Detonation velocities are of limited value in assessing the reaction rates of nitrocellulose propellants formulated to avoid detonation. Although the slower reaction is often described as burning because of similar gaseous end products at elevated temperatures, the decomposition differs from combustion in an oxygen atmosphere. Conversion of nitrocellulose propellants to high-pressure gas proceeds from the exposed surface to the interior of each solid particle in accordance with Piobert's law. Studies of solid single- and double-base propellant reactions suggest reaction rate is controlled by heat transfer through the temperature gradient across 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. The "foam zone" acts as an insulator slowing the rate of heat transfer from the "flame zone" into the unreacted solid. Reaction rates vary with pressure; because the foam allows less effective heat transfer at low pressure, with greater heat transfer as higher pressures compress the gas volume of that foam. Propellants designed for a minimum heat transfer pressure may fail to sustain the "flame zone" at lower pressures.