SURFACE CHARGED SMART SKIN TECHNOLOGY FOR HEAT

SURFACE CHARGED SMART SKIN TECHNOLOGY FOR HEAT
PROTECTION, PROPULSION AND RADIATION SCREENING
B. Göksel1, I. Rechenberg2
Institute of Bionics and Evolutiontechnique, TU Berlin
Ackerstr. 71-76, Secr. ACK1, D-13355 Berlin
Email: berkant.goeksel@elektrofluidsysteme.de
1 PhD Student, Founder of Future Workshop Electrofluidsystems
2 Professor, Head of Institute of Bionics and Evolutiontechnique
INTRODUCTION
In January 1962 the Apollo command module heatshield
requirements for several design trajectories were established.
In March 1962 AVCO-Everett was selected by NAA
to design and install an ablative material heatshield on the
Apollo spacecraft outer surface.
Parallel to the Apollo development of ablative materials the
AVCO-Everett Research Laboratory studied active plasma
shields for heat protection by hydromagnetic braking in reentry
and radiation screening [1]-[4], [7].
In 1969 Wernher von Braun published his paper “Will
Mighty Magnets Protect Voyagers to Planets?” and discussed
the general principle of magnetic shielding and the
application of superconductors in plasma shields [5]. In
1970 Ali Bülent Cambel published his article “MHD for
Spacecraft” and discussed the idea of hydromagnetic
braking in reentry [6]. Cambel wrote in [6]: “Astrophysical
data signify that heavenly bodies behave in accordance
with the principles of MHD. A well-known example of
man‘s attempts to exploit such phenomena is controlled
thermonuclear fusion, and another is his attempt to
generate electricity in an MHD power device. It might be
suggested philosophically that, throughout his endeavours,
the innovative engineer attempts to imitate nature.
Most of the suggestions I will describe are but manifesttions
of the phenomena constituting cosmic electrodynamics
which our spaceship, Earth, obeys.”
FIG. 1: The Magnetosphere - produced by the terrestrial
magnetic field and plasma from the ionization of the upper
layers of the atmosphere.
FIG. 2: Earth’s ionospheric and magnetic radiation shield
that protects us from cosmic and solar particles. The
arrows show the outflow of plasma from the ionosphere
into the magnetosphere.
In 1982 Birch revived the idea for radiation shields for
ships and settlements [8]. But it was Landis from NASA
who attracted more attention with his article “Magnetic
Radiation Shielding – An Idea Whose Time has
Returned?” published in 1991 [9]. In 1999 Sussingham et
al. summarized “Forty Years of Development of Active
Systems for Radiation Protection of Spacecraft” [10].
In 2000 Winglee et al. published a concept for a Mini-
Magnetospheric Plasma Propulsion [14] (see Figure 3).
FIG. 3: Radiation Shielding Produced by Mini-Magnetosphere
[15].
The Mini-Magnetospheric Plasma Propulsion (M2P2)
(Figure 3) is ejecting plasma or ionized gas which then is
trapped on the magnetic field lines generated onboard by
a solenoid coil. The plasma can drag the magnetic field
lines out and form a plasma bubble. This is similar to the
Earth's magnetic field trapping a large volume of electrified
gas - thus forming the magnetosphere - and forcing
the solar wind to flow around it (Figure 1 and Figure 2).
1. SURFACE CHARGED SKIN TECHNOLOGIES
An alternative to pure magnetic shielding is surface
charged skin shielding which is a form of electrostatic
shielding.
1.1. AVCO-Everett Research Laboratory,
Plasma Shielding Studies from 1961 - 1969
Von Braun wrote in [5]: “If a spaceship’s exterior could be
kept positively charged, at a potential of some 300 million
volts, that would repel the positively charged protons. The
catch is that the negative-charged electrons in space,
irresistibly lured by the positive charge, would flow to the
ship and rapidly discharge it…. But a way around that is
now seen: Superconducting rings, encircling the ship,
would create a magnetic barrier that attracted electrons
couldn’t cross. Instead, they would orbit around the ship in
a cloud or plasma-for all the world like a circling swarm of
voracious mosquitoes, eager to “bite” the craft (discharge
it) but kept at a distance by its “Citronella” (magnetic
field)”. This “plasma shielding” should be even more
weight-saving than “pure” magnetic shielding, says its
proponent, Dr. Richard H. Levy of Avco-Everett Research
Laboratory, Everett, Mass. A lower-strength magnetic field,
probably less than 3,000 gauss should prove sufficient.”
