Scramjet engine performances were evaluated by several simple analyses employing single efficiency or assuming constant pressure combustion cycle (Brayton cycle). A new specific impulse formula was derived from the Brayton cycle analysis which suitably included effects of mass, momentum and energy addition by fuel injection as well as combustor drag. Ideal values of specific impulse were calculated by the analyses and compared to find suitable one to represent that of the ramjet/scramjet. Analytical equations for fuel mass ratio of a spaceplane utilizing air breathing engines were derived from some of single efficiency analyses and compared with that of the rocket. A performance index for the scramjet combustors was proposed to evaluate relative importance of combustion and drag in the combustors.