Electrical Propulsion Seminar Report

Electrical Propulsion

Seminar 2012

SEMINAR REPORT On

ELETRICAL PROPULSION Submitted in partial fulfillment of the requirements For the award of the degree of Bachelor of Technology In Electrical and Electronics Engineering Of COCHIN UNIVERSITY OF SCIENCE AND TECHNOLOGY Submitted by

Rishal Mathew

Department of Electrical & Electronics Engineering School of Engineering Kochi-22 SEPTEMBER 2012

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SCHOOL OF ENGINEERING COCHIN UNIVERSITY OF SCIENCE AND TECHNOLOGY DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING This is to certify that the report

Electrical Propulsion

IS A BONAFIDE RECORD OF THE WORK RELATED TO THE PAPER EEE 706 SEMINAR DONE BY

Rishal Mathew REGISTER NO: 19100046

Awarded On __________, 2012

Staff in Charge: Sheena K M

ELECTRICAL & ELECTRONICS DEPT.

Head Of the Department: DR C A BABU

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ABSTRACT Satellites are the peak of human technical expertise which has enabled many more technical innovations which are taken for granted. To put these satellites into the earth’s orbit, the spacecraft has to escape earth’s gravity to put a satellite in orbit or send a space mission. To accomplish this, the spacecraft has to be propelled upwards. Various propulsion systems have been designed to achieve the purpose depending on the environment the spacecraft operates. Electric or ion propulsion is the newest propulsion system that NASA has put into successful practical operation. The Deep Space 1 mission used the ion engine as its primary propulsion system and tested its capabilities for the 21st century. Its advantages over conventional propulsion include lower fuel weight, much higher fuel efficiency, and longer operational life. The NSTAR engine that operated on Deep Space 1 used electric propulsion. In propulsion systems, fuel efficiency is technically referred to as specific impulse or the amount of momentum increase for a given amount of fuel consumption. Given a sufficiently long mission time, an ion engine is able to achieve speeds far greater than any chemical rocket. The use of the ion engine will undoubtedly be the best choice of propulsion for space probes in the 21st century.

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CONTENTS 1. INTRODUCTION .................................................................................................... 5 2. GENERAL PRINCIPLES.........................................................................................6 3. OVERVIEW OF ELECTRIC PROPULSION………………………………………8 4. ELECTRO-THERMAL PROPULSION………………………………………….....9 a. Resistojet………………………………………………………………………….9 b.Arcjets……………………………….……………………………………………10 5.ELECTROSTASTIC PROPULSION………………………………………………..11 a. Ion Thruster……………………………………………………………………11 b. Hall Effect Thruster…………………………………………………………13 6. ELECTROMAGNETIC PROPULSION…………………………………………..14 a. Pulsed Plasma Thruster…………………………………………………...14 b. Magneto-Plasma Thruster……………………………………………….15

7. OTHER TERMS RELATED WITH PROPULSION…………………………..16 5. CONCLUSION…………………………………………………………………………...20 6. REFERENCE…………………………………………………………………..……....21

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INTRODUCTION What is propulsion? The word is derived from two Latin words: pro meaning before or forwards and pellere meaning to drive. Propulsion means to push forward or drive an object forward. A propulsion system is a machine that produces thrust to push an object forward. On airplanes, thrust is usually generated through some application of Newton's third law of action and reaction. A gas, or working fluid, is accelerated by the engine, and the reaction to this acceleration produces a force on the engine. Rocket propulsion is any method used to accelerate spacecraft and artificial satellites. There are many different methods. Each method has drawbacks and advantages. However, most spacecraft today are propelled by forcing a gas from the back/rear of the vehicle at very high speed through a supersonic de Laval nozzle. Ion propulsion is a new technology that has been fully tested and implemented in experimental space craft. Research in to this unique type of propulsion began in 1950’s. Commonly referred to as electric propulsion, ion propulsion system are particular types of electric propulsion. The most noticeable difference between a fully loaded conventional rocket and an electric propulsion system would be the mass of fuel required to produce thrust. While conventional chemically fueled rocket require millions of kilograms of propellant, ion propulsion system require only a

