Torpedo explosive charges

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José M. Rico
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Torpedo explosive charges

Post by José M. Rico » Tue Dec 28, 2004 1:44 am

This topic has been moved here from the old forum. Feel free to continue the discussion.
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08 Jun 2004 02:59:43 - Cesar

I learned that these navies used different types of explosives on their torpedoes during WWII.

The U.S. Navy used Torpex.
The Royal Navy used TNT.
The Kriegsmarine used hexanite.

Question: what is the difference among them in terms of explosive power? Are 300kg of TNT equivalent to 300kg of Torpex or Hexanite?

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08 Jun 2004 05:28:21 - Tiornu

The British developed Torpex in 1942, getting it into service right around the end of the year and sharing it also with the Americans who adopted it in 1943. Torpex is considered 50-100% more powerful than TNT. Japanese Type 97 explosive is considered 7% more powerful than TNT. The various explosives used by the Germans were more powerful than TNT, but I don't have any figures for them. The French used a sweetened TNT for a little added oomph, but again, I don't have any figures for that.

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08 Jun 2004 15:17:25 - Ulrich Rudofsky

Basically, all the explosives in torpedos were TNT enhanced in various ways to increase the efficiency of the detonation and to propagate and prolong the shock wave (eg. Torpex contained 18% aluminum powder). Hexanite is considered by most to be only slightly more effective than plain TNT, the military mix being generally 60% TNT and 40% hexanitrodiphnylamine. Torpex was made from about 40% TNT plus 42% cyclomethylenetrinitroamine and the aluminum kicker).

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08 Jun 2004 16:20:14 - José M. Rico

Also if this is of any interest to you, it is generally accepted that the torpedo defence system of the Bismarck Class battleships was designed to resist explosive charges of 250 kg (550 lbs) of TNT. However, According to “Technical Report No. 222-45. Loss of the Battleship Tirpitz on 12 November 1944” (available from this site: http://www.kbismarck.com/tirpitz-technical-report.html ), the torpedo defence system on the Tirpitz was designed to withstand about 660 pounds (300 kg) of German hexanite.

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08 Jun 2004 23:01:29 - Ulrich

Can anyone explain to me the meaning of the numbers given for various TNT-based explosive types compared to plain TNT and how they were measured and quantified?: % over TNT, ratio to TNT, effectiveness over TNT etc. The Dutch Navy, for example, http://www.dutchsubmarines.com/specials ... _mines.htm says that Torpex was 1.61 times as effective as TNT. How is this measurement derived? What kind of mathematical relationship is there between armor thickness and type, and the TNT (or enhanced TNT) force applied to it. Hexanit, for example, has been described as being slightly better than TNT to being 3 times as powerful as TNT (British Coast Guard). I am aware that Hexanit (Hexanite, German plural) comes in many formulations depending on use and year of deployment (military, naval, and industrial). As I said I am puzzled by the numbers given for Torpex and Hexanit. How the two compare in effectiveness against a particular armor plate and construction system is still a mystery to me.

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09 Jun 2004 08:20:36 - phil gollin

The general British comparison figures are from the wartime "Static Explosion Committee" and are for specific mixes and (as the name suggests) are for charges compared in free air.

There are some British figures for underwater comparisons which are not so easily tied down, and are inconsistent. This MAY be a result of changes in test criteria in the evolution of UNDEX, but I haven't found any specific papers.

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13 Jun 2004 00:33:05 - thomas

Well, the subject can actually fill volumes. Generally, the statement that say torpex is two to three times more powerful than straight TNT is a little misleading. More powerful does not always mean more effective. Effectiveness depends largely on the application you are applying the explosive to. In some instances less powerful is actually better. Generally speaking, RDX, explosive mixed with TNT to form Torpex, is rated by itself as giving a detonation wave of 8700m/s, while TNT yields 6900m/s. By itself, RDX is therefore about 21% more power than TNT, but combine the two and they compliment eachother. The RDX is too sensitive to use on its own in most applications, but mixing RDX powder into molten TNT allows the TNT to desensitize it. TNT is relatively insensitive, and powerful on its own, but it often does not detonate cleanly and is therefore not quite so efficient; with RDX in the TNT matrix, the efficiency of detonation is increased. The advantage, at least in warheads and depth charges, is that the power of the explosion can be increased with out materially increasing the weight of the warhead. Adding aluminium powder to the explosive increases the generation of heat and increases the brisance by allowing the explosive to convert itself into gas pressure more rapidly. Brisance is the ability of the explosive through heat, shock and pressure to destroy structures it is in contact with.

The choice of the british and US navies in adopting aluminized RDX/TNT explosives was simply to increase the power of a given weight of explosives in a warhead without having to adopt a whole new delivery system, ie, design and produce a larger more powerful torpedo body. As a HBX type explosive, it became pretty much a standard in post war years both as an contact and proximity type explosive because of its ability to rapidly generate maximum pressure and shock waves. During WW2, in British service at least, Torpex began to replace earlier explosives from about 1943, with 18in arial torpedos having having a higher priority than the larger 21 inchers of surface ships and subs. The percentage of explosive was 40-45% TNT, 40-45% RDX, and 10-20% aluminium powder.

In the case of German torpedo warheads, the basic explosive was 50% TNT, to which was added 10% or so Hexinit to improve the power and efficiency of detonation, a small percentage of ammonium nitrate as combustable filler oxidizing agent, and a large percentage of aluminium powder to increase the heat and brisance of the detonation. This was largely an economy explosive which was rather cheaper to produce than Torpex, particularly since German production of RDX was limited to only about eight concerns with a similar number involved in processing the RDX into other types of explosive combinations; the majority of German RDX went to use in 20, 30, and 37mm cannon shells in light flak and aircraft cannon munitions, and shaped charge anti-tank munitions.

