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IEEE TRANSACTIONS ON MAGNETICS, VOL. 31, NO. I , JANUARY 1995

The electromagnetic chemical propulsion concept

Yuri A. Dreizin DYUAR Inc., 1916 IDS Center, Minneapolis, MN 55402

To the memory of Nikolai Mikhailovich Kolyadin

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I Abstmct - A new concept of propulsion to high velocities is I presented. I t combines the strong points of electrothermal chemical and electromagnetic methods.

The projectile is accelerated by electromagnetic forces. Similar to other methods of electromagnetic propulsion, this overcomes the common limitations of conventional and electrothermal guns, caused by limited expansion speed of hot gases a t acceptable temperatures. As with electrothermal chemical technology, most of the energy required for propulsion is produced during the launch in the launcher itself from chemical energy of propellant or fuel, and only a fraction of the projectile muzzle energy is provided by an external electric source.

potential advantage of the devised method over other methods of electromagnetic propulsion in which much larger amounts of energy (as a rule, exceeding the muzzle energy several times) need to be stored in external electric sources, making electromagnetic acceleration less appealing for potential users because these sources are too heavy, bulky, expensive and vulnerable.

Other merits of the new propulsion method are lower pressure in the barrel, lower heating of the armature and nearly stress free acceleration of long launch packages by distributed forces. The acceleration is approximately constant during most of the launch time resulting in high piezometric efficiency combined with low jerk start and exit.

Potential applications of the EMC propulsion are briefly discussed

This determines the major

I. INTRODUCTION

Extensive research in electromagnetic propulsion is driven by several important potential applications including advanced armament systems, low cost access to space and, as a more remote prospect, impact nuclear fusion. But even the most advanced electromagnetic launcher - the rail launcher, or railgun - is still not in practical use, and serious and difficult technical challenges remain.

Manuscript received December 15, 1993.

Yu. A. Dreizin, formerly with Kurchatov Institute for Atomic Energy. Tel. 612/332-2561, fax 612/334-3348

The author gratefully acknowledges moral and financial support of William Fine, Norman Dann, Irving Mise1 and Melvin and Paul Mooty

The major problems in railgun development are associated with the following issues (listed in the order of significance):

High mass, volume, complexity and cost of pulsed power sources capable of energizing railguns to launch payloads of practical interest.

Damage of bore walls associated with high current density electric discharge through contacts between the rails and fast moving armatures

High barrel stiffness required to maintain reliable contact between rails and metal armatures under heavy magnetic loading.

High parasitic mass of sub-caliber launch packages used to propel long and narrow payloads, for example, rod penetrators.

This paper introduces a new propulsion concept which addresses these problems and outlines a way to their practical solution. It describes a launcher of the railgun family incorporating three novel features. The first one is a resistive link between the armature and the rails. The resisitive link distributes the current over armatures elongated in the direction of motion. This reduces the Joule heating of armatures and improves survivability of sliding contacts. This also makes possible to propel spear-like payloads in small caliber bores by means of longitudinally distributed forces. The second is a non-explosive built-in magnetic flux compression generator (MFCG) internally matched with the launcher. It generates most of the electric energy required for the launch. The third is a considerable change in the launcher geometry, resulting in a reduced pressure level in the launcher. Figures 1-3 illustrate these concepts.

Instead of rails functioning in railguns as electric busbars, mechanical guides and contactors, a fly tube made of insulating material guides the projectile. The current is conducted along the barrel by wide busbars having no immediate contact with the armature. The busbars are spaced from the bore, and ballast resistors connect the contactors with the busbars. To distinguish the launcher with busbars distributing the current to contactors from conventional railguns, it will be referred to as the bus launcher, or busgun. As

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will be discussed ,later in detail, due to the wide span of the busbars, the magnetic pressure can be significantly reduced without compromising the driving force.

Fig. 1. Illustration of the principle of EMC propulsion. 1. 4 - membranes, 2 - fly tube, 3 - magnetic field lines, 5 - projectile, 6 - current flow lines, 7 - imploded membranes, 8 - augmentation coil, 9 - gas pressure. Combustion chambers and a containment structure are not shown.

