The Hypervelocity Expansion Tube (HET) located at the California Institute of Technology is an impulse facility that uses gas dynamic processes to accelerate a test gas to hypervelocity conditions, reaching up to 9 MJ/kg, for the study of thermochemical molecular processes that are typical in hypersonic flight. Accurate and non-intrusive measurements of the freestream are needed to reduce uncertainty in interpreting the data obtained from the test article. In particular, time-resolved freestream information is critical due to the unsteady wave interactions that accelerate the test gas and the short test times present in expansion tubes/tunnels compared to equivalent reflected shock tunnels. In collaboration with the Hanson Group at Stanford University, Tunable Diode Laser Absorption Spectroscopy (TDLAS) targeting the D1 line of nascent atomic potassium was used to obtain measurements of freestream velocity and temperature in the HET. The experimental results are compared to 2D axisymmetric, reacting Navier-Stokes numerical simulations using Eilmer4 [1, 2] and an in-house 1D analytical gas dynamics solver, LETS [3]. Reasonable agreement is found, but the simulated results fall below the experimental values, just out of the range of the experimental error bars. Potential non-idealities in expansion tube operations are discussed in the context of ongoing experimental and numerical work.