Supersonic Combustion Ramjets, commonly known as Scramjets (Jacobs et al 2005), are aerospace engines that make use of the free air in the atmosphere with on-board fuel such as methane or hydrogen to propel the vehicle. At very high Mach numbers, the combustion time becomes very limited resulting in a decrease in mixing efficiency. The main focus of this thesis is to investigate some ways of improving this combustion efficiency so as to provide thrust over the Mach range required to form part of a more economical access-to-space system. A lot of research has been done on the viability of enchancement techniques over the past couple of years and three of them of particular interest to this thesis. The work done here builds on the previous research efforts on oxygen enrichment (Hardy 2012), cascade injectors (Collins 2012) and rectangular cross-sectional slot injectors (Jacobs and Wheatley 2010), which have all been simulated near-field (within 50mm of the injector) and have been shown to significantly improve mixing efficiency. However, the performance of the above techniques is yet to be assessed for the full length of the combustor grid which is the primary objective of this thesis. By quantifying the overall performance enhancement of the above techniques through the computer simulations, the goal is that these results should help determine the types of flow structures that provide long-lasting mixing enhancement, and also the types whose benefits rapidly fade.