The objective of this program is to develop magnetohydrodynamic (MHD) power generators for use on hypersonic vehicles. MHD generators could provide orders of magnitude more power than current technologies and enable missions using directed energy weapons. The program would design, build, and test a 1 megawatt MHD generator in a hypersonic wind tunnel to demonstrate the technology's viability. Additional plasma technologies like inlet compression and combustion may also be investigated to help overcome challenges preventing scramjet development. Non-rotating MHD generators are well-suited to efficiently produce high voltage power for hypersonic vehicles propelled by non-rotating scramjet engines producing gigawatts of power.

1. 2002-2109
Hypersonic Vehicle Electric Power System Technology
Rene Thibodeaux
Air Force Research Laboratory
1950 Fifth Street
Dayton, OH 45433
937-255-6016
Rene.Thibodeaux@wpafb.af.mil
Abstract-The objective of this program is to
develop magnetohydrodynamic (MHD) and other
plasma technologies for hypersonic vehicles. The
focus of this effort is to design, fabricate, and test a
hypersonic MHD generator. In addition, MHD inlet
compressors and plasma combustors for scramjets
may be investigated. Hypersonic engines will need
to produce power up to 10 Gigawatts to achieve the
high speeds, so power extraction of l00 Megawatts s
reasonable. The extreme temperatures and flow
speeds at hypersonic conditions will eliminate any
rotating turbines for propulsion, so non-rotating
MHD power generation technology is ideal for
hypersonic vehicles. The enormous kinetic energy of
the hypersonic flow can produce MHD generators of
extremely high power densities on the order of l00’s
Megawatts per cubic meter. The high-speed flow
reduces the magnetic field requirements compared to
previous MHD generator efforts. These technologies
will provide unprecedented mission capabilities for
future military hypersonic aerospace vehicles. MHD
power preserves the versatility of air-breathing
hypersonic engines by avoiding the need to carry
independent oxygen supplies for high power turbine-
based auxiliary power units or fuel cells.
Additionally, MHD and plasma technologies may be
the breakthrough technologies that enable sustained
combustion in scramjets, a goal that has prevented
hypersonic engine development for decades.
BACKGROUND
Magnetohydrodynamics is a concept dating
back to the time of Sir Humphrey Davy and Michael
Faraday who built the first motor using a wire
moving in liquid mercury. Concepts for conducting
fluid MHD generators began to appear in patents as
early as 1910; however, no method of ionization was
suggested. The first work on a conducting gas MHD
generator began with Karlovitz and Halasz in 19381
.
The natural high temperature produced by the drag at
hypersonic speeds lead to speculation of MHD
reactive control of the vehicle by Krantowitz in
19552
. The Air Force initiated a large research
program from 1960 to 1980 to develop MHD for
aircraft, focusing on subsonic and supersonic flows.
The largest MHD power generator proposed by the
Air Force was a 30 MW /Mach 2/LOX/JP-4 MHD
that was to be tested at the Arnold Engineering
Development Center3
. The program was cancelled in
1978. The last known Air Force MHD effort was the
testing of a PAMIR-3U unit purchased from Russian
in 1995. This generator produced 10-14 MW and
weighed 18,000 kg.
The combustion-driven MHD power unit is
the oldest MHD technology. The disadvantage of the
direct combustion unit is the need for internal
oxidizer and the limited operating time. Russian
work on inlet ram air-driven MHD “AJAX” concept
has been recently published44
. To date scramjet
driven MHD has remained a theoretical concept due
to the simple fact that a long duration working
scramjet engine has not been developed. Ironically,
MHD and plasma combustion technology may be the
breakthrough technologies that produce sustained
combustion in the scramjet. An MHD inlet
compressor can also control the angle of the inlet
shock and eliminate the need for mechanical variable
geometry inlets5
. Russians research has been
ongoing in these areas since the 1990’s6
.
TECHNICAL DISCUSSION
MHD generators produce a current in an
ionized gas by passing the gas through a magnetic
field. The current density is governed by the law,
J = s(E + UxB),
where J is current density, sigma is the conductivity
TT is the gas velocity, and B is the magnetic field in
Tesla. The electric field, UxB, consist of a
perpendicular (Faraday) component and an axial
(Hall) component relative to the direction of motion.
