Piobert's law applies to the reaction of solid propellant grains to generate hot gas. It is stated: "Burning takes place by parallel layers where the surface of the grain regresses, layer by layer, normal to the surface at every point." Model rocket engine diagram

History

The law was devised by French

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Guillaume Piobert in 1839 to explain the behavior of

gunpowder Gunpowder, also commonly known as black powder to distinguish it from modern smokeless powder, is the earliest known chemical explosive. It consists of a mixture of sulfur, carbon (in the form of charcoal) and potassium nitrate (saltpeter). Th ...

, but it has subsequently been applied to other solid propellants. Description of the reaction as ''burning'' may cause confusion with simple atmospheric

combustion Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combus ...

of solid materials where a similar reaction progression may be attributed to availability of the

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...

reactant only at the surface of the solid being consumed by the reaction. In the case of single-phase propellant grains, the progression is attributed to

heat transfer Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, ...

from the surface of the solid of energy necessary to initiate the reaction. The heat transfer rate increases with pressure; and

smokeless powder Finnish smokeless powderSmokeless powder is a type of propellant used in firearms and artillery that produces less smoke and less fouling when fired compared to gunpowder ("black powder"). The combustion products are mainly gaseous, compared t ...

reaction rates vary with pressure as described by

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in 1893.

Mechanism

Studies of solid single- and double-base propellant reactions suggest a series of zones or phases as the reaction proceeds from the surface into the solid. The deepest portion of the solid experiencing heat transfer melts and begins phase transition from solid to gas in a ''foam zone''. The gaseous propellant decomposes into simpler molecules in a surrounding ''fizz zone''. Energy is released in a luminous outer ''flame zone'' where the simpler gas molecules react to form conventional combustion products like steam and

carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...

. Propellants designed for a minimum heat transfer pressure may fail to sustain the ''flame zone'' at lower pressures.

References

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Bibliography

G. Piobert, ''Mémoire sur les poudres de guerre, des différents procédés de frabrication'' (1844)
Firearm propellants Ballistics Physical chemistry