Heat Shield Technology

Couldn’t limit cycle yaw cause what you are seeing with the drag coefficient at supersonic velocities?

Not with anything other than a grossly under-stabilized projectile. If limit cycle yaw was the root cause of the observed increase in drag short range, we would have seen the same drag signature regardless of tip used. Limit cycle yaw is a phenomenon associated with some projectiles in which, because of their shape and their stability, they begin to fly with some persistent level of yaw. This phenomenon occurs almost exclusively at long ranges at low supersonic or subsonic Mach numbers. The shape of the boattail plays into this because of the aerodynamic changes that are occurring to a bullet as it gets nearer to Mach 1.0. Short or steep boat tails tend to exacerbate this problem. However, we are talking about low supersonic Mach numbers when a projectile is a long way from the muzzle. We are also talking about projectiles that do not have short or steep boattails. Figure 2 shows the radar-generated drag coefficient (Cd) vs. distance graph at supersonic Mach numbers for the same 6.5 mm 140-grain ELD™ Match projectiles discussed above. As can be seen from the graph, the Heat Shield™-tipped projectile has a very normal looking drag curve. The Delrin-tipped projectile immediately sees a rapid rise in drag as compared to the Heat Shield™ projectile and maintains this higher level of drag offset until low supersonic Mach numbers, where the Meplat diameter has much less of an effect on drag. The two plots do not come back to the same starting point because of the separation, offset, between the radar head and the rifle.

Drag Coefficient vs Distance

The yaw necessary to produce the drag effects seen in the Delrin-tipped bullet would have to occur at the muzzle and be nearly 10 degrees in order to produce the initial performance observed. This type of yaw would have been readily evident on yaw cards placed downrange, and it is not. Furthermore, with the gyroscopic stability (Sg) this projectile was fired at, by using a 6 degree of freedom trajectory calculator, this initial yaw would be damped to less than 0.5 degree by 250 yards. The drag curve for the Delrin-tipped projectile would show an initial convex “bubble” on the curve out to about 200 yards and return to essentially the same curve as the Heat Shield™-tipped projectile by 300 yards if in-flight yaw near the muzzle was the source of the observed drag increase. The phenomenon being observed can only be accounted for by a rapid and sustained change in the shape of the projectile, the polymer tip.