Scaling and simulation issues in ram accelerator projectile design

An attractive feature of ram acceleration is that there are no conceptual obstacles to increasing tube diameters and projectile masses. However, scaling does affect ram accelerator operation and design through gasdynamics. In this paper, previous efforts at determining the role of scaling on projectile thrust are reviewed. To alleviate the shortcomings of the earlier analyses, quasi-steady-state, axisymmetric flowfield simulations are performed. Preliminary results fall into two classes depending on the initialization of the flowfield: steady solutions in which positive thrust and full-tube combustion are not achieved, and unsteady solutions in which thrust is generated. The steady solutions are suspect because the initialization of the flowfield is not realistic; the unsteady solutions are questionable because of uncertainty in the exit boundary condition. Other classes of solutions may be possible for different conditions or faster kinetics, but these preliminary calculations call into question the existence of quasi-steady flow around ram accelerator projectiles.