Water Tunnel Tests of the 60mm. Mortar Projectile

1. The projectile as ·submitted is quite unstable due to the location of the center of gravity so far aft of the nose. This condition could be greatly improved by making the nose of metal instead of plastic. The tests illustrate how intimately the position of the G.G. is related to stability. Only one of the changes tested failed to increase the stability of the projectile. 2. Extending the tail aft by means of a 1-1/2" boom will add greatly to stability without increasing the drag materially. The effect of this on the interior ballistics of the mortar has not been investigated. 3. A 1-5/16" diameter disc at the rear of the fins would give greater stability than the 1-1/2" boom, but the drag would be increased 70%. This increased drag would probably reduce the range from 10% to 25% depending on the velocity. A 1-1/8" diameter disc would increase the drag about 30% and would have a materially stabilizing effect due to the increased moment and greater center of pressure eccentricity but would not be as stable as the 1-5/16" disc. 4. The stability of projectiles that are now on hand could be increased by the addition of a thin disc held in place by the primer assembly. 5. It is suggested that a number of projectiles be equiped with discs as described in paragraph 4 in order to determine performance under actual conditions. Both the plain and notched discs should be tried. 6. All tests show that the center of pressure location is very sensitive to any changes in the projectile with consequent effect on the stability. It is imperative, therefore, that no other changes than these herein recommended can be made in the projectile if the test results are to apply. If additional changes seem desirable, a new series of tests should be made in order to predict performance