Pyrotechnic White Papers

Explosives are expected to release large energy and expand greatly in volume to generate force in the time scale of μs. To achieve high power output, it is necessary to propagate reaction rapidly through the whole material, as known as detonate. Detonation, deflagration, and regular fuel combustion are different phenomena distinguished by their rate-determining-step and propagation rate. For regular fuel combustion, the reaction rate is limited by diffusion of reactive species (mass transfer), which is relatively slow, leading to low propagation rate. In the case of deflagration, the oxidizer and fuel are premixed, therefore the diffusion of reactive species is no longer the rate-determining-step. Instead, the propagation of reaction zoom is controlled by heat transfer, resulting in its faster rate than regular fuel combustion. When energetic material detonates, the shock wave propagates through the material. At the wave front the material is highly compressed, leading to the temperature rise, which triggers exothermic chemical reactions and create a chemical reaction zoom after the wave front. The exothermic reactions increase the temperature and pressure to the point higher than the condition before the passage of shock wave, which provide energy to sustain the propagation of shock wave. Therefore detonation is in the speed of shock wave, which is supersonic, in contrast to the cases of deflagration and regular fuel combustion, which are subsonic.

Propellants are not expected to detonate, but combust in a controlled manner, i.e.,DDT is not desired for propellants, different than explosives. The most important performance parameter of propellants is specific impulse (Isp), which is defined as the gain of impulse (impulse=force × time, or mass × velocity)4when one unit mass of propellants is consumed, and it can be roughly perceived as the exhaust velocity. Since Isp is normalized to per unit mass, it is a material-specific parameter and not dependent on the burning rate of propellant if the thrust comes from only the exhaust gas. Propellants can be in liquid or solid form. Common solid propellants are mixtures of oxidant (nitrate or perchlorate salts) and reductant powder (C, Al, etc.). Explosives, such as RDX or HMX, can also be used as propellants, as long as there is no shock wave generated during the combustion to start the detonation. Rocket motors powered by solid propellants have high propellant fraction in weight because there is no liquid pump or cryogenic tank, and they are more reliable to operate. The drawback is that once the motor starts, there is little control over the combustion of the solid propellants.

Pyrotechnics is the art and science of creating and utilizing the heat effects and products from exothermically reacting, predominantly solid mixtures or compounds when the reaction is, with some exceptions, nonexplosive and relatively slow, self-sustaining, and self-contained.