Arrays of contactors/electrodes are mounted on the inner surfaces of the fly tube walls. They commutate the current to the armature either by means of direct metal-to-metal contact or, alternatively, by means of "plasma brush" discharges. (Technically, the contactors may be connected together forming a continuous contactor rail, which, however, do not carry any appreciable current along the bore; the same is valid for the ballast resistors). Each contactor/electrode is connected to the busbar in series with a ballast resistor. This resistive link distributes the current over the armature, even if the armature length-to-diameter ratio is large. A schematic electric diagram of the busgun is shown in Fig. 2.

Electric energy is generated by a non-explosive MFCG which may be integrated in the barrel and driven by a primitive combustion drive. The flux compressor and the drive extend continuously along the barrel and operate coherently with the projectile motion. They generate a magnetic field wave traveling behind the projectile with increasing speed. The magnetic flux of the traveling wave is compressed by flexible metal membranes pushed by hot gas pressure

as shown by the blunt arrows in Fig. 1. The membranes form "wings" of the busbars. Each of the membranes moves towards the other with relatively low speed displacing the magnetic flux towards the projectile. Finally the membranes collide forming a traveling crowbar (impact electrical contact). The busgun flux compressor can be considered as a slow version of the explosive strip (or bellow) type MFCG with very wide strips.

Note that the hot gas does not flow along the barrel. I t is generated by the properly phased combustion of a fuel or propellant in sections of combustion chambers. The combustion chambers are mounted adjacent to the membranes and their outlets form a fine rack facing the membranes so that hot gas ejected from the outlets pushes the membranes (in the interests of clarity the chambers and a containment structure are not shown in Fig. 1). The combustion can be initiated by fuses, or by spark plugs, distributed in sections of the combustion chambers. The membrane velocity and the flame propagation speed need not be a t all close to the projectile velocity. A typical membrane velocity a t collision, for example, may be as low as 70-100 m/s.

The flux compression generates electromagnetic energy which is immediately used to accelerate the projectile. Initial magnetic flux is produced by an augmentation field coil (as an alternative, the seed current source can be connected directly to the busbars to produce initial flux; in this case the busbars should be short-circuited a t the muzzle or at the breech). Seed electric energy required for that is only a fraction of the muzzle energy.

f Fig. 2. Schematic electric diagram of the busgun. 1 , 3 - busbars, 2 - ballast resistors, 4 - contactors, 5 - armature, 6 - crowbar

The busgun converts the chemical energy into the projectile kinetic energy in a "nearly direct" way, with compressed magnetic flux being just an instant intermediate agent. The flux can be considered as "two-dimensional working fluid" consisting of magnetic field lines, This conversion process can be called electromagnetic chemical (EMC) propulsion.

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Any fuel or propellant initiating and burning fast enough and producing high enough pressure can drive the membranes of the flux compressor. Presumably, diesel fuel combusted in compressed air can be used.

More than ten years ago experiments with railguns powered by explosive MFCG were successful in accelerating small projectiles to high velocities [ 11. At that time a possibility of integration of MFCG strips with railgun rails was discussed. One-shot hybrid devices explosively compressing the flux right in the bore were considered in [2,3]. Somewhat more conventional approaches to combustion-based pulse power generators (such as inverse railguns and other types of piston generators [ 4-61 were also considered.

An important difference between the busgun and these previous proposals is that in the busgun the magnetic flux passing through the bore is compressed outside the bore over the entire barrel length. This is reflected in the busgun design - the flux compression generator has a laterally distributed connection to the fly tube where the projectile is propelled. The idea is quite simple but, due to this difference, repetitively operating and easily rechargeable launchers can be designed. They are automatically matched with the non-explosive MFCG powering them.

The EMC concept can be implemented in several essentially different designs. An associated paper [ 71 describes an example of a busgun design capable of launching a 3.3 kg projectile (consisting of a 1 m long tungsten rod embedded in the aluminum armature of the same length) to 3 km/s muzzle velocity a t acceleration length 6 m. The muzzle energy of the launch package in this example amounts to 15 MJ, but only 7 MJ is required from an external electric source. The paper [7] also discusses flux and energy losses and scaling.