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small amount of propellant by comparison. Also all electric engines are highly efficient and reliable, making them excellent choices for long, unattended operation. One of the most applicable areas for electric propulsion is space exploration

GENERAL PRINCIPLES The fundamental physical principle involved in rocket propulsion was formulated by Sir Isaac Newton. According to his third law of motion,

the

rocket

experiences

an

increase

in

momentum proportional to the momentum carried away in the exhaust,

where M is the rocket mass, ΔvR is the increase in velocity of the rocket in a short time interval, Δt, m° is the rate of mass discharge in the exhaust, ve is the effective exhaust velocity (nearly equal to the jet velocity and taken relative to the rocket), and F is force. The quantity m°ve is the propulsive force, or thrust, produced on the rocket by exhausting the propellant,

Evidently thrust can be made large by using a high mass discharge rate or high exhaust velocity. Employing high m° uses up the propellant supply quickly (or requires a large supply), and so it is

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preferable to seek high values of ve. The value of ve is limited by practical considerations, determined by how the exhaust is accelerated in the supersonic nozzle and what energy supply is available for the propellant heating. Most rockets derive their energy in thermal form by combustion of condensed-phase propellants at elevated pressure. The gaseous combustion products are exhausted through the nozzle that converts most of the thermal energy to kinetic energy. The maximum amount of energy available is limited to that provided by combustion or

by

practical

considerations

imposed

by

the

high temperature involved. Higher energies are possible if other energy are used in conjunction with the chemical propellants on board the rockets, and extremely high energies are achievable when the exhaust is accelerated by electromagnetic means

. Fig 4: action-reacton concept

The effective exhaust velocity is the figure of merit for rocket propulsion because it is a measure of thrust per unit mass of propellant consumed—i.e.,

Values of ve are in the range 2,000–5,000 metres (6,500–16,400 feet) per second for chemical propellants, while values two or three times ELECTRICAL & ELECTRONICS DEPT.

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that are claimed for electrically heated propellants. Values beyond 40,000 metres (131,000 feet) per second are predicted for systems using electromagnetic acceleration.

OVERVEIW OF ELECTRIC PROPULSION There are basically two types rockets. They are chemical rockets and electrical systems. There are some limitations for each systems. Energy is limited in the case of chemical rockets. They are limited to energy contained in propellants and they have high power(W=J/s) is due to rapid conversion of energy . Power is limited in the electrical systems. There is no limit to energy added in propellant ( In Theory). However , rate conversion of energy to power limited by mass of conversion equipment which must be carried ,Melectrical. It is possible to achieve very high exhaust velocities at cost of high power consumption. Electric propulsion is broadly defined as acceleration of propellants by 1. Electrothermal 2. Electrostatic 3. Electomagetic

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Eletrothermal PROPULSION Propellent is electrically heated through wall (resistojet) or by electrical arc discharge (arcjet). The model for thermal thrust chamber is applicable. It uses electricity to heat propallent ( resistojet,arcjet). Energy is limited by wall temperature heating. Specific impulse not be much greater than H2-O2 chemical rocket . Its uses unless pure H2 as propellant.

Resisojet:-In the resistojet subclass of devices, chamber temperature is necessarily limited by the materials of the walls and/or heater coils to some 3000◦ K or less, and hence the exhaust velocities, even with equilibrated hydrogen, cannot exceed 10,000 m/sec, which is nonetheless a factor of two or three beyond that of the best chemical rockets. In contemporary practice, lower performance but more readily space storable propellants, such as hydrazine and ammonia, along with biowaste gases such as water vapor and carbon dioxide, are more commonly employed because of their overall system advantages. Beyond the frozen flow kinetics, the major practical challenge facing resistojet technology is retaining the integrity of the insulator and heater surfaces at the very high temperatures the concept demands, while still minimizing the viscous and radiative heat losses that further decrease thruster efficiency.

