An Investigation of the Influence of Charcoal on BP

Transcription

1 An Investigation of the Influence of Charcoal on BP Part 2 Ballistic Performance Richard Harrison, July 2014 Following my earlier work on BP burn rates, I wanted to see how the ballistic properties something that is related to what others have called the explosive power varied with the charcoal used in the powder. Would the trends be similar to, or different from, those seen for the burn rate, and would any different information come to light? The Powder Samples The BP samples used in this investigation were those made from the twenty one charcoals, as described in part one 1. Each had been milled for a total of eight hours and had all reached a plateau of performance, as measured by their rates of burn. A 50 gram sample was carefully mixed by hand with 3% of deionised water, left in a covered container for one hour and then mixed again to achieve a reasonably uniform distribution of moisture. The sample was placed in a 65 mm diameter mould and pressed at a pressure that was slowly increased to a value of 184 kg/sq. cm (about 2600 lb/sq. in) and held at that pressure for a dwell time of one minute, after which there was no further evidence of plastic flow. The resulting puck was air-dried at room temperature until there was no further detectable decrease in weight. Each puck was weighed and its diameter and thickness were measured, using a digital caliper. Four measurements were made for each value, at the points and diameters as indicated in the diagram, and an average calculated. These results enable a reasonably accurate calculation of the puck density to be made. The puck was then granulated by repeatedly striking it with the textured face of a meat hammer until it all passed a 4 mesh screen. Passing the resultant grains through a set of screens produced a series of 4-10 mesh, mesh, mesh, mesh and -60 mesh samples. The Eprouvette I decided to use a mortar-style device for testing the powder. The requirement was to have a simple device with a range limited to no more than 50 m, so that it could be fired in my garden. Given the location, it should ideally also be relatively quiet in operation. 1 n%20bp.pdf

2 I built a mortar from 22mm copper tube (I.D. approximately 20mm), as shown in the diagram. The end is closed by a 15 mm plug of epoxy resin and the assembly is removably mounted in a laminated wood block, which holds it at an angle of approximately 28 degrees to the horizontal. The fixed firing position is 60 cm above level ground. Initially, I tried firing a length of 19mm dowel but found that it not only tumbled in the air, but also bounced on landing, making consistent and accurate range measurements impossible. I therefore designed and built a custom dart that was a comfortable sliding fit in the tube. The body is made from a length of 12mm hardwood dowel, to which was added a leading steel spike and a cylindrical aluminium head. The tail fins are made from 2mm aluminium sheet, inserted into grooves cut into the dowel. All the parts are held in place with epoxy resin and the assembly has a total mass of 30 grams. The charge is ignited with a short length of 2 mm Visco fuse, inserted through the touch-hole until it touches the opposite wall of the tube. Initial tests showed that the dart flew true, embedding its spike in the ground on landing, but the range appeared to be somewhat erratic. Further trials suggested that at least part of the cause of this variation was random differences in the amount of gas escaping past the dart. I therefore spent some time investigating a variety of wads, placed between the BP charge and the dart. The most consistent behaviour was with the use of paper wads, with a thickness of 2 to 3 mm, pressed while wet into a short length of the mortar tubing, then carefully ejected and left to dry. Slight differences in fit still seem to have a significant effect but, as will be seen in the following section, average results appear to be physically significant. Initial Testing The eprouvette was fired, using a variety of charges and grain types, in order to determine the best conditions under which it should be used. The BP for these tests was from stock, with a different milling history from that of the test samples. The first set of tests was with mesh grains, to determine the effect of the amount of charge. I made up to five tests for each charge, with charges varying from 50 to 150 mg. I made it a rule to brush out the mortar tube between individual firings, and to thoroughly clean and dry it after each set of five firings. As the graph shows, the range measurements display a significant variation, despite all the efforts made to standardise the conditions.

3 However, when the same data was used to plot the mean range from each set of measurements against the charge, a different picture emerged. The points are now a surprisingly good fit to a straight line. Statistically, the results seem to be more consistent than appears at first sight, with a standard deviation on an individual range measurement of around 5.0 m, and about 2.0 m on the mean of five measurements. According to the standard ballistic equations (ignoring the effects of air resistance) the range is proportional to the square of the muzzle velocity and, therefore, proportional to the kinetic energy transferred to the projectile. I would therefore expect the graph to be linear as appears to be the case. The best fit line has an x-intercept at a charge of 43.0 mg, which represents the minimum amount of powder required to cause the projectile to exit the mortar. It therefore approximates to the amount of powder needed to overcome the inevitable energy losses due to friction, blow-by and escape of gases from the touch-hole. As another way of looking at this data, I used the ballistic equations to calculate the muzzle velocity from the measured range. In theory, this velocity should be proportional to the square root of the range, and therefore since range and the powder charge are linearly related the mass of the charge should be proportional to the square of the muzzle velocity. Bearing in mind the fact that some of the charge will be used to overcome energy losses, a correction was subtracted from the charge s mass before plotting it against muzzle velocity. The value of the correction was adjusted until the best fit curve was a pure quadratic relationship, with the results as shown. The fit is extremely good, and corresponds to a charge correction of 43.8 mg, which is in excellent agreement with the value of 43.0 mg that was determined from the linear range/charge relationship. The next set of measurements was made to determine the effect of powder grain size. The standard charge was set to 100 mg and the range was determined for a number of charges at each of the mesh size ranges mentioned earlier. Plotting a graph of the calculated muzzle velocity versus the average mesh size (with the -60 mesh sample given an arbitrary average value of 70) for each sample gave the results as shown. The values for the 4-10 mesh samples need additional comment. Given the large size of the grains, it was impossible to create a sample that was even reasonably close to a mass of 100 mg. For these measurements, I used 200 mg samples and corrected the measured results by means of the linear range/charge relationship that was described above.

