REBAR Theory and Approach

Obviously the goal of projectile design is to successfully launch, fly and impact a target downrange. REBAR allows the user to analyze the stress and strain generated in bore due to launch accelerations. The interface geometry (Buttress Grooves) between the Sabot and Penetrator can than be modified to not only ensure successful launch, but also the lightest weight Sabot.

The REBAR analysis to optimize projectile mass is an iterative process, with each iteration divided into two parts. First, the projectile section aft of the pressure seal or obturator is analyzed. Next, the section forward of the obturator is solved. If incompatibilities arise, the solution is repeated until convergence is achieved.

The acceleration load for the projectile is gun pressure acting on the projectile surface aft of the obturator. The equilibrating reaction load is provided by the inertial force developed as the projectile accelerates. The total net force at any axial location is simple to determine by adding the total applied and inertial forces forward (or aft) of that station. The portion of the total force carried by each part in the cross section is unknown. Computation of these forces requires calculation of the shear load transferred between parts, which can only happen between threaded joints or bearing surfaces. There is no shear transfer between parts unless these mechanisms are employed, and until such surfaces are used, it is a simple matter to calculate loads and stresses for the individual parts.

In the analysis, mass and accumulated inertial forces are calculated at each node point, or station, selected by the user. This information, along with the description of connecting threads (if any), is used to perform a more detailed analysis between the stations. The latter calculations are not apparent to the user. The connected parts are assumed to have the same axial strain. REBAR can correctly compute the stress and strain in the projectile parts as long as there are no axial clearances between the threads and the axial node is not at the end of a thread length.

The Following Assumptions Apply

The axial acceleration due to gun gas pressure loading is uniform throughout the projectile.
The projectile base pressure is applied uniformly across the projectile base.
All threaded and/or buttressed interfaces between the sabot and core are considered “welded” (e.g. there is no relative movement at the interface).
Shear load transfer between piece parts takes place only at threaded joints or bearing surfaces.
Uniform axial stress and strain distribution is assumed throughout each piece part cross-section (frequently referred to as composite action of materials through a cross-section). That is, plane sections remain plane, with no transverse shear present.
The effects of multi-directional strain are usually ignored. However, the presence of pressure forces aft of the obturator seal can be included as an option.