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Author: Subject: Cap design

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Registered: 29-2-2008
Location: NE USA
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[*] posted on 6-8-2011 at 17:03
Cap Design

Quicksilver says: “It takes so little azide to initiate a compressed gram of RDX or PETN that it (the azide) does not even need to be exposed to the same compression levels.”

Urbanski, V3 p177, has a table confirming part of this. Compressing primaries above 7,000 psi does not help their initiation performance (despite increasing peak pressure).

However, the commonly used US M213 fuze employs 0.3g azide, which is a large quantity considering that tables show, e.g., .03g azide initiating TNT. But this TNT is finely powdered, whereas the RDX or PETN in a blasting cap is compressed to a density of 1.4 (per Gerald Hurst).

A cap in which the output explosive is compressed to an average of 1.4 in one stage will have a density below 1.4 at the output end, and a density above 1.4 at the compression end. Possibly the densities at the two ends are as disparate as 1.2 and 1.6, and the density of 1.6 may require a significant amount of azide, 0.3g, for reliable initiation. I do not know how many pressing stages are typical in the manufacture of a cap. At any rate, it is the contention of the US Bureau of Ordnance that this large amount of azide is required for RDX at a nominal density of 1.4, and I go with that.

I checked tables of compression pressure vs. density for cyclonite in PATR, Urbanski, and Cooper, and they differ significantly, and I suspect it is because the lengths of the columns being compressed varies significantly, and in no case do the authors state the length.

Of interest is that the US military is considering replacing the RDX with a PBX consisting of 97% HNIW and 3% Viton binder. Although HNIW is more sensitive than RDX, in PBX form it may well be less sensitive than a compressed column of RDX. The density of the PBX is about 2.0, generating a pressure over twice that of RDX at 1.4. No pressing necessary.

Though the military can’t afford silver azide, I (we) can - still only a dollar a gram. Too much silver causes one to turn grayish-silver color, but that is the sole symptom of argyria.

I yearn for a cap with a closed end, but can’t get one. One solution is to leave it open, with the naked output charge at the end.

Also, in regard to sealing, I would like to know for sure whether curing epoxy next to azide leaves the azide unaffected. I would hate to have it increase sensitivity. How about silicone, which has acetic acid? Anybody know anything about this?

[Edited on 7-8-2011 by Recessive]
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