efficient
than thrust produced by the core.
• Turbofans are generally more efficient than turbojets at subsonic
speeds, but they have a larger frontal area which generates more
drag
25. Turbofan Engine
• Advantages:
• High take off thrust
• Thrust developed is higher than turbojet engine
• Weight per unit thrust is lower than turbo prop
engine
• Disadvantage:
• Fuel consumption is high compared to turbo prop
engine
• Increased frontal area
• Engine construction is more complicated.
29. Ramjet Components
• Diffuser,
• Combustion chamber
• Nozzle to accelerate the exhaust gases.
• Compression for combustion comes from the
diffusion of the air stream and shock waves
from the nose cone.
30. Ramjet
• A ramjet is designed around its inlet. An object
moving at high speed through air generates a high
pressure region in front and a low pressure region to
the rear. A ramjet uses this high pressure in front of
the engine to force air through the tube, where it is
heated by combusting some of it with fuel. It is then
passed through a nozzle to accelerate it to supersonic
speeds. This acceleration gives the ramjet
forward thrust.
• A ramjet is sometimes referred to as a 'flying
stovepipe', a very simple device comprising an air
intake, a combustor, and a nozzle.
• Only moving parts are those within the turbopump,
which pumps the fuel to the combustor in a liquid-
fuel ramjet.
31. Ramjet Advantages & disadvantages
Advantages
• Low Weight
• High Thrust to Weight Ratio.
• No moving parts keep initial and maintenance costs down.
• Large Thrust to Unit Frontal Area.
• Provides best specific fuel consumption of all air breathing
engines at supersonic speeds.
Disadvantages
• Does not work well at off design Mach numbers without a
variable geometry diffuser and supersonic spike.
• By the nature of air compression, does not provide static
thrust.
• Fuel consumption at subsonic speeds is very high compared to
other air breathing engines.
32. Ramjet Thrust
• Compression and expansion phases are assumed to be
isentropic and combustion is at constant pressure.
• The ideal engine thrust is given by
where f is the fuel-air ratio, ue is the exhaust velocity
and u is the flight velocity
34. • A scramjet (supersonic combustion ramjet) is a variant of
a ramjet air breathing combustion jet engine in which the
combustion process takes place in supersonic airflow.
• It relies on high vehicle speed to forcefully compress and
decelerate the incoming air before combustion
(hence ramjet), but whereas a ramjet decelerates the air
to subsonic velocities before combustion, airflow in a
scramjet is supersonic throughout the entire engine.
• Allows the scramjet to efficiently operate at extremely high
speeds.
Advantages:
• Does not have to carry oxygen
• No rotating parts makes it easier to manufacture
• Has a higher specific impulse than a conventional engine;
could provide between 1000 and 4000 seconds, while a
rocket only provides 600 seconds or less
• Higher speed could mean cheaper access to outer space in
the future
.
35. • Disadvantages
• Cannot be started from rest
• Hypersonic flight within the atmosphere generates
immense drag, and temperatures found on the aircraft and
within the engine can be nearly six-times greater than that
of the surrounding air.
• Maintaining combustion in the supersonic flow presents
additional challenges, as the fuel must be injected, mixed,
ignited, and burned within milliseconds
• The scramjet is composed of three basic components:
• a converging inlet, where incoming air is compressed and
decelerated;
• a combustor, where gaseous fuel is burned with
atmospheric oxygen to produce heat
• a diverging nozzle, where the heated air is accelerated to
produce thrust.
36. Additional components
• Fuel injectors, a combustion chamber, a thrust nozzle and
an intake,
• Flame holder, or an area of focused waves
or pyrophoric fuel additives, such as silane, to aid
combustion at supersonic speed (A pyrophoric substance
is a substance that will ignite spontaneously in air)
• An isolator between the inlet and combustion chamber to
improve the homogeneity of the flow in the combustor
and to extend the operating range of the engine.
• Isolator protect the inlet flow from the pressure changes
in the combustion chamber. This compression is the result
of the shock train present in the isolator that, depending
on the flight regime, may extend in the core of the
combustion chamber surrounded by regions of subsonic
flow or end with a normal shock
37. COMPARISONS
• RAMJETS & SCRAMJETS
• Both relies on high vehicle speed to forcefully compress and decelerate
the incoming air before combustion and must be accelerated to the
required velocity by some other means of propulsion, such as turbojet,
rail gun, or rocket engines.
