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I promised in another thread to go in and speak with one of my professors in the Aerospace Engineering Department at SDSU regarding the aerodynamics of OTM and polymer tipped bullets. Some of the faculty directed me to Dr. Nagy Nossier, who oversees the graduate research into hypersonic and supersonic flow at the university. This is a short summary of his input, as well as what I have learned from the limited empirical data available to me.
To better understand how the open tip of OTM bullets affect aerodynamics, I would like to first direct your attention to FIGURE 1 below, which is a simple diagram of the shock waves experienced by a conventionally shaped spire-tip bullet. In this type of bullet, the air directly strikes the sharpened tip of the bullet and produces what is known as an "oblique shock wave" that is attached to the very tip of the projectile. The oblique shock wave slows the oncoming flow, creating drag and deflecting the surrounding flow away from the tip. However, in the case of a supersonic projectile, the flow remains above Mach 1. As it travels around the bullet, it produces a series of "expansion waves" that accelerate the fluid flow and direct it around the body of the projectile. If the flow is still attached, it then produces another oblique wave at the tail of the bullet, which once again slows the fluid flow surrounding the bullet and adds drag.
EDIT: There is an error in the paragraph above and drawings below. The aft shock actually occurs behind the bullet where the flow is turned in on itself by the wake, and does not appear to require the flow to remain attached. For more details and a shadowgram showing the phenomena, see post #35.
The OTM bullet, on the other hand, is shown in FIGURE 2. The shockwaves surrounding the body and tail of the bullet are similar, but the tip shows a significant variance. The open tip of the bullet creates a pressure riser immediately in front of it, pushing the formation of the shockwave away from the tip itself. It essentially acts like a blunt nose, creating a detached normal shockwave that slows the air around the tip to subsonic speeds. Increasing the meplat diameter increases this effect.
So, you may ask, what are the expected consequences of the above? They are as follows:
1. The detached shockwave of the OTM bullet creates increased aerodynamic drag, as the massive deceleration of the the supersonic flow to subsonic dissipates kinetic energy. This is empirically confirmed by the fact that spire-pointed bullets frequently show higher ballistic coefficients than equivalent OTM counterparts.
2. The attached shockwave of the polymer tipped bullet has a tendency to destabilize the projectile, creating perturbations as the supersonic flow makes direct contact with the bullet nose.
3. The problem in #2 is further exacerbated by the fact that the polymer tip moves the center of pressure further forward than the shift it induces in center of gravity.
4. The subsonic flow at the tip of the OTM bullet, on the other hand, protects the stability of the bullet by moving the shockwave away from the tip of the bullet itself.
5. Thus, you would expect OTM bullets to be more stable for a a given cross section, as well as being more stable for a given ballistic coefficient. I do have an empirical study I can site showing that real-world OTM bullets consistently are more stable than would be expected for an equivalent spire tipped bullet.
6. However, the greater instability of the spire-tipped projectile can be countered by increased twist rate, so the practical impact of the stability reduction is dependent upon the individual firearm being used.
CONCLUSIONS:
1. If the goal is maximum downrange energy retention, and stability is not a limiting factor, the spire tipped bullet offers more potential for a given bullet weight and length.
2. However, if projectile stability is the limiting factor in terms of bullet cross section, then the OTM bullet offers better aerodynamic performance by allowing the same gun to fire a longer projectile while maintaining bullet stability.
3. If your goal is maximum precision at moderate ranges, the OTM bullet likely offers some advantages in terms of consistency due to the stability concerns cited above. This factor is enhanced by the fact that the polymer tip introduces another possible source of manufacturing variance.
4. If your needs, on the other hand, require minimized bullet drop and wind drift above pure mechanical precision, the polymer tipped bullets may be more accurate in real world conditions.
5. As the meplat diameter increases, the effects noted above likewise increase. Thus, for consistent performance, the meplat diameter should be as similar as possible from projectile to projectile.
To better understand how the open tip of OTM bullets affect aerodynamics, I would like to first direct your attention to FIGURE 1 below, which is a simple diagram of the shock waves experienced by a conventionally shaped spire-tip bullet. In this type of bullet, the air directly strikes the sharpened tip of the bullet and produces what is known as an "oblique shock wave" that is attached to the very tip of the projectile. The oblique shock wave slows the oncoming flow, creating drag and deflecting the surrounding flow away from the tip. However, in the case of a supersonic projectile, the flow remains above Mach 1. As it travels around the bullet, it produces a series of "expansion waves" that accelerate the fluid flow and direct it around the body of the projectile. If the flow is still attached, it then produces another oblique wave at the tail of the bullet, which once again slows the fluid flow surrounding the bullet and adds drag.
EDIT: There is an error in the paragraph above and drawings below. The aft shock actually occurs behind the bullet where the flow is turned in on itself by the wake, and does not appear to require the flow to remain attached. For more details and a shadowgram showing the phenomena, see post #35.
The OTM bullet, on the other hand, is shown in FIGURE 2. The shockwaves surrounding the body and tail of the bullet are similar, but the tip shows a significant variance. The open tip of the bullet creates a pressure riser immediately in front of it, pushing the formation of the shockwave away from the tip itself. It essentially acts like a blunt nose, creating a detached normal shockwave that slows the air around the tip to subsonic speeds. Increasing the meplat diameter increases this effect.
So, you may ask, what are the expected consequences of the above? They are as follows:
1. The detached shockwave of the OTM bullet creates increased aerodynamic drag, as the massive deceleration of the the supersonic flow to subsonic dissipates kinetic energy. This is empirically confirmed by the fact that spire-pointed bullets frequently show higher ballistic coefficients than equivalent OTM counterparts.
2. The attached shockwave of the polymer tipped bullet has a tendency to destabilize the projectile, creating perturbations as the supersonic flow makes direct contact with the bullet nose.
3. The problem in #2 is further exacerbated by the fact that the polymer tip moves the center of pressure further forward than the shift it induces in center of gravity.
4. The subsonic flow at the tip of the OTM bullet, on the other hand, protects the stability of the bullet by moving the shockwave away from the tip of the bullet itself.
5. Thus, you would expect OTM bullets to be more stable for a a given cross section, as well as being more stable for a given ballistic coefficient. I do have an empirical study I can site showing that real-world OTM bullets consistently are more stable than would be expected for an equivalent spire tipped bullet.
6. However, the greater instability of the spire-tipped projectile can be countered by increased twist rate, so the practical impact of the stability reduction is dependent upon the individual firearm being used.
CONCLUSIONS:
1. If the goal is maximum downrange energy retention, and stability is not a limiting factor, the spire tipped bullet offers more potential for a given bullet weight and length.
2. However, if projectile stability is the limiting factor in terms of bullet cross section, then the OTM bullet offers better aerodynamic performance by allowing the same gun to fire a longer projectile while maintaining bullet stability.
3. If your goal is maximum precision at moderate ranges, the OTM bullet likely offers some advantages in terms of consistency due to the stability concerns cited above. This factor is enhanced by the fact that the polymer tip introduces another possible source of manufacturing variance.
4. If your needs, on the other hand, require minimized bullet drop and wind drift above pure mechanical precision, the polymer tipped bullets may be more accurate in real world conditions.
5. As the meplat diameter increases, the effects noted above likewise increase. Thus, for consistent performance, the meplat diameter should be as similar as possible from projectile to projectile.
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