Standard upward-burning promoted ignition tests (“Standard Test Method for Determining the Combustion Behavior of Metallic Materials in Oxygen-Enriched Atmospheres,” ASTM G4-124  or “Flammability, Odor, Offgassing, and Compatibility Requirements and Test Procedures for Materials in Environments that Support Combustion,” NASA-STD-6001, NASA Test 17 ) were performed on cylindrical iron (99.95% pure) rods in various oxygen purities (95.0–99.98%) in reduced gravity onboard NASA JSC's KC-135 to investigate the effect of gravity on the regression rate of the melting interface. Visual analysis of experiments agrees with previous published observations showing distinct motions of the molten mass attached to the solid rod during testing. Using an ultrasonic technique to record the real-time rod length, comparison of the instantaneous regression rate of the melting interface and visual recording shows a non-steady-state regression rate of the melting interface for the duration of a test. Precessional motion is associated with a higher regression rate of the melting interface than for test periods in which the molten mass does not show lateral motion. The transition between the two types of molten mass motion during a test was accompanied by a reduced regression rate of the melting interface, approximately 15–50% of the average regression rate of the melting interface for the entire test.