inches, and employing a special polyethylene obturator to allow use of 

 the partially tapered bore, Figure 7. Since the piston is muzzle 

 loaded, the obturator disc must be loaded with the cartridge case. 

 This is accomplished by fashioning the disc to serve also as the cart- 

 ridge case closure plug. Pressurization of the case forces the 

 obturator/plug against the piston base. As the piston is accelerated 

 down the tube, the plastic flows to provide obturation over the entire 

 firing cycle. Figure 8. 



Existing components were utilized as follows: 



Army 90nmi M41 tube - shortened and smooth-bored 



Army M108B1 Cartridge Case - shortened 



Army M58 primer - shortened and reduced charge 



Navy smokeless propellant (PYRO) 

 The breech block was designed to mate to the existing breech 

 threads on the M41 tube. A high-strength steel ring was incorporated 

 into the base of the cartridge case to strengthen the unsupported region 

 where the safe and arm device (S&A) is inserted into the breechblock. 

 Explosive interface tests were conducted to mate the S&A output charge 

 to the primer percussion element and the primer black powder charge to 

 the main propellant charge. 



An interior ballistics computer simulation was utilized in the 

 design of the overall propulsion system. "Fine-tuning" the system is 

 then achieved by slight variations in the charge weight and grain con- 

 figurations (see Appendix B) . Similar anchor velocities are desired 

 independent of firing depth. However, external pressure can vary by 

 more than 8000 psi over the operating range, resulting in significant 

 effects on the gun performance. Fortunately, by designing the propul- 

 sion package to provide sufficient velocity at the shallow end of the 

 range without generating excessive pressures at the deep end, success 

 is virtually achieved. As shown in tabular form below, at progressively 

 increased depths. Increased outside water pressure impedes anchor motion 

 more, resulting in higher chamber pressures, more propellant burned, and 

 ultimately more energy transferred to the anchor. 



Depth 

 (ft) 



Outside 



pressure 



(psi) 



Chamber 



pressure 



(psi) 



Fraction 

 burned 



Velocity 

 (ft/sec) 





 10,000 

 20,000 





 4,400 

 8,800 



22,000 

 33,000 

 45,000 



0.66 

 0.89 

 1.00 



330 

 365 

 385 



Indeed, velocities are higher, rather than lower, at increased firing 

 depths. If necessary, even these variations can be reduced by the use 

 of several propulsion packages, to be used selectively for various 

 firing depth ranges. 



