Temperature resistant igniter
Technical solution proposal J2014-002
Corporate entity: Junghans Microtec GmbH
The proposed technical solution describes a temperature-resistant fuze for projectiles with a particularly long range and high velocity. According to the proposed solution, the temperature resistance, which is particularly important in the case of high velocities, is achieved by a targeted selection of materials and an advantageous design of individual areas of the fuze.
Compared to detonators known in the prior art, such as those described in DE 10 2008 026 245 A1, the detonator according to the proposed solution handles the heat energy better and thus increases the durability and the associated reliability of the detonator in use. The fuze according to the proposed solution is particularly suitable for use with long-range, high-velocity projectiles that have to withstand a high energy input over a longer period of time.
The projectile fuze known from the previously mentioned DE 10 2008 026 245 A1 comprises a metallic case 1, to which a bonnet 2 made of metal or polyphenylene ether (PPE) with a gradation 3 is connected (see Figure 1). To compensate for this gradation 3, a shrinkable ring 4 made of metal or PPE is provided (see Figure 2). The ring 4 serves to avoid a reduction in range due to the gradation. A further disadvantage of this projectile fuze is the high heating of the tip of the canopy 2 at high and long-lasting airspeeds.
The task of the proposed technical solution is to specify a fuze that can be used for modern projectiles of long range and high velocity, such as base-bleed projectiles, and has a radome to realise a radar approach function.
The detonator according to the proposed solution has a radome made of polyetheretherketone (PEEK) and a detonator housing part made of steel or stainless steel, wherein a) the detonator housing part is connected to a lower stepped part of the radome and b) a metallic ring extending partially over the radome and the detonator housing part is provided in the a) the fuze housing part is connected to a lower stepped part of the radome, and b) a metallic ring extending partially over the radome and the fuze housing part is provided in the connecting region.
The proposed solution is based on the consideration that in order to realise a radar proximity function, the radome must be constructed of a non-metallic, non-conductive material.
Furthermore, the proposed solution is based on the realisation that after firing a projectile at a higher velocity due to air friction, the fuze experiences a high temperature load, whereby such a temperature load can lead to the complete destruction of a radome made of plastic.
Furthermore, the proposed solution is based on the knowledge that a high overall thermal load on a fuze also causes a temperature increase inside the fuze. As a result, there is a risk that electronic components of the igniter are destroyed and/or an igniter malfunction occurs due to an excessive temperature load.
In a further step, the proposed solution is based on the knowledge that surface inhomogeneities of a fuze, such as larger depressions or protruding areas, impair the aerodynamics and thus reduce the range and additionally lead to a further heat input, i.e. heating of the fuze and thus to possible damage to the fuze. Heating of an area 10 comprising such an inhomogeneity in the form of a flanged edge, which was created by flanging a lower part of a plastic radome 12 with a metallic fuze housing part 14, and heating of a radome tip 16 can be seen in Figure 3. Figure 3 shows an IR image taken in a wind tunnel at a load of approximately Mach 1. A uniformly high temperature load can be seen both on the radome tip 16 and in the area 10 of the flanging edge.
The problem of reduced durability of a radome or damage to fuze components due to a high energy input into the fuze tip in the case of long-range projectiles flying at high speed is solved in the proposed technical solution in particular by the following three measures:
The first measure to increase the durability of the radome consists in the choice of a more stable plastic material for the radome. In addition to a constant permeability for electromagnetic waves, the plastic should be more temperature-stable and at the same time have a lower thermal conductivity value than commonly used materials. Furthermore, it is necessary that the plastic has an increased heat penetration coefficient. A material that meets these requirements is polyetheretherketone. Due to the previously mentioned advantages of the material, less heat is generated inside the radome. The advantageous behaviour of the material PEEK under realistic conditions is confirmed with the help of simulations and measurements, see figures 4, 5 and 6.
The second measure to increase the durability of the radome is the application of a metallic ring, in particular made of steel or stainless steel, in the connection area. On the one hand, this achieves an aerodynamic optimisation and, on the other hand, the entire area of the connection point between the radome and the housing is shielded by the metal surface of the ring. Since this area is of no significance for the radar function, the use of metal at this point is harmless. The metallic ring conducts the entire heat input that occurs in the area of the joint to the fuze housing part and thus supports the cooling of this joint-sensitive area. The radome is preferably pressed axially through the ring onto the fuze housing part with a radial clearance fit. The ring itself is then screwed or adequately connected to the fuze housing part.
The third measure to increase the durability of the radome is to manufacture the fuze housing part from a material whose thermal properties are such that, on the one hand, the material can absorb the heat in the area of the ring well and, on the other hand, releases the thermal energy only slowly to the interior of the fuze. The aim is to significantly delay the thermal response of the fuze interior to the external thermal load. The risk of damage to components inside the detonator can thus be reduced. A suitable material that meets the requirements is steel or stainless steel. The heat penetration coefficient is approx. 2.5 times and the thermal diffusivity approx. 20 times lower compared to aluminium commonly used for fuze casings (see table). The use of steel or stainless steel leads to a significantly lower heat conduction and thus to a lower heating of internal components, such as holders for electronic components and thus indirectly to the heating of the electronic components themselves.
The interaction of these three measures achieves the following positive effects: The durability of the radome is increased and thus the reliability of the igniter is enhanced. Through the use of materials optimally adapted to the requirements and the advantageous design comprising a metallic ring, the generation of heat itself is reduced. In addition, it should also be emphasised that usual basic designs, as used in the field of fuze technologies, can be largely retained: The basic design of the radome and the internal structure do not have to be changed within the framework of the present proposed technical solution. The additional effort and costs for retrofitting existing fuze systems are thus kept within limits - and yet the design achieves the thermal robustness to withstand even a significantly higher heat input, such as occurs with base-bleed ammunition.
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