The bus launchers implementing EMC propulsion can be also designed for space applications requiring the launch of massive bodies (one stage rockets, typically in the 1-10 ton range) to velocities 5-10 km/s, or smaller payloads, for example capsules with nuclear wastes, up to 20 km/s. The earth-to-orbit and earth-to-space bus launchers require only several percent of the muzzle energy from an external electric source. The rest comes directly from the distributed combustion drive. For example, 2-3 GJ of electric energy released during -0.5-1 s seems to be sufficient to launch a 1 ton one-stage rocket to 7 km/s (about 100 GJ is required with a conventional rail launcher, see [8]). This may radically reduce the cost of hardware and launch costs for payloads which can tolerate high accelerations.

The busgun design is based, essentially, on two associated ideas. The first is the current distribution over elongated armatures and the second is the magnetic pressure reduction. These effects are discussed below.

11. CURRENT DISTRIBUTION OVER LONG ARMATURES

Electromagnetic acceleration is limited by heating of the armature material, especially near the contact interface. While the total driving force in railguns does not depend on the current distribution in the armature, the Joule heating is sensitive to it. To lower the maximum current density it is desirable to increase the current carrying cross section. For elongated payloads this can be done by extending the armatures in longitudinal direction.

Usually, however, an increase in the armature length does not change the maximum current density at the contact, because the current in the armature is "focused" - that is, most of it flows through a very narrow zone at the rear edge of the contact. This is a consequence of the velocity skin effect [9-111, which results in fast destruction of high-velocity sliding contacts and finally leads to arcing transition in the contacts [ 11-13].

To extend the current distribution over the entire contact area, rails laminated with a layer of resistive material interfacing the armature were proposed in [14,15]. The magnetic field diffuses faster along the resistive layer, spreading the current over the armature and lowering its maximum density at the contact.

The resistive rail concept was analyzed in [13] using a two-dimensional model. It was found that the ability of the resistive layer to spread the current is determined only by the resistivity of its material, and does not depend on the thickness of the layer as was assumed in [ 151.

The implementation of the resistive rail idea, however, may be a difficult problem. For example, in conventiodkrailguns at the velocity 2 km/s, the resistivity rguired to spread the current over a contact zone only 5 cm long is about several thousand times greater than that of copper. This is far above the metal resistivity range. In addition, the thermal regime of the resistive layers for a realistic rail and armature geometry is noticeably worse than was predicted by the two-dimensional model [ 131.

To spread the current along the contact, the busgun implements a similar idea, but its technical realization is far simpler. Ballast resistors are used to connect the busbars with an array of contactors/electrodes

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mounted along the bore walls. They facilitate the penetration of the magnetic field into the gaps between the armature and the busbars. Note, that the magnetic field easily penetrates into the bulk of the armature from these gaps, and therefore transient skin effect does not necessarily limit the electromagnetic propulsion as was postulated in [ 161.

The array of the ballast resistors can be described by the resistance R of the unit length of the array ( R is measured in Ohmm). For long enough armatures this parameter can be estimated according to the following simplified treatment. The linear flux density aq in the gap between the armature and the busbar (magnetic flux per unit length of the gap) is proportional, at any given point, to the current I in the adjacent part of the busbar ai= L i I . The "gap inductance gradient" L, is proportional to the gap width d,. The voltage drop between the armature and the busbar is Ri , where i = d I / d x is the current through resistors per unit length, and the balance of flux in the gap is given by

I

d(Ri ) /dx = - d @ i / d t (the voltage drop in the longitudinal direction in the armature and in the busbar is negligible). The current distribution travels with the same velocity V as the projectile, and therefore d / d t can be replaced with V(d /dx) . Using also a,= L,I, the equation for the linear current density i through the resistors becomes

I f

I

d(Ri) /dx = VL, i . This equation shows that the current is distributed

more or less uniformly over the length 1 of the armature if

R - IVL,.

The thermal capacity of ballast resistors should be sufficient to withstand the heat dissipated in the resistors by the current pulse. The heat per unit length of resistor array can be estimated as

Ri21/V - Li12. This energy loss is a necessary and acceptable

trade-off for the advantage of nearly uniform current distribution over long sliding contacts.

The resistors can be readily made from folded metal sheets or strips as illustrated in Fig. 3. Choosing the thickness and length of the strips i t is possible to control independently their thermal capacity and resistance and to match the resistance with the local projectile speed according to the derived formula.

