Structure And Purpose Of Ion Engines Engineering Essay

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An ion engine is one of the propulsion systems used in spaceflight. To sum it up it essentially it is a propulsion system that works by accelerating plasma, a charged gas to produce thrust for the space rocket or vehicle. Ion engines are better known by the term ion thrusters.

The principle was first theorized by an American pioneer in rocket science, Dr. Robert H. Goddard in 1906. He later carried out his first experiments on ion thrust in the period of 1916-1917.  The theory was later mentioned by Herman Oberth in a paper he wrote in 1923. Ion engines presently fall under two categories: the first is electrostatic and the second electromagnetic.  Each of these engines uses different principles of electromagnetic forces to make ion thrust possible.

Electrostatic ion engines work on the principle of Coulomb's Law. This principal states that an electric charge moved across a distance creates an electric field. The strength of this electric field or the potential difference is what accelerated the propellant gas in this type of ion engine. The most common example of this type of engine is the Gridded Electrostatic Thruster. It has a relative simple design. First, the propellant is a neutral gas like Xenon. At one end of the thruster is a cathode which bombards the propellant gas with electrons until it's charged becoming ionized gas or plasma. The plasma is then directed to the grids. The first grid is called a sheath and the second grid is the accelerator. The potential difference between the plasma in the two grids creates an electrostatic field which accelerates the plasma out of the thruster creating thrust for the rocket. Another cathode ray is used at the end of the thruster to maintain balance between positive and negative ions in the propelled plasma preventing it from going back into the thruster.

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Electromagnetic ion engines work on the principle of Lorentz law. Essential the force that accelerates the ionized particles is generated by an electric field being perpendicular with an alternating magnetic force. A simple example is the Pulsed Inductive Thruster. It is basically a cone with an induction coil around it and a group of capacitors touching the coil; it's a big electromagnet.  The gas is emitted into the cone while an electric current runs through the induction coil. This creates a magnetic force on the outside circle of the cone that creates a circular electric current on the inside which moves opposite of the first current. This ionizes the gas and the positive ion particles are propelled by the second current perpendicular to the magnetic force.

The ion thrust engines have pros and cons. On the plus side they are efficient. On the downside they decay quickly and have weak thrust. Sometimes an ion thruster needs to be running continuously for days and even weeks to build up significant speed.

We have written many articles about ion engines for Universe Today. Here's an article about how flat thrusters could save weight and fuel for spacecraft, and another article about NASA testing a new ion engine.

Ion Engines are the most exciting new rocket propulsion system since the Chinese invented the rocket about a thousand years ago.

Most rocket engines use chemical reactions for power. They combine various gases and liquids to form chemical explosions which push the rocket through space. Chemical rocket engines tend to be powerful but have a short lifetime.

Ion Engines use electric fields instead of chemical reactions. Ion Engines tend to be much less powerful, but they are so efficient, they can last for years before running out of fuel.

These activities should help you understand how Ion Engines work. [1, 2]

ION THRUSTERS:

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. Note that the term "ion thruster" frequently denotes the electrostatic or gridded ion thrusters, only.

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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.

(An ion thrusters for NASA's deepspace)

Due to their relatively high power needs, given the specific power of power supplies, and the requirement of an environment void of other ionized particles, ion thrust propulsion is currently practical only in outer space.

ELECTROSTATIC ION THRUSTER:

Gridded electrostatic ion thrusters:

Figure 2: A diagram of how a gridded electrostatic ion engine (Kaufman type) works

Gridded electrostatic 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 a hot cathode filament and accelerated in the electrical field of the cathode fall to the anode (Kaufman type ion thruster). Alternatively, the electrons can be accelerated by the oscillating electric field induced by an alternating magnetic field of a coil, which results in a self-sustaining discharge and omits any cathode (radiofrequency ion thruster).

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.

Gridded electrostatic ion thruster research (past/present):

NASA Solar electric propulsion Technology Application Readiness (NSTAR)

NASA's Evolutionary Xenon Thruster (NEXT)

Nuclear Electric Xenon Ion System (NEXIS) (Project Canceled)

High Power Electric Propulsion (HiPEP)

EADS Radio-Frequency Ion Thruster (RIT)

Dual-Stage 4-Grid (DS4G)

Schematic of a Hall Thruster

Hall Effect thrusters:

Hall HYPERLINK "http://en.wikipedia.org/wiki/Hall_effect_thruster"Effect thrusters accelerate ions with the use of an electric potential maintained between a cylindrical anode and negatively charged plasma which forms the cathode. The bulk of the propellant (typically xenon or bismuth 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 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.

