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Ion Propulsion: Farther, Faster, Cheaper

12.07.04

Ion thrusters, the propulsion of choice for scientific discipline fiction writers accept become the propulsion of option for scientists and engineers at NASA. The ion propulsion arrangement's efficient apply of fuel and electric power enable mod spacecraft to travel farther, faster and cheaper than any other propulsion technology currently available. Chemical rockets have demonstrated fuel efficiencies upwards to 35 percent, simply ion thrusters have demonstrated fuel efficiencies over 90 percentage. Currently, ion thrusters are used to keep communication satellites in the proper position relative to Earth and for the primary propulsion on deep space probes. Several thrusters can be used on a spacecraft, but they are ofttimes used only 1 at a time. Spacecraft powered by these thrusters can accomplish speeds up to 90,000 meters per second (over 200,000 mph). In comparison, the Infinite Shuttles tin accomplish speeds around eighteen,000 mph.

The merchandise-off for the high top speeds of ion thrusters is depression thrust (or low acceleration). Electric current ion thrusters tin can provide only 0.5 newtons (or 0.1 pounds) of thrust, which is equivalent to the force you would feel by holding 10 U.South. quarters in your paw. These thrusters must exist used in a vacuum to operate at the available power levels, and they cannot be used to put spacecraft in space because big amounts of thrust are needed to escape Earth's gravity and atmosphere.

Image right: Artist's concept of Deep Infinite 1 probe with its ion thruster operating at full ability.
Credit: NASA

To compensate for low thrust, an ion thruster must be operated for a long time for the spacecraft to reach its height speed. Dispatch continues throughout the flight, however, then tiny, abiding amounts of thrust over a long time add up to much shorter travel times and much less fuel used if the destination is far away. Deep Space ane used less than 159 pounds of fuel in over sixteen,000 hours of thrusting. Since much less fuel must be carried into space, smaller, lower-cost launch vehicles tin be used.

Propulsion

Sir Isaac Newton's third Police force states that every activeness has an equal and opposite reaction. This is similar air escaping from the end of a balloon and propelling it forrad. Conventional chemic rockets burn a fuel with an oxidizer to make a gas propellant. Large amounts of the gas push out at relatively low speeds to propel the spacecraft.

Prototype left: Xenon ion belch from the NSTAR ion thruster of Deep Space one. Credit: NASA

Modern ion thrusters use inert gases for propellant, so at that place is no gamble of the explosions associated with chemical propulsion. The majority of thrusters use xenon, which is chemically inert, colorless, odorless, and tasteless. Other inert gases, such as krypton and argon, as well tin be used. Simply relatively minor amounts of ions are ejected, just they are traveling at very high speeds. For the Deep Space 1 probe, ions were shot out at 146,000 kilometers per hour (more than than 88,000 mph).

Making Ions and Plasma

Ion thrusters eject ions instead of combustion gases to create thrust: the force practical to the spacecraft that makes it move forrard. An ion is merely an atom or molecule that has an electrical charge because it has lost (positive ion) or gained (negative ion) an electron. With ion propulsion, the ions have lost electrons, so they are positively charged. A gas is considered to be ionized when some or all the atoms or molecules independent in it are converted into ions.

Plasma is an electrically neutral gas in which all positive and negative charges--from neutral atoms, negatively charged electrons and positively charged ions--add upward to zero. Plasma exists everywhere in nature (for example, lightning and fluorescent low-cal bulbs), and it is designated equally the fourth state of matter (the others are solid, liquid and gas). It has some of the properties of a gas but is affected by electric and magnetic fields and is a good conductor of electricity. Plasma is the building block for all types of electrical propulsion, where electric and/or magnetic fields are used to accelerate the electrically charged ions and electrons to provide thrust. In ion thrusters, plasma is made upward of positive ions and an equal amount of electrons.

NASA'south conventional method of producing ions is chosen electron bombardment. The propellant is injected into the ionization bedroom from the downstream end of the thruster and flows toward the upstream end. This injection method is preferred because it increases the time that the propellant remains in the sleeping accommodation.

In such ion thrusters, electrons are generated past a hollow cathode, called the discharge cathode, located at the center of the thruster on the upstream end. The electrons catamenia out of the discharge cathode and are attracted (similar hot socks pulled out of a dryer on a cold day) to the discharge chamber walls, which are charged highly positive past the thruster's power supply.

