K.L. and B.J Kosanke
The time chosen for the interval between a shell firing and its burst is sometimes given less thought than it deserves. By carefully choosing the delay interval provided by the time fuse, it may be possible to produce undistorted bursts, with a higher level of safety. When an aerial shell bursts, while it is nearly stationary, its stars are propelled outward, each experiencing nearly the same aerodynamic drag.
Thus the symmetry of the burst is determined only by the construction of the shell, and the pattern will appear to be suspended in the air for its duration. That is to say, a properly made peony will appear as an expanding, near-perfect sphere and will seem to hang motionless in the air as it spreads. See the left column of Figure 1, which is intended to appear as a timed sequence of the burst and expanding pattern of stars from a near stationary spherical shell. On the other hand, if the same shell were to burst while it was in rapid motion, the star pattern would be distorted. This is because the spreading stars would be subjected to a little different aerodynamic force depending on which way they were traveling relative to the motion of the shell. The star pattern will appear smaller and somewhat elliptical. Also the star pattern will be slightly more sparse on the bottom than on the top. Perhaps, most noticeably, the developing star pattern will move in the direction of the original shell motion, and will appear to expand from a point which is not at the center of the pattern. See the right column of Figure 1 for an illustration of the case where the upward motion of the shell approximately equals the burst velocity of the stars. (Readers wishing to learn more about star ballistics are referred to Reference 1.) Thus there are aesthetic reasons why aerial shells are normally intended to burst near their apogee, when their upward motion has essentially stopped.
Ref: Selected Pyrotechnic Publication of K.L. and B.J Kosanke, Part 3, (1993-1994), pp 1-3
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