A 2D numerical investigation was undertaken utilising transient, time-accurate, RANS computational fluid dynamics, with the aim of investigating the influence of viscous and unsteady effects when attempting to start a hypersonic scramjet inlet. A variable geometry inlet starting mechanism incorporating sliding doors was utilised, with the two doors initially extending upstream from the scramjet leading edge. After establishing a steady-state solution, the doors were retracted up and over the inlet and the resulting flow field captured. A started flow regime was achieved only when retracting the doors at such a speed (0.1ms total retraction time) that at no point could quasi-steady Kantrowitz assumptions be applied to the internal flow field. Viscous effects were seen to play a significant role in the inlet starting process, with unstart being obtained earlier for lower Reynolds number conditions. Shockwave boundary layer interactions (SWBLI) were found to be the key driver behind inlet unstart.