4 Once again the individual values showed significant scatter, but plotting the average of up to five results gave a much clearer relationship. The dependence on mesh size is obvious, and shows that using either mesh or mesh would give the most sensitivity. Of the two, my granulation technique gives a greater yield in the mesh range. As a result of these measurements, I decided that the standard charge to use in testing the samples made with a variety of charcoals should be 100 mg of mesh powder. That charge gives a range that comfortably fits in the available space, allowing a generous margin to allow for the likelihood that some of the samples would be more effective than the one used in these tests. The charge is sufficiently small that it also matches the second requirement, that the tests should not generate too much noise. These tests also gave me confidence that, even if the individual measurements showed significant scatter, the average of five results would be likely to have real physical meaning. Results Five firings were made for each of the BP samples, made from the twenty one different charcoals. I decided to abandon my previous policy of not associating the data with the charcoal types, and to quote the results in full, together with the apparent densities, ash contents and mass burn rates from my earlier measurements. Charcoal Type Charcoal App. Density (g/cc) Ash (%) Mass Burn (g/s) BP Density (g/cc) Mean Range (m) Pine A ± 2.2 Acacia (genus Albizia) ± 7.0 Mix A ± 3.2 Willow A ± 3.3 Vine ± 3.4 Buddleia ± 1.2 BBQ A (recooked) ± 3.9 Paulownia ± 1.8 Leylandii ± 5.5 Willow B ± 3.1 Willow C ± 2.9 Willow D (commercial, unverified) ± 0.3 BBQ B (recooked) ± 1.3 Jelutong ± 2.8 Pine B ± 4.7 Knotweed ± 3.7 Pine C ± 3.0 Mix B ± 2.1 Balsa ± 2.8 Alder ± 3.7 BBQ C ( hardwood, recooked) ± 0.3 Willow B (validation) ± 3.8

5 The range varied from the lowest value of 0.3 m to 25.0 m, with the majority of the results lying between 10 and 15 m. The standard deviation, calculated for each set of five data values was, on average, around ±3.0 m, which gives an idea of the worst case likely error over the whole data set. As discussed earlier, the results are likely to be more physically meaningful than the formally calculated standard deviations would suggest. To get another measure of the reliability of the measured values, an additional set of measurements was taken. The BP for this set was made using Willow B charcoal, by repeating the whole milling and granulating processes that was used to make the first sample. A comparison of some of the results is given below. Willow B Linear Burn (mm/s) Mass Burn (g/s) Density (g/cc) Range (m) Batch ± 3.1 Batch ± 3.8 The agreement between these two, totally independent sets of results is very good. In particular, the range values differ by only 0.9 m, which is significantly less than the calculated standard deviations would suggest. In retrospect, I see that I overpressed the pucks, since a BP density of about 1.7 g/cc is generally accepted as the desired value. I used a pressure that had previously given me densities around that value, but with powders that were less thoroughly milled. In the interests of consistency, I continued with the same pressure throughout all the tests. Analysis Graphs of the range, plotted against either the linear or mass burn rates show a reasonably strong correlation. So, a powder that burns fast is likely to be effective in ballistic terms. A broadly similar trend is observed in the relationship between range and the measured density of the BP pucks. In both cases, however, there is a significantly greater scatter than in the corresponding burn rate relationships. The graph of range against the original charcoal s apparent density should, therefore, follow a pattern similar to the graphs of burn speed against density, and that is found to be the case.

