EP0870166B1

EP0870166B1 - Launch pack

Info

Publication number: EP0870166B1
Application number: EP96938553A
Authority: EP
Inventors: Willem Johannes Kolkert; Willem Karthaus
Original assignee: Nederlandse Organisatie voor Toegepast Natuurwetenschappelijk Onderzoek TNO
Current assignee: Nederlandse Organisatie voor Toegepast Natuurwetenschappelijk Onderzoek TNO
Priority date: 1995-11-21
Filing date: 1996-11-21
Publication date: 2000-03-01
Anticipated expiration: 2016-11-21
Also published as: IL124504A0; NL1001710C2; NO982216D0; AU7590796A; PL326749A1; KR19990071501A; NO982216L; AU702837B2; DE69606889D1; IL124504A; WO1997019315A1; DE69606889T2; JPH11515083A; EP0870166A1

Description

The present invention relates to a launch pack for accelerating projectiles to hypervelocities in an electromagnetic accelerator, comprising a casing in which at least one electric sliding contact is arranged, which sliding contact is formed by multiple fibres of an electrically conducting material onto which an electrically insulating material has been applied, the fibres running from the top to the bottom of the casing. A similar launch pack is known from "Behaviour of multi-tiered Copper Wire Solid Armatures", Keith A. Jamison et al., IEEE Transactions on Magnetics 31 (1995) Jan. N°1. Part1 NY US P174-179.

A launch pack of this type is known in practice from research and can, for example, be used to launch projectiles for military purposes at velocities of up to 4 km/s. In this context use is often made of an electromagnetic rail accelerator which operates repeatedly.

A known problem is that spark erosion occurs in the first part of the launch process, in which the launch pack is accelerated from stationary to the so-called transition velocity. During this phase the ends of the fibres wear away. When the transition velocity is reached, a fluid and/or metallic plasma boundary layer is then produced by the high generation of heat at the contact surface between the electric sliding contact and the rails. During this phase the ends of the fibres wear away further, the length of the fibres is reduced and the solid-solid electric sliding contact surface becomes a hybrid sliding contact surface.

The abovementioned effects give rise to inefficient energy transfer between the accelerator and the launch pack and, furthermore, lead to undesired damage to, and excessive wear of, the accelerator. The result of this is that the maximum achievable velocities with launch processes of this type are limited and the accelerator is usable only a limited number of times.

The aim of the invention is to overcome the abovementioned disadvantages.

To this end the launch pack according to the present invention is characterised in that the length of the fibres is essentially greater than the height of the casing to such an extent that a feed mechanism is provided to compensate for wearing away of the ends of the fibres.

The nature of the feed mechanism is that there is compensation, during acceleration, for the length of the fibres lost as a result of the ends of the fibres wearing away by erosion. With this arrangement the extra length of the fibres also ensures that the mechanical pretensioning at the contact surface is maintained, postponing the occurrence of spark erosion. Consequently, with the launch pack according to the invention there is a controlled, gradual transition from a solid-solid moving electric sliding contact to a sliding contact with a fluid and/or metallic plasma boundary layer with the stationary electrical conductor. In addition, current is fed through the contact surface virtually free from spark erosion.

In a preferred embodiment of the launch pack two or more electric sliding contacts are, viewed in the longitudinal direction of the launch pack, placed essentially one after the other and at least partially spatially separated from one another, such that the orientation of the electric sliding contacts within the casing is essentially identical. With this arrangement the electrical part of the launch pack can be extended in an advantageous manner, it being possible to reduce the consequences of the "velocity skin effect" and further to optimise the current distribution in the electric sliding contacts.

In a further embodiment of the launch pack two or more electric sliding contacts, viewed in the transverse direction of the launch pack, are positioned essentially alongside one another and at least partially spatially separated from one another. With this arrangement the electric sliding contacts placed alongside one another will attract one another, as a result of which the mechanical pretensioning of the sliding contacts on the rails will be further intensified.

