Delay composition


Delay composition

Delay composition, also called delay charge or delay train, is a pyrotechnic composition, a sort of pyrotechnic initiator, a mixture of oxidizer and fuel that burns in a slow, constant rate that should not be significantly dependent on temperature and pressure. Delay compositions are used to introduce a delay into the firing train, e.g. to properly sequence firing of fireworks, to delay firing of ejection charges in e.g. model rockets, or to introduce a few seconds of time between triggering a hand grenade and its explosion. Typical delay times range between several milliseconds and several seconds.

A popular delay charge is a tube of pressed black powder. The mechanical assembly prevents the outright detonation of the charge.

While delay compositions are principally similar to other fuel-oxidizer compositions, larger grain sizes and less aggressively reacting chemicals are used. Many of the compositions generate little or no gas during burning. Typical materials used are:

The burn rates are dependent on: [1]

  • nature of fuel - fuels that release more heat burn faster
  • nature of oxidizer - oxidizers that require less heat to decompose burn faster
  • the composition ratio - stoichiometric mixtures burn the fastest, also slight excess of metallic fuel also increases burn rate, probably due to heat transfer
  • particle sizes - smaller particles burn faster, but too small particles may lead to incomplete or interrupted burn due to too narrow heating zone
  • mechanical assembly and housing - charge diameter and thermal conductivity of housing influence lateral heat losses
  • ambient temperature - ideally this dependence is very low but extremely low or extremely high temperatures may have influence

Examples of some compositions are: [2]



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