Fireworks Stars Blown Blind
Originally published in Best of AFN III:
When an aerial shell bursts, those stars that fail to burn are often said to be "blind stars", or more descriptively as having been "blown blind." This detracts from the beauty of the shell and contributes to debris fallout. The problem can be caused by any of a combination of factors, the most important of which are, the degree of violence of the shell burst and the burn characteristics of the stars.
In simplest of terms, a star will ignite when its surface has been raised to its ignition temperature. The star will continue to burn only so long as the burning surface feeds sufficient energy to the next layer of the star, to raise that unignited composition to its ignition temperature. (See Figure 1.)
One way in which thermal energy is fed to the next (unignited) layer s for radiant energy from the flame to be absorbed by the star and conducted into the star. When a burning star is moving through the air, the flame will be deflected down wind. (See Figure 2.)
Thus, in this case, the flow of energy to unignited composition is impeded. During some recent tests, this effect was captured on film. Photo 1 shows the explosion of an 8-inch aerial shell suspended in a test stand. Photo 2 is an enlargement of a portion of Photo 1, showing tars (dark spots) with their flames (light areas) trailing behind.
Photo 1. Explosion of an aerial shell in test stand
Photo 2. Enlargement of Photo 1 showing the flames trailing the burning stars.
If the amount of energy being fed back is no longer sufficient to raise the next layer of the star to its ignition temperature, the burning star will be extinguished. Among those factors of importance in determining whether this will is the speed of the star through the air. The faster the star is moving, the more its flame trails behind, and less radiant back. For a given star size and mass, its initial energy is fed speed is determined by the violence of the shell burst. Thereafter the star quickly slows down, due to aerodynamic drag forces. Thus, if a star manages to stay ignited during the first brief moments after the shell bursts, it will generally burn completely.
Other important factors determining whether a star will be extinguished upon shell burst depend on the chemical nature of the star. For example, one factor is the amount of heat being produced by the burning composition; another is the amount of energy needed to raise a composition to its ignition temperature.
Often star priming is only thought of in terms of aiding star ignition. However, it is also an important aid in the continuation of burning during and just after shell burst. When the authors manufactured spherical stars commercially, it was learned that the optimum amount of rough meal prime to use was, as much as possible without noticeably delaying the visual appearance of the star after the shell burst. Generally this was 10-15% of prime (by weight) for stars larger than 3/8 inch, and 15-25% for stars smaller than 3/8 inch. This felt to be optimum for two reasons. First, with this amount of prime, perchlorate color stars and even strobe stars would stay ignited even after emerging from hard-breaking shells. Second, rough meal prime (75% potassium nitrate, 15% charcoal, 10% sulfur and +5% dextrin) is the least expensive composition used in making stars. The more of it that could be used without detracting from the star's performance, the less expensive the stars could be made.
Blind stars are often thought of as failing to ignite before the shell bursts. However, as can be seen above, the stars may have ignited, only to be blown blind by the explosion of the shell. Two easy solutions to the problem are break the shells more softly or to prime the stars more heavily.
-KL & BJK