Figure 4 shows a design of a space vehicle with a plasma
shield utilizing a four-coil superconducting magnet system.
Ejection from the vehicle must be accomplished at a
velocity greater than E/B velocity, or about 300 keV [2].
Levy wrote in [2]: “Further, any space vehicle configuration
will possess a certain amount of solid shielding in the form
of its skin and other equipment. This shielding may be
estimated roughly at 2-4 g/cm² aluminium. Suppose, for
example, that it is required to stop 100 MeV protons. If the
skin thickness is 2 g/cm², reference to the range-energy
tables shows that this thickness will just stop a 40 MeV
proton. It is therefore only necessary to provide 60 million
V of potential in the plasma radiation shield in order to
achieve the desired effect. The incident 100 MeV proton
crosses the plasma radiation shield voltage, losing 60
MeV. The remaining 40 MeV are then absorbed in the 2
g/cm² of skin. If the thickness is 4 g/cm², reference to the
range-energy tables shows that this thickness will stop a
60 MeV proton. Thus a 40 MV plasma radiation shield
outside of 4 g/cm² of skin would also suffice to stop 100
MeV incident protons… For example, to stop a 100 MeV
proton requires 10 g/cm² of solid shielding. But we saw
previously that 40 MeV plasma radiation shielding ahead
of 4 g/cm² of skin will also stop a 100 MeV proton. In a
sense, the 40 MeV plasma radiation shield is the equivalent
of 6 g/cm² of solid shielding.
FIG. 4: Design of a space vehicle with a plasma shield [2].
FIG. 5: Electric and magnetic field lines around a shield [2].
1.2. Diamond Film Layers as Cold Cathode
May wrote in [13]: “Diamond has some of the most extreme
physical properties of any material, yet its practical
use in science or engineering has been limited due its
scarcity and expense. With the recent development of
techniques for depositing thin layers of diamond on a
variety of substrate materials, we now have the ability to
exploit these superlative properties in many new and
exciting applications.”
1.2.1 Principle of Cold Field Electron Emission
Fowler-Nordheim Equation is the classic relation showing
emission current's dependencies:
(6.1)
2 R
V
E =
(6.2) 1 x 10 V/cm
100 Angstrom
100 Volt
E = = 14
(6.4) E = βV
(6.5) 



 


=
βV
- bφ
exp
φ
α a β
V
1 2 3/2
2
with
φ = work function
β = enhancement factor
a = emission area
E = electric field strength
R = tip radius
V = gate voltage
I = emission current
α, b = constants FIG. 6: Field Lines on Tip.
– Electric fields strongest where minimum radius of
curvature
– High electric fields distort potential energy well for
electrons in tip
– Electrons at Fermi Energy can tunnel out in a vacuum
or gas
1.2.2 Specialities of Diamond Film Layers
– IDEAL cathode since diamond naturally repels e-'s
– Very low work function (0.2 to 0.3 eV)
– Negative electron affinity and wide band gap
– Donor e-'s need to have ~0.01 eV to eject (< KbT,
0.025 eV)
– Hence, emits electrons at much lower E-fields than Si
or metals
– Simple, no need to microfabricate sharp tips
– Can operate at higher pressures
In general diamond cold cathodes offer higher stability,
larger emission site density, much higher lifetime than
other existing cold cathodes. The higher thermal conductivity,
higher thermal difusivity and lower sputter yield
discourages thermal evaporation of emission sites, and
thus reduces the possibility of anode-cathode (A-K) gap
closure. In addition, diamond negative electron affinity
allows extremely high currents (~100 A/cm²) which can be
emitted at low electric fields.
1.2.3 Further Properties of Diamond Film Layer
– Hardness 8,000 - 10,000 Kg/mm^2
– Electrical Resistivity 10*3 - 10*13 ohm-cm
– Dielectric Constant ~5.7
– Breakdown Voltage 100 - 300 volts/micron
– Loss Tangent (~15GHz) < 0.05
– Thermal Conductivity > 1,200 w/m-K
1.3. Diamond Layers as Superconductors ?
Physics Web announced in April 2003 that “a physicist (J.