• Ramjet decelerates the air to subsonic velocities before combustion,
whereas airflow in a scramjet is supersonic throughout the entire
engine.
• High speed in a Scramjet makes the control of the flow within the
combustion chamber more difficult. Since the flow is supersonic, no
upstream influence propagates within the free stream of the combustion
chamber. Thus throttling of the entrance to the thrust nozzle is not a
usable control technique. In effect, a block of gas entering the
combustion chamber must mix with fuel and have sufficient time for
initiation and reaction, all the while traveling supersonically through the
combustion chamber, before the burned gas is expanded through the
thrust nozzle. In Ramjet combustion is relatively simpler on account of
subsonic velocity and use of flame holder
38. • Fuel injection and management is also potentially complex in a
Scram jet. One possibility would be that the fuel be pressurized to
100 bar by a turbo pump, heated by the fuselage, sent through the
turbine and accelerated to higher speeds than the air by a nozzle.
The air and fuel stream are crossed in a comb like structure, which
generates a large interface. Turbulence due to the higher speed of
the fuel leads to additional mixing. Complex fuels like kerosene need
a long engine to complete combustion.
• RAM JET& SCRAM JET WITH TURBO JETS
• Ramjet &scramjet does not use rotating, fan-like components to
compress the air & no moving parts are needed
• The achievable speed of the aircraft moving through the atmosphere
causes the air to compress within the nozzle & simplifies both the
design and operation of the engine.
• In comparison, typical turbojet engines require inlet fans, multiple
stages of rotating compressor fans, and multiple
rotating turbine stages, all of which add weight, complexity, and a
greater number of failure points to the engine.
39. • Due to the nature of their design, Ramjet & scramjet operation is
limited to near-hypersonic / supersonic velocities. As they lack
mechanical compressors, scramjets require the high kinetic
energy of a hypersonic flow to compress the incoming air to
operational conditions. Thus, a scramjet-powered vehicle must be
accelerated to the required velocity by some other means of
propulsion, such as turbojet, rail gun, or rocket engines.
• Scramjets are designed to operate in the hypersonic flight regime,
beyond the reach of turbojet engines, and, along with ramjets, fill
the gap between the high efficiency of turbojets and the high
speed of rocket engines.
• Turbo machinery-based engines, while highly efficient at subsonic
speeds, become increasingly inefficient at transonic speeds, as the
compressor fans found in turbojet engines require subsonic
speeds to operate.
• While the flow from transonic to low supersonic speeds can be
decelerated to these conditions, doing so at supersonic speeds
results in a tremendous increase in temperature and a loss in the
total enthalpy of the flow. Around Mach 3–4, turbo machinery is
no longer useful, and ram-style compression becomes the
preferred method.
40. • RAM JET& SCRAM JET WITH ROCKETS
• Scramjet engines are a type of jet engine, and rely on the
combustion of fuel and an oxidizer to produce thrust. Similar
to conventional jet engines, scramjet-powered aircraft carry
the fuel on board, and obtain the oxidizer by the ingestion of
atmospheric oxygen (as compared to rockets, which carry
both fuel and an oxidizing agent). This requirement limits
scramjets to suborbital atmospheric flight, where the oxygen
content of the air is sufficient to maintain combustion,
• The specific impulse of a rocket engine is independent of
velocity, and common values are between 200 and 600
seconds The specific impulse of a scramjet varies with
velocity, reducing at higher speed,
• ds, starting at about 1200s,
41. • A rocket has the advantage that its engines have very high
thrust-weight ratios (~100:1), while the tank to hold the liquid
oxygen approaches a tankage ratio of ~100:1 also. Thus a
rocket can achieve a very high mass fraction which improves
performance. By way of contrast the projected thrust/weight
ratio of scramjet engines of about 2 mean a very much larger
percentage of the take off mass is engine
• Thrust weight disadvantage is compensated for in scramjets
partly because the weight of the vehicle would be carried by
aerodynamic lift rather than pure rocket power (giving
reduced 'gravity losses'), but scramjets would take much
longer to get to orbit due to lower thrust which greatly offsets
the advantage.
• The takeoff weight of a scramjet vehicle is significantly
reduced over that of a rocket, due to the lack of onboard
oxidizer, but increased by the structural requirements of the
larger and heavier engines
42. Pulse jet engines:
• The construction of Pulsejet engine is similar
to ramjet engine.
