Diameter effect in the initiation process of combustion in a ram accelerator in subdetonative mode

A large variety of initiation results in a ram accelerator have shown that a series of interactive parameters play a key role, such as the design of the facility, tube diameter, shape of both the projectile and sabot and the choice of the reactive propellant mixture. Two different ram accelerators were used at ISL for this study, namely a 30-mm and a 90-mm-caliber. In addition to the diameter effect, it was observed that the dimension of the sabot must be adapted not only to the caliber of the tube but also to both the launching device and vent section that are located before the ram accelerator tube. For instance, a sensitive mixture yielded a superdetonative mode in the larger tube diameter facility at ISL. Comparative experiments were conducted in these two caliber ram accelerators at ISL. Important transient effects were observed as the projectile enters the tube which could explain the discrepancies between the data derived from similar devices. The aim of the present paper is to provide a series of data that show the effect of the tube diameter on the initiation process of the propellant mixtures that are currently used in this facility. The design of the sabot is also discussed.