The Effect Of Low-Angle Yaw On The Armour Penetration Of Light Armour-Piercing Projectiles

Introduction

It is generally recognised that for maximum armour penetration a projectile should impact with little or no yaw so that kinetic energy density is maximised. However there is some evidence of anomalous effects at small angles of obliquity [1] which might imply an effect at small angles of yaw. This would be an important effect as small yaw angles are difficult to detect in routine tests. In this work the performance of 7.62×54r B32 armour-piercing incendiary (API) projectiles against ceramic-faced armour was determined for slightly yawed (>5°) with normal impacts (<5°). This projectile was chosen as it is a standard test round defined in NATO STANAG 4569 [2]. To test the effect of yaw the method developed by Coburn [3] was used to induce a small yaw angle in the projectiles. Trials were then conducted with a target placed at 10m from the muzzle. High-speed video was used to record the projectile yaw in two orthogonal planes immediately before impact. Firings were then conducted with both yawed and un-yawed projectiles against a silicon-carbide-faced aramid composite target and the ballistic limit was determined for both cases.

Ammuniton

The 7.62×54R B32 is a boat-tailed API projectile with a hardened steel core, surrounded by a lead sheath and encased within a gilding metal-coated steel jacket. The magnesium-based incendiary composition is positioned both in the nose cavity and in a gilding metal-coated steel cup in the base of the projectile.

Figure 1 shows a complete projectile and components from a sectioned projectile (left to right: whole projectile, half of jacket, steel core above incendiary cup, half of jacket).

Three projectiles from batches of 7.62×54R ammunition, manufactured by two factories (identified from headstamp) from 1952–1983 were dismantled to confirm projectile construction and to allow measurement and hardness testing of the steel cores. Vickers hardness on each core was determined using a micro-hardness tester (INDENTEC HWDM-7) with a load of 2 kg. Average dimensions, masses, and Hv values are listed in Table 2.

Radiographs were taken of all 69 projectiles in the trial to assess consistency of production within a single batch and across a number of batches (with different factory and date information on the headstamp). Figure 2 shows the radiograph of the projectiles from Factory 21 ammunition used for all yawed test firings (PANTAK Industrial Radiograph equipment and KODAK Industrex C Film settings: energy 120 kV, current 3 mA, exposure time 2 minutes). The length of the lead sheath in all batches of B32-API tested was found to vary, this feature combined with the lack of concentricity of the lead sheaths may be sufficient to alter significantly the centre of gravity position (along and away from the projectile axis) and potentially affect the level of stability in flight. The variability in the turnover can easily be seen in Figure 2, as can the inconsistent incendiary density in the rear cup.

Inducing yaw

Of the potential methods for inducing yaw in the order of 5–15°, chamfering the base of the round had been shown by Coburn [3] to achieve the majority of these requirements, with the exception of the amount of yaw induced. The induced yaw from a 45° chamfer across the whole diameter of the projectile’s base (as shown in the left-hand image of Figure 3) was shown to be approximately 30° for a 7.62-mm ball round fired at a velocity of 800 ms^–1. So the 45° chamfer was limited to a cut width of 3 mm from the outer edge of the jacket as shown in Figure 3. The reduction in projectile mass was less than 1.5% of the original projectile mass.

Test firings

A proof mount with a gun barrel length of 720 mm and a twist length of 240 mm (1 in 9.45 in) was used to fire the projectiles. Velocity was measured by two MS Instruments Optical Detectors with yaw cards placed in the path of the projectile throughout the range as shown in Figure 4.

Measurement of yaw

All test firings were recorded using a Phantom high-speed video camera at a frame rate of 4,700 pictures per second and exposure time of 2 µs, In order to detect yaw in orthogonal planes, a mirror was erected at an angle of 45° to the plane of the camera, within the field of view of the camera, thus capturing a side view (pitch) and top view (yaw) of the projectile prior to impact with the target, Figure 5.

The projectile tip and centre of the base of the projectile were used as measurement points for evaluating the pitch and yaw angles from individual frames of video files. The top image of the projectile in Figure 6 is seen as the ‘top view’ in the mirror, and the bottom image is the ‘side view’. The projectile is travelling from right to left and yawing with its tip right 11.5° (Figure 6a ) and down 6° (Figure 6b) relative to the direction of flight.

The overall yaw of the projectile is:

where:

α’ is the yaw in one orthogonal plane, and

αꞌꞌ is the yaw is the second orthogonal plane.