The current distribution can be analyzed taking into account the increase in strip resistances due to

Joule dissipation. This effect helps distribute the armature current i more evenly along the projectile. With the armature thickness adjusted proportionally to i, the armature current density j becomes constant In addition, the proper distribution of the payload mass makes possible nearly stress-free acceleration.

4

Fig. 3. Fly tube and ballast resistors. 3 - ballast resistors and contactors

1 - bore, 2, 4 - busban,

In the next section i t will be shown that in busguns the total current through the armature is considerably higher than in comparable railguns. Nevertheless, with the armature extending over the entire length of the long payload, the current density is relatively low. Simple numeric examples may help understand the advantage of uniform and low current density. Using the formula for the projectile acceleration a = j B / p , , where j is a constant current density, B is the average magnetic field in the armature and perf is the effective mass density (the ratio of the launch package mass to the armature volume), it is easy to verify that the projectile can be accelerated to a high enough velocity even in a moderate magnetic field and at a short acceleration length without overheating the armature. For example, with B = 10 T, j = 5.108 A/m2 and peff = 6.7.103 kg/m3 (the parameters are taken from [7]) the projectile can reach the velocity of 3 km/s passing 6 m of the barrel length. The accumulated electric action, 1015A2s/m4, is well below the action to melt for aluminum alloys (-1 .7.1016A2s/m4). For space-to-orbit launchers accelerating a I ton rocket with an armature mass of 600 kg to 7 km/s along a 1 km guideway, the armature current density around 3.107 A/m2 (not much different from densities used in welding! ) interacting with the average magnetic field 6 T is sufficient, and this results in an even lower action, 2.5.1014 A2s/m4.

111. EFFECT OF PRESSURE REDUCTION

Another remarkable effect associated with the longitudinally extended current distribution is the

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possibility of decreasing the magnetic pressure. The pressure in busguns can be considerably lower than in railguns generating the same driving force. In railguns, as in gas guns, the driving force can be estimated as F p>b, where S, is the bore cross-section area and p,, the pressure maximum. To increase the driving force sub-caliber launch packages and the highest acceptable pressure levels (close to the barrel strength limit) are required. The state-of-the-art railguns, for example, have a maximum magnetic field about 30-35 T or higher. This corresponds to a magnetic pressure of about 400-500 MPa. In busguns generating the same driving force the maximum magnetic field may be 2-3 t i e s lower, and its pressure 4-9 times lower.

This effect can be explained in the following way. The electromagnetic driving force can be expressed as an integral of the magnetic stresses over the projectile surface. Unlike isotropic gas pressure which always acts perpendicular to the surface, magnetic stresses, described by the Maxwell stress tensor, have components tangential to the surface (similar to elastic stresses in solids). Hence, the electromagnetic driving force may be proportional to the side area S, of the elongated projectile, and the magnetic pressure can be reduced considerably provided that S, >> Sb. I t can be said that magnetic field is able to provide a natural sub-caliber effect. The busgun takes advantage of this effect.

Another explanation of the pressure reduction can be given by considering the recoil force. In busguns the recoil force is due to the magnetic pressure acting on the curved parts of the membranes. The recoil force equals the product of the cross-section area between the busbars and the magnetic pressure. This cross-section area is large due to the wide busbars; therefore the magnetic pressure can be lower to provide the same recoil force (and, hence, the same driving force). Note, that for the simplest busgun design shown in Fig. 1 the total magnetic load separating the busbars remains approximately the same as in a comparable railgun, and only the magnetic pressure is lower. In a slightly more complex design the load on the containment structure can also be reduced (see more in [7]).

The pressure reduction effect can also be illustrated by energy considerations. The driving force is equal to the magnetic energy per unit length of the barrel. With the lower magnetic pressure and, consequently, the lower density of magnetic energy, the barrel cross-section should increase to maintain the driving force. The trick is that in the busgun most of the energy is stored outside the bore between the wide wings of the busbars. For narrow payloads the bore diameter may even decrease because the natural

magnetic sub-caliber effect makes conventional sub-caliber launch packages unnecessary.

