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An experimental investigation on the effects of buoyancy on opposed flow smolder is presented. Tests were conducted on cylindrical samples of open-cell, unretarded polyurethane foams at a range of ambient pressures using the Microgravity Smoldering Combustion (MSC) experimental apparatus. The samples were tested in the opposed configuration, in which the flow of oxidizer is induced in the opposite direction as the propagation of the smolder front. These data were compared with opposed forced-flow tests conducted aboard STS-69, STS-77, and STS-105 and their ground based simulations. Thermal measurements were made of the smolder reaction to obtain peak reaction temperatures and smolder velocities as a function of the ambient pressure in the MSC chamber. The smolder reaction was also observed using high-frequency ultrasound pulses as part of the Ultrasound Imaging System (UIS). The UIS measurements were used as a second means of providing smolder propagation velocities as well as to obtain permeabilities of the reacting samples. Results of forced flow testing in normal gravity were compared to results in microgravity at a range of ambient pressures and forced flows. Results indicate that a critical oxidizer mass flux of roughly 0.5 to 0.8 g/m 2 s is required in normal gravity for a self-sustaining propagation in this configuration. In microgravity tests, self-sustained smolder propagation was observed at a significantly lower oxidizer mass flux of 0.30 g/m 2 s. Analysis suggests that the removal of buoyancyinduced heat losses in microgravity allows for self-sustained propagation at an oxidizer mass flux below the critical value observed in normal-gravity testing. Normal-gravity tests also show that the smolder propagation velocity is linearly dependent on the total oxidizer mass flux in an oxidizer-limited regime. Pressure effects on the chemical kinetics of a smolder reaction are inferred by comparison of normalgravity and microgravity tests and believed to be only weakly dependent on pressure (~P 1/3 ).

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