Contraj7DOF Overview

Contraj 7DOF adds the 7th degree to the Contraj analysis' capability. It enables modeling a round with a despun nose.

This tutorial explains how to use Contraj7DOF to model a spin-stabilized projectile with a secondary spin axis (7 Degrees of Freedom).

The most important thing to remember is that Contraj utilizes the Fin Aero tables from Finner which splits out the contribution of the body and the maneuvering fins separately allowing the user to customize and create various aerodynamic states for the simulation such as deploying fins during flight etc.. For spin stabilized projectiles the aerodynamic state becomes a hybrid system between the dynamic coefficients particular to spin stabilized projectiles (such as magnus moment), gyroscopic effects such as Yaw of Repose (Precession) and subtleties in aero performance.

The user will need to predict both the body aerodynamic state without fins (basic spinner), store those aero coefficients and then model and add the fin alone aerodynamics and rebuild the aerodynamic state with the fin aeros table.

This exercise will require the user to be familiar with both the Spinner and Finner aeropredictors and resulting tables, the Aero Direct input interface, and how to setup the Fin aeros table so that the full aerodynamic state is recreated and output in Contraj.

Projectiles fall generally into two different types, statically stable projectile where the center of pressure is behind the center of gravity (when measured from the nose) and spin stabilized projectiles (where the center of pressure is ahead of the center of gravity when measured from the nose). The spin stabilized projectile requires a gyroscopic stabilizing moment to keep the round traveling straight as it goes down range. This is why rifling is introduced. Spin stabilized projectiles fired from a gun historically have been around longer than gun launched fin stabilized projectiles. It is highly recommended that the user reviews the Stability analysis output to see the particular requirements for both types of projectiles.

The first part of this presentation will focus on the aerodynamic modeling required for spin-stabilized projectiles, using the 105mm M1 projectile model included in the Large Caliber 105mm M1 Projectile module provided with PRODAS