The problem discussed is that of theoretically predicting the normal force induced in the boundary layer separation region immediately ahead of a secondary injection port. Attention was limited to the two-dimensional problem of sonic or supersonic normal injection into supersonic or hypersonic mainstreams. A critical review of available experimental studies was conducted in order to isolate those data best representing a two-dimensional state. The resulting compilation of experimental results was thereafter employed as a basis of comparison for available theoretical and empirical flow models. As a result it was found that analytical means are only capable of rough quantitative estimates of control jet effectiveness and that a more detailed flow model is needed to further explain observed phenomenon. The basic features of such a model are formulated and its conceptual validity demonstrated through comparison with experimental results. Problem areas requiring further research are also discussed in detail. (Author).