Electric propulsion

THEORY OF PROPULSION Electric Propulsion

Theory of Propulsion 2

Limitations of Chemical RocketsLimitations of Chemical Rockets

• Chemical rocket: exhaust ejection velocity Chemical rocket: exhaust ejection velocity intrinsically limitedintrinsically limited by the propellant-oxidizer by the propellant-oxidizer reactionreaction

• Larger velocity increment of the spacecraft could Larger velocity increment of the spacecraft could be obtained only with a be obtained only with a larger ejected masslarger ejected mass flow. flow.

• Mission Mission practical limitationpractical limitation: exceedingly large : exceedingly large amount of propellant that needs to be stored amount of propellant that needs to be stored aboardaboard


“The acceleration of gases for propulsion by electric heating and/or by electric and magnetic body forces.”

The general classes of systems for electric propulsion are:

•Electrostatic propulsion devices

•Electrothermal propulsion devices

•Electromagnetic propulsion devices.

Electric propulsion


Advanced (Electric) PropulsionAdvanced (Electric) Propulsion

Features:Features:• High exhaust speed (High exhaust speed (i.e.i.e. high specific impulsehigh specific impulse), ),

much greater than in conventional (chemical) much greater than in conventional (chemical) rocketsrockets

• Much Much less propellant consumptionless propellant consumption (much higher (much higher efficiency in the fuel utilization)efficiency in the fuel utilization)

• Continuous propulsionContinuous propulsion: apply a smaller thrust for : apply a smaller thrust for a longer timea longer time

• Mission Mission flexibilityflexibility (Interplanetary travel, defense) (Interplanetary travel, defense)


Electric Propulsion ConceptsElectric Propulsion Concepts

• Variety of designs to accelerate Variety of designs to accelerate ions or plasmasions or plasmas• Most concepts utilize grids or electrodes: Most concepts utilize grids or electrodes: power power

and endurance limitationsand endurance limitations• Ion EngineIon Engine• Hall ThrusterHall Thruster• RF Plasma Thrusters (ECR, VASIMR, Helicon RF Plasma Thrusters (ECR, VASIMR, Helicon

Double Layer)Double Layer)• Magnetoplasma Dynamic (MPD) ThrustersMagnetoplasma Dynamic (MPD) Thrusters• Plasmoid Accelerated ThrustersPlasmoid Accelerated Thrusters


Electrothermal rocket engines are very similar in principle to chemical and nuclear thermal rockets, differing only in using electrical heating to raise the temperature of the propellant prior to accelerating it in a nozzle.

•Electric resistance heating, as in the “resistojet”

•Heating of the propellant by a high energy arc discharge passing through it, as in the “arc jet”

•Heating by passing radio frequency (RF) electromagnetic waves through the propellant to heat it.

Fairly high F at reasonably high Isp, but thermal limitations are the same as in chemical and nuclear thermal rocket engines.

Electrothermal rockets


Resistojet

• Propellant: Nitrogen, Xenon,butane & most gases• Thrust: up to 100 mN• Feed pressure: up to 10 bar• Operation temperature to 500°C• Redundant heaters• A resistojet works by super-heating a propellant fluid, such as water or nitrous oxide, over an electrically-heated element and allowing the resulting hot gas to escape through a converging-diverging nozzle. Thrust and specific impulse (a measure of the engine's efficiency) are limited by the material properties of the resistor.

http://www.daviddarling.info/encyclopedia/N/nitrous_oxide.html


Resistojet

Heat exchanger

Electric power supply

Valve Nozzle


Arcjet

A simple, reliable form of electrothermal propulsion used to provide brief, low-power bursts of thrust, such a satellite needs for station-keeping. A nonflammable propellant is heated, typically changing state from liquid to gas, by an electric arc in a chamber. It then goes out the nozzle throat and is accelerated and expelled at reasonably high speed to create thrust. Arcjets can use electrical power from solar cells or batteries, and any of a variety propellants. Hydrazine is the most popular propellant, however, because it can also be used in a chemical engine on the same spacecraft to provide high thrust capability or to act as a backup to the arcjet.

http://www.daviddarling.info/encyclopedia/E/electrothermalprop.html

http://www.daviddarling.info/encyclopedia/S/station-keeping.html

http://www.daviddarling.info/encyclopedia/H/hydrazine.html


Arcjet


Electrothermal : Arc Jet

~

Propellant tank Pump

Radiator


Negative electrode

Positive electrode

Pump

Circulating nozzle wall coolant

Accelerated plasma

Arc

Propellant cooled chamber wall


1800K 1900K 2000K 100K 2200K

2300K

Anode attachment zonePropellant flow

Arcjet nozzle block temperatures


Arcjet

Type Propellant

Energy Isp,vac (s) Thrust(N)

Density(g/cc)

Resistojet

N2, NH3,N2O4, H2

Resistive heatingη=0.9

150-700

0.005-0.5 0.28, 0.60,1.0, 0.019

Arcjet NH3, H2,N2H4

Arc heatingη=0.3

450-1500

0.05-5 0.60, 0.0191.0


Electrostatic rockets

The temperature limitations of electrothermal rockets may be avoided if the acceleration of the propellant is achieved by electric body forces. The ion rocket accomplishes this by using -

•An ion source to produce a stream of positively charged particles•A negatively charged grid electrode to electrostatically accelerate the ions•An electron source to neutralize the accelerated ions

Thus there is no physical nozzle or pressure chamber and the only temperature limitations are on the ion source device.


