Flammability tests of iron using the Lewis Research Center's (LeRC) 2.2 s drop tower are modeled. Under the conditions of the test, after ignition, about 2.0 s of burning in reduced gravity (0.01-0.001 g) occurs. Observations (film and video) show the accumulating product mass to be well-mixed; therefore the system is modeled as a semi-batch reactor, that is, reactants continuously fed with the product accumulating in the reactor. The regression of the melting sample is considered steady-state. Real-time temperature and pressure measurements of the chamber gas provide measurements for model validation. The model consists of a set of 22, non-linear, first-order differential equations which are solved using MATLAB®. The model predicts, for 0.32-cm-diameter iron rods burning at 4300 kPa, an average reaction temperature of 3600 K and a molten oxide temperature of 3400 K. The system experimental parameters are the thermal conductivity of the molten liquid, kFe(ℓ), the thermal conductivity of the molten iron oxide mixture, kFeO(ℓ), and the heat transfer coefficient, h, between the molten oxide and the oxygen in the chamber and chamber itself. These model parameter values are: kFe(ℓ)=1.4 J/s cm K, 1.8 ⩽ kFeO(ℓ) ⩽ 35.0 J/s cm K, and 0.24 ⩽ h ⩽ 2.24 J/s cm2 K. It is suggested that the internal circulation within the molten ball formed during burning decreases as the ball grows. More work is necessary to understand the chemical nature of the reacting oxygen and determine the species formed during burning.