What structural flaw in a solid propellant grain causes a rapid burn acceleration?

Answer

An internal void or crack acting as extra surface area

The catastrophic failure mechanism unique to solid rocket motors relates directly to the homogeneity of the propellant grain. Because the fuel and oxidizer are physically bound together, the burn rate is governed by the exposed surface area of the grain exposed to combustion. If an internal flaw exists, such as a void or a crack formed during manufacturing or storage, this defect acts as an unintended, localized increase in burning surface area. This increased area leads to a higher production of hot gases, which further heats and accelerates the burn rate across that imperfection—a positive feedback loop that can quickly exceed the structural limits of the motor casing, leading to explosive failure.

What structural flaw in a solid propellant grain causes a rapid burn acceleration?

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