Water Tunnel Tests of the 2 1/4" AA Rocket Projectile

The High Speed Water Tunnel is operated by the California Institute of Technology under Contract OEMsr-207 with the Office of Scientific Research and Development, and is sponsored by Division Six, Section 6.1, of the National Defense Research Committee. The report covers Water Tunnel tests of 1 - 1/2 " and 2" diameter models of the 2-1/4" AA Rocket Projectile. The drag, cross force, and moment acting on the models were measured and the position of the center of pressure relative to the center of gravity was calculated for various yaw angles. These results were compared with prototype field test data. The main findings are summarized as follows: i. The rocket is statically stable as indicated by a stabilizing moment coefficient and a center-of-pressure eccentricity of more than 0.26. Furthermore, the large area of 1he tail fins will probably provide sufficient damping to make it dynamically stable also. 2. The tail fins cause very large cross force coefficients compared to values for other cylindrical projectiles with folding fin or ring tails. Consequently, unless the rocket is rotated in flight, small misalignments of the tail fins can cause drifting and increase the dispersion seriously . 3. Both the cross force and moment coefficients increase with yaw at a greater than linear rate. 4. Comparison of Water Tunnel and field test data shows good agreement for the moment coefficient 5. The drag coefficient from Water Tunnel tests is 9% lower than the value of 0.46 measured during field tests in air. Scale effects, oscillation of the projectile during free flight tests, and compressibility effects on the drag in air are factors that could account for this difference. 6. The drag is nearly independent of yaw for small angles and increases rapidly for angles greater than about 4°. 7. The high drag coefficient for this projectile is caused by skin friction on the relatively large area of the body and fins and by pressure drag due primarily to a large eddying wake behind the blunt body