Electrons moving because of this field form surface
charges at of the MHD channel and develop an
opposing electric field E. Electrodes are positioned
to use the Faraday or Hall fields, but diagonally
1

2. connected electrodes produce the optimum
configuration that takes full advantage of both the
Faraday and Hall electric fields simultaneously.
Most MHD generators have been diagonally
connected generators, although some pulse power
generators utilized continuous Faraday electrode
designs are possible. The higher voltage
requirements of DEW favor the diagonal
configuration.
The extracted power density of a diagonal
MHD is determined by the equation,
P = k (l-k)s(UB)2
,
where p is the power density, k is the load factor.
The conductivity itself is actually a three dimension
tensor usually simplified to a scalar value, s = K ne,
and composed of the mobility K, the electron (or ion)
number density n, and the electron charge e.
At the low subsonic and even at supersonic
speeds, large power was only possible with large
magnetic fields or large conductivity. The former
was a major technical problem and the latter was a
major physics problem. Even though MHD
generators were built and worked, the difficulty of
solving the magnet or conductivity properties limited
the application of MHD. Operating in high-speed
flow dramatically changes these requirements,
reducing the required magnetic field and taking
advantage of the “natural” ionization produced by
high temperature hypersonic flows.
Thermal ionization is a natural byproduct of
the high collision rate of the atoms in the high
temperature gas. Achieving natural thermal
ionization in molecular gases requires extremely high
temperature. For example, ionization of pure air
requires temperatures over 4,000 °K. To enhance the
thermal ionization, techniques of artificial
nonequilibrium ionization such as adding high
electron affinity alkali metals, high electric fields,
particle beams, radio frequency inductive heating, or
high power microwaves can be used. The techniques
strip electrons from any neutral atoms or to absorb
free electrons to prevent recombine with any ions.
Hypersonic MHD generators can easily
extract large amounts of electric power from the
tremendous kinetic and thermal energy available in a
hypersonic flow. Typically, the MHD becomes more
efficient with size since most irreversible associated
with it are wall losses that proportionally as the
surface-to-volume decreases. Demonstrated enthalpy
extraction MHD generators in the 0.5-2.0 MWe range
has been at the order of 1.0-5.0 % and for larger units
in the 10-500 MWe range, enthalpy extraction
increases to the 10.0-20.0 % range. For large-scale
MHD power systems, isentropic efficiency
approaching 80% is theoretically possible.
Load
Working
Fluid Out
Anode Wall
Working
Fluid In
Cathode
Wall
Electric
Current
Flow of Electrically Conductive Fluid (Plasma) Through a Magnetic Field
Electric Potential (u x B) is Induced Orthogonal to the Flow and Magnetic Field Direction
Tapping Across Potential through External Load Extracts Electric Power
Principle of MHD Power Generation
Insulating
Wall Magnetic Field
u x B
u
Four generator configurations are possible
for a hypersonic vehicle. The first is a ram air driven
MHD auxiliary power units using free-stream air.
This either can be located at the inlet of the
hypersonic engine or placed side-by-side with the
hypersonic engine using an independent combustion
source if necessary. The second is a scramjet
integrated MHD power unit using the scramjet's
combustor flow. For a space vehicle, a direct
(rocket) combustion-driven MHD power unit can be
fueled with traditional propellants and oxidizer. A
trans-atmospheric vehicle can use a re-entry plasma-
drive MHD power unit for trans-atmospheric vehicles
can be integrated with the external skin.
ADVANTAGES OF HYPERSONIC MHD
Advantages of Hypersonic Speeds
An aircraft with the combined capability of
high speed and long distance weaponry would
perform missions not possible before. A hypersonic
vehicle cruising in the atmosphere at speeds up to
Mach 12 could travel anywhere on the globe within
two hours at altitudes above 100,000 ft. At that
altitude, the directed energy weapon system or
surveillance radar would have a virtually turbulent
free atmosphere and extremely long line-of-sight
distance to the horizon.
A trans-atmospheric space vehicle has wider
global range and is virtually a self-contained
platform, capable of launching from the achieving
orbit, and returning to ground. This alone is a
significant improvement over the permanently
orbiting weapon platforms proposed under the
Strategic Defense Initiative. A major drawback of
this concept was always the requirement for heavy
launch vehicles and maintenance shuttles (or more
likely no maintenance at all).