The present invention also relates to a method for the production of an electric sliding contact, as described, as a component of the launch pack according to the invention, the method comprising the following steps:

(a) applying an adhesive to a winding core, the shape of which is matched to the shape desired for the rear of the electric sliding contact;

(b) winding a layer of fibres of an electrically conducting material, onto which an electrically insulating material has been applied, essentially parallel around the winding core;

(c) applying the adhesive to the wound fibre layer;

(d) winding a further fibre layer on the preceding fibre layer;

(e) repeating steps (c) and (d) until a desired thickness for the electric sliding contact has been obtained;

(f) clamping the fibre layers on the winding core essentially symmetrically around the axis of the winding core in such a way that the outermost fibre layers are shaped into the shape desired for the front of the electric sliding contact;

(g) heating the clamped fibre layers to a temperature which is higher than or equal to the softening temperature of the adhesive;

(h) cooling the clamped fibre layers;

(i) subdividing the wound product into a number of electric sliding contacts of essentially equal dimensions.

In this context the method according to the invention has the advantage that the winding technique leads to an electric sliding contact of an approximately uniform density. No further filling is needed for homogenisation of the electric sliding contact produced in this way, with the result that wastage of material is counteracted.

The present invention also relates to an electric sliding contact obtained by use of the method, wherein the fibres make up 75 % to 85 % of the volume of the electric sliding contact.

The present invention also relates to a method for the production of a launch pack, wherein a casing of, preferably, fibre-reinforced material is arranged around one or more electric sliding contacts obtained by use of the method described above, the dimensions of said casing being matched to the dimensions of the accelerator, and wherein empty spaces inside the launch pack are filled with the aid of a filler material of high compressive strength and high temperature stability.

The present invention will now be described in more detail with reference to the appended figures, wherein:

Figure 1 shows a simplified diagram showing the principle of a rail accelerator;

Figure 2 shows a longitudinal section of a first embodiment of a launch pack according to the present invention;

Figure 3 shows a cross-section along the line III-III in Figure 2;

Figure 4 shows a cross-section of a rail accelerator in which a second embodiment of the electric sliding contacts of a launch pack according to the present invention is shown diagrammatically; and

Figure 5 shows a top view of a possible installation for carrying out the method according to the present invention.

Figure 1 shows a simplified diagram showing the principle of a rail accelerator 1. Rail accelerator 1 is an electromagnetic accelerator provided with rails 2 which serve as electric guides for the projectile 4 to be launched. With the aid of a pulsed electric power supply 3, a current I of the order of magnitude of a few MA is passed through rails 2 and launch pack 5 for the projectile 4. Current I, in conjunction with magnetic field B, provides a resultant Lorentz force F on the projectile 4, which is accelerated as a result.

In the future it will, for example, be possible to use a rail accelerator of this type for repeated electromagnetic launches of projectiles to hypervelocities (up to 4 km/sec) for military tactical applications. When launch packs known from laboratory practice are used, spark erosion occurs during the acceleration process on the contact surface between the stationary rails 2 and the launch pack 5, which is moving very rapidly. On reaching the so-called transition velocity, that is to say the velocity at which a fluid and/or metallic plasma boundary layer is formed at the contact surface of the electrical part of the launch pack 5 as a result of the high temperature, the solid-solid electric sliding contact surface becomes a hybrid contact surface. Said effects can lead to undesirable damage to, and excessive wear of, the electromagnetic accelerator 1, which is an obstacle to repeated use of the accelerator. Moreover, said effects give rise to an inefficient energy transfer between the accelerator 1 and the launch pack 5 and consequently said effects limit the maximum achievable velocities with launch processes of this type.

