An explanation of spin-up instabilities for a 155mm binary projectile

In 1971 during the development testing of the 155mm XM687 binary projectile, liquid-induced flight instabilities were observed. Tests were conducted that indicated unstable behavior for fill ratios of 80 to 100%. However, stable flights occurred for a fill ratio of 65%. A solid, cylindrical spacer was fitted to the interior of the aft end of the rear payload canister, thus shortening the overall interior length of the payload compartment. This simple modification yielded stable flights and was incorporated into the standard projectile. This report presents yawsonde data and analytical models that explain the stabilizing effect of the spacer which was eventually employed in the M687 projectile.