Field emission electric propulsion (FEEP):

Field emission electric propulsion (FEEP) thrusters use a very simple system of accelerating liquid metal ions to create thrust. Most designs use either caesium or indium as the propellant. The design consists of a small propellant reservoir that stores the liquid metal, a very small slit that the liquid flows through, and then the accelerator ring. Caesium and indium are used due to their high atomic weights, low ionization potentials, and low melting points. Once the liquid metal reaches the inside of the slit in the emitter, an electric field applied between the emitter and the accelerator ring causes the liquid metal to become unstable and ionize. This creates a positive ion, which can then be accelerated in the electric field created by the emitter and the accelerator ring. These positively charged ions are then neutralized by an external source of electrons in order to prevent charging of the spacecraft hull.

ELECTROMAGNETIC THRUSTERS:

Pulsed inductive thrusters (PIT):

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Pulsed inductive thrusters (PIT) use pulses of thrust instead of one continuous thrust, and have the ability to run on power levels in the order of Megawatts (MW). PITs consist of a large coil encircling a cone shaped tube that emits the propellant gas as shown in the diagram. Ammonia is the gas commonly used in PIT engines. For each pulse of thrust the PIT gives, a large charge first builds up in a group of capacitors behind the coil and is then released. This creates a current that moves circularly in the direction of jθ as seen in the diagram. The current then creates a magnetic field in the outward radial direction (Br), which then creates a current in the ammonia gas that has just been released in the opposite direction of the original current. This opposite current ionizes the ammonia and these positively charged ions are accelerated away from the PIT engine due to the electric field jθ crossing with the magnetic field Br, which is due to the Lorentz Force.

Magnetoplasmadynamic (MPD) / lithium Lorentz force accelerator (LiLFA):

Magnetoplasmadynamic (MPD) thrusters and lithium Lorentz force accelerator (LiLFA) thrusters use roughly the same idea with the LiLFA thruster building off of the MPD thruster. Hydrogen, argon, ammonia, and nitrogen gas can be used as propellant. The gas first enters

the main chamber where it is ionized into plasma by the electric field between the anode and the cathode. This plasma then conducts electricity between the anode and the cathode. This new current creates a magnetic field around the cathode which crosses with the electric field, thereby accelerating the plasma due to the Lorentz Force. The LiLFA thruster uses the same general idea as the MPD thruster, except for two main differences. The first difference is that the LiLFA uses lithium vapor, which has the advantage of being able to be stored as a solid. The other difference is that the cathode is replaced by multiple smaller cathode rods packed into a hollow cathode tube. The cathode in the MPD thruster is easily corroded due to constant contact with the plasma. In the LiLFA thruster the lithium vapor is injected into the hollow cathode and is not ionized to its plasma form/corrode the cathode rods until it exits the tube. The plasma is then accelerated using the same Lorentz Force.

Electrode less plasma thrusters:

Electrode less plasma thrusters have two unique features: the removal of the anode and cathode electrodes and the ability to throttle the engine. The removal of the electrodes takes away the factor of erosion which limits lifetime on other ion engines. Neutral gas is first ionized by electromagnetic waves and then transferred to another chamber where it is accelerated by an oscillating electric and magnetic field, also known as the ponderomotive force. This separation of the ionization and acceleration stage give the engine the ability to throttle the speed of propellant flow, which then changes the thrust magnitude and specific impulse values.

Electro thermal thrusters:

Electro thermal thrusters use electric power to accelerate propellant. There are several types:

1. Resistojet

2. Arcjet

3.Microwave electro thermal thrusters

4. Ion Cyclotron Heating thrusters (VASIMR)

Helicon double layer thrusters:

A helicon double layer thruster is a type of plasma thruster, which ejects high velocity ionized gas to provide thrust to a spacecraft. In this thruster design, gas is injected into a tubular chamber (thesource tube) with one open end. Radio frequency AC power (at 13.56 MHz in the prototype design) is coupled into a specially shaped antenna wrapped around the chamber. The electromagnetic waveemitted by the antenna causes the gas to break down and form a plasma. The antenna then excites a Helicon wave in the plasma, which further heats the plasma. The device has a roughly constantmagnetic field in the source tube (supplied by Solenoids in the prototype), but the magnetic field diverges and rapidly decreases in magnitude away from the source region, and might be thought of as a kind of magnetic nozzle. In operation, there is a sharp boundary between the high density plasma inside the source region, and the low density plasma in the exhaust, which is associated with a sharp change in electrical potential. The plasma properties change rapidly across this boundary, which is known as a current free electric double layer. The electrical potential is much higher inside the source region than in the exhaust, and this serves both to confine most of the electrons, and to accelerate the ions away from the source region. Enough electrons escape the source region to ensure that the plasma in the exhaust is neutral overall.