Figure 2 shows a longitudinal section of a first embodiment of a launch pack 5 according to the invention. Figure 3 shows a top view of a cross-section, along the line III-III, of the launch pack according to Figure 2. In this example launch pack 5 comprises multiple electric sliding contacts 6 around which a casing 7 is arranged, such that the electric sliding contacts 6 make contact with rails 2. Empty spaces between the electric sliding contacts 6 have been filled with the aid of filler material 8. Each electric sliding contact 6 consists of a number of fibres of an electrically conducting material onto which an electrically insulating material has been applied. Examples of a suitable electrically conducting material are copper, aluminium and molybdenum. The electrically insulating material used is, for example, polyester-imide. The fibres preferably run from the top to the bottom of the casing 7 essentially parallel to one another, said fibres having the shape of the electric sliding contact 6. In a further preferred embodiment the fibres are at an angle to one another (not shown). This leads to a reduction in the consequences of the so-called "velocity skin effect", that is to say the effect which results in the current I passing through only part of the launch pack 5, the current-carrying part becoming smaller as the velocity of the launch pack 5 increases. Specifically, it has been found from research that in order to prevent the "velocity skin effect" the resistance must be relatively high in the direction of movement of the launch pack (see arrow) and must be low in the direction perpendicular thereto (preferably a ratio of ≥ 100 : 1). This anisotropic resistance distribution can, for example, be obtained by the use of fibres running parallel to one another. In connection with an optimum heat balance, the thermal separation between the fibres must preferably be as small as possible. Furthermore, in order to prevent the formation of microsparks in the contact surface, an optimum number of contact spots per unit surface area in the contact surface is desirable. This can be achieved, for example, by means of a suitable choice of the fibre diameter (preferably between 40 and 100 µm) and a suitable choice of the insulating material.

It is clear from Figures 2 and 3 that the length of the fibres has been chosen to be sufficiently greater than the height of the casing 7 in order to provide a feed mechanism to compensate for wearing away of the ends of the fibres during the launch process. Preferably, the length of the fibres is chosen such that the feed mechanism is available in use essentially throughout the entire launch process. The feed mechanism produced is further supported by the fact that the thermally softened fibres stretch as a consequence of the Lorentz force acting on them. This has the result that the transition from a solid-solid moving electric sliding contact to a sliding contact with a fluid and/or metallic plasma boundary layer takes place gradually. In this context the fluid/plasma mixture produced has an advantageous lubricating action.

Each electric sliding contact 6, and thus all fibres in an electric sliding contact, makes (make) an angle α with the casing 7, angle α not being equal to 90°. Preferably angle α is approximately the same for all fibres in an electric sliding contact 6. Preferably α = 45 ± 5°. In the preferred embodiment shown each electric sliding contact 6 has a mid section 6a which runs essentially perpendicular to the casing 7, arms 6b being fixed to, respectively, the top and the bottom of the mid section 6a, which arms 6b run at an angle β in a direction opposite to the direction of movement. β is preferably given by 15° ≤ β ≤ 25°.

The launch pack 5 according to the invention comprises at least one electric sliding contact 6, but, if desired, can be extended by placing multiple electric sliding contacts one after the other, and at least partially spatially separated from one another, in the longitudinal direction of the launch pack 5, the orientation of the electric sliding contacts 6 inside the casing 7 being essentially identical, as is shown in Figure 2. An optimum current distribution can be obtained by extending the launch pack in the abovementioned manner by means of segmentation. Said length optimisation also makes it possible to use a longer rail accelerator, as a result of which spark erosion-free acceleration can be achieved over a longer path.

The optimum length l₁ of each electric sliding contact 6 is related to the diffusion length of the magnetic field B in the electric sliding contact as a result of the "velocity skin effect" and the temperature dependence of the specific resistance of the fibre material used and the shape of the electric sliding contact. For example, for copper the optimum length l₁ = 11 mm and for molybdenum l₁ = 34 mm. It will be clear that the dimensions of the electrical part of the launch pack 5 are dependent on, inter alia, the calibre and the shape of the bore of the accelerator concerned as well as on geometrical requirements which are imposed in connection with fitting the launch pack in a projectile. An optimum length can, if desired, be obtained by placing a number of sliding contacts one after the other. It can be pointed out that the height of the casing 7 is preferably approximately equal to the distance between the rails 2 of accelerator 1.

