Supplement A: Electrohydrodynamics
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"Proposal to Develop Practical Military, Space and Research Applications
of Electrohydrodynamics as related to:
(Prepared by Seabrook Hull & Associates, March 4, 1960)
I - EHD Definition:
Electrohydrodynamics (EHD) is the study of electric field
phenomena and their relationship to dielectric media and ion plasma.
II - Outline:
This proposal consists essentially of:
(a) A review of experimental work performed to date and empirical results
obtained thereby;
(b) A discussion of known theory and possible further theoretical
development;
(c) A series of project outlines aimed at developing practical design criteria
for the exploitation of EHD phenomena.
III - EHD General Description:
In essence, EHD is the study of high-intensity electric
field phenomena and their influence on non-conducting (dielectric) media.
It is in many respects analogous to the study of magnetic field phenomena
and their influence on conducting media and electric currents - through which
we have developed the host of electromagnetic devices we enjoy today.
There is a fundamental difference, however: Magnetic
fields influence or are influenced by certain conductors, electric currents,
etc. In contrast, high-intensity electric fields interact with dielectric
media, including gasses such as air, and, according to the fundamental laws
of electricity and magnetism, a vacuum. Even as a magnetic field can be "shaped"
by the geometric design of the field-producing components, so can electric
fields be shaped by paying careful attention to the geometric design of the
electrodes. In fact, in this way it has been possible to establish a considerable
differential force between the device - essentially a condenser - and its
dielectric environment.
The establishment of such a force with respect to air,
a vacuum, or any other dielectric medium results in thrust being imparted
to the device with respect to its ambient medium. If the device is held fixed,
the medium moves and the effect of a pump is achieved. If the device is free
to move, it accelerated in the direction of the force according to Newton's
Third Law of Motion, and the effect of propulsion is achieved. It is towards
the exploitation of these potentials that this proposal is primarily
directed.
IV - Qualifying Considerations:
Empirical data developed through years of privately financed
research indicate that:
(a) Proposed applications of EHD should result in energy
conversion efficiencies greater by many orders of magnitude than those now
possible or contemplated from any other competitive technology.
(b) The validity of the claims made for EHD can be
reconfirmed by a relatively small expenditure of research and development
funds.
(c) Development of fundamental design criteria for the
construction of full-scale devices can be programmed into a series of relatively
inexpensive projects.
(d) Construction of full-scale equipment will result
in a major break-through in space vehicle propulsion, pumping, particle
acceleration, etc.
(e) Development of full-scale equipment is not contingent
on a completed theoretical explanation of the phenomena involved.
(f) Full theoretical justification of EHD phenomena may
provide the basis for a new appreciation of fundamental physical laws in
the area of field-energy/mass relationships.
V - EHD Project Objectives:
The proposed end-result objectives of any research and
development effort in EHD are as follows:
(a) An electrically-powered space vehicle propulsion
system with a specific impulse reckoned in millions of seconds, the life
of which would be limited only by the duration of its electric power source
- i.e., a propulsion system that requires no expendable working
fluid.
Laboratory devices weighing 100 grams (~3.5 ounces) less
power source have produced a thrust of 110 grams, for an electrical power
expenditure of 500 watts (250,000 volts @ 2.0 milliamperes). This experiment
was performed in air (1 atmosphere). Supplementary research indicates much
greater efficiency (same thrust for less power input) results when operated
in a vacuum (10-4 mm. Hg. or better), when the current drops to
about 2.0 microamperes.
This performance compares with the ion propulsion unit
being operated at NASA's Lewis Research Center, which weighs several pounds
(kilograms) and produces 28.35 grams of thrust for a power input of 1,200
watts (10,000 volts @ 120 milliamps). It produced these results in a
10-8 mm. Hg. vacuum.
Even disregarding the disadvantage it suffers due to
its environment (air @ 1 atmosphere), the EHD device shows an energy conversion
efficiency many times that of the ion unit.
(b) Direct translation of electrical energy into mechanical
(rotary) energy has been achieved in a simple demonstration turbine. This
EHD plasma turbine is about 10 inches in diameter, and when energized with
a 50,000 volt power supply it accelerated to something over 100 revolutions
per minute. Current used is of the order of 0.5 milliamperes.
(c) Effective pumps for dielectric fluids - including
cryogenic liquids, gases, etc. - containing no moving parts have been built
and demonstrated. One of these about 18 inches long and six inches in diameter
pumps air through a one-half-inch diameter pipe at a rate of 250 feet per
minute for a power consumption of 38,000 volts and less than one milliampere.