B
J
Electric
Current
2

3. A trans-atmospheric vehicle may be a very
large single-state to orbit vehicle or a two-stage
launcher and orbiter. In either case, the system is
likely a multi-billion platform. A more economical
platform may be a boost-glide re-entry vehicle
equipped with MHD generators imbedded in the
vehicle skin. Launch methods range from the old
expendable vehicles similar to the 1960’s Dynasoar
concept to a two-state hypersonic cruiser launcher
concept. The reentry heat easily produces very high
conductivity plasma flowing at extremely high
velocities so that magnetic fields of less than one
Tesla could be sufficient. Although the reentry
plasma would restrict the use of radio and microwave
devices, the plasma frequencies at reentry conditions
are below the range of visible light, making visible or
UV lasers viable candidates as weapons. Second, the
resulting effects of turbulence and hypersonic shock
waves can be moderated or even eliminated through
the very application of external high magnetic fields7
.
The vehicle then glides back to the launch point or a
recovery area for late re-boosting.
It must be considered that the great
capabilities of hypersonic speeds also cause extreme
environmental problems for a human pilot. The
extreme temperature environment, high centrifugal
turns, and the high angle of attack needed to optimize
the inlet shock angle make uninhabited vehicle
technology more appropriate for hypersonic vehicles.
Advantages of MHD in Hypersonic Flows
Due to the unique nature of hypersonic
flows, MHD has several advantages over competing
power generating systems. First, the high velocity
and associated extreme temperatures of hypersonic
flows prohibit the use of rotating turbines and instead
require the use of non-rotating ramjets, scramjets, and
rocket engines. The fact that an MHD generator is
nothing more than a flow channel with external
magnets makes it a natural companion of hypersonic
engines. The lack of moving parts in an MHD
generator eliminates the inertia limits inherent in
heavy turbo-generators, allowing rapid repetitive
turn-on and turn-off, ideal for directed energy
weapon applications.
The advantage of ramjets and scramjets is
the fact that they are air breathing and therefore avoid
the extreme weight penalty of carrying any oxidizer.
An MHD generator retains the air breathing
advantage of the hypersonic engine. A turbine based
auxiliary power unit or fuel cells could supply power
to the hypersonic vehicle, but an oxidizer must be
carried on the vehicle. This would quickly become a
problem if the vehicle must fly for extended time and
produce Megawatts of electrical power for a directed
energy weapon.
MHD is an excellent match to directed
energy weapons in terms of achievable power levels
and duty cycles. MHD is competitive with most
storage and rotating devices over durations of 1 to
1000 seconds. Chemically driven MHD pulse power
systems with power density on the order 100’s
MW/m3
have been demonstrated over the pass 40
years. Russian work in 1980’s was reported8
to have
produced an output of 500 MW at a power density of
approximately 600 MW/m3
. Power densities of
hypersonic MHD generators can be made extremely
high on the order from hundreds to thousands of
megawatts per cubic meter. No other non-nuclear
power technology can compete with this technology
for hypersonic air and space vehicle power sources
using directed energy weapons.
PROGRAM OBJECTIVE AND GOALS
The overall program objective is to perform
a ground test demonstration of up to a one-megawatt
MHD generator in a hypersonic wind tunnel. This
ground test will determine whether MHD generators
can operate effectively in hypersonic flows and if
MHD adversely affect the performance of a scramjet.
The effort will demonstrate insulation techniques
needed for superconductor magnets to operate in the
extreme temperature environment expected under
hypersonic conditions. Plans for a flight
demonstration of MHD technology in a prototype
hypersonic test vehicle will be developed from the
results of this effort, Other plasma-aerodynamic
effects utilizing MHD principles such as inlet
compression control, plasma combustion, and
exhaust thrust vectoring could be tested in this effort.
Initial work will be trade studies, analysis,
modeling, and simulation of hypersonic vehicles
equipped with MHD power generators. Different
integration configurations between the MHD
generator, engine, and vehicle will be identified and
characterized. The important issue of power
conditioning will be addressed in this program.
particularly the optimum distribution voltage needed
for DEW systems. Since the preferred fuel
hypersonic vehicle above Mach 10 is 20 °K liquid
hydrogen and hydrocarbon fuels below Mach 10,
different thermal requirements must be considered in
concert with the overall vehicle/weapon system
thermal management. This will reduce the large
external magnetic field leakage as well as the need
for a homogeneous internal field will be important
for the design of a flyable superconducting magnet.
