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Precision Thrusters
Task Objective
Development of a Miniature Xenon Ion (MiXI) thruster development
that will enable precision spacecraft positioning and formation
maneuvers for formation-flying spacecraft. The current MiXI thruster
prototype
will provide 0.5 – 3 mN thrust at 3000 sec specific impulse
and efficiencies around 50% or better. The MiXI thruster will use
Xenon propellant, a noble gas, minimizing spacecraft contamination.
Task Description
Future interferometer-based missions aimed at detecting Earth-like
planets around distant stars, for example, require multiple spacecraft
to fly in a precisely controlled formation. Even though these spacecraft
may be separated over relatively large distances (100s of meters),
they may have to be controlled to a relative position with respect
to each other within only microns (10-6 m)
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JPL Miniature Xenon
Ion Thruster (MiXI) |
Although other concepts are being explored, positioning
control of spacecraft typically requires thrusters to move them about.
For the very fine positioning requirements to be encountered for
future formation flying missions, this requires very precisely controllable,
low thrust values, even for relatively sizable spacecraft in the
500-kg wet mass range or above. Several thrusters are being studied
for formation flying and constellation missions. In the past, so
called colloid or field emission electric propulsion devices have
been considered. However, current designs of these devices deliver
thrust levels of only sub-milli-Newton levels, although very precisely
controlled. Cold gas thrusters have also been considered. However,
here a gas is simply vented to space through a valve at very low
exhaust velocities, or specific impulses. A low specific impulse
means that one requires a lot of propellant to accomplish the spacecraft
maneuver.
A new concept being explored for formation flying applications is
MiXI. Actually, the basic concept of this thruster is not new. Ion
thrusters have been developed since the 1960s, and have flown in
space for the first time in the early 1970s. In an ion thruster,
a plasma is generated. This plasma is basically a very hot gas in
which electric charges are separated into positive ions and negative
electrons. Although very hot, the plasma in an ion thruster is not
very dense, having a pressure about a ten millionth of atmospheric,
and thus does not pose a severe thermal problem. The plasma is generated
by injecting a neutral gas into a so-called discharge chamber, then
bombarding this gas with electrons ejected from a cathode. The electrons
collide with the gas atoms, and “ionize” them, i.e. split
electrons off its outer shells, leaving positively charges ions and
negatively charged electrons. The ions are then extracted from the
discharge chamber via a set of electrostatic grids and accelerated
in an electrostatic field to very high velocities of approx. 40,000
m/s. To maintain charge neutrality, i.e. avoiding the spacecraft
charging itself negative after emitting the positive ions, a neutralizer – a
device very similar to the cathode - emits electrons back into the
ion beam.
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2-m Dia. UHV Chamber for
Plume and Contamination Testing
inside MPL
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After a long hiatus, ion thrusters have found a market niche in
North-South stationkeeping maneuvers for Earth-orbiting satellites
beginning in the 1990s – i.e. preventing spacecraft orbit drifts
due to gravity actions of the sun and moon. Most notably, in the
NASA New Millennium DS1 mission launched in 1998, a 30-cm dia. ion
thruster was used as the main propulsion device to fly a spacecraft
past the comet “Borrelly”.
State of the art ion thruster technology, such as the DS1 thruster,
is relatively large, and scaled for main propulsion applications,
i.e. the need to move a spacecraft through space at high speeds.
For formation flying applications, however, ion thrusters would be
used in a unique attitude control mode, accurately maintaining the
relative position of several spacecraft, or the rate at which they
are moving with respect to each other. This will require multiple
thrusters per spacecraft facing in different directions to allow
3-axis positioning control in space. Also, the required thrust levels
will only be on the order of a few milli-Newtons, as opposed to the
90-mN thrust levels of the DS1 thruster. Hence, a miniaturized ion
thruster is required.
The MiXI thruster being developed at JPL is only 3-cm in diameter – one
tenth the diameter of the DS1 thruster. It will require less than
100 W of power, provide a precisely controllable thrust (in its current
configuration) of 0.5 – 3 mN and a specific impulse of over
3000 sec. As noted above, this high specific impulse will allow for
the conservation of propellant, enabling this mission to either last
longer, or the spacecraft to be lighter. The propellant used is xenon – a
noble gas. It is very non-reactive, and thus poses no significant
contamination risks. Propellant contamination is of major concern
to future formation flying/interferometer spacecraft due to sensitive
optical lenses on board the spacecraft.
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CeB6Cathode |
A laboratory version of the MiXI thruster has
already been successfully operated at JPL at thrust levels up to
1.5 mN, specific impulses of 3100 sec and a thruster efficiency of
56% (excluding the cathode and neutralizer). This represents a major
advance in the long history of ion thruster development, since small
thrusters due to a higher surface-to-volume ratio have a greater
tendency to loose ions to the thruster walls in recombination reactions,
decreasing thruster efficiency. Currently, an optimized thruster
configuration is in fabrication, and will be ready for extensive
performance, plume and contamination tests shortly. Several areas
of technology development still exist, most importantly low power
cathode technologies, magnetic field and ion accelerator grid optimization,
all with the MiXI thruster scale in mind. Plume (propellant exhaust)
studies will also be conducted in a new JPL laboratory called “MPL” (Microthrust
Propulsion Laboratory). Here we will determine if plasma exhaust
could pose a danger to a spacecraft through “plume impingement”,
and what design steps would have to be taken to avoid it.
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