The basis of obtaining high quality gas in an underground gas generator is the establishment of certain conditions for the interaction between the blast and the coal's carbon. Generally it is attempted to bring the blast stream to the reaction surface by varying the scheme of gasification, by selecting the points and the method of leading in the blast, by making use of the laws governing the streams motion, by packing the gasified space, etc.; in other words, apart from other factors, the gas formation process is determined by the aero-gas dynamics of the flow, which in turn depend partly on the dimensions of the gasification channel. In addition to this, the determination of the mean cross section of the fire channel makes it possible to clarify the behavior of the surrounding rocks and the nature and form of the combustion focus, to determine the optimal length of the gas generator, and so on.