Hornady's hot tip: here's an aerospace engineer's perspective on Hornady's game-changing C bullet.
By now you've probably read colleague Joseph von Benedikt's article on the new bullet beginning on page 36 of this magazine. I'm here to give you my engineer's perspective on the new bullet. Some of you may know that I am an aerospace engineer by training and profession and spent much of my career working on the Space Shuttle program.
I attended the same technical presentation hosted by Hornady as von Benedikt, and I also conferred with Hornady Chief Ballistician Dave Emary about the bullet's development. Three years ago the Hornady engineers and ballisticians had been tasked with designing a long-range hunting bullet that's better than everyone else's. It was a lengthy process, and there were some bumps along the way, but the end result is a bullet that outperformed all other polymer-tipped bullets in the lab, at the shooting range, and in the field.
Gathering the Data
The first rule an engineer learns when solving a tough problem is you can't have too much data--as long as it's good data! With the ELD-X, Hornady designers used Doppler radar measurements every 2 to 3 feet to generate precise drag coefficient (Cd) versus Mach number plots. Eventually, they were able to characterize an often-suspected-but-unproven (until now) event that occurs while a tipped ' bullet travels downrange. What happens is the plastic tip gets too hot and begins to fail, which adversely changes the bullet's ballistic performance.
Maximizing the ballistic coefficient (BC) of the new bullet was the most important technical parameter driving the design. Mathematically speaking, BC is a function of a bullet's form factor and sectional density. Minimizing time of flight and retaining velocity provides for a flatter trajectory and reduced wind drift. Less drag means better terminal ballistics, so the sleeker the bullet, the better. Pointed tip, tapered heel, smooth body, and nose shape all contribute to optimizing the bullet's form factor. To get the desired performance at extended ranges, the bullet must also operate at supersonic velocities. Cd increases significantly as velocity drops from, say, 3,000 fps to 1,700 fps, and it changes even more dramatically when going from supersonic to subsonic.
When you consider sectional density, we have conflicting criteria. On one hand, the bullet needs to be heavy, but at the same time, the smaller the diameter, the better. That may be true theoretically; however, practical considerations often trump purely scientific factors. That's where the engineer excels--bending the theoretical constraints to eventually develop the best product. Hornady s engineers concluded early on that using tried-and-true conventional cup-and-lead-core construction was the most practical starting point.
Why? Because the accuracy exhibited by a nonbonded, drawn-jacket, swaged-lead-core bullet is typically better than any bonded bullet.
Additionally, monolithic designs often employ relief grooves to reduce friction but which also increase aerodynamic drag. Bonded bullets require a softer lead core, so they may not provide effective and consistent terminal performance at one or both ends of the velocity spectrum.
Hollowpoint, tapered-heel bullets provide excellent aerodynamic performance at all ranges, but terminal performance is unpredictable and often unreliable. Adding a tip ensures the meplat isn't easily damaged under recoil. When the tip is properly designed, it provides a reliable mechanism to initiate expansion, especially at lower velocities and extended range impacts.
So Hornady's engineers redesigned the jacket, tweaked the profile parameters, and incorporated the company's more pliable Flex Tip instead of the harder Delrin material that's typically used for the tips.
Identifying a Problem
Then, late in the design stage, the team gained access to a Doppler radar. They fired round after round at ranges out to 1,200 yards and were able to precisely measure speed and position every 2 or 3 feet. The resulting flight data allowed them to plot actual Cd versus Mach number charts instead of correlating limited velocity data with conventional ballistics charts to establish precise BC values.
Studying those charts, Emary noticed a glitch in the Cd versus Mach number plots at ranges beyond 400 yards (shown in the accompanying graph). The curve should be concave at all points along the x-axis. But it wasn't. The only reasonable conclusion was the bullet profile was changing in flight. And the only thing that could physically change shape was the plastic nose tip.
They speculated the stagnation heating caused by the shock wave formed at the bullet nose must have altered the shape of the tip in flight. Thermal modeling predicted bullets traveling at velocities over 2,100 fps see stagnation temperatures above 700 degrees Fahrenheit. This is exactly the same phenomenon experienced by the Apollo capsule and Space Shuttle protective shields during reentry--the thermal tiles glow red from severe aerodynamic heating.
The Delrin material used to make everyone's bullet tips softens and begins to melt at 400 degrees Fahrenheit. Apparently, the tips were slumping or even ablating, which caused the bullets' profiles to change in flight. The result was increased drag, and the effective ballistic coefficients suffered. In layman's terms, the bullet was slowing down quicker, and the predicted trajectory and points of impact at extended ranges changed significantly.
Finding the Solution
Identifying the effect of stagnation heating on the bullet tip dictated a material change. That turned out to be the easy part of the tip redesign. They sourced an existing, alternate material with a much higher melting point. Additional Doppler testing conclusively demonstrated the suspected cause and effect. Firing identical SST bullets with both Delrin tips and the new-material Heat Shield tips improved BCs by at least 10 percent. But the engineers soon determined that simply substituting materials wasn't the total solution. They took the opportunity to revamp the proven InterLock bullet's design features to further assure optimal terminal ballistic performance at both ends of the range scale. In order to assure effective and consistent terminal performance at all hunting ranges and velocities, the tip was upsized, and the meplat diameter was optimized to increase heat capacity, structural capability, and accuracy. The shank diameter was also increased to improve extended range lower velocity expansion.
The ELD-X provides the most versatile performance at both close and extended ranges. Doppler radar data demonstrated extraordinary aerodynamic performance and the highest actual BCs to date for tipped bullets. The new Heat Shield tip material coupled with the enhanced InterLock design ensures outstanding accuracy. Bottom line: The new ELD-X bullet will provide consistent and effective terminal performance at any practical hunting range, assuming the rifle and the one firing it are fully capable of doing their parts.
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|Title Annotation:||Extremely Low Drag-eXpanding|
|Date:||Feb 1, 2016|
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