low thrust propulsion is essential for
all space missions and NASA's Lewis
Research Center is conducting programs
to provide a broad range of low thrust
propulsion concepts for both auxiliary
and primary functions auxiliary
propulsion is used for keeping space
systems in desired locations or for
orientation typical examples are the
reaction control system for earth to
orbit vehicles drag makeup and attitude
control for low Earth orbit systems such
as Space Station freedom station-keeping
for higher orbit systems such as
geosynchronous satellites and finally
retropropulsion functions near planetary
bodies primary propulsion functions
include the moving of space vehicles
from point to point in earth space as
well as propulsion between Earth space
and various planetary bodies to
understand these various propulsion
applications specific auxiliary and
primary missions will be discussed on
earth to orbit vehicles such as the
shuttle orbiter Auxiliary low thrust
propulsion systems are used to control
vehicle orientation or to perform small
orbit changes examples are orientation
of the shuttle orbiter to face the Sun
and rendezvous with low Earth orbit
systems such as the long-duration
exposure facility led F the low thrust
devices for these applications are
called reaction control systems which
generally operate at thrust levels from
25 to a few hundred pounds space station
freedom requires low thrust propulsion
for both orbit and attitude control
orbit control includes atmosphere drag
makeup and collision avoidance attitude
control includes damping of disturbances
such as shuttle docking and momentum
management small electric rockets called
resistojet
with thrust levels less than a pound and
25 to 100 pound chemical rockets are
being considered for orbit and attitude
control for Space Station freedom
communication satellites are usually
first placed in a geosynchronous
transfer orbit or GTO
a 100 to 200 pound thrust apogee
propulsion system is used to change the
gto to a circular geosynchronous orbit
at an altitude of about 20 2,300 miles
in the geosynchronous orbit small
station-keeping Rockets are used to
overcome gravitational forces from the
Sun Moon and Earth to maintain the
satellite in the desired position a
final example of auxiliary propulsion is
the use of 100 to 500 pound retro
rockets for orbit change or capture of
satellites near planetary bodies low
thrust systems are also useful for
primary propulsion applications for
Earth orbit and planetary missions
examples in Earth space include the
transport of communication satellites
from low-earth orbit into geosynchronous
orbit and the placement of weather
satellites in polar orbits for earth
observations planetary missions include
transferring of systems beyond earth
space to planets comets and asteroids
low thrust electric propulsion is
particularly valuable for very energetic
planetary applications such as cargo
vehicles for major moon Mars missions
the Lewis Research Center is developing
low thrust chemical and electric
propulsion systems chemical propulsion
includes rockets which use hydrogen
oxygen and storable propellants although
chemical rockets use various propellants
all involved heating the propellant and
its subsequent expansion through a
nozzle to produce thrust chemical
rockets using gaseous hydrogen oxygen
have been developed for possible use on
the space station designs ranging from
25 to 50 pounds thrust have been built
and life tested studies indicate that
future launch vehicles would benefit
from liquid hydrogen oxygen reaction
control systems and optimal approaches
are being defined a breakthrough in
storable chemical propulsion technology
has been verified with a 5-pound rocket
100 to 200 pounds storable rockets are
now under development which will provide
major increases in the life and
performance of Apogee retro and orbit
change propulsion systems specific
electric rockets have very different
operating principles but all electric
propulsion systems share many features
energy is derived from a solar or
nuclear power source and is converted
into electricity and then conditioned
for use by the electric rockets which
use the power in various ways to
accelerate the propellant to produce
thrust resistojet s' the simplest
electric rockets add energy to a
propellant via heat transfer from an
electrically heated resistor
a version which uses waste gas from the
station modules as the propellant is
being developed for drag makeup on Space
Station freedom in our Jets
the propellant is heated by an electric
arc and is then expelled through a
nozzle arc Jets which use about one
kilowatt of power and hydrazine
propellant are under intense development
for station keeping on commercial
geosynchronous satellites electrostatic
or ion thrusters and magneto plasma
dynamics or MPD Rockets are being
developed for Earth orbit and planetary
primary propulsion functions ion Rockets
first emit electrons from a cathode to
create positively charged ions in a
discharge chamber and then
electrostatically accelerate those ions
through two perforated plates called ion
optics
this is a typical discharge chamber
along with the ion optics which contain
over 20,000 holes to accelerate the ions
this scene shows an ion thruster
operating at approximately 10 kilowatts
MPD Rockets produce thrust by using an
electromagnetic field to accelerate a
plasma direct measurements of ion and
MPD performance and exhaust plumes are
necessary these measurements require
large frost stands and state-of-the-art
plume Diagnostics ion and MPD Rockets
both require large space simulation
facilities with high gas pumping speeds
in order to obtain space like vacuums
during testing
in summary advanced low thrust
propulsion provides benefits for many
applications
these range from reaction control
systems for Earth orbit vehicles such as
the Space Shuttle and drag make up and
attitude control devices for large
platforms in low-earth orbit such as
Space Station freedom it also includes
significantly reducing the propellant
for Apogee propulsion and
station-keeping propulsion for
geosynchronous satellites and then
advanced electric propulsion systems
enable an order of magnitude reduction
in the propellant required for primary
propulsion in both earth orbital and
planetary missions the lowest Research
Center is conducting a low thrust
propulsion program utilizing an in-house
university and industrial team this
blend of skills assures that the program
will develop practical devices for
near-term applications and also produce
more advanced concepts for the
longer-term higher payoff national space
missions