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Arcjets:- If an electrothermal thruster is to attain exhaust speeds substantially higher than 10,000 m/sec, interior portions of the propellant

flow

through

the heating chamber must

reach

temperatures as high as 10,000K, while being restrained from direct contact with the chamber and nozzle walls. Thus, steep radial gradients in temperature must be sustained, which renders the entire flow pattern explicitly two-dimensional. The most effective and straightforward means for achieving such profiles is by passing an electric arc directly through the chamber in some appropriate geometry . Direct currents of tens or hundreds of amperes are passed through the gas flow between an upstream conical cathode and a downstream annular anode integral to the exhaust nozzle, generating a tightly constricted arc column that reaches temperatures of several tens of thousands of degrees on its axis. The incoming propellant is usually injected tangentially, then swirls around, along, and through this arc, expanding in the anode/ nozzle to average velocities of tens of thousands of meters per second. Properly designed and operated, the chamber and nozzle walls remain tolerably cool under the steep radial gradients, and even the arc attachment regions on the cathode and anode are somewhat protected by the electrode sheath processes, even though the cathode tip must reach incandescent temperatures to provide the requisite thermionic emission of electron current.

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Electrostatic PROPULSION Charged particles (ions) accelerated by electrostatic forces (Ion Thrusters and Hall Effect type thrusters)

Ion Thruster:- An ion thruster is a form of electric propulsion used for spacecraft propulsion that creates thrust by accelerating ions. Ion thrusters are categorized by how they accelerate the ions, using either electrostatic or electromagnetic force. Electrostatic ion thrusters use the Coulomb force and accelerate the ions in the direction of the electric field. Electromagnetic ion thrusters use the Lorentz force to accelerate the ions. The term "ion thruster" by itself usually denotes the electrostatic or gridded ion thrusters. The thrust created in ion thrusters is very small compared to conventional chemical rockets, but a very high specific impulse, or propellant efficiency, is obtained. This high propellant efficiency is achieved through the very frugal propellant consumption of the ion thruster propulsion system. Ion thrusters use beams of ions (electrically charged atoms or molecules)

to

create thrust in

accordance

with momentum

conservation. The method of accelerating the ions varies, but all designs take advantage of the charge/mass ratio of the ions. This ratio means that relatively small potential differences can create very high exhaust velocities. This reduces the amount of reaction mass or fuel required, but increases the amount of specific power required

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compared to chemical rockets. Ion thrusters are therefore able to achieve extremely high specific impulses. The drawback of the low thrust is low spacecraft acceleration because the mass of current electric power units is directly correlated with the amount of power given. This low thrust makes ion thrusters unsuited for launching spacecraft into orbit, but they are ideal for in-space propulsion applications. Ion thrusters commonly utilize xenon gas. This gas has no charge and is ionized by bombarding it with energetic electrons. These electrons can be provided from hot cathode filament and accelerated in the electrical field of the cathode fall to the anode. Alternatively, the electrons can be accelerated by the oscillating electric field induced by an alternating magnetic field of a coil, which results in a selfsustaining discharge and omits any cathode The positively charged ions are extracted by an extraction system consisting of 2 or 3 multi-aperture grids. After entering the grid system via the plasma sheath the ions are accelerated due to the potential difference between the first and second grid (named screen and accelerator grid) to the final ion energy of typically 1-2 keV, thereby generating the thrust. Ion thrusters emit a beam of positive charged xenon ions only. In order to avoid charging-up the spacecraft, another cathode is placed near the engine, which emits electrons (basically the electron current is the same as the ion current) into the ion beam. This also prevents the beam of ions from returning to the spacecraft and thereby cancelling the thrust.