Efficiency of this device improves materially at fluid working pressures
above one atmosphere and below 10-3 mm. of Hg. In the region between,
the glow discharge causes high-to-prohibitive ohmic losses.
(d) A high vacuum pump is readily visualized as a special
application of (c). This should be capable of producing a vacuum on the order
of 10-9 mm. of Hg or better - thus enabling the hard vacuum of
outer space to be simulated in the laboratory.
(e) A meteorite simulator is yet another special application
of (c). The same pump principle can be utilized to accelerate sand (or other
dielectric particles) in a continuous stream to velocities of 100,000 feet
per second or better. This compares to 18,000 feet per second now achieved
through the use of light gas guns such as those employed by NASA's Ames Research
Center.
(f) Electric Power Generation - particularly at very
high voltages (several million) - essentially the converse of the thrust
production principle. The name for the device is Flame-Jet Generator. In
this instance a shaped electrostatic field constricts and excites a high
velocity powered flow of a medium, as from a jet flame. This results in a
high free electron density which increases along the length of the flame
jet. When this electron concentration is picked off by specially designed
electrodes, the structure acts as an electric power generator.
A flame jet generator has yet to be built. However, empirical
data supports its feasibility. The main problem would be the selection of
materials of construction with adequate electrical and high-temperature
properties, though it seems likely that a satisfactory solution to this problem
could be found in current development work in the refractory metals (tungsten,
columbium, etc.), graphite and certain ceramics (such as silicon nitride
which has excellent electrical, chemical and mechanical high temperature
properties).
VI - EHD Theoretical Discussion:
The theory behind observed EHD phenomena is not entirely
clear, but appears to be related to the laws governing electrostatic fields.
These include:
Electrophoresis - the force exerted on a charged particle
in the presence of electric fields (Coulomb force), and which is proportional
to the electric charge and the field strength.
Dielectrophoresis - the force exerted on dielectric materials
in non-uniform electric fields. This force is approximately proportional
to the dielectric constant, field strength, and field gradient, with the
force tending to move the dielectric in the direction of increasing field
strength.
Stress Systems in Dielectric Media - (First formulated
by Helmholtz). As outlined by Sir James Jeans "The Mathematical Theory of
Electricity and Magnetism" (Cambridge University Press, 1951, Page 177) these
consist of:
A tension KR
2/8
per unit area in
the direction of the lines of force;
A pressure KR
2/8
per unit area
perpendicular to the lines of force;
A hydrostatic pressure R
2/8
2K/2
of amount in
all directions."
(Where K = dielectric constant, R = electric field density,
and
= density of the dielectric medium)
No serious effort has been made to equate these theories
and formulae to observed EHD phenomena. However, it seems feasible that careful
vector analysis of the interrelated field/dielectric forces and stresses
may in part at least explain the observed results.
Graphic -
Figure 1 (111.3 Kb).
Among the observations are these:
Device thrust (EHD force) increases directly as K (dielectric
constant of the medium).
Device thrust (EHD force) increases as the square of
the voltage (in some special cases this has been observed to be a cubic
function), starting with a minimum of observable effect thought to be at
about 10,000 volts.
Thrust and current in air are found to vary with pressure
according to the following relationship:
Note that with a moderate reduction in pressure below
one atmosphere, current rises catastrophically and thrust terminates. This
is the region of so-called "glow discharge" in which the air ionizes and
becomes a conductor, virtually "shorting" the electrodes.
A significant feature of the curves is that, except for
this limitation, thrust remains constant with the reduction in pressure down
to 10
-6 mm. of Hg, while current consumption falls off sharply.
- demonstrating the system's improved efficiency as a hard vacuum is
approached.
An additional consideration in the development of EHD
theory is the effect of so-called "ion winds" - plasmas accelerated by the
electric field. In the operation of EHD lift devices, a toroidal flow of
the medium (air, for example, or dielectric oil) is clearly evident.
Though procedures have been developed to calculate thrust
purely on the basis of ion wind, their validity has neither been proved nor
disproved. They appear to be generally reliable under the limiting condition
of an appreciable atmosphere from which plasma can be generated, but fail
to explain the continuing constancy of thrust at very low pressures - such
as 10
-6 mm. of Hg, which is the lowest pressure a which measurements
have been taken.
For this reason the strong indication remains that thrust
is results primarily from electrostatic field stresses - rather than plasma
flow. Thus EHD may prove more efficient in a hard vacuum (10
-12
mm. of Hg) than in air, where the induced plasma actually seems to result
in unnecessary power consumption. It seems certain that fundamental EHD
theory will become clearer with further experimentation.
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