Defense Intelligence Reference Document MHD Air Breathing Propulsion And Power For Aerospace Applications

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electric (plasma) thrusters have been around for decades, their use in space was
limited by the electric power available onboard the spacecraft. 2 The advent of
high-power solar arrays has made systems from a few kW to tens of kW
practical. Chemical systems will probably always be the primary choice for
getting vehicles into space. The thrust levels for electric systems are too low to
be practical for that purpose.
Chemical and electric (or electromagnetic) propulsion systems have intrinsic
differences. For example, chemical propulsion is "energy limited" because the
chemical reactants have a finite amount of energy per unit mass (i.e., their
enthalpy of combustion or reaction), which ultimately limits their achievable
exhaust velocity. However, because the propellants are their own energy source,
the rate at which energy is supplied to the propellant (which is ultimately limited
by the reaction kinetics) is independent of the mass of propellant, so very high
powers and thrust levels can be achieved. By contrast, electric propulsion
systems are typically not energy limited; an arbitrarily large amount of energy
can be delivered (from the external solar, nuclear or chemical power system) to
a given mass of propellant so that the exhaust velocity can be an order-of-
magnitude larger than that of a chemical system. Instead, electric propulsion
systems are "power limited" because the rate at which energy from the external
source is supplied to the propellant is proportional to the mass of the power
system. This has the result of limiting the thrust of the electric propulsion system
for a given vehicle mass. Because of this, electric propulsion vehicles are
typically low thrust-to-weight (T/W) ratio (i.e., low acceleration) vehicles.
WEAKLY IONIZED PLASMAS FOR PROPULSION APPLICATIONS
This review is devoted to a group of emerging technologies centering on weakly
ionized plasmas for propulsion and power. 3 Charged particles (ions and
electrons) must be present in the flow so that it can interact with applied electric
and magnetic fields. Space thrusters operate at very low pressures ( < 100 mTorr
or < about 2 psi) with a significant fraction of the working fluid/gas being
partially ionized (from a few percent to nearly 100 percent). In contrast, air-
breathing systems operate at much higher pressures and have low ionization
fractions. The ionization fraction of concern (i.e., the fraction of gas molecules
that are ionized) ranges from as low as 10-s to 10-2, hence the term "weakly
ionized." The gas pressure in the plasmas can take almost any value. In
applications to high-altitude flight, the static pressure is on the order of 10-100
Torr, whereas combustion applications demand near-atmospheric (~760 Torr) or
above-atmospheric pressures. The temperature of the gas can be near-ambient
in low-pressure glow discharges, rising to 5,000-10,000K in arc or high-pressure
microwave discharges, or even 20,000-30,000K in laser-generated sparks. The
plasmas can be generated by electric or electromagnetic fields, from DC to RF,
short pulses, microwaves, and optical (laser) beams, or by various combinations
of the above. In general, low pressure plasmas tend to be uniform (diffuse) and
nonequilibrium. The temperature of electrons and internal molecular modes can
be very high, while the gas as a whole stays relatively cold. As the pressure and
power loading increase, plasmas tend to become hotter, getting closer to
thermal equilibrium, and also break into channels (streamers and arcs). The
reality, however, is more complex. In some devices, such as dielectric barrier
discharges, nonequilibrium plasmas are generated even at atmospheric pressure,
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