In this study a performance tool using simplified methods for evaluating the aerodynamic coefficients as well as assuming a constant flight path angel has been evaluated. These design choices make it possible to instantaneously obtain, among other things, range, time to target, and maximum velocity. Results are presented for an academic missile using two different engines, a boost engine bringing up the missile to a desired speed, and a sustain engine for keeping that speed under the sustain phase. During the boot phase a rocket is modeled; in the sustain phase both a rocket and a scramjet are evaluated. The results from this exercise show that a near ten fold increase in range can be achieved by going from a rocket to a scramjet in the sustain phase. However, the results have to be considered with care; it has become apparent that in the models for air breathing engines the thrust is never calculated, the only measure of the performance of the engine being calculated is the specific impulse. When estimating the range of the vehicle an expression is used where it is assumed that the thrust exactly balances the drag of the vehicle, something which implies that the results can be completely wrong if that assumption is not fulfilled -- which is also illustrated when the fuel air ratio of a ramjet driven missile is varied.