6 Given the known densities of potassium nitrate and sulphur, and the measured BP puck densities, it is possible to calculate the density of the compressed charcoal in the pucks (assuming that all remaining air spaces within the puck are associated with the charcoal). A graph of the measured range, plotted against this calculated density, appears very similar to the graph of the range as a function of BP density, with a modified density scale. This is not surprising, since the calculated charcoal density is approximately linearly related to the BP density in the measured ranges. One further density-related correlation may be of interest; the dependence of the ballistic range on the ratio of densities between the compressed charcoal and the corresponding uncompressed apparent density. As the diagram on the right shows, there does appear to be a correlation; the charcoals with a higher compression ratio tending to be the ones that give a greater ballistic range. Given the reasonably good correlation between ballistic properties and burn rates, it would not be a surprise if the range and/or muzzle velocity were to show a similar relationship with charcoal ash content as does the burn speed. As shown in the accompanying graph, that is indeed the case for the muzzle velocity (the measured ranges show a similar relationship). The peak of the best fit curve is at an ash content of 7.1%, which is remarkably consistent with the corresponding values (6.8% for the linear rate and 7.1% for the mass rate) determined from burn speed measurements. I tried looking at any possible correlations between the ballistic and puck density data on the one hand, and the time-related milling properties 2 on the other. Although the relationships are clearly not random, nothing stood out as being worthy of comment at this stage. Conclusion The eprouvette described here appears to be highly sensitive to differences in BP performance, which is a useful property for it to have. However, the corresponding downside is that individual firings show significant variation, presumably caused at least in part by small differences in the loading conditions. As mentioned earlier, the results seem to imply that the average of five readings gives a value that is more physically significant than the spread of the individual measurements would suggest. Several repeat tests on one or more of the BP samples would be needed to give a better idea of the accuracy and consistency of the measured values. 2 See

7 The results seem to agree reasonably well with the previous measurements of burn rates. In particular, they confirm the earlier findings that BP performance appears to vary with the apparent density and the ash content of the charcoal; fluffier charcoals with an ash content of about 7% being the ones that tend to give the best result. At this stage in the analysis of the data, it is hard to see what other new insights there are to find. Perhaps the most significant point may be that, given the correlation between the ballistic and burn rate properties, it might not be essential to run ballistic tests when trying to determine the suitability of a charcoal for making BP! There is, however, a significant scatter in the graphs of ballistic range against burn rate, which suggests that the relation between these two properties may vary from charcoal to charcoal. As yet, I have not been able to find any consistent patterns in these variations (nor in the relationships with the time-dependent milling data) but maybe some will emerge with further study of the data. I should point out that the ballistic testing described here ought not to be taken as a true indicator of the different powders performances under different conditions. The intention of these measurements was merely to provide a comparison of a range of powders that were made under closely similar conditions. It would be unwise to expect the results to transfer to their performances with differing confinement conditions and grain sizes for example, as lift charge for a shell. I suspect that in such a case the order of their performances would be likely to be preserved, at least in broad terms, but that the actual difference between the best and worst is likely to be less extreme. I would expect that any of these powders, with the exception of the small number that show exceptional (both good and bad) performance, would be suitable for most pyrotechnic uses. Some Thoughts on Barbecue Charcoals I have seen it stated many times that barbecue charcoal is overcooked and therefore not suitable for pyrotechnic use, especially for making BP. Of the three BBQ charcoal samples that I used in these tests, only one (BBQ C) seems to be truly unsuitable, another (BBQ B) is modest in performance, and the third (BBQ A) turns out to have as good a ballistic performance as some of the better charcoals I have measured. It is clear that, as with many other charcoals, there is no hard and fast rule about the suitability of a charcoal that is sold for use in barbecues. All three charcoals were recooked in retorts before being used in my tests, and they all showed significant outgassing of volatiles and a reduction in weight. I tested one of these charcoals before and after recooking and noticed a small, but significant, increase in performance. It seems, therefore, that none of these three were overcooked before being sold quite the reverse, in fact. From a manufacturer s point of view, undercooking with its higher yield and lower fuel consumption would appear to make commercial sense.

8 A Bit of Speculation The charcoal labelled as Willow D is the only one whose origin is not certain. It was a commercial purchase that the seller advertised as being willow charcoal, but its performance is far below the other willow charcoals that I tested. It most closely resembles BBQ C, which was the worst performer of all. The resemblance is close in all respects and I am tempted to suspect that the two are cooked from closely similar, unknown woods that I will call BBQ Hardwood. On the assumption that this is the case, I redrew the muzzle velocity vs. compressed charcoal density graph, this time identifying those charcoals that are made from the same type of wood (willow = green triangles, pine = red squares, BBQ hardwood = purple crosses). I noticed that the slope of the best fit lines for the three pine, and for the two BBQ Hardwood samples were close to being equal in slope. Clearly, this might just be coincidence, bearing in mind (a) the small number of samples involved (b) the dubious identification of the two poor performers and (c) that the relationship certainly doesn t hold for the three known willow samples (the two overlapping green triangles represent the two independent tests I made on the Willow B charcoal). On the other hand, the slope of the best fit straight line to all the points (ignoring the two poor performers) is also very similar, at a value of In addition, the point relating to Mix B charcoal which I have recently learned is probably made from pine or spruce lumber also lies very close to the pine best fit line. These coincidences are, if nothing else, intriguing. For a BP made from a given charcoal, I would expect the performance to depend on density, with harder pressing producing a denser and slower burning grain. It is interesting to see that this relationship also seems to apply, in broad terms, to BPs compacted under equal pressures, but made from different charcoals.