Image left: Ion thruster functioning: Step 1--Electrons (shown every bit small, pale dark-green spheres) are emitted by the discharge hollow cathode, traverse the discharge chamber, and are collected by the anode walls. Step 2--Propellant (shown in green) is injected from the plenum and travels toward the discharge cathode. Step 3--Electrons touch the propellant atoms to create ions (shown in blue). Step 4--Ions are pulled out of the discharge sleeping accommodation by the ion optics. Pace five--Electrons are injected into the beam for neutralization. Credit: NASA

When a loftier-energy electron (negative charge) from the discharge cathode hits, or bombards, a propellant atom (neutral accuse), a second electron is released, yielding two negative electrons and 1 positively charged ion. Loftier-strength magnets are placed along the belch chamber walls so that as electrons approach the walls, they are redirected into the discharge sleeping room by the magnetic fields. Maximizing the length of time that electrons and propellant atoms remain in the discharge chamber, increases the chances that the atoms will be ionized.

NASA also is researching electron cyclotron resonance to create ions. This method uses loftier-frequency radiation (ordinarily microwaves) coupled with a high magnetic field to add energy to the electrons in the propellant atoms. This causes the electrons to break gratuitous of the propellant atoms and create plasma. Ions tin and so be extracted from this plasma.

In a gridded ion thruster, ions are accelerated by electrostatic forces. The electric fields used for this acceleration are generated past ii electrodes, called ion optics or grids, at the downstream end of the thruster. The greater the voltage deviation between the two grids, the faster the positive ions move toward the negative charge. Each grid has thousands of coaxial apertures (or tiny holes). The ii grids are spaced close together (but not touching), and the apertures are exactly aligned with each other. Each set up of apertures (reverse holes) acts like a lens to electrically focus ions through the optics.

NASA's ion thrusters employ a two-electrode arrangement, where the upstream electrode (chosen the screen grid) is charged highly positive, and the downstream electrode (called the accelerator filigree) is charged highly negative. Since the ions are generated in a region that is highly positive and the accelerator grid's potential is negative, the ions are attracted toward the accelerator grid and are focused out of the discharge chamber through the apertures, creating thousands of ion jets. The stream of all the ion jets together is chosen the ion axle. The thrust is the force that exists between the upstream ions and the accelerator grid. The exhaust velocity of the ions in the beam is based on the voltage practical to the optics. Whereas a chemical rocket's height speed is limited by the heat-producing adequacy of the rocket nozzle, the ion thruster's top speed is limited by the voltage that is applied to the ion eyes, which is theoretically unlimited.

Because the ion thruster ejects a large amount of positive ions, an equal amount of negative charge must be ejected to go on the total charge of the frazzle beam neutral. Otherwise, the spacecraft itself would concenter the ions. A second hollow cathode called the neutralizer is located on the downstream perimeter of the thruster and pushes out the needed electrons.

Electric Propulsion System

The ion propulsion system consists of v main parts: the power source, the power processing unit, the propellant direction organisation, the control computer, and the ion thruster. The power source can exist whatsoever source of electrical power, just solar or nuclear are commonly used. A solar electric propulsion system (like that on Deep Infinite 1) uses sunlight and solar cells to generate ability. A nuclear electrical propulsion system (similar that planned for the Jupiter Icy Moons Orbiter) uses a nuclear heat source coupled to an electrical generator.

Epitome right: HiPEP ion thruster beingness tested at twenty kilowatts in Glenn's Vacuum Facility half-dozen. For comparison, a household microwave operates at about i kilowatt. Credit: NASA

The power processing unit converts the electrical ability generated by the power source into the power required for each component of the ion thruster. Information technology generates the voltages required by the ion eyes and discharge chamber and the loftier currents required for the hollow cathodes. The propellant management system controls the propellant period from the propellant tank to the thruster and hollow cathodes. It has been developed to the point that it no longer requires moving parts. The control estimator controls and monitors organisation functioning. The ion thruster then processes the propellant and power to propel the spacecraft.

The first ion thrusters did not final very long, only the ion thruster on Deep Space ane exceeded expectations and was used more than 16,000 hours during a period of over 2 years. The ion thrusters existence developed at present are being designed to operate for vii to x years.

Find out more than about ion propulsion:
>  Glenn Ion Propulsion Research and Evolution
>  Deep Infinite 1 Technologies From Glenn

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Source: https://www.nasa.gov/centers/glenn/technology/Ion_Propulsion1.html

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