F. Prins) in South Africa claims to have created a new
superconducting state of matter at room temperature.
Johan Prins of the University of Pretoria observed the
superconducting state in experiments with diamonds that
had been doped with oxygen… Diamond is a semiconductor
and Prins has long been interested in using n-type
diamond as a "cold" cathode to replace the "hot" cathodes
found in television tubes and many other devices. Moreover,
he believes that the results of his experiments on ntype
diamond surfaces - made by exposing the diamond to
energetic oxygen ions - can only be explained by a new
type of superconducting state. "If it is not superconductivity
then it must be violating the second law of thermodynamics,"
he says.“
Prins wrote in [17]: “It is generally believed that if an n-type
semiconductor with negative electron affinity could be
found, it would act as an ideal ‘cold cathode’. A model is
proposed to describe the conditions at an ideal surface
between such a semiconductor and the vacuum. When
such an interface is created, electrons will have to exit the
semiconductor owing to the difference inenergy X between
the conduction band and the vacuum level. They
leave a positively charged depletion layer behind, within
which a barrier to further electron egression is generated.
A self-consistent potential well has to form, which bounds
the emitted electrons within quantum states such that they
remain within an ‘electron-charge’ layer adjacent to the
surface. Together with the depletion layer, the electroncharge
layer forms a dipole that screens the field caused
by the initial offset X between the energies of the
conduction band and vacuum level. When applying an
electric field to extract electrons, the barrier in the
depletion layer increases. This impedes electron flow
through the semiconductor into the vacuum.“
Prins continued in [18]: “It is shown experimentally that ntype
diamond is a negative electron affinity material from
which electrons can be extracted at room temperature.
This is achieved by generating an 'ohmic' tunnelling contact
to the vacuum. It is found that the extracted electrons
within the gap between the diamond surface and the
anode are able to form a stable, highly conducting phase.
Band theory, combined with the equations that describe
electron transport in a vacuum diode, unequivocally show
that the distances between these electrons, as well as
their speeds, must keep on decreasing as long as there is
an electric field between the diamond surface and the
anode. This implies that steady-state current flow, as experimentally
observed, can only occur if this field becomes
zero while still allowing a current to flow from the
diamond to the anode. The only way to achieve such a
situation is for the extracted electrons within the gap to
form a superconducting phase. Because electrons are fermions,
an unabated decrease in their nearest-neighbour
distances as well as their speeds should eventually force
them to violate the Heisenberg uncertainty relationship. At
this limit, they become restricted, as pairs, within volumes
or 'orbitals' which in turn fill the whole space between the
diamond and the anode. Because these 'orbitals' have
zero spin, they are boson-like charge carriers, and because
they are as near to each other as is physically possible,
they automatically constitute a Bose–Einstein condensate;
i.e. they constitute a superconduc-ting phase.”
2. ELECTROHYDRODYNAMIC PROPULSION BY
POLYPHASE SURFACE CHARGED SMART
SKIN PLASMA ACTUATORS FOR TAKE-OFF
AND LOW SPEED CRUISE MODE
In the NASA online article Riding the Highways of Light -
Science mimics science fiction as a working model flying
disc - a "Lightcraft" - takes to the air, Prof. Leik Myrabo
from the Rensselaer Polytechnic Institute explains the
design for a hypersonic trans-atmospheric vehicle (Figure
7 and 8). Myrabo has been building on the idea to use
lasers to launch satellites since 1972, when it was developed
by founder and former CEO of AVCO-Everett,
Prof. Arthur Kantrowitz.
The article says: “The concept that evolved is a part
airship, microwave receiver, and (the smallest part) jet and
rocket engine, and as green as any space concept. The
12-person, 20-meter (66 ft) craft would be powered from
the Earth's surface to the Moon by sunlight captured by an
orbiting power station (1 km diameter, 20 GW power),
converted to microwaves, and beamed to rectennas
(rectifying antennas) that turn it back into electricity on the
Lightcraft. That's where the saucer shape comes from.
The airship part is a pressurized helium balloon-type structure
made of advanced silicon carbide film (transparent to
microwaves) to make the craft partly buoyant and to
provide for a large parabolic reflector for the energy
beamed from space. The craft would be encircled by two
superconducting magnet rings and a series of ion engines,
and topped with solar cells (Figure 7).
