Anti-RPG Warhead Phases II and III: An Aircraft Protection Solution

By: Vincent Schuetz

This article, which is an update to work previously published in the summer 2018 issue of Aircraft Survivability [1], presents test and analysis results for warheads designed to dismember and/or dud incoming rocket-propelled grenades (RPGs) while simultaneously meeting stringent active protection system (APS) collateral damage requirements. As discussed [1], the near-field-velocity liner warhead technologies previously developed were found to be successful at minimizing collateral damage away from the RPG intercept point; however, only one design inflicted sufficient damage to the RPG to be considered a viable solution. Thus, it was determined that a new warhead concept was needed to provide the same limited level of collateral damage previously demonstrated while also having a significant increase in lethality to the warhead. What was needed were self-consuming fragments, which have the potential to improve lethality against RPGs over the previous lethality mechanism tested in Phase I by introducing larger fragment masses that are still capable of meeting collateral damage requirements.

PHASE II – DESIGN CONSIDERATIONS AND MANUFACTURING

The concept of consumable fragmentation is that fragments will begin burning and consuming mass as they are explosively dispersed from a warhead. A case/liner wrapped around an explosive will undergo shock heating from the detonation wave, mechanical heating from case expansion (assuming natural fragmentation), heating from exposure to the explosive gases, potential aerodynamic heating, and finally additional shock and mechanical heating from impacting the target. For a consumable fragment to be most effective as a collateral damage reduction agent, combustion needs to occur as early in the heating process as possible.

There are many manufacturing methods and techniques to create a fragment that is conducive to consumption. The manufacturing method selected for this project is known as cold spray [2]. With this method, micro-sized particles of material are deposited onto a substrate and chemically bond from the high-velocity impact. The resulting build-up of particles can be capable of approaching near the theoretical maximum density of the material(s). A mixture of materials can also be combined together into one consolidated matrix, which has with the advantage of customizing the weight percentage of each material.

Figure 1 shows an example of cold-sprayed material pre- and post-machining. Note that cold-spray experts at the U.S. Army Research Laboratory facilitated the manufacturing and processing of all consumable fragmentation material used in this effort.

Figure 1. Cold-Sprayed Material Pre-Machining (Top) and Post-Machining (Bottom).

Four different warhead configurations (shown in Table 1) were generated using different materials and varying the consumable fragmentation liner thickness. As a risk reduction to the collateral damage effectiveness of some of the materials, two of the designs included multiple thin consumable fragmentation liners instead of a single thicker liner. The smaller fragments produced by the thinner liner were expected to provide less collateral damage, and consequently also to be less lethal.

PHASE II – TEST SETUP

The Phase II test setup (shown in Figure 2) remained largely the same as that used in Phase I testing except for the exclusion of the horizontal RPG test condition. For Phase II, both of the RPGs surrounding the thermite warhead were in the vertical orientation, as this arrangement produces the most likely engagement scenario. Witness panels at differing distances were used to record collateral damage effectiveness.

Figure 2. Anti-RPG Phase II Test Setup.

Table 2 shows a breakdown of the 11-test matrix executed in Phase II. Testing comprised four experimental warhead designs statically detonated at three RPG standoff distances (close, medium, and far). Two RPGs were used for each test, allowing for two differing miss distances per test.

PHASE II – TEST RESULTS

As expected from the research, test results of consumable fragmentation detonations (a picture of which is given in Figure 3) showed that the thicker liners produced larger fragments. The dense liners also produced deeper penetration into the RPG, as compared to the rare liners of the same design.

Figure 3. Consumable Fragmentation Warhead Effects.

Post-test inspection of witness panels assessed low collateral damage due to fragment effects for each of the experimental warhead designs. A sympathetic detonation by the RPG due to fragment impacts would understandably negate the desired low-collateral-damage nature of the intercept. However, the experimental warheads did not induce an RPG detonation in any test, including those at the closest miss distances tested.

PHASE II – SUMMARY

The low-collateral-damage aspect of consumable fragmentation worked out extremely well for the materials and manufacturing method selected. In all four designs, the lethal radius of the warhead did not exceed the minimum collateral damage radius. Figure 4 provides a series of test photos showing fragments consuming themselves after detonation of the explosive. Based on this project’s findings, it is believed consumable fragmentation is a viable and promising solution for low collateral damage and lethality effectiveness in an RPG APS engagement scenario.

