Sensitivity of munitions to lightning strikes: The effect of building lay-out, munition stacking, type of EED and packaging
conference paper
The effect of lightning strikes on munition storage sites is being studied extensively by the Dutch MoD, DNV, and TNO, and comprises site reviews, risk assessments, modelling of storage sites and understanding of the link with munition sensitivity. This paper focuses on one specific part of the study: the munition sensitivity to electromagnetic effects of lightning strikes. The approach is physics-based, and aims to model the electromagnetic field distribution in a magazine from a lightning strike attachment to the structure, the effect of packaging on the damping of the electromagnetic field, the electromagnetic coupling into the electro-explosive devices (EEDs) in
a munition, in order to make a comparison to known EED initiation characteristics.
The response of EEDs to an electric field, assumed proportional to the lightning strike current, was studied. It was found that the ignition is governed by the No-Fire Threshold (NFT) energy rather than the NFT power. The NFT energy is specific to each EED, which requires a worst-case approach, as a munition magazine should be able to store a wide variety of munitions.
A 3-dimensional model was constructed for a storage magazine constructed with reinforced concrete, with rebar acting as natural lightning protection system (LPS). A comparison was made to a situation with an added external LPS (air-termination, masts as down-conductors, and earth termination), not being isolated from the storage magazine. A direct comparison of electric field strength is used for the assessment of flash-over probability. It was noted that the limiting electric field strength needed careful consideration. A further initiation route is the direct coupling of the electromagnetic field into an EED (i.e. without a flash-over from the wall). The EED initiation was related to its NFT energy and dimension, assuming a maximum coupling of the electromagnetic field.
Typically, a storage magazine is modelled as an empty structure. However, the high metallic content of munitions and packaging, will influence the electromagnetic field distribution inside a magazine. A specific 3-dimensional model was therefore made to study this effect. It was found that the ratio of electric field strength and magnetic field vector in the magazine does not equal the 377 Ω impedance of air in the far field approach, and therefore limits the damping of the electromagnetic field by the ammunition boxes.
a munition, in order to make a comparison to known EED initiation characteristics.
The response of EEDs to an electric field, assumed proportional to the lightning strike current, was studied. It was found that the ignition is governed by the No-Fire Threshold (NFT) energy rather than the NFT power. The NFT energy is specific to each EED, which requires a worst-case approach, as a munition magazine should be able to store a wide variety of munitions.
A 3-dimensional model was constructed for a storage magazine constructed with reinforced concrete, with rebar acting as natural lightning protection system (LPS). A comparison was made to a situation with an added external LPS (air-termination, masts as down-conductors, and earth termination), not being isolated from the storage magazine. A direct comparison of electric field strength is used for the assessment of flash-over probability. It was noted that the limiting electric field strength needed careful consideration. A further initiation route is the direct coupling of the electromagnetic field into an EED (i.e. without a flash-over from the wall). The EED initiation was related to its NFT energy and dimension, assuming a maximum coupling of the electromagnetic field.
Typically, a storage magazine is modelled as an empty structure. However, the high metallic content of munitions and packaging, will influence the electromagnetic field distribution inside a magazine. A specific 3-dimensional model was therefore made to study this effect. It was found that the ratio of electric field strength and magnetic field vector in the magazine does not equal the 377 Ω impedance of air in the far field approach, and therefore limits the damping of the electromagnetic field by the ammunition boxes.
TNO Identifier
977682
Source title
PARARI 2022 - Australan Explosive Ordnance Safety Symposium - Supporting Australian Manufacture, Canberra, Australia, 8-10 November 2022
Collation
Paper 5C - 16 sheets
Place of publication
Rijswijk