According to the information I have gathered on ramjets, a ramjet uses a diffuser to slow down the air thus increasing the pressure. The nozzle is used to maintain pressure in the combustion chamber. So my question is, why can't a ramjet (operating at low speeds) simply not have a diffuser at all? If the goal were to make a ramjet that works at the lowest possible speeds and the larger the diffuser to combustion chamber ratio (cross sectional area), the higher the required operating speeds. wouldnt it make the most sense to eliminate the diffuser altogether? Also I have not been able to come across any actual equations for calculating diffuser to combustion chamber ratios, nozzle to combustion chamber ratios, etc.