There is no single predominant challenge to conducting simulations, either for those currently conducting them or for those who are not. However, the two groups face different themes of challenges. Those who are not conducting Concept Simulations were stonewalled by executives, the hardware and software costs or some combination of both. Those conducting Concept Simulations face cultural issues, in the form of knowledge of skills of engineers, as well as simulation quality issues, such as generating the right mesh.