The Ram accelerator (RAMAC) is an aerothermochemical device aimed at accelerating a projectile up to an hypersonic speed. It is based on the firing of an under-calibrated and biconical body in a tube filled with a dense gaseous reactive mixture. The shape of the projectile must be designed in order to trigger the combustion and control its appropriate location at the rear of the projectile, where the flow expands, once it has been compressed on the conical nose. The pressure increase due to the combustion of the gaseous mixture generates a thrust that accelerates the projectile. The different types of combustion, namely deflagration or detonation, yield two different propulsion modes termed subdetonative and superdetonative, respectively. Depending on whether the projectile speed is lesser or greater than the theoretical Chapman-Jouguet detonation velocity, the main difficulty lies on the control and possibly the prevention of the transition from deflagration to detonation. A phenomenon called unstart occurs when a detonation is initiated and overtakes the projectile, yielding a reverse thrust on the forebody. The knowledge of both the real aerothermal conditions in the flow, i.e. the pressure and temperature distributions, and the kinetic behavior of the combustible gaseous mixture under various conditions, is required, in order to avoid such phenomenon. Experimental and numerical tools, i.e., a non reactive and steady Navier-Stokes code, are used at ISL to provide helpful data in order to operate the ram accelerator in the subdetonative propulsion mode. Pressure and temperature can be estimated in specific locations in the flow when the projectile enters the tube. The relevance of these calculations is being highlighted by available data obtained experimentally in the 30mm ram accelerator. Projectile shape is examined and a variety of results are provided, ranging from noninitiation of the mixture to direct transition to detonation. Experimental investigations on detonation of gaseous mixtures were also conducted in a 90mm caliber ram accelerator. A normal shock was generated by a supersonic piston entering a section of the detonation tube filled with an inert gas, yielding a transmitted normal shock wave into the reactive mixture located in the adjacent test section.