P14.pdf

SURFACE CHARGED SMART SKIN TECHNOLOGY FOR HEAT

PROTECTION, PROPULSION AND RADIATION SCREENING

B. Göksel 1 , I. Rechenberg 2

Institute of Bionics and Evolutiontechnique, TU Berlin

Ackerstr. 71-76, Secr. ACK1, D-13355 Berlin

Email: berkant.goeksel@elektrofluidsysteme.de

INTRODUCTION

In January 1962 the Apollo command module heatshield

requirements for several design trajectories were estab-

lished. 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 re-

entry and radiation screening [1]-[4], [7].

In 1969 Wernher von Braun published his paper “Will

Mighty Magnets Protect Voyagers to Planets?” and dis-

cussed 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 endea-

vours, the innovative engineer attempts to imitate nature.

Most of the suggestions I will describe are but manifest-

tions of the phenomena constituting cosmic electrodyna-

mics which our spaceship, Earth, obeys.”

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. 1: The Magnetosphere - produced by the terrestrial

magnetic field and plasma from the ionization of the upper

layers of the atmosphere.

FIG. 3: Radiation Shielding Produced by Mini-Magneto-

sphere [15].

1 PhD Student, Founder of Future Workshop Electrofluidsystems

2 Professor, Head of Institute of Bionics and Evolutiontechnique

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 electri-

fied gas - thus forming the magnetosphere - and forcing

the solar wind to flow around it ( Figure 1 and Figure 2 ).

SURFACE CHARGED SKIN TECHNOLOGIES

An alternative to pure magnetic shielding is surface

charged skin shielding which is a form of electrostatic

shielding.

FIG. 4: Design of a space vehicle with a plasma shield [2].

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.”

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 ex-

treme 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.”

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 equiva-

lent of 6 g/cm² of solid shielding.

1.2.1

Principle of Cold Field Electron Emission

Fowler-Nordheim Equation is the classic relation showing

emission current's dependencies:

(6.1)

E =

100

Volt

(6.2)

V/cm

100

Angstrom

(6.4)

E =

βV





3/2

bφ





(6.5)

exp

βV





with

1.3.

Diamond Layers as Superconductors ?

φ = work function

β = enhancement factor

a = emission area

E = electric field strength

R = tip radius

V = gate voltage

I = emission current

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 semicon-

ductor 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. More-

over, he believes that the results of his experiments on n-

type 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 thermodyna-

mics," he says.“

α,

= 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

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 bet-

ween 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 electron-

charge 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.“

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

Prins continued in [18]: “It is shown experimentally that n-

type diamond is a negative electron affinity material from

which electrons can be extracted at room temperature.

This is achieved by generating an 'ohmic' tunnelling con-

tact 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 ex-

perimentally observed, can only occur if this field be-

comes 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 fer-

mions, 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 be-

cause they are as near to each other as is physically pos-

sible, they automatically constitute a Bose–Einstein con-

densate; i.e. they constitute a superconduc-ting phase.”

–

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 con-

ductivity, 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

2. ELECTROHYDRODYNAMIC PROPULSION BY

POLYPHASE SURFACE CHARGED SMART

SKIN PLASMA ACTUATORS FOR TAKE-OFF

AND LOW SPEED CRUISE MODE

Synthetic diamond and silicon carbide are the best mate-

rials 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 micro-

structured 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 ).

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 de-

veloped 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 struc-

ture 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. 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. 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 de-

velop 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 recent-

ly 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”.

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 magnetohydro-

dynamic 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.”

3.1.

Laser-Driven Water-Powered Propulsion

"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 .”

FIG. 11: Lightcraft flying atop a beam of laser light.

“At least initially, during the prototype phase, it won't be for

everyone, just NASA and military test pilots. The hyper-

energetic 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.”

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.

“It all sounds a bit too much like science fiction, but My-

rabo points out that most of the technologies or principles

have been demonstrated. Faculty and students at Rens-

selaer 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 )

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 condi-

tions [16].

4. HYDROMAGNETIC BRAKING IN RE-ENTRY

In 1967, at the height of AVCO-Everett’s research on

plasma radiation shielding utilizing superconductive mag-

nets 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 bra-

king 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. ...

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 cau-

ses the shock wave ahead of the model to be pushed a-

way 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.”

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 superconduc-

ting magnets would have to be used, because only these

promise to be light enough. …

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