For years Ry and I tested new reactive target recipes for Boomershoot. When our hypothesis for making an explosive which could be easily detonated with long distance rifle fire were proven false Ry would lament that we didn’t have enough columns on our spreadsheet. There was some variable, which we didn’t know existed, that was critical to our understanding of explosive detonation. Literally it was true that I had (have) a spreadsheet with lots of different variables that we thought might be critical to make our explosives better. Some of those included:
- Flammability limits (acceptable ratios of fuel to oxygen where ignition can occur)
- Heat of vaporization
- Specific heats (including those for phase changes)
- Flash point
- Auto ignition temperature
- Heat of combustion per unit mass
- Heat of combustion per unit of oxygen
- Heat of combustion relative to specific heat of the materials
- Temperature of decomposition of the oxidizer
Our experiments yielded no obvious corelation between any of our hypothesises and the real world–until the last couple of days.
The title for the column on the spreadsheet we apparently were looking for is Ω. In explosive engineering terms (rather than electrical engineering terms which is what first comes to mind with that symbol) this is the weight ratio, expressed as a precentage, of the oxygen remaining or required (expressed as a negative number) for complete combustion of all the fuel in the explosives. For example, TNT, C7H5N3O6 has end products of CO, H2O, and N2. That carbon monoxide (CO) could have been converted into carbon dioxide (CO2) and more heat if there had been enough oxygen around. It turns out that Ω for TNT is -74%. For RDX (the active ingredient in C-4), C3H6N6O6, Ω is -21.6%. From my earliest attempts at reactive target explosives I started out with stoichiometric ratios. This would give me the most bang for a given mass of components. That is, no excess fuel and no excess oxygen left over after the reaction was complete. It was ultimately discovered via both experimental results and hints found on the Internet that maximum sensitivity was not achieved with stoichiometric ratios. It was more sensitivity when the explosive was oxygen rich. From some of my “new” books on explosives I found that “Ω” is a measure of that “richness” or “poverty”. I modified my spreadsheet to calculate Ω for various recipes.
Here is a partial (I have three times this number of recorded experiments) table of various Boomerite recipes and my best approximation of Ω:
There were other variables that changed as well such as packaging materials, fuel used, ratios of oxidizers, catalysts, size of the particles, and packing density which also affected the sensitivity. But the correlation with Ω is very strong. Each year the sensitivity increased and Ω, a measure of the excess oxygen, was a significant component of that increase in sensitivity. It also can be too high–obviously if there is no fuel at all and only oxiderizer it’s going to be a minor explosion at best. But this gives me a reason to revisit old fuels and try something a little bit different this time.
Side note: The most recent recipe on the web is not what we actually use. What I publish is always at least one “generation” behind our “latest and greatest”. Ω for the web recipe is 20.1%.
Is the omega for Boomerite increasing or decreasing in that table? (That is, TNT is -74%, and the table looks like we’re moving away from that)
TNT and RDX are oxygen starved. Boomerite is oxygen rich. We are moving the Ω of Boomerite away from RDX and TNT. RDX and TNT are not particularly sensitive explosives and also rely on a different mechanism than ammonium nitrate to generate their explosive power. RDX and TNT both have their fuel in same molecular as their oxygen. AN, NH4NO3, does supply hydrogen as a fuel but in most cases needs an external fuel supply and supplies the oxygen for that reaction. I suspect the oxygen rich environment makes the reaction easier just as other fuels in our ordinary lives burn faster and easier in a very oxygen rich environment. I’ve read, but haven’t personally tested it, that two pieces of hardwood struck together in a 100% oxygen environment will burst into flames.
One of the more interesting things to me about the Ω charting is that AN by itself has an Ω of 20.0. Hence one could hypothesize that it’s only the KClO3 that supplies the oxygen for the reaction and that perhaps other oxidizers could be used instead.
Also for those with detailed knowledge of Boomerite’s chemical history, Boomerite 2001 was the mixture we called 441 and Boomerite 2002 we called 631. 631 was a cheaper version of 441 which we tested and thought to be “almost as good” as 441. In essence we removed oxygen by reducing the KClO3 and added most of it, but not quite all, back in with the AN. Perhaps if we had reduced the fuel just a bit (951 or some such thing) it would have been just as cheap and perhaps more sensitive.
Boomerite 2006 is still cheaper and better in other ways but because we weren’t treating Ω as the independent variable we weren’t doing the right experiments to determine its optimal value. It could be that we could use diesel, sawdust, lactose, sucrose, or some other very cheap hydrocarbon as our fuel if we keep Ω in the range of 15 to 20.