• Consists of Diffuser, a Valve grid (contains
springs that close on their own spring
pressure), combustion chamber, spark plug,
and tail pipe (nozzle).
• Two main types of pulsejet engines, both of
which use resonant combustion and harness
the expanding combustion products to form a
pulsating exhaust jet which produces thrust
intermittently.
43. Pulse jet engines:
Valved engines use a mechanical valve to control the flow of
expanding exhaust, forcing the hot gas to go out the back of the
engine through the tailpipe only, and allow fresh air and more fuel
to enter through the intake.
Valve less pulsejets, have no moving parts and use only their
geometry to control the flow of exhaust out of the engine. Valve
less engines expel exhaust out of both the intakes and the
exhaust, most try to have the majority of exhaust go out the
longer tail pipe, for more efficient propulsion.
Advantages:
• Light weight and Less maintenance
• Unlike ramjet engine the pulsejet engine develops thrust at zero
speed.
Disadvantages:
• High rate of fuel consumption.
• Low propulsive efficiency than turbojet engines.
• High degree of vibration leads to noise pollution.
44. Classification of rocket engines
• type of energy source (chemical, nuclear, or solar),
• the basic function (booster stage, sustainer, attitude
control, orbit station keeping, etc.),
• the type of vehicle (aircraft, missile, assisted take-off,
space vehicle, etc.),
• size,
• type of propellant,
• type of construction, or number of rocket propulsion
units used in a given vehicle.
• Another way is to Classify by the method of producing
thrust.
45. Classification of rocket engines
• Thermodynamic expansion of a gas is used in the majority of
practical rocket propulsion concepts.
• The internal energy of the gas is converted into the kinetic
energy of the exhaust flow and the thrust is produced by the
gas pressure on the surfaces exposed to the gas.
• This same thermo-dynamic theory and the same generic
equipment (nozzle) is used for jet propulsion, rocket
propulsion, nuclear propulsion, laser propulsion, solar-
thermal propulsion, and some types of electrical propulsion.
• Totally different methods of producing thrust are used in
other types of electric propulsion or by using a pendulum in
a gravity gradient.
• These electric systems use magnetic and/or electric fields to
accelerate electrically charged molecules or atoms at very
low densities.
• It is also possible to obtain a very small acceleration by
taking advantage of the difference in gravitational attraction
as a function of altitude
46. Classification of rocket engines
• The energy from a high-pressure combustion
reaction of propellant chemicals, usually a fuel
and an oxidizing chemical, permits the heating
of reaction product gases to very high
temperatures (2500 to 4100°C).
• These gases subsequently are expanded in a
nozzle and accelerated to high velocities (1800
to 4300 m/sec).
• Since these gas temperatures are about twice
the melting point of steel, it is necessary to cool
or insulate all the surfaces that are exposed to
the hot gases.
47. Liquid propellant rocket engines
• use liquid propellants that are fed under pressure from tanks
or from a large turbo pump into a thrust chamber.*
• The liquid bipropellant consists of a liquid oxidizer (e.g.,
liquid oxygen) and a liquid fuel (e.g., kerosene).
• A monopropellant is a single liquid that contains both
oxidizing and fuel species; it decomposes into hot gas when
properly catalyzed.
• Gas pressure feed systems are used mostly on low thrust,
low total energy propulsion systems, such as those used for
attitude control of flying vehicles, often with more than one
thrust chamber per engine.
• Pump-fed liquid rocket systems are used typically in
applications with larger amounts of propellants and higher
thrusts, such as in space launch vehicles.
48. Liquid propellant rocket engines
• In the thrust chamber the propellants react to form hot
gases, which in turn are accelerated and ejected at a high
velocity through a supersonic nozzle, there by imparting
momentum to the vehicle.
• A nozzle has a converging section, a constriction or throat,
and a conical or bell-shaped diverging section as further
described in the next two chapters.
• Some liquid rocket engines permit repetitive operation and
can be started and shut off at will. If the thrust chamber is
provided with adequate cooling capacity, it is possible to run
liquid rockets for periods exceeding 1 hour, dependent only
on the propellant supply.
• A liquid rocket propulsion system requires several precision
valves and a complex feed mechanism which includes
propellant pumps, turbines, or a propellant-pressurizing
device, and a relatively intricate combustion or thrust
chamber.
52. Solid propellant rocket motors
• The propellant to be burned is contained within the
combustion chamber or case.
• The solid propellant charge is called the grain and contains
all the chemical elements for complete burning.
