Repeat Firing from HDPE Mortars

K.L. and B.J Kosanke

ABSTRACT: High Density Polyethylene (HDPE) mortars are beginning to find wide use because of their desirable characteristics. They are relatively inexpensive, lightweight, have a long service life, and some consider them to be among the safest mortars presently in use. Initially HDPE mortars were only recommended for use in electrically fired displays, where each mortar is fired only once. This was done because of a desire to take a cautious approach with this relatively new mortar material, even though the mortars had successfully passed some repeat firing tests. Now, it is becoming increasingly common to use HDPE mortars for manually discharged displays, in which individual mortars are fired repeatedly. The mortars heat up during firing, and, being a thermoplastic, they lose strength with increasing temperature. If mortar temperature rises sufficiently high, they will burst during use, venting the lift gases needed to propel shells to safe altitudes. To date there has been no systematic study of HDPE mortars under conditions of repeat firings, in order to determine the safe limits for their use. Thus it is appropriate to more carefully examine the performance of HDPE mortars under conditions of repeat firing and to offer guidance for their use. Measurements were made of the thermal energy deposited in a mortar during the process of firing 10.2 cm (4 in.) aerial shells and of the distribution of that thermal energy along the length of mortars for typical aerial shell firings. Then, measurements were made of the rate of heat dissipation from HDPE mortars freely exposed to air and when buried in dry sand. Finally, data was collected regarding the ability of HDPE mortars to survive shell firings as a function of temperature. With this information, very rough guidelines are proposed for repeat firing of thick-walled, 10.2 cm (4 in.), SDR–17, HDPE mortars.

Ref: Selected Pyrotechnic Publication of K.L. and B.J Kosanke, Part 2, (1990-1992), pp 55-66

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