APPLICATIONS:

Ion thrusters have many applications for in-space propulsion. The best applications of the thrusters make use of the long lifetime when significant thrust is not needed. Examples of this include orbit transfers, attitude adjustments, drag compensation for low earth orbits, transporting cargo such as chemical fuels between propellant depots and ultra fine adjustments for more scientific missions. Ion thrusters can also be used for interplanetary and deep space missions where time is not crucial. Continuous thrust over a very long time can build up a larger velocity than traditional chemical rockets. [6]

DS4G ion engine:

The dual-stage 4-grid (DS4G) thruster is a new design for a highly efficient ion engine designed and built at the Australian National University and sold to the European Space Agency (ESA) which laid down some basic conceptual requirements. According to the results of tests, announced in January 2006, DS4G achieved an exhaust velocity of 210 kilometers per second - more than 10 times faster than possible with the ion engines used on Deep Space 1 and SMART-1, and four times faster than the latest prototype ion engine designs.

How it works:

Traditional ion engines use three closely separated perforated grids containing thousands of millimeter-sized holes attached to a chamber containing a reservoir of charged particles. These systems effectively extract and accelerate the ions in one stage, which because of physical constraints limits the extraction potential applied between the first and second grids to 5 000 V. The DS4G ion engine solves this limitation by effectively decoupling the acceleration from the extraction process into a two-stage system. This allows for independent throttling of the exhaust velocity but more importantly allows very high accelerating fields to be applied to the second stage without adversely affecting the extraction field. The test model has reached total acceleration potentials as high as 30,000 V, resulting in the high exhaust velocity noted above. [3]

Future missions using DS4G engines:

"Using a similar amount of propellant as SMART-1, a future spacecraft using our new engine design wouldn't just reach the Moon, it would be able to leave the Solar System entirely," according to an ESA press release. Once developed into full flight ready devices, these engines will propel spacecraft to the outermost planets, the newly discovered planetoids beyond Pluto and further into interstellar space, all with-in the working lifetime of a mission scientist.

Closer to home, these supercharged ion engines could figure prominently in the human exploration of space. With an adequate supply of electrical power, a small cluster of larger, high power versions of the new engine design would provide enough thrust to propel a crewed spacecraft to Mars and back.

ION PROPULSION:

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 (see nuclear-electric propulsion).

Two types of ion propulsion have been investigated in depth over the past few decades: electron bombardment thrusters and contact ion thrusters. Of these, the latter remains in the research stage while the former has already been used on a number of spacecraft. Specifically, the variety of electron bombardment thrusters known as XIPS (a Hughes/Boeing product) is used for station-keeping by some geosynchronous satellites, while the NSTAR ion engine (developed by NASA and Hughes) propelled the Deep Space 1 interplanetary probe.

One of the most promising new developments in ion propulsion is the DS4G (dual-stage 4-grid) ion engine, developed by the European Space Agency and a group at the Australian National University. This was first tested by ESA in 2005. The DS4G thruster achieves much higher voltages to be used than previously thought possible, resulting in a more powerful post acceleration of the extracted ions. The thruster was tested in a large space simulation chamber in the ESA Technology centre in the Netherlands at a remarkable 30,000 V and produced an ion exhaust plume that travelled at 210 km/s - over four times faster than state-of-the-art ion engine designs achieve.

History of ion propulsion:

A NASA engineer prepares an early ion engine for a vacuum chamber test in 1959. Lined up at right are the major electrical parts.

Among the most difficult challenges in the early development of ion engines was proving that injecting electrons could neutralize an ion beam. Continually spewing positively charged ions will leave a spacecraft with a negative charge so great that the ions are attracted back to the spacecraft. The solution is an electron gun that dumps the electrons into the ion stream, thus neutralizing both spacecraft and exhaust. But the beam's interaction with the walls of even a large vacuum chamber makes it very difficult to conduct meaningful beam neutralization experiments on Earth. These uncertainties led to considerations for flight testing electric engines. Another challenge of electronic propulsion involved developing an efficient technique to produce ions. Working at NASA's Lewis, Harold Kaufman invented an electron-bombardment technique to ionize mercury atoms. At NASA/Marshall, a process was under development whereby cesium atoms would become ionized upon contact with a hot tungsten or rhenium surface. Marshall's major development in electrical propulsion centered, however, on a 30-kilowatt ion engine development contract, initiated in September 1960 with Hughes Research Laboratory in Malibu, California. At first, Marshall directed Hughes to design a laboratory model of an ion engine. The 0.01 lb.-thrust model would be followed by the development of a 0.1 lb.-thrust engine. Marshall later modified the Hughes contract to include a flight test model ion engine, primarily to determine whether a beam neutralization problem existed in space.