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Hall Effect Thrusters:-Hall effect thrusters accelerate ions with the use of an electric potential maintained between a cylindrical anode and a negatively charged plasma which forms the cathode. The bulk of the propellant (typically xenon gas) is introduced near the anode, where it becomes ionized, and the ions are attracted towards the cathode, they accelerate towards and through it, picking up electrons as they leave to neutralize the beam and leave the thruster at high velocity. The anode is at one end of a cylindrical tube, and in the center is a spike which is wound to produce a radial magnetic field between it and the surrounding tube. The ions are largely unaffected by the magnetic field, since they are too massive. However, the electrons produced near the end of the spike to create the cathode are far more

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affected and are trapped by the magnetic field, and held in place by their attraction to the anode. Some of the electrons spiral down towards the anode, circulating around the spike in a Hall current. When they reach the anode they impact the uncharged propellant and cause it to be ionized, before finally reaching the anode and closing the circuit.

Electromagnetic PROPULSION Electrically conducting accelerated by electromagnetic and pressuforces.

Pulsed Plasma Thrusters:-Pulsed plasma

thrusters are

a

method

of spacecraft propulsion also known as Plasma Jet Engines in general. They use an arc of electric current adjacent to a solid propellant, to produce a quick and repeatable burst of impulse. PPTs are excellent for attitude control, and for main propulsion on particularly small spacecraft with a surplus of ELECTRICAL & ELECTRONICS DEPT.

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electricity (those in the hundred-kilogram or less category). However they are also one of the least efficient electric propulsion systems, with a thrust efficiency of less than 10%. At present they are deployed in space vehicles and probes as space does not offer any frictional force when compared to that on earth. The extremely quick and repetitive thrust accelerates the space probe continuously. Thus it eventually reaches and goes beyond the speeds of conventional propulsion systems. The electrical energy required to operate the arc mechanism is abundantly available by harnessing the solar energy via self-adjusting solar panels on the probe. PPTs have much higher exhaust velocity than chemical propulsion engines.

Magneto-plasma-dynamic-Thruster:-The Magneto-plasmadynamic thruster (MPDT) is a form of electrically powered spacecraft propulsion which uses the Lorentz force (the force on a charged particle by an electromagnetic field) to generate thrust. It is sometimes referred to as Lorentz Force Accelerator (LFA) or (mostly in Japan) MPD arcjet. Generally, a gaseous fuel is ionized and fed into an acceleration chamber, where the magnetic and electrical fields are created using a power source. The particles are then propelled by the Lorentz force resulting from the interaction between the current flowing through the plasma and the magnetic field (which is either externally applied, or induced by the current) out through the exhaust chamber. Unlike chemical propulsion, there is no combustion of fuel. As with other electric-propulsion

variations,

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both

specific

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impulse and thrust increase with power input, while thrust per watt drops.

Electrostatic and Electromagnet are not influenced in that way. In their condition propellant is not heated. Ionized gas is accelerated by electric and magnet fields. They have higher exit velocities , but very limited by power.. In simplest form ,a plasma conductor carries a current in the direction of an applied electric field but perpendicular to a magnetic field ,with both of these vectors in turn normal to the direction of plasma direction. Thus they have only very low amount of thrust and thus they are used for long mission times

ROCKET NOZZLES The large bell or cone shaped expansion nozzle gives a rocket engine its characteristic shape. In rockets the hot gas produced in the combustion chamber is permitted to escape from the combustion

chamber

through

an

opening (the "throat"), within a high expansion-ratio 'de Laval' nozzle. Provided sufficient pressure is provided to the nozzle (about 2.5-3x above ambient pressure) the nozzle chokes and a supersonic jet is formed, dramatically accelerating the gas, converting most of the thermal energy into kinetic energy.