3

4. The generator design effort includes fluid
dynamic, thermodynamic and plasma analysis of
MHD generators as well as scaling studies.
Conceptual designs of MHD generator power
systems will be produced for airborne and spaceborne
systems considering the integrated support structure
for the generator/magnet/engine. Using the analytical
tools developed, generator modeling and simulation
of both spaceborne and airborne MHD systems with
parallel laboratory verification tests will be
performed.
The magnet development effort will begin
with the analysis of various conceptual designs.
Using state-of-the-art magnet field codes, the magnet
conceptual designs will be modeled, leading to the
eventual fabrication and testing of samples magnets.
The conductors of interest include low
temperature niobium based superconductors with
advanced helium cryocoolers, high temperature
superconducting BSCCO and YBCO, as well as
cryogenic high purity aluminum.
The MHD generator development effort will
initially utilize smaller supersonic testing facilities.
Supersonic wind tunnel test include material testing,
component testing, seeding scheme testing, gas
dynamic physics testing and develop diagnostics for
the final hypersonic facility tests. A plasma torch
device will be used to simulate the high-temperature
plasma environment for testing electrodes and
insulator materials.
After sufficient verification tests of the
conceptual designs are accomplished, the detailed
design, and fabrication of the prototype generator
components will begin. Preparation for the
hypersonic MHD generator tests will be conducted at
a hypersonic wind tunnel test facility depending on
availability. Test data will include basic power
extraction, component lifetimes, electrode wear rates,
magnet performance, and off-design behavior.
SUMMARY
The objective of this program is to develop
magnetohydrodynamic (MHD) power generators for
integrated power production on hypersonic vehicles.
MHD will provide order-of-magnitude improvement
in the power capabilities of future Air Force
hypersonic aerospace vehicles, enabling global
missions using directed energy weapons. Proposed
scramjet engines will be non-rotating engines
producing Gigawatts of power. Therefore, non-
rotating MHD generators are a natural match to
produce efficient, high voltage power for many
directed energy weapons concepts.
Recent developments in lightweight
superconductors and structural material now make
flight-weight MHD power systems for hypersonic
aircraft feasible. In addition, an inlet MHD
compressor can control the engine pressure and
temperature along with a plasma technique to sustain
hypersonic combustion, may overcome the problem
that has prevented the development of scramjet
engines.
REFERENCES
1
B. Karlovitz, “Process for the Conversion of
Energy,” U. S. Patent No. 2,210,918 (August 13, 1940).
2
Krantrowitz, A., “A Survey of Physical
Phenomena Occurring in Flight at Extreme Speeds,”
The Proceedings of the Conference on High Speed
Aeronautics, 1955
3
Swallom, D., et al, “Results from the
PAMIR-3U Pulsed Portable MHD Power System
Program,” IEEE Paper 96467 31st Intersociety
Energy Conversion Engineering Conference
(IECEC), August 11-16 1996.
4
Goovitchev, V. I., Hansson, J., “Some
Trends in Improving Hypersonic Vehicles
Aerodynamics and Propulsion,” AIAA Paper IS-090
14th International Symposium on Air-Breathing
Engines, September 5-10 1998
5
Lineberry, J. T., et al, “Prospects of MHD
Flow Control for Hypersonics,” AIAA Paper 2000-
3057 35th IECEC, July 24-28 2000.
6
Bityurin, V. A., Zeigarnik, V. A. ,Kuranov,
A. L., “On A Perspective of MHD Technology In
Aerospace Applications,” AIAA Paper No.1996-
2355 2nd
AIAA Plasmadynamics and Lasers
Conference, June 17-20 1996.
7
Miles, R. B., et al, “Plasma Control of
Shock Waves in Aerodynamic and Sonic Boom
Mitigation,” AIAA Paper No.2001-3062 33rd
AIAA
Plasmadynamics and Laser Conference and Weakly
Ionized Conference, June 11-142001.
8
Velikov, E. P., et al., “Pulsed MHD Power
System ‘Sakhalin’ -The World Largest Propellant
Fueled MHD Generator of 500 Electric Power
Output,” International Conference on MHD Power
Generation and High Temperature Technologies,
October 1999.
4