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low thrust propulsion is essential for

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all space missions and NASA's Lewis

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Research Center is conducting programs

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to provide a broad range of low thrust

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propulsion concepts for both auxiliary

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and primary functions auxiliary

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propulsion is used for keeping space

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systems in desired locations or for

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orientation typical examples are the

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reaction control system for earth to

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orbit vehicles drag makeup and attitude

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control for low Earth orbit systems such

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as Space Station freedom station-keeping

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for higher orbit systems such as

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geosynchronous satellites and finally

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retropropulsion functions near planetary

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bodies primary propulsion functions

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include the moving of space vehicles

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from point to point in earth space as

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well as propulsion between Earth space

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and various planetary bodies to

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understand these various propulsion

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applications specific auxiliary and

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primary missions will be discussed on

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earth to orbit vehicles such as the

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shuttle orbiter Auxiliary low thrust

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propulsion systems are used to control

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vehicle orientation or to perform small

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orbit changes examples are orientation

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of the shuttle orbiter to face the Sun

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and rendezvous with low Earth orbit

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systems such as the long-duration

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exposure facility led F the low thrust

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devices for these applications are

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called reaction control systems which

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generally operate at thrust levels from

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25 to a few hundred pounds space station

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freedom requires low thrust propulsion

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for both orbit and attitude control

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orbit control includes atmosphere drag

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makeup and collision avoidance attitude

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control includes damping of disturbances

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such as shuttle docking and momentum

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management small electric rockets called

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resistojet

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with thrust levels less than a pound and

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25 to 100 pound chemical rockets are

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being considered for orbit and attitude

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communication satellites are usually

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first placed in a geosynchronous

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transfer orbit or GTO

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a 100 to 200 pound thrust apogee

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propulsion system is used to change the

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gto to a circular geosynchronous orbit

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at an altitude of about 20 2,300 miles

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in the geosynchronous orbit small

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station-keeping Rockets are used to

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overcome gravitational forces from the

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Sun Moon and Earth to maintain the

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satellite in the desired position a

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final example of auxiliary propulsion is

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the use of 100 to 500 pound retro

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rockets for orbit change or capture of

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satellites near planetary bodies low

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thrust systems are also useful for

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primary propulsion applications for

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Earth orbit and planetary missions

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examples in Earth space include the

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transport of communication satellites

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from low-earth orbit into geosynchronous

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orbit and the placement of weather

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satellites in polar orbits for earth

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observations planetary missions include

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transferring of systems beyond earth

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low thrust electric propulsion is

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particularly valuable for very energetic

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planetary applications such as cargo

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vehicles for major moon Mars missions

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the Lewis Research Center is developing

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low thrust chemical and electric

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propulsion systems chemical propulsion

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includes rockets which use hydrogen

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oxygen and storable propellants although

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chemical rockets use various propellants

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all involved heating the propellant and

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its subsequent expansion through a

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nozzle to produce thrust chemical

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rockets using gaseous hydrogen oxygen

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have been developed for possible use on

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the space station designs ranging from

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25 to 50 pounds thrust have been built

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and life tested studies indicate that

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future launch vehicles would benefit

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from liquid hydrogen oxygen reaction

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control systems and optimal approaches

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are being defined a breakthrough in

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storable chemical propulsion technology

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has been verified with a 5-pound rocket

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100 to 200 pounds storable rockets are

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now under development which will provide

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major increases in the life and

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performance of Apogee retro and orbit

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change propulsion systems specific

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electric rockets have very different

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operating principles but all electric

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propulsion systems share many features

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energy is derived from a solar or

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nuclear power source and is converted

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into electricity and then conditioned

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for use by the electric rockets which

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use the power in various ways to

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accelerate the propellant to produce

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thrust resistojet s' the simplest

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electric rockets add energy to a

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propellant via heat transfer from an

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electrically heated resistor

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a version which uses waste gas from the

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station modules as the propellant is

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being developed for drag makeup on Space

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Station freedom in our Jets

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the propellant is heated by an electric

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arc and is then expelled through a

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nozzle arc Jets which use about one

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kilowatt of power and hydrazine

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propellant are under intense development

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for station keeping on commercial

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geosynchronous satellites electrostatic

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or ion thrusters and magneto plasma

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dynamics or MPD Rockets are being

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developed for Earth orbit and planetary

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primary propulsion functions ion Rockets

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first emit electrons from a cathode to

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create positively charged ions in a

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discharge chamber and then

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electrostatically accelerate those ions

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through two perforated plates called ion

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optics

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this is a typical discharge chamber

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along with the ion optics which contain

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this scene shows an ion thruster

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operating at approximately 10 kilowatts

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MPD Rockets produce thrust by using an

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electromagnetic field to accelerate a

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plasma direct measurements of ion and

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MPD performance and exhaust plumes are

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necessary these measurements require

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large frost stands and state-of-the-art

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plume Diagnostics ion and MPD Rockets

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both require large space simulation

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facilities with high gas pumping speeds

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in order to obtain space like vacuums

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in summary advanced low thrust

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propulsion provides benefits for many

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applications

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these range from reaction control

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systems for Earth orbit vehicles such as

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the Space Shuttle and drag make up and

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attitude control devices for large

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platforms in low-earth orbit such as

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Space Station freedom it also includes

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significantly reducing the propellant

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for Apogee propulsion and

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station-keeping propulsion for

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geosynchronous satellites and then

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advanced electric propulsion systems

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enable an order of magnitude reduction

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in the propellant required for primary

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propulsion in both earth orbital and

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planetary missions the lowest Research

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Center is conducting a low thrust

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propulsion program utilizing an in-house

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university and industrial team this

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blend of skills assures that the program

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will develop practical devices for

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near-term applications and also produce

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more advanced concepts for the