With NASA’s new proposed spacecraft atmosphere having higher oxygen concentration and lower pressure than here on earth, the Narrow Channel Apparatus (NCA) is one of the only ways to test how thick fuels combust in a microgravity environment under conditions altering the oxygen concentration and pressure. This thesis presents experimentation conducted in an NCA involving the combustion of thermally intermediate to thermally thick samples of poly(methyl methacrylate) (PMMA) while altering the oxygen concentration and pressure. An NCA simulates a microgravity environment by suppressing buoyancy forces via a very narrow gap height. Ten testing points of differing oxygen concentration and pressure were selected, nine of which were on the normoxic curve, and the tenth being a NASA proposed spacecraft atmosphere. A 9.87 mm gap height was used for experimentation. 3, 5, and 10 mm thick samples of PMMA and 7.6 cm/s, 10.1 cm/s, and 12.7 cm/s opposed flow velocities were used during experimentation. It was found that as oxygen concentration increased and pressure decreased, a small change in the opposed flow velocity had a greater impact on the flame spread rate. A comparison of the two NCAs at San Diego State University was conducted, highlighting differing experimental outcomes and the variations between the two NCAs that could have contributed to these results. Results from a computational model of PMMA combustion in an NCA at a single point on the normoxic curve, 21% oxygen, were processed and analyzed for a 10 mm thick sample with 1D heat conduction. The results had been obtained using Fire Dynamics Simulator (FDS) coupled with Gpyro. These results were compared to other model outputs involving both 1D and 2D heat conduction, as well as experimentally obtained data. The comparison found that the flame spread rate for the 1D and 2D heat conduction cases converged around 10 mm thickness. Additionally, the computational model was outputting unrealistic trends, with flame spread rates that increased with material thickness and that were faster than experimentally obtained. An updated investigation into the source of these unrealistic trends was conducted, but a definitive conclusion of the source was not able to be drawn.