To cause ionization there are mainly 5 different procedures, they are :

• electron bombardment

• radio frequency

• field emission

• microwave

• ion contact

Types Of Electostatic Propulsion Thrusters::


Ion rocket propulsion

Def:- A form of electric space propulsion in which ions are accelerated by an electrostatic field to produce a high-speed (typically about 30 km/s) exhaust. An ion engine has a high specific impulse (making it very fuel-efficient) but a very low thrust. Therefore, it is useless in the atmosphere or as a launch vehicle, but extremely useful in space where a small amount of thrust over a long period can result in a big difference in velocity.

This makes an ion engine particularly useful for two applications: (1) as a final thruster to nudge a satellite into a higher orbit and or for orbital maneuvering or station-keeping, and

(2) as a means of propelling deep-space probes by thrusting over a period of months to provide a high final velocity. The source of electrical energy for an ion engine can be either solar (see solar-electric propulsion) or nuclear

http://www.daviddarling.info/encyclopedia/E/electricprop.html

http://www.daviddarling.info/encyclopedia/I/ion.html

http://www.daviddarling.info/encyclopedia/S/specificimpulse.html

http://www.daviddarling.info/encyclopedia/T/thrust.html


Electrostatic: Ion Rocket in space

Ion source

Accelerating electrode

Neutralizer: Electron emitter

Electrons

Ions

Battery

Propellant line


Ion EngineIon Engine

• Scheme of a gridded ion engine with neutralizationScheme of a gridded ion engine with neutralization


V=V0

x=0

V=0x=xa

Ion source Accelerating grid

Electric fieldE=dV/dx

Ion stream

Ion rocket capabilities

a am N MU A=2

00

8

9a aa

VF MmU j U

A q xε

= = = ÷

Ion mass flow

Thrust

Ua


In an electron bombardment thruster, a gas propellant enters a discharge chamber at a controlled rate. A hot, hollow cathode (negative electrode) at the center of the chamber emits electrons, which are attracted to a cylindrical anode (positive electrode) around the walls of the chamber. Some of the electrons collide with and ionize atoms of the propellant, creating positively-charged ions.

Electron Bombardment


Hall Thruster Electrons are generated by a hollow cathode (negative electrode) at the downstream end of the thruster. The anode (positive electrode) or "channel" is charged to a high potential by the thruster's power supply. The electrons are attracted to the channel walls and accelerate in the upstream direction.

As the electrons move toward the channel, they encounter a magnetic field produced by the thruster's powerful electromagnets. This high-strength magnetic field traps the electrons, causing them to form into a circling ring at the downstream end of the thruster channel. The Hall thruster gets its name from this flow of electrons, called the Hall current.


Hall Thruster (II)Hall Thruster (II)

The Hall thruster scheme


Hall ThrusterHall Thruster

The Hall effect


ELECTROMAGNETIC:PPT

• PPTs use solid Teflon propellant to deliver specific impulses in the 900 - 1,200 s range and very low, precise impulse "bits" (10-1,000 μNs) at low average power (< 1 to 100 W)

• PPTs inherently inefficient (η ~5%)– Simplicity and low impulse bits provide highly useful– Precision-flying of a spacecraft constellation

• PPT consists of a coiled spring that feeds Teflon propellant bar, an igniter plug to initiate a small-trigger electrical discharge, a capacitor, and electrodes through which current flows

• Plasma is created by ablating Teflon from discharge of capacitor across electrodes

• Plasma is then accelerated to generate thrust by Lorenz force that is established by current and its induced magnetic field

Theory of Propulsion 25Courtesy NASA GRC

Pulsed plasma thruster


MagnetoPlasma AccelerationMagnetoPlasma Acceleration


MagnetoPlasma Dynamic ThrusterMagnetoPlasma Dynamic Thruster


ELECTROMAGNETIC: MPD• Electromagnetic devices pass a large current through a small amount of gas

to ionize propellant• Once ionized, plasma is accelerated by electromagnetic body force called

Lorentz force which is created by interaction of a current (j) with magnetic field (B):

F=j x B

• Current provided between energized positive and negative electrodes, while magnetic field is either induced by (created from) current itself, applied externally via an electromagnet or both

• Strength of Lorentz force for an MPD thruster with a self-induced magnetic field is roughly proportional to ratio J2 / mdot, where J is total thruster current

• While gas-phase propellants like hydrogen and lithium (after vaporization) can be used, solid propellants can also be used in pulsed electromagnetic accelerators called pulsed plasma thrusters (PPTs).


~

Electromagnetic: MHD enginePropellant tank Pump


Negative electrode

Positive electrode

Arc

Anode

Cathode

Electric current

Accelerating plasma

J

B

J X BElectromagnetic force (J X B)

Magnetic field coils


ELECTROMAGNETIC: MPD


Electric Propulsion ApplicationsElectric Propulsion Applications

1.1. ISSISS

2.2. Interplanetary MissionsInterplanetary Missions

3.3. Commercial/DefenseCommercial/Defense


REFERENCES ::

• http://www.daviddarling.info/encyclopedia.hmtl

• http://www.mypptsearch.com/index.php

• http://www.2dix.com

• http://www.jetaerospace.org/


THANK YOU……..

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