It is well known that the burning rate of a solid propellant during a rapid pressure excursion is not given by the steady state relation, r = apn, but instead is generally a function of the pressure change rate, the pressure magnitude change, and properties of the propellant. An approximate but explicit relation for the non-steady burning rate as a function of these parameters is derived and compared with other treatments. Use of a current model for composite propellant combustion is made (Krier,et al., 1968). Discussion of the applicability of such explicit equations to predict burning extinction as a function of the pressure decay rate is also included.