FIG. 7: Trans-atmospheric vehicle design from Rensselaer
Polytechnic Institute.
At launch, the Lightcraft would use electricity from its solar
cells (powered by an infrared space-based laser at night)
to ionize the air and move the craft through electrostatic
discharges. The craft could move at 80 to 160 km/h (50-
100 mph).”
Myrabo gives no details about the electroaerodynamic
propulsion. It is an idea of the corresponding author to develop
a solitary electrostatic wave propulsion by polyphase
plasma actuators based on bionic principles by mimicking
the flapping wing propulsion of birds and insects [19]. To
test this idea, the Future Workshop Electrofluidsystems of
the Institute of Bionics and Evolutiontechnique has recently
acquired a new 300 Watt polyphase high-frequency (5-
50 kHz) high-voltage (3-6 kV) power supply. The same
electrostatic wave propulsion concept is currently in the
review process for the BMBF (German Research Ministry)
concept contest “Bionics – Innovations from Nature”.
Synthetic diamond and silicon carbide are the best materials
to develop cold-emission electron guns powerful to
ionize air under atmospheric conditions (see section 1.2).
Presently, such cold cathodes are under development for
the next generation of flat screen monitors. The same
technology could be used to develop a synthetic diamond
and silicon carbide based smart skin technology applicable
on wing and fuselage surfaces of several square meters.
In the case of silicon carbide the skin has to be microstructured
with protected tips whereby synthetic diamond
is independent from sharp tips and may be the most
promising smart skin material for applications on future
hypersonic trans-atmospheric flight vehicles (Figure 8).
FIG. 8: Microwave Lightcraft Design.
3. MAGNETOHYDRODYNAMIC PROPULSION IN
HYPERVELOCITY MODE
As Myrabo says: “That (the EHD mode) is just low gear.
Switching on the microwave transmitter would make the
Lightcraft disappear in less than an eye blink. The
microwaves would be focused by the internal reflector to
heat the air on one side or the other of the craft and push it
in the opposite direction.”
The principle of the hot plasma spike is shown in Figure 9.
FIG. 9: Principle of air plasma spike for hypervelocity mode.
"This is used to climb out to a good altitude and beyond
the speed of sound where you use the magnetohydrodynamic
drive," Myrabo continued. Now the craft tilts from
flying edgewise to flying flat into the air stream. That
seems wrong but for another trick. The microwaves are
reflected forward to create a superhot bubble of air above
the craft and form an air spike that acts as the nose cone
as the Lightcraft accelerates to 25 times the speed of
sound.”
"This cleans up the aerodynamics of a vehicle that does
not look like it should fly in that direction," Myrabo said.
Even better, when the load is properly balanced the craft
sails through the air without leaving a shock wave and
virtually no supersonic wake. Water is used by the craft to
cool the rectennas and as a propellant in the last stages of
ascent.”
“At least initially, during the prototype phase, it won't be for
everyone, just NASA and military test pilots. The hyperenergetic
performance will require that the crew ride in
liquid-filled escape pods to protect them from g-forces
greater than even fighter pilots occasionally endure. In
some Air Force Space Command schemes, the crew
would breath an oxygenated fluid to protect their lungs.”
“It all sounds a bit too much like science fiction, but Myrabo
points out that most of the technologies or principles
have been demonstrated. Faculty and students at Rensselaer
have demonstrated the MHD slipstream accelerator
and the air spike concept in a high-speed wind tunnel, and
will test new models of other parts of the propulsion
system later this year.” (Figure 10)
FIG. 10: Principle of the MHD slipstream accelerator.
In his paper “MHD for spacecraft” [6] Ali Bülent Cambel
already wrote: “Our experiments have demonstrated that
operating an electromagnet inside a re-entry model causes
the shock wave ahead of the model to be pushed away
from the body at a steeper angle. Clearly the magnet
influences the flow field. It is not difficult to imagine that a
magnet three dimensionally gimballed could be used to
modify the flow field in any desired way and to vary the lift
and drag upon command.”
3.1. Laser-Driven Water-Powered Propulsion
FIG. 11: Lightcraft flying atop a beam of laser light.
