Jet injection into a confined supersonic cross flow is studied using experiments and numerical simulations, with the goal of identifying the conditions that cause the flow to unstart. This is accomplished by varying the jet momentum ratio and studying the evolution of the complex pattern of shocks and shock-boundary layer interactions that ultimately lead to unstart. Reynolds-Averaged Navier-Stokes simulations and experimental data (Planar Laser Rayleigh Scattering) are used to identify the upstream location of the shock system, and the distance between this location and the inlet is used to define the safety margin. A preliminary investigation is performed on the sensitivities of the unstart margin to various uncertainties associated with the geometry and flow conditions of the experiment, as well as the choice of turbulence model in the simulations. This work reports initial progress towards the larger goal of quantifying margins and uncertainties in scramjet unstart and also evaluates the ability of steady RANS simulations to predict precursor conditions leading to the highly unsteady unstart process.