During the sea-level ignition process of rocket motors, the nozzle is subjected to an overexpanded e ow condition that can cause high side loads. Prediction of the symmetrical separation location is the e rst key step to a determination of the range of possible side-load magnitudes. The mechanisms responsible for causing the e ow to separate from the nozzle wall are demonstrated, and the theory for a new solution of the separation location is presented. The model is also correlated with historical rocket data, a new approximate solution, and an empirical curve e t that has been in use for 35 years. Nomenclature A = cross-sectional area of nozzle B = constant in approximate solution C = constant in Schilling curve e t CD = discharge coefe cient E = exponent in Schilling curve e t F = thrust M = Mach number P m = mass e ow rate p = pressure V = velocity ® = ratio of Mach disk area to nozzle area at separation point AMd=A3