Figure 11 shows an illustration of Myrabo’s lightcraft flight
tests from July 1996 to Sept. 1999 where the lightcraft
vehicle was launched with a 10 kW military laser at White
Sands Missile Range in New Mexico [12]. The laser
energy is converted into propulsive thrust as it strikes a
parabolic condensing reflector mounted on the bottom of
the lightcraft. This area has a thin coating of propellant
which after struck by laser pulses detonates and thrusts
the lightcraft upward.
Recent publications reveal that the overlay structure is
using water which is insulated from vacuum by an “air
curtain” to avoid evaporation or freeze in vacuum conditions
[16].
4. HYDROMAGNETIC BRAKING IN RE-ENTRY
In 1967, at the height of AVCO-Everett’s research on
plasma radiation shielding utilizing superconductive magnets
made of the silvery metal niobium with tin, Sampson
et al. wrote in [3]: “A large magnetic field could produce
hydromagnetic drag in the cloud of ionized air the vehicle
produces as it enters the atmosphere. With hydromagnetic
braking the kinetic energy of the vehicle would be
absorbed through the magnetic field rather than through
heating of the vehicle itself, with the result that the total
weight required to protect the vehicle from overheating
and destruction could be markedly reduced.“
The concept of hydromagnetic braking in re-entry was
worked out in detail by Ali Bülent Cambel who made
extensive computer analysis and calculated that MHD braking
can increase drag appreciably, in first experiments by
almost 40 percent [6]. In his paper “MHD for spacecraft”
[6], Cambel wrote: “Re-entry to the Earth‘s atmosphere
must always be made at the correct angle. Too steep and
the spacecraft may burn up; too shallow and it may skip
out again into space and be unable to return home. ...
The key feature of the proposed space vehicle is that it
would carry one or more electromagnets which could be
adjusted in field strength and in direction. Strong fields
would be required and so it is certain that superconducting
magnets would have to be used, because only these
promise to be light enough. …
Magnetic braking is as important as electric propulsion,
since it offers a solution to the difficult problem of slowing
down during re-entry to the Earth‘s atmosphere. Space
vehicles do not use wings, because space is practically a
vacuum and they would be of no use. ... A spacecraft
travelling through space has an enormous amount of kinetic
energy. A small one tonne vehicle travelling at 15,000
m/s has a kinetic energy corresponding to 23 MW-h; at a
re-entry speed of 6000 m/s it would have energy
corresponding to 4.9 MW-h. In today‘s spacecraft this
energy is not used profitably but is dissipated by firing
retro-rockets or in aerodynamic heating. But an MHD
generator could convert part of this enormous energy to
useful electricity. When a vehicle enters or re-enters a
planetary atmosphere at high velocity the gas in front of it
is carried along with it; relative to the vehicle it is drastically
slowed down. The vehicle‘s kinetic energy is largely converted
into thermal energy, resulting in an extremely high
temperature which could consume the vehicle. …
Aerodynamic drag can appear in at least two ways: friction
drag, which heats the vehicle, and pressure drag which
heats the stagnating gas. Heat transfer from the gas demands
that the vehicle be protected. The present method
of using a heat shield has shortcomings. It adds a weight
penalty. It melts and results in asymmetrics which can
cause instabilities. It introduces a variety of chemical species
into the flow field surrounding the vehicle. The ablating
shield concept is being pushed to its limits and is of
questionable merit when the atmosphere of a planet is not
accurately known. Finally, the vehicles with ablative
shields are not readily re-usable without extensive refurbishing.
As an alternative to a heat shield it is possible to repel the
incandescent gas away from the vehicle and thus prevent
contact between the two. As the temperature of a gas is
raised its degrees of freedom increases with its energy
level. As a gas molecule is heated it assumes first a rotational
mode, then a vibrational mode and then, if it is diaatomic,
it dissociates. If its temperature is raised still further,
the gas becomes ionized. In this state the positively
charged ions and the negatively charged electrons constitute
an electrically conducting plasma. Like a metal, the
plasma will be susceptible to the influence of a magnetic
field and so can be manipulated with a magnet. Indeed,
this is what the Earth’s magnetic field does in creating the
Van Allen belts which surround it.”
