Many countries are developing technologies for future hypersonic air breathing cruise missiles. These missiles are foreseen to be employed against, amongst others, deeply buried targets. The main technological challenges are related to severe aerodynamic heating and complex physical processes of aerodynamics and combustion at hypersonic flight speed. This paper reports on a study of the flight mechanics of these systems. Engineering methods for both aerodynamics and propulsion were extracted from literature and have been integrated in a so-called system engineering tool which allows for a performance analysis of the complete system. Two baseline configurations have been defined for this performance analysis (an axi-symmetric and waverider type of configuration). The system engineering tool allows for studying design choices (e.g. ramjet or scramjet propulsion) and for studying the effect of subsystem model uncertainties on overall system performance. These studies can be done for a wide range of mission parameters (like cruise flight Mach number, weapon dimensions, etc). The first system analyses that have been performed have yielded insight in the effects of design choices and model uncertainties on system performance. More exhaustive system analyses will help in determining the directions for follow-on research by narrowing down the broad range of possible designs for hypersonic air breathing cruise missiles and by focussing on developing more detailed prediction models for subsystem aspects that influence system performance most. Copyright © 2005 by TNO.