• Once ignited, burns smoothly at a predetermined rate on all
the exposed internal surfaces of the grain. Initial burning
takes place at the internal surfaces of the cylinder
perforation and the four slots. The internal cavity grows as
propellant is burned and consumed.
• The resulting hot gas flows through the supersonic nozzle to
impart thrust.
• Once ignited, the motor combustion proceeds in an orderly
manner until essentially all the propellant has been
consumed.
• There are no feed systems or valves
53. Hybrid propellant rocket
• Hybrid propellant rocket
propulsion systems use
both a liquid and a solid
propellant.
• When liquid oxidizing
agent is injected into a
combustion chamber
filled with solid
carbonaceous fuel grain,
the chemical reaction
produces hot combustion
gases
55. Advanced propulsion systems
Chemical rockets
• Relatively low values of
specific impulse<500
• relatively light machinery
(i.e., low engine weight)
• very high thrust
capability,
• high acceleration
• high specific power
• upper limit on specific
impulse as energy linked
to propellant mass .
Advanced Propulsion
devices
• have a very high specific
impulse>2000
• low thrust <1N
• Longer operation period
• heavy electric power
• no upper limit on specific
impulse as any amount of
electrical energy can be
added
56. Classification
Electric Rocket
Propulsion
Electro Thermal
• Resist to jet
• Arc jet
Electro Static
• Ion Propulsion
• Arc jet
Electro Magnet
• Pulsed Plasma Thruster
• Magneto Plasma Dynamic
Thruster
• Hall Thruster
Nuclear Rocket
Propulsion
• Nuclear Thermal
• Nuclear Electric
Solar Propulsion
• Solar Thermal
• Solar Sail
57. Types of Electric propulsion
• Electrothermal. Propellant is heated electrically and
expanded thermodynamically, the gas is accelerated
to supersonic speeds through a nozzle, as in the
chemical rocket.
• Electrostatic. Acceleration is achieved by the
interaction of electrostatic fields on non-neutral or
charged propellant particles such as atomic ions,
droplets, or colloids.
• Electromagnetic. Acceleration is achieved by the
interaction of electric and magnetic fields within a
plasma. Moderately dense plasmas are high
temperature or non equilibrium gases, electrically
neutral and reasonably good conductors of electricity.
58. Applications
• Overcoming translational and rotational perturbations in
satellite orbits, such as north-south station keeping (NSSK) of
satellites in geosynchronous orbits (GEO) or aligning telescopes
or antennas or drag compensation of satellites in low (LEO) and
medium earth orbits (MEO).
• For a typical north-south station-keeping task in a 350-km orbit,
a velocity increment of about 50 m/sec every year might be
needed .For a 15 years mission chemical rockets may require
around 750 kg of propellant where as electric propulsion with
Isp >2800 requires < 100 kg of propellant .Considering the
heavy electrical system weight a propellant weight saving of
450 can be achieved which goes as useful satellite mass.
• Increasing satellite speed such as orbit raising from a low earth
orbit (LEO) to a higher orbit or even to a geosynchronous orbit
(GEO).
• Potential missions such as interplanetary travel and deep space
probes, return to the moon, missions to Mars, Jupiter, and
missions to comets and asteroids
59.
60.
61. Electric propulsion thruster
• Use electrical energy for heating and/or directly ejecting
propellant, utilizing an energy source that is independent of
the propellant itself.
• Subsystems
• Raw energy source such as solar or nuclear energy with its
auxiliaries such as concentrators, heat conductors, pumps,
panels, radiators, and/or controls
• Conversion devices to transform this energy into electrical
form at the proper voltage, frequency, pulse rate, and
current suitable for the electrical propulsion system
• Propellant system for storing, metering, and delivering the
propellant;
• One or more thrusters to convert the electric energy into
kinetic energy of the exhaust.
62. Resisto jet
• Simplest type of electrical thruster &the technology is based
on conventional conduction, convection, and radiation heat
exchange.
• The propellant is heated by flowing over an ecnomically
heated refractory-metal surface, such as coils of heated wire,
heated hollow tubes, over heated knife blades, and over
heated cylinders.
• Power requirements range between 1 W and several
kilowatts; a broad range of terminal voltages, AC or DC, can be
designed for, and there are no special requirements for power
conditioning.
• Thrust can be steady or intermittent as programmed in the
propellant flow
• Material limitations presently cap the operating temperatures
to under 2700K, yielding maximum specific impulses of about
300 sec.