In general, l₂ for arms 6b is preferably equal to or less than 7 mm. In the embodiment shown in Figure 2 the dimensions are matched to a rail accelerator which has a bore of 20 mm.

Casing 7 is preferably produced from a fibre-reinforced plastic material and materials of this type are generally known to those skilled in the art. In general they comprise reinforcing fibres, such as carbon fibres, aramide fibres (Kevlar®, Twaron®) or other reinforcing fibres, which are incorporated in a matrix or are glued by means of an adhesive or binder, such as epoxy adhesive. The casing is preferably produced from a fibre material which is obtained by winding, preferably at a winding angle of between 80 and 85°, the wound layer being bonded to itself by means of a minimum amount of adhesive, for example an epoxy adhesive. Preferably, the casing is fitted using a force fitting.

The filler material 8 used is preferably a material of high compressive strength and high temperature stability, such as boron nitride, aluminium oxide, glass fibre-reinforced epoxy material (G10) or materials to influence the penetration of the magnetic field, such as µ-metal.

Figure 4 shows a diagrammatic representation of a possible second embodiment of a launch pack according to the invention. The only parts of the electrical part of the launch pack which have been drawn in cross-section are the two electric sliding contacts 6' positioned alongside one another in the transverse direction thereof. The electric sliding contacts 6' are at least partially spatially separated from one another. It can clearly be seen that the fibres in the electric sliding contacts 6' are curved outwards and taper together towards the rails 2. A current I runs through the two electric sliding contacts 6', as a result of which opposing Lorentz forces F₁ and F₂, respectively, act on the two electric sliding contacts. The electric sliding contacts 6' thus attract one another, a pressure being exerted on the rails 2 as a result of the curved shape of said sliding contacts. The effect of this is to promote the abovementioned feed mechanism.

It will be clear that the first embodiment (shown in Figure 2) and the second embodiment (shown in Figure 4) of the electric sliding contacts 6 and 6', respectively, can be integrated.

Figure 5 shows a top view of an installation to illustrate the method according to the present invention. Installation 10 comprises a winding core 11, the shape of which is matched to the shape desired for the rear of an electric sliding contact. An adhesive, preferably a thermoplastic two-component adhesive having a softening temperature of ≥ 120 °C, is first applied to winding core 11. A layer of fibres 12 of a suitable material (preferably having a diameter of between 40 and 100 µm) is then wound around winding core 11. Preferably, a layer about 0.5 mm thick is wound. The adhesive is then applied to the wound layer, after which a subsequent layer of fibre material is wound in the same way on top of the layer which has already been wound, etc. This process is repeated until a desired thickness for the electric sliding contact has been obtained. Clamping plates 13, the shape of which is matched to the shape desired for the front of the electric sliding contact, are then applied approximately symmetrically around the axis of the winding core 11 in order to clamp the fibre material 12. The whole is then heated to a temperature which is higher than or equal to the softening temperature of the adhesive (for example 120 °C). The whole is then cooled to room temperature. The resultant wound product consisting of fibre material 12 is then removed from installation 10 and can be subdivided into a number of electric sliding contacts of essentially the same dimensions. In this example the electric sliding contacts 6 as shown in Figure 2 are produced by dividing the wound product into four equal parts.

To prevent the fibre material becoming stuck to the components of installation 10, inter alia the winding core 11 and the clamping plates 13 can be provided with a protective layer, for example a 0.1 mm thick layer of Teflon tape. To provide symmetrical clamping, use can be made of a four-part claw 14, so that the clamping plates 13 abut well and form a closed mass. The clamping plates can optionally be fixed in place with the aid of bolts. The dimensions and shape of the components of installation 10 can, of course, be adapted to the desired final shape of the electric sliding contacts to be produced. Thus, for example, after adaptation of installation 10 it is also possible to make electric sliding contacts by this method which are suitable for use in a rail accelerator having, for example, a circular bore.

