A examination of two major innovations in military aeronautics--turbojet propulsion and the variable-sweep wing--using the classical economic investment model. Three phases of the innovation process are distinguished: invention or conception, demonstration of feasibility, and acceptance or adoption. Patterns of innovation characterizing the evolution of jet engines and of variable-sweep wings tend to resemble one another. Both devices showed deficiencies in the feasibility demonstration phase, but efforts to overcome those for jets began immediately, while those of the variable sweep were neglected until alternative technologies had been exhausted. Even then, the military was slow to acknowledge the value of an operational application of the swing wing. All evidence suggests that once an innovation reaches the stage where appraisal is appropriate, technical feasibility demonstrations should be conducted as quickly and cheaply as possible. Feasibility should not be subordinated to an existing requirement, but the requirement should be built around the demonstrated capability of the innovation. Wartime stresses encourage early exploitation of innovations, but during peacetime the military must have more compelling evidence of technical feasibility before investing in novel devices.