63.
64. Arc Jet Propulsion
• Propellant is heated
to high temperature
in an electric arc
and then expanded
in a conventional
nozzle
• Hydrogen and
Helium are the
general propellant
used
• Helium is preferred
over Hydrogen as it
is monoatomic and
does not dissociate
65. Electrostatic thrusters
• Electron bombardment thrusters. Positive ions from a
monatomic gas are produced by bombarding the gas or
vapor, such as xenon or mercury, with electrons
emitted from a heated cathode. Ionization can be
either DC or RF.
• Ion contact thrusters. Positive ions are produced by
passing the propellant vapor, usually cesium, through a
hot (about 1100°C or 2000°F) porous tungsten contact
ionizer. Cesium vapor was used extensively in the
original ion engines.
• Field emission or colloid thrusters. Tiny droplets of
propellant are charged either positively or negatively
as these droplets pass through an intense electric field
discharge. The stability of large, charged particles
remains a challenge.
68. Electron bombardment ion thruster
• Ionization of a gas by electron bombardment is a well-established
technology
• Electrons are emitted from a thermionic (hot) cathode or the more
efficient hollow cathode and are forced to interact with the gaseous
propellant flow in a suitable ionization chamber.
• The chamber pressures are low, typically 10 -3 torr or 0.134 Pa.
• Figure depicts a typical electron bombardment ionizer which
contains neutral atoms, positive ions, and electrons.
• Emitted electrons are attracted toward the cylindrical anode but are
forced by the axial magnetic field to spiral in the chamber, causing
numerous collisions with propellant atoms which lead to ionization.
A radial electric field removes the electrons from the chamber and
an axial electric field moves the ions toward the accelerator grids.
• These grids act as porous electrodes, which electrostatically
accelerate the positive ions.
69. Electromagnetic Thrusters
• Electromagnetic Thrusters are the propulsion device s which accelerates
propellant gas that has been heated to a plasma state.
• Plasmas are mixtures of electrons, positive ions, and neutrals that readily
conduct electricity at temperatures usually above 5000 K or 9000 R.
• According to electromagnetic theory, whenever a conductor carries a
current perpendicular to a magnetic field, a body force is exerted on the
conductor in a direction at right angles to both the current and the
magnetic field.
• Unlike the ion engine, this acceleration process yields a neutral exhaust
beam. Another advantage is the relatively high thrust density, or thrust
per unit area, which is normally about 10 to 100 times that of the ion
engines.
• For all of these devices the plasma is part of the current-carrying electrical
circuit and most are accelerated without the need for area changes.
• Motion of the propellant, a moderate-density plasma or in some cases a
combination of plasma and cooler gas particles, is due to a complex set of
interactions.
• Basically, the designer of an electromagnetic thruster tries to (1) create a
body of electrically conductive gas, (2) establish a high current within by
means of an applied electric field, and (3) accelerate the propellant to a
high velocity in the thrust vector direction with a significantly intense
magnetic field (often self-induced).
70. Pulsed plasma thruster (PPT)
• Figure shows the simplest plasma accelerator, employing a
self-induced magnetic field.
• This is a pulsed plasma thruster (PPT), accelerating plasmas
"struck" between two rail electrodes and fed by a capacitor,
which is in turn charged by a power supply.
• The current flow through the plasma quickly discharges the
capacitor and hence the mass flow rate must be pulsed
according to the discharge schedule. The discharge current
forms a current loop, which induces a strong magnetic field
perpendicular to the plane of the rails.
• Analogous to a metal conductor in an electric motor, the
Lorentz force acts on the plasma, accelerating it along the
rails. For a rail width s, the total internal accelerating force
has the value F = slB, where I is the total current and B the
magnitude of the self-induced field.
• Hence no area changes are required to accelerate the
propellant.
73. Plasma Propulsion- Rail accelerator
• Simplified diagram of a rail accelerator for self-induced
magnetic acceleration of a current-carrying plasma.
• When the capacitor is discharged, an arc is struck at
the left side of the rails. The high current in the plasma
arc induces a magnetic field.
• The action of the current and the magnetic field causes
the plasma to be accelerated at right angles to both the
magnetic field and the current, namely in the direction
of the rails.
• Each time the arc is created a small amount of solid
propellant (Teflon) is vaporized and converted to a
small plasma cloud, which (when ejected) gives a small
pulse of thrust. Actual units can operate with many
pulses per second.