So taking αꞌ= 6° and αꞌꞌ = 11.5° the overall yaw of this projectile was calculated to be 12.9°.

A total of 16 chamfered projectiles were fired during the experimental investigation 15 of which yawed by more than 5°. In addition to the high speed video recording, yaw cards were placed at measured intervals (A–F shown in Figure 4) in the path of the projectile to record projectile yaw angle (overall yaw rather than pitch and yaw components), monitor the yaw rate / period and to confirm the round was at least neutrally stable during flight. An additional yaw card was placed 100 mm from the front face of the ceramic armour to confirm overall projectile yaw at impact (position T shown in Figure 4). The witness holes left in the yaw cards provided a means of determining the amount of yaw, as well as the relative tip orientation about the line of flight, as it rotated due to precession.

Witness holes on the yaw cards and recorded video showed that all rounds were neutrally stable and were not tumbling. Chamfered rounds were fired at velocities in the range 680–820 ms^–1, with the only round failing to yaw being recorded at a velocity of 811 ms^–1. The angles of yaw recorded by the yaw cards are summarised in Table 3.

Ballistic test results

The V₅₀ ballistic limit was determined for unmodified and un-yawed rounds against the ceramic-faced targets. The ballistic limit velocity of the B32-API against the candidate ceramic faced composite armour was calculated to be 735.5 ms^–1, being the average of six shots, three perforations and three stops. The date for an individual test is shown in Table 4. Seven of the chamfered rounds were fired against the ceramic faced composite targets at velocities below the V₅₀, Table 4. The highlighted figure represents a shot where the velocity was calculated based on the distance travelled by the projectile between successive frames of the recorded video. The results show that five of six of the yawed rounds were able to perforate the armour system at velocities below the V₅₀ for normal impact. The slowest chamfered round (685 ms^–1) which failed to perforate the target may not reflect the ‘ballistic limit’ for yawed impact as it was shown that this particular round, whilst yawing early in its flight, impacted the target with minimal yaw, and can therefore be assessed as a non yawed impact. Additional non-yawed impacts at velocities in the range 695–707 ms^–1 showed the armours’ ability to stop the B32-API round, establishing that the reduced velocity perforations were not happening as the result of the shatter gap phenomena.

Due to the small sample size of yawed impact test results it is difficult to establish the statistical significance of the perforations at velocities below the V₅₀ for normal impacts. Figure 7 shows a logistic regression plot of the normal and yawed impact results. The full binomial distribution curve for the normal impact V₅₀ test firings corresponds well to the calculated V₅₀ of 735.5 ms^–1. The partial curve fitted to the yawed impact results is based on the limited samples available, and shows that a ballistic limit velocity for the yawed impact results would fall below that for normal impacts. As the data points to assess the statistical significance of the yawed impact curve are limited it can only be indicative, however it shows that the V₅₀ for yawed impact may be as much as 50 ms^–1 below the normal impact V₅₀.

The relatively high occurrence of yaw greater than 5° in the unmodified B32-API rounds is most likely as a result of effects during intermediate ballistics, possibly due to variations of turnover affecting the symmetry of reverse flow of propellant gas as the projectile leaves the muzzle. The level of consistency of the reverse gas flow in each batch is also likely to vary with the shape of the rear edge of the boat tail

Whilst there is noticeable variation in the length and symmetry of the lead sheath around the hardened core, this is unlikely to have been a significant cause of instability (and hence yaw) when firing B32-API projectiles from the test gun.

Conclusions

The expectation that maximum penetration is achieved with an ideal normal impact does not always hold true, particularly in the case of the B32-API round against a silicon carbide / KM2 Kevlar armour system. Small angles of yaw have been shown to enhance the penetrative ability of the armour piercing round. The mechanism by which yawed rounds show improved penetration is not known and will require further work to quantify the effect.

References

Authors

Celia Watson is a Principal Research Fellow for Cranfield Defence and Security at the Defence Academy UK. Her MSc was on Aluminium Armour alloys and her work on body armour systems for police, security and the military is internationally recognised.

Professor Ian Horsfall is the Head of the Impact and Armour Group at the Defence Academy UK with more than 20 years experience in ballistics and composite armour systems. His PhD research was on stab resistant body armours and he was a major contributor to both the HOSDB and NIJ Stab resistant Standards.

Lt Cmdr Lawrence Bates RNZN was an MSc student on the Explosives Ordnance Engineering MSc Course at the Defence Academy UK, and this work was completed as his MSc research project in 2008.