Figure 4. Test Showing Successful Consumable Fragmentation.

PHASE III – DYNAMIC TESTING

The next step was to test the most promising experimental warhead designs from Phases I and II against a dynamic RPG setup. Four designs from Phases I and II were selected to move on to Phase III. These included Warhead B from Phase I (small diameter with brisant explosive) and Warheads E, F, and H from Phase II (see Table 1).

The test setup included an RPG launcher pointed downrange at a large block of witness panels. The experimental warheads were then detonated statically at the medium-range miss distance. To trigger the warheads, a break screen with a delay generator was placed in the line of sight of the RPG.

PHASE III – TEST RESULTS

Two of the four designs were considered successful in defeating the dynamically launched RPG. The two designs not deemed successful allowed the RPG to survive as an artifact of the test setup, where large variations in the RPG velocity resulted in nonoptimal intercept locations. These locations were generally at the extreme aft of the RPG payload since a faster-than-expected RPG would fly farther during the preprogrammed delay used to initiate the warheads.

It is believed that had the locus of fragments impacted the warhead for the unsuccessful designs, the RPG would have been defeated. Figure 5 shows an overall view of a dynamic RPG warhead engagement.

Figure 5. Dynamic RPG Warhead Engagement.

Figure 6 is an example of defeating an RPG with a consumable fragmentation warhead (in this case, Warhead E). Likewise, Figure 7 shows an overall view of the Warhead B dynamic engagement test.

Figure 6. View of a Dynamic Engagement Showing Combusting Fragments From Warhead E.

Figure 7. Warhead B Dynamic Engagement.

Finally, Figure 8 shows the locus of fragmentation effects to the aft of the RPG warhead. Warhead E also impacted aft of the RPG warhead, yet it still resulted in defeating the RPG. Post-test inspection of the RPG from test 3 (i.e., Warhead E) was unavailable since it was damaged from impacting a steel plate downrange.

Figure 8. Warhead B Fragmentation Location on Dynamic RPG.

CONCLUSIONS

Analysis from the aforementioned fragmentation testing indicated that Warheads B and F produced the most damage during static testing. Unfortunately, dynamic testing was skewed by differing RPG flight velocities. The test setup could have been improved by measuring the velocity of the RPG out of the gun and using that velocity to calculate a delay time for the fuzing. In a real-life engagement, a target-detecting device on an APS countermeasure could produce fuzing accuracies to provide fragmentation well within the intended intercept point of an RPG.

Lessons learned from the test results indicated that there are definitely areas where both Phase I and Phase II experimental warhead designs could be optimized. However, the project did complete its goal—namely, that warhead countermeasures that can fit inside an ALE-47 flare dispenser can successfully defeat a dynamic RPG while meeting or exceeding low-collateral-damage requirements of the engagement.

For a more comprehensive analysis of all three phases of this project, see the full technical report published by the Naval Air Warfare Center Weapons Division [3].

ABOUT THE AUTHOR

Mr. Vincent Schuetz is currently a warhead design engineer at the Naval Air Warfare Center Weapons Division – China Lake. For the past 6 years, he has developed warheads for both aircraft defeat and protection. He is also an annual instructor in warhead effects for the Joint Combat Assessment Team. Mr. Schuetz has a B.S. in mechanical engineering from Gonzaga University.

References

Schuetz, Vincent R. “Anti-RPG Warhead: An Aircraft Protection Solution.” Aircraft Survivability, summer 2018.
Trexler, Matthew, and Muge Fermen- Coker. “A Brief Summary of ARL’s R&D Program on Consumable Fragments.” ARL-SR-0338, U.S. Army Research Laboratory, Aberdeen Proving Ground, MD, September 2015.
Schuetz, Vincent. “Low-Collateral Anti-Rocket-Propelled Grenade (RPG) Warheads.” NAWCWPNS TM 8826, Naval Air Warfare Center Weapons Division, China Lake, CA, March 2019.