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The exhaust speeds vary, depending on the expansion ratio the nozzle is designed to give, but exhaust speeds as high as ten times the speed of sound of sea level air are not uncommon. Rocket thrust is caused by pressures acting in the combustion chamber and nozzle. From Newton's third law, equal and opposite pressures act on the exhaust, and this accelerates it to high speeds. About half of the rocket engine's thrust comes from the unbalanced pressures inside the combustion chamber and the rest comes from the pressures acting against the inside of the nozzle (see diagram). As the gas expands (adiabatically) the pressure against the nozzle's walls forces the rocket engine in one direction while accelerating the gas in the other.

PROPELLANT EFFICIENCY For a rocket engine to be propellant efficient, it is important that the maximum pressures possible be created on the walls of the chamber and nozzle by a specific amount of propellant; as this is the source of the thrust. This can be achieved by all of: 

heating the propellant to as high a temperature as possible (using a high energy fuel, containing hydrogen and carbon and sometimes metals such as aluminium, or even using nuclear energy)



using a low specific density gas (as hydrogen rich as possible)

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using propellants which are, or decompose to, simple molecules with few degrees of freedom to maximise translational velocity

Since all of these things minimise the mass of the propellant used, and since pressure is proportional to the mass of propellant present to be accelerated as it pushes on the engine, and since from Newton's third law the pressure that acts on the engine also reciprocally acts on the propellant, it turns out that for any given engine the speed that the propellant leaves the chamber is unaffected by the chamber pressure (although the thrust is proportional). However, speed is significantly affected by all three of the above factors and the exhaust speed is an excellent measure of the engine propellant efficiency. This is termed exhaust velocity, and after allowance is made for factors that can reduce it, the effective exhaust velocity is one of the most important parameters of a rocket engine.

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POWER REQUIREMENTS

SPECIFIC IMPULSE The most important metric for the efficiency of a rocket engine is impulse per

unit

of propellant,

impulse (usually written

this

is

called specific

). This is either measured as a speed

(the effective exhaust velocity

in meters/second or ft/s) or as a

time (seconds). Specific impulse is a ratio of thrust and mass flow rate. Thrust is the force for the forward motion Mass flow rate is the product of density, cross sectional area of nozzle and velocity of

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exhaust ions or propellant. An engine that gives a large specific impulse is normally highly desirable. The specific impulse that can be achieved is primarily a function of the propellant mix (and ultimately would limit the specific impulse), but practical limits on chamber pressures and the nozzle expansion ratios reduce the performance that can be achieved.

CONCLUSION The electric ion propulsion system is the next step in mankind's forward progress into space exploration. NASA's goal of the New Millennium Program was to test new technologies that could be used in the 21st century. In the near future, electric propulsion has the potential to fuel interplanetary manned missions to Mars and possibly even missions to the moons of Jupiter. The simple fact that the fuel efficiency of electric propulsion is so great makes it ideal for communication satellites, an area in which electric thrusters are already being widely used. Still, electric propulsion is only a small step forward. On the grand scheme of space exploration, new technologies like solar sail , magneto sail , beam-powered propulsion, Alcubierre drive that would cause the seemingly infallible principles of physics to break down would have tobe invented in order for local interstellar travel to be possible.

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REFERENCES 1.R.K John , Physics of Elecctric Propulsion, McGraw –Hill Company , New York. 2.E.Stuhlinger .Ion Propulsion for space Flight , McGraw Hill Books, New York. 3. Introduction to Plasma Physics by C.F.Jen. 4. Electric propulsion devices http://www.scribd.com/document_downloads/direct/104279831?e xtension=pdf&ft=1348688161<=1348691771&uahk=OTz6OZpWL ++GloWIqCsxVDZnEyU 5.ELECTRIC PROPULSION http://www.scribd.com/document_downloads/direct/77512867?ex tension=pdf&ft=1348686533<=1348690143&uahk=5LZ2B+Qh/h5 tG/Uc9vnDu21XFhg 6.Fundamentals of space electric propulsion http://www.scribd.com/document_downloads/direct/104279831?e xtension=pdf&ft=1348688161<=1348691771&uahk=OTz6OZpWL ++GloWIqCsxVDZnEyU

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