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

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Chapter 4: Summary and Predictions
The principal reason for attractiveness of plasma/MHD propulsion concepts is
that they could in principle reach beyond the limits of conventional propulsion,
power, and aerodynamic technologies. It is thus near or beyond those limits that
novel plasma technologies will likely find their application.
The principal difficulties or flaws associated with the plasma and MHD
technologies are as follows:
• Weight and complexity, especially if there is a need for a strong (>1 Tesla)
magnetic field in large volumes, and if electron beams are needed for
ionization.
• For MHD accelerator/thruster, power requirements could be overwhelming.
• MHD operation is accompanied not only by the work of ampere forces, but
also by irreversibilities and entropy generation due to Joule dissipation. This
reduces thrust and l sp of propulsion systems.
• In the absence of thermal ionization (i .e., at Mach< 12), complexity and
power budget associated with nonequilibrium ionization all but make MHD
propulsion systems impossible.
In contrast, applications to reentry, global-strike hypersonic gliders, and
aeroassisted orbital maneuvering look very promising in the near future. The
"free" thermal ionization enables MHD devices with very modest B field and the
ability of plasma/MHD system to provide L/D far beyond that possible
conventionally; together, it makes these applications both feasible and desirable
for national defense. However, these types of applications are also likely to
attract attention of other nations, including (but not limited to) Russia, China,
and Japan, that have proven knowledge and experience required to accomplish
such missions and technologies. These nations have the capability to develop
such novel technologies within several years and deploying those technologies
perhaps within 10 years.
The fortunes of MHD propulsion could increase dramatically if high-speed
(hypersonic) vehicles begin to carry powerful onboard electricity sources, such as
nuclear (fission or fusion) reactors. Since deployment of onboard nuclear power
is mostly a political rather than a technological issue, it is difficult to predict if
this going to occur and, if yes, when.
For spacecraft, the current trend of replacing chemical rockets with electric
propulsion systems will continue and probably will become the standard. Electric
systems can provide a much wider ra nge of operation (e.g., low-thrust fine
positioning/pointing, more frequent or nontraditional maneuvers, and longer
times on station) than chemical systems can. The trend to larger spacecraft
power levels will further accelerate this trend. Ultimately, the high power levels
required (hundreds of kW to multi-megawatts) for certain missions will lead to
revisiting the use of nuclear fission reactors in space.
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