Developing Responsive and Agile Space Systems - Space-Library

Research Horizons
Independent R&D at Aerospace
Low-Power Electric Propulsion
The concept of operationally responsive access to space is leading to
an increased interest in small, low-power spacecraft. Research scientist
Kevin Diamant of The Aerospace Corporation’s Propulsion
Science department said that these spacecraft—severely limited in
mass and power—could benefit from the use of low-power electric
propulsion, increasing useful payload mass and enhancing mission
duration and flexibility.
“The most important figure of merit for satellite propulsion
is the velocity at which propellant exits the thruster. For a given
amount of momentum imparted to the exhaust stream, and thereby
to the spacecraft, larger exhaust velocity results in reduced exhaust
mass. Reduced propellant mass can translate to reduced launch cost
or additional payload. Or, for a given propellant load, larger exhaust
velocity permits a greater total impulse (momentum change), which
equates to greater maneuverability or longer life on station,” Diamant
explained.
The vast majority of satellites have relied on chemical thrusters,
but chemical rocket exhaust velocity is limited by the heat released
by the combustion or decomposition of the propellant. Diamant,
principal investigator of an Aerospace team exploring the Hall
thruster—a type of plasma-based propulsion system—said that this
limitation can be removed by supplying power to the propellant
from an external source, which in electric propulsion is any external
electrical power source. Rostislav Spektor, senior member of the
technical staff, Propulsion Science department, is a coinvestigator
on the team.
“The Hall thruster may be an attractive option due to its compactness
and relatively simple construction,” Diamant said. “Typical
Hall thruster exhaust velocities range from 15,000 to 20,000 meters
per second; whereas practical values for chemical thrusters lie in the
range of 2000 to 4500 meters per second.”
The Hall thruster is an electrostatic thruster—a type of ion
thruster that operates on the principle that a charged particle accelerates
in an electric field, Diamant explained. Hall thrusters typically
consist of an annular (ring-shaped) ceramic discharge channel
with an electrode (anode) at one end. Propellant (usually xenon)
enters through ports in the anode, and is ionized in a high-voltage
discharge struck between the anode and another electrode (cathode)
placed externally to the channel. The ionized gas—plasma—
consists of neutral atoms, free electrons, and positively charged ions.
A radial magnetic field is applied close to the channel exit. This
magnetic field impedes the motion of electrons to the anode, resulting
in the presence of a large electric field in the plasma. Ions are
accelerated by this field, producing thrust.
Hall thrusters in the 0.4–1.4-kilowatt power range have extensive
flight heritage. Approximately 250 Hall thrusters are in space,
mostly on Russian satellites launched since the early 1970s, but also
on several recently launched satellites built by Space Systems/Loral.
The AEHF (Advanced Extremely High Frequency) satellites will
carry 4.5-kilowatt Hall thrusters for orbit raising, stationkeeping,
and repositioning. A 200-watt Hall thruster recently performed
drag compensation for the Air Force’s TacSat-2 spacecraft.
Diamant pointed out, however, that scaling Hall thrusters to
power levels below a few hundred watts while preserving high average
exhaust velocity and efficiency is challenging because of the
need to reduce channel size to preserve ionization efficiency. Small
Rostislav Spektor and Xuan Eapen, senior research associate, investigate the origin
of Hall thruster electromagnetic emission in the EMI facility.
size leads to difficulty in generation of magnetic fields with appropriate
magnitude and topology, and to increased power loss from
the plasma due to the larger surface area-to-volume ratio.
“The cylindrical Hall thruster (CHT), invented by researchers at
Princeton University Plasma Physics Laboratory, eliminates most,
or all, of the inner wall of the annulus [area between two concentric
circles], with the intent of boosting efficiency and life at low power,”
Diamant said. “A drawback of the CHT is its relatively wide plume
divergence. Ions accelerated at high angles detract from thruster
performance and can potentially heat and erode nearby structures.”
“In an ongoing Aerospace collaboration with Princeton in the
use of CHTs, we have verified that the CHT plume divergence can
be reduced by approximately 25 percent and ion energy increased
by 10 percent by operating an auxiliary discharge to an electrode
(known as a ‘keeper’) placed just in front of the cathode,” Diamant
said. The researchers also found that efficiency from 30 to 40 percent
may be achievable at power levels from 100 to 200 watts. Measurements
of multiple charged ions in the CHT plume found them
to be correlated with the presence of thruster erosion products.
Diamant said that in the coming year, the researchers will examine
the feasibility of using a low-power Hall thruster to perform
drag compensation in low orbit using propellant ingested from the
atmosphere. “By decreasing altitude, smaller, lower-power, and less
massive instruments can deliver capability similar to that achieved
by larger satellites in higher orbits. For example, the resolution of
Earth imagery is proportional to the ratio of orbital altitude to
optical aperture diameter. Lower orbits enable finer resolution, or
smaller, and therefore lighter and cheaper, optics.”
Today’s commercial Earth-observing satellites operate at altitudes
from 400 to 800 kilometers and are able to resolve objects less
than a meter in size. “An analysis based on assumptions appropriate
Crosslink Summer 2009 • 43