Regarding the Figure 12, Cambel wrote that the “MHD
spacecraft is depicted opposite during entry to a planetary
atmosphere, causing interactions between the electrically
conducting plasma sheath over the blunt nose and the
magnetic field produced by the powerful electromagnet
(shown larger than true size in relation to a typical manned
vehicle). In the presence of Hall currents, which in the
case of a conducting spacecraft return through the vehicle
structure itself, the magnet experiences forces in the
radial, polar and azimuthal (roll) directions. By suitably
controlling the magnetic field strength and orientation the
vehicle drag and/or flight path can be altered in a powerful
way. Moreover, by using the principle of the homopolar
generator, the magnet can be made to produce electric
current; in this example the magnet is spun on its axis by
the Hall current reaction and power is produced at a fixed
disc armature.“
FIG. 12: Principle of Hydromagnetic Braking [6].
5. BLACKOUT CONTROL IN RE-ENTRY
According to Ali Bülent Cambel there is also the possibility
of blackout control during re-entry [6]: “The charged particles
are in constant agitation, jumping back and forth.
With this oscillatory motion is associated the ‘plasma
frequency’, which is a characteristic parameter of a plasma
depending on its density. For an electromagnetic signal to
pierce the plasma its frequency must be much higher than
the plasma frequency. This means that its wavelength
must be very small, and this is directly related to the size
of signal source equipment. … When a magnet is applied
to a plasma the plasma be-comes anisotropic because the
charged particles assume preferential motion just as do
electrons in a cyclotron. In addition to their oscillatory
motion the charged particles rotate around the magnetic
field lines. They do so at the so-called cyclotron frequency,
the magnitude of which is governed by the magnetic field
strength. Whereas an isotropic homogeneous plasma can
be characterized by its plasma frequency alone an
anisotropic plasma has both its plasma frequency and
cyclotron frequency. It can be shown that the propagation
of a signal through such a plasma depends on the relative
magnitudes of the signal frequency and both the plasma
and cyclotron frequencies. So if a magnetic field is applied
there is no longer a necessity for a microwave system
which operates at an exceedingly high frequency. Depending
on the combinations of frequencies used, signals can
propagate through the plasma even when the plasma
frequency is higher than the signal frequency, provided
that the applied magnetic field is sufficiently strong or that
the cyclotron frequency has the proper value. This can be
achieved for various plasma and cyclotron frequencies,
and for a useful range of signal frequencies. The frequency
combinations at which the signal can penetrate are
called passbands or magnetic windows. The existence of
such ‘windows’ suggests the possibility of eliminating
communications blackout. They also offer the potential for
changing their position, according to varying ambient conditions,
allowing the ground station to use a single frequentcy.
If the spaceship should encounter hotter plasmas,
the plasma frequency would change. This could be
compensated for by changing the magnetic field strength;
as long as the correct ratio of the cyclotron to plasma
frequency is maintained, the magnetic window will allow
the signal to penetrate.”
6. SUMMARY AND OUTLOOK
In the dense air of low altitude up to the rarefied atmosphere
at the edge of space, microwaves are reflected
forward to create a superhot bubble of air above the craft
and form an air spike that acts as the nose cone as was
shown in Figure 9 and 10. The MHD craft is propelled by
accelerating the ionized air rapidly over the lip of the edge
forcing with superconducting magnets placed around the
circumference of the ring of the disc shaped vehicle.
But outside the earth atmosphere, the MHD slipstream
ionized air accelerator would not work. MHD propulsion
would be restricted to propulsion by magnetic field interaction
with the solar wind which is moving at 300-800 km/s
(see also [14], [15]). Alternatively the craft could us ablative
coatings of chemical propellant ignited by intense
masers or lasers from ground or orbiting power stations
(see Figure 11).
To overcome the need for chemical propellants in the
future, one could speculate about unknown natural principles
to be discovered which could strongly use collective
neutrino-plasma interactions and the ponderomotive force
of neutrinos in a magnetized plasma [11]. Presently there
is intensive research in the new field of ‘neutrino plasma
physics’.
Without knowing more about neutrino plasma physics it is
speculative but fascinating to think about how to create a
superhot bubble of neutrino plasma above a MHD craft
and how to accelerate this exotic plasma by using collective
neutrino plasma interactions on a surface charged
smart skin. In analogy to the “MHD propeller” such a
system could be called a “vacuum propeller”. A breakthrough
in these cutting edge technologies could open new
dimensions for space flight.
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