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%.