The method according to the invention has the advantage that the fibre material 12 can be wound so compactly that the fibres in the electric sliding contacts are able to make up 75 % to 85 % of the volume thereof. The electric sliding contacts produced in this way are therefore virtually homogeneous, but nevertheless have the requisite anisotropic resistance distribution. A further advantage of the method according to the invention is that virtually all of the wound fibre material is used; there is thus hardly any question of material wastage.

The use of fibres in combination with an adhesive gives the electric sliding contacts according to the invention the necessary give to be able to absorb slight deformations of the accelerator during the acceleration process and, as a result, to allow the feed mechanism to function in an optimum manner as the ends of fibres wear away during the acceleration process. In order to improve the electrically insulating action of the adhesive, a fine pulverulent ceramic material can be added to the adhesive.

The present invention has been explained with reference to a launch process in an electromagnetic rail accelerator. However, it will be abundantly clear to a person skilled in the art that many other applications are conceivable, for example as a launch pack for an electromagnetic linear induction accelerator, electric sliding contact for a magnetic flux compressor and as electric sliding contact for electrical machines in general.

Claims

Launch pack (5) for accelerating projectiles (4) to hypervelocities in an electromagnetic accelerator (1), in which at least one electric sliding contact (6) containing multiple fibres of an electrically conducting material is arranged, the fibres running from the top to the bottom of the launch pack (5), having a length essentially greater than the height of the launch pack (5) and being at least at one side of the launch pack (5), at an angle α with respect to the top and bottom face of the launch pack (5), angle α not being equal to 90 degrees and essentially constant over the entire length of the at least one electric sliding contact (6), characterized in that an electrically insulating thermoplastic material has been applied along the entire length of each fibre and in that the at least one electrical sliding contact is accommodated in a casing (7) of fibre reinforced material.
Launch pack according to Claim 1, wherein, viewed in cross-section of the launch pack (5), the fibres of at least two electric sliding contacts (6) placed alongside one another are curved outwards.
Method for the production of an electric sliding contact (6) as a component of a launch pack (5) according to one of the preceding claims, the method comprising the following steps:

(a) applying an adhesive to a winding core (11), the shape of which is matched to the shape desired for the rear of the electric sliding contact (6);

(b) winding a layer of fibres (12) of an electrically conducting material, onto which an electrically insulating material has been applied, essentially parallel around the winding core (11);

(c) applying the adhesive to the wound fibre layer (12);

(d) winding a further fibre (12) layer on the preceding fibre layer (12);

(e) repeating steps (c) and (d) until a desired thickness for the electric sliding contact (6) has been obtained;

(f) clamping the fibre layers (12) on the winding core (11) essentially symmetrically around the axis of the winding core (11) in such a way that the outermost fibre layers (12) are shaped into the shape desired for the front of the electric sliding contact (6);

(g) heating the clamped fibre layers (12) to a temperature which is higher than or equal to the softening temperature of the adhesive;

(h) cooling the clamped fibre layers (12);

(i) subdividing the wound product into a number of electric sliding contacts (6) of essentially equal dimensions.
Method for the production of an electric sliding contact according to Claim 3, wherein the fibres make up 75 % to 85 % of the volume of the electric sliding contact (6).
Method according to claim 3 comprising the additional step of arranging a casing (7) of fibre-reinforced material around one or more of the electric sliding contacts (6) in order to obtain a launch pack (5), where the dimensions of said casing (7) are matched to the dimensions of the accelerator (1), and wherein empty spaces inside the launch pack (5) are filled with the aid of a filler material (8) of high compressive strength and high temperature stability and/or a filler material (8) which influences the penetration of a magnetic field.