Flame-spreading over the surface of a flat solid propellant in a turbulent boundary layer has been investigated experimentally. Nitrogen gas was used as a main flow gas, and noncatalyzed and catalyzed double-base propellants were used as specimens. Experimental measurements of the flame-spreading were conducted as a function of the main stream velocity in a small scale wind tunnel under atmospheric pressure. After the upstream edge of the specimen was ignited, the flame-spreading phenomena were investigated photographically. The temperature of the unburned propellant surface and the heat flux through the gas-phase to the unburned surface were measured with thermocouples and a calorimeter. It is found that the flame does not always spread downstream continuously but a secondary (and/or tertiary) ignition occurs on the unburned surface far downstream of the main burned region at the lower main stream velocity or for a propellant with higher normal burning rate, and that in general the history of the unburned surface temperature is divided into three stages; (a) the surface temperature rises with time, (b) it reaches a plateau, and (c) it rises again with time up to the critical temperature.
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