On Aug. 1, 1961, NASA awarded a contract to the Astro-Electronics Division of RCA to design and build a payload capsule for flight-testing electric propulsion engines. The program called for seven capsules, three for ground tests and four for actual flight tests. Each capsule was expected to carry two electric engines. The first was expected to carry one cesium-fueled ion-engine representing Stuhlinger's design with the Hughes engine. The second was expected to carry one mercury-fueled ion engine representing Kaufman's design with the Lewis engine. Plans called for the engines to operate from 1 to 2 kW of power. Hughes demonstrated an ion engine on Sep. 27, 1961, at its research laboratories in Malibu. Stuhlinger was among those on hand to greet the scientific and technical writers who attended the event.

Ion propulsion in science fiction:

Frequent mention of ion propulsion has been made in works of science fiction for several decades. It was featured, for example, in a September 1968 episode of Star Trek called "Spock's Brain," in which invaders steal Spock's brain and flee in an ion-powered spacecraft.

A NASA engineer prepares an early ion engine for a vacuum chamber test in 1959. Lined up at right are the major electrical parts. [4]

NSTAR Ion Engine:

In October 2000, The Boeing Company acquired three units within Hughes Electronics Corporation: Hughes Space and Communications Company, Hughes Electron Dynamics, and Spectrolab, Inc., in addition to Hughes Electronics' interest in HRL, the company's primary research laboratory. The four are now part of Boeing's newest subsidiary, Boeing Satellite Systems, Inc. 

NSTAR Engine:

30 centimeters

17.6 lbs (8 kg)

3100 seconds ISP

20 to 92 mN of thrust

Hughes' Ion Engine Serving as Primary Propulsion to NASA's Deep Space

In 1995, Hughes Electron Dynamics, today known as Boeing Electron Dynamic Devices, Inc., located in Torrance, Calif., was awarded a $9.2 million contract to design and manufacture the NASA Solar Electric Propulsion Technology Application Readiness (NSTAR) 30-centimeter system for validation on the New Millennium Deep Space 1 project. This would be the first time an ion engine would be used as the primary method of propulsion in a deep space mission. The system consists of an ion thruster, power processor, and digital control and interface units.

Deep Space 1 was launched on Oct. 24, 1998 from the Cape Canaveral Air Station, the first mission in NASA's New Millennium Program. The purpose of the New Millennium Program is to test and validate new technologies in a series of deep space and Earth-orbiting missions. This is the first deep space NASA mission to focus on technology, rather than science.

As one of the 12 new technologies being tested on Deep Space 1, the ion engine performs the critical role of spacecraft propulsion. It is the primary method of propulsion for the 8-1/2-foot, 1,000-pound spacecraft, and its use is preparing it for possible inclusion in future NASA space science missions.

Still on its planned 11-month technology validation mission, as of the end of February 1999, the Deep Space 1 spacecraft has traveled more than 28 million miles from Earth.

An ion engine relies on electrically charged atoms, or ions, to generate thrust. Xenon, an inert, noncombustible gas, is electrically charged and the ions are accelerated to a speed of about 62,900 miles per hour (30 kilometers per second). The ions are then emitted as exhaust from the thruster, creating a force, which propels the spacecraft in the opposite direction.

Unlike its chemical counterpart, the ion engine produces a gentle thrust, but for a very long duration. The Deep Space 1 spacecraft carried about 81.5 kilograms of xenon propellant, which will provide about 20 months of continuous thrusting, more than enough to propel Deep Space 1 throughout its entire mission. The 30-centimeter ion thruster on Deep Space 1 will eventually change the spacecraft's speed by 4.5 kilometers per second, the equivalent of 10,000 miles per hour.

Hughes and NASA began investigating the use of xenon as a propellant alternative back in the early 1960s. Other materials, such as cesium and mercury, were also investigated, but xenon was preferred because it would generate the greatest thrust and, as an inert gas, would not be hazardous to handle and process.

The NSTAR engine was designed for operation in deep space. Prolonged periods of operation in low levels of sunlight required a unique design for deep space missions. The NSTAR engine is remotely programmable from the ground, enabling ground stations to adjust the thruster's operation as needed. The spacecraft's on-board autonomous software can also adjust the operation of the thruster.

Boeing Electron Dynamic Designs also produces a commercial xenon ion propulsion system, XIPS, for use on Boeing 601HP and Boeing 702 spacecraft in geosynchronous orbit. The first satellite to fly with an onboard XIPS system was PAS-5, which was launched in August 1997.

Boeing Electron Dynamic Designs is a world leader in the design and manufacture of microwave, traveling wavetube amplifiers, and ion thrusters for commercial and military applications. [5]

View of ion engine on Deep Space 1

Deep Space  1 spacecraft during testing