The bore diameter, the magnetic fi$d and, consequently, the linear flux density Qr in the busgun are lower than in railguns accelerating the same mass at the same length. To compensate for that, the bysgun current is higher and the inductance gradient L is significantly less than in conventional railguns. This is the trade-off for the lower pressure level, and this would be a serious disadvantage if the busgun had to be energized by an *external source. However, with the magnetic flux compressor integrated in the barrel the high current is produced internally. (Currents up to 100 MA were generated by magnetic compression as early as 1953 1171, two years after Sakharov had invented flux compression generators.) Note, that the traveling crowbar and the armature itself are the only means for switching the high current in the busgun.

IV. CONCLUSION

The busgun has an important common feature with energy recuperating coilguns - in both devices the projectile is accelerated by a well-localized traveling wave of magnetic field. The busgun sliding contacts work at considerably lower current densities than in conventional railguns. At higher velocities i t may be preferable to use plasma brush contacts [18] instead of metal-to-metal sliding contacts. The ballast resistors help stabilize the extended plasma discharge.

The bus launcher may be viewed as an effective energy concentrator. It collects the energy of the fuel distributed along the guideway and converts i t into the projectile energy with reasonable overall efficiency, presumably in a 515% range. Even with low 5% efficiency, only ten gallons of diesel fuel per kilogram of the launch package are needed to reach escape velocity. This example helps understand that the long standing difficulties in hypervelocity launch technology were caused not by the amount of energy to deal with, but, rather, by a lack of the direct and efficient method for the energy conversion that could be implemented within realistic design constraints and costs. While conventional approaches include several stages of the energy conversion and storage, the EMC propulsion concept shows that there is a short one stage way. The associated paper [7] demonstrates that practically all the important parameters (magnetic field, pressure, current density in the armature) which determine the level of technical challenges in EM launchers are considerably lower in busguns as compared to railguns. In addition, the busgun design is flexible due to a number of possible technical tradeoffs, and can be adjusted to various missions.

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This together with the simplicity of the underlying physical principles gives certain grounds to believe that the EMC propulsion concept will be viable. h efficient method of electromagnetic propulsion

will have a very strong impact in several areas of commercial and military technology. It will give rise to low cost earth-to-orbit launch of various payloads, including communication satellites. Also, it will lead to revolutionary progress in anti-armor guns, field artillery, air and missile defense, with a potential to change today's balances in armament systems, making some of them outdated and, possibly, generating new ones.

In particular, strategic intercontinental artillery, not existing today, may become a reality in just several years. It will be far less expensive than ballistic missiles and technologically accessible to underdeveloped countries. Unlike missiles, its launches (characterized by very low thermal and other signatures in the upper atmosphere and in space) would be extremely difficult to detect and monitor. The biggest threat is that intercontinental artillery could easily deliver unguided projectiles charged with nuclear wastes to densely populated regions. When such projectiles disperse and burn up in the lower atmosphere like meteors the radioactive dust would deposit and accumulate in large areas. Several tons of nuclear wastes delivered by several hundreds of intercontinental projectiles to a 1,000 square miles target area may result in huge environmental and economic damage. When such like actions become a realistic possibility, new approaches to international security would be required.

Hypervelocity technology will bring forth new experimental capabilities in high energy density physics. This includes impact sources of soft X-rays and, at a farther horizon, impact ignition of nuclear fusion, which is currently possible only in nuclear tests based on fission. If "heavy duty" hypervelocity launchers are eventually developed (capable of accelerating thousands of 1-10 kg projectiles to escape velocity and higher), an increase in the effective specific impulse will advance rocket propulsion, making flight times of interplanetary missions shorter, and their costs significantly lower [ 191.

In view of the above mentioned prospects electromagnetic chemical propulsion holding the promise of several important advantages over traditional propulsion methods should be comprehensively evaluated as a candidate for advanced propulsion technology. Its evaluation may take advantage of the vast knowledge and technology base which has been developed for electromagnetic and electrothermal propulsion, internal combustion and rocket engines.

ACKNOWLEDGMENTS

Helpful and encouraging discussions with John Barber, Dave Bauer, Tony Challita, Max Fowler, Bob Marinos, George Chryssomallis, Vitaly Shapovalov and Vitaly Sitko are greatly appreciated. The author thanks Dr. Harry Fair and Mr. Patrick Sullivan for the opportunity to present the concept a t the workshop on magnetic compression in EML technology.

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