The German torpedo warhead was slightly more powerful than an equivalent TNT only warhead, but had a much greater brisance than TNT. Torpex, on the other hand would have yielded approximately 20% more power making a 500 lbs warhead the equivalent yield of a 600-lb TNT warhead. The inclusion of aluminium powder to increase brisance would have given a greater destructive power against standard ship hull structures, though not necessarily having an effect on heavy armour.

As I mentioned earlier, more powerful is not always better. In some instances a lower power yield is actually better. Most military explosives are relatively expensive commodities and not always ideally suited for certain tasks. Depth charges are a good example. These tended to be dropped in huge numbers. High brisance was not nearly so important because the chances were that you would never score a direct hit anyway. Instead, you wanted an explosive that was both cheap, because you might drop scores of the ash cans, and yet which would give a good prolonged shock wave pulse. Consequently amatol, TNT/ammonium nitrate, fit the bill quite well. Likewise in artillery shells and airplane bombs, bombardments and barrages required prodigious quanties of shells and bombs in any given major battle. TNT filled shells required a relatively high quality alloy shell body which would shatter into useful anti-personnel fragments. However, alloy steels could be hard to come by in wartime because of the need to use it in other applications such as gun production, armour, etc, so frequently cast steel was used. It was cheap, easily obtained and manufactured, but straight TNT would shatter the shell or bomb body into almost iron filings, so you decreased the brisance by adding ammonium nitrate which once more gave your shells and bombs good fragmentation. you lost a little in explosive power, but you wound up the better for it by economizing on exotic steels and explosives.

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13 Jun 2004 07:28:52 - thomas

I should add, to my previous post, that the torpedo warheads employed by the Kriegsmarine during WW2 were generally SW36, SW39 or SW39a, the numbers indicating year of adoption. SW36 was 67% TNT, 8% Hexinit, and 25% aluminium powder. SW39 was 45% TNT, 5% Hexinit, 30% ammonium nitrate, and 20% aluminium powder. The final version, SW39a was 50% TNT, 10% Hexinit, 5% ammonium nitrate, and 35% aluminium powder.

I would suspect that the change between SW39 and SW39a was basically to increase blast yield as the earlier version would have had rather modest capabilities in terms of shock wave. Part of the problem in determining exactly how powerful German torpedo warheads were centers around the various changes in torpedo warhead strength. This probably accounts for the stated figures which are at variance or contradictory. Certainly the decrease in the percentage of ammonium nitrate and the increase in TNT, aluminium and hexinit in SW39a would have represented a considerable increase in power and brisance over SW39. It might be that economy measures, which probably dictated the substitution of ammonium nitrate for a large proportion of the TNT in SW36 might have been taken too far and only shown up in battle during the crucial 1939-40 period when so many torpedo faults were uncovered. The small percentage of ammonium nitrate in SW39a was not an economy, but an oxidizing agent to increase efficiency of gas pressure propagation during the initial detonation of the warhead.

This can lead one to speculate that perhaps one set of figures for resistance to torpedo blast might be for SW36 while the larger figure quoted for Tirpitz might be for warheads with SW39. In all probability, SW39a should have been very nearly as destructive as SW36, if not more so by the addition of more aluminium powder to the formula.

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13 Jun 2004 18:12:06 - José M. Rico
This can lead one to speculate that perhaps one set of figures for resistance to torpedo blast might be for SW36 while the larger figure quoted for Tirpitz might be for warheads with SW39. In all probability, SW39a should have been very nearly as destructive as SW36, if not more so by the addition of more aluminium powder to the formula.
I think Bill Jurens also mentioned once that Bismarck's torpedo protection system was tested with a full charge of 300 kg. of Type S1 explosive. It would be interesting to know the result of these tests and when they were carried out.

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14 Jun 2004 06:08:56 - thomas

José

What was the composition of Type S1 explosive?

I did a little checking and Sw39 was an economy explosive, (Amatol 39) with Hexinit and aluminium powder to increase brisance.

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14 Jun 2004 17:31:27 - José M. Rico
What was the composition of Type S1 explosive?
I have no idea. We should ask Bill Jurens, perhaps he knows.

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14 Jun 2004 18:51:51 - billjurens

I've published the detailed data on the Bismarck tests a couple of times now; try "Underwater Protection in Capital Ships -- A Technical Approach", which, as I recall was published in Warship International about 1985 or so. I also published this material again, in more expanded form regarding Bismarck per se, in the Warship International article "Bismarck's Final Battle", published about 10 years ago or so now, I guess.

Too much is generally made regarding the exact relative strength of explosives used in torpedoes. The amount of damage, though of course not entirely unrelated to explosive strength, can be surprisingly disconnected from this variable. Relative strength in and of itself can be a very difficult thing to pin down in any sort of objective way, or -- more properly -- the relative strength of explosives can vary somewhat depending upon exactly what you are measuring. Both overpressure and total impulse are important, and in some cases, if the charge bubble period is in resonance or near-resonance with the structure, a smaller charge of 'weaker' explosive can actually do more physical damage.

Also, keep in mind that due to scaling laws, even a relatively large difference in charge weight only increases the damage radius a relatively small amount. In air, the radius of equivalent overpressure is proportional to the cube root of the charge weight, so doubling the damage radius (using overpressure as a measuring stick) requires multiplying the charge weight by eight. The situation in water is much more complex, but basically quite similar. In a nutshell, changes of even 15%-20% in charge weight tend to have relatively little effect on the amount of damage observed.

Hope this helps...

Bill Jurens