AAWSAP DIRD, MHD Air Breathing Propulsion and Power for Aerospace Applications, November 2010

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Electrothermal thrusters use electric energy to
heat the propellant and add additional enthalpy.
This can be done with simple resistive heating or
by passing the propellant gas through an arc
plasma discharge. The plasma can be generated
through a high-current discharge or by
absorption of microwaves. The hot pressurized
gas is then accelerated out of the thruster using
a conventional converging-diverging gas-dynamic
nozzle. An example of an electric arc heated
thruster or arcjet thruster is shown in Figure 1.
Electrostatic thrusters use an applied static
electric field to accelerate propellant ions. Strong
electric fields are created in the engine which
then accelerate the (positive) ions to high velo­
cities. The accelerating field can be applied using physical grids such as those
used in ion engines or using "virtual grids" generated by an applied magnetic
field that traps the electrons as is done in Hall-effect thrusters. A photograph of
the NASA ion engine used on the Deep Space One spacecrah is shown in Figure
2. While gridded electrostatic thrusters like ion thrusters are cap­
able of very high l sp (1,000 to >20,000 seconds) values they have very low
thrust densities (1-5 N/m2) due to the space-charge current limit in the
accelerator system. Hall-effect thrusters do not have this space-charge limit but
also have thrust density limits due to the annular geometry (tens of N/m2).
Typical power levels are from watts to 50 kW.
In the Hall field orientation, the electric field causes electrons to flow upstream
and the ions to drih toward the exhaust as shown in Figure 3. 15 The electrons
and ions transfer equal and opposite amounts of momentum to the air, resulting
in zero thrust when no magnetic field is present. However, with the application of
a transverse magnetic field, the forward flow of electrons is slowed while the ah
flow of ions is nearly unaffected. Consequently, there is a net momentum
transfer resulting in thrust on the vehicle.
Figure 2. Electrostatic Gridded
Ion Thrusters. Photo-
graph of a gridded 30 cm diam­
eter ion thruster being tested at
the Jet Propulsion Laboratory .14
E
Hall Field
Orientation
Figure 3. Field Orientation for Hall Field Systems and PS Hall Thruster. Left: Figure
shows the Hall field orientation. Right: PS Hall Effect thruster being tested at the University of
Michigan.
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