ANTI-RPG WARHEAD: AN AIRTCRAFT PROTECTION SOLUTION

By: Vincent Schuetz

Rocket-propelled grenades (RPGs) pose a significant and growing threat to military aircraft.  RPGs are heavily proliferated in all military theatres and are increasingly used to target U.S. and allied aircraft.  To counter this prolific threat, multiple solutions are being researched by the Joint Aircraft Survivability Program Office (JASPO).  The research project described herein investigates several warhead damage mechanisms against RPGs that also meet stringent active protection system (APS) requirements for effectiveness and collateral damage mitigation.

TO KILL AN RPG

The greatest weakness of an RPG is also arguably one of its greatest strengths.  An RPG is an unguided weapon, meaning it contains no seeker or other electronics designed to guide the weapon toward a target.  This weakness has typically required operators to position themselves close enough to ensure a direct hit on a moving targeted aircraft.

Figure 1. Depiction of a Notional RPG With Launcher (Top) and Cross Section of a Notional RPG Variant Warhead (Bottom).

A weapon of this nature also brings forth many challenges to defend against it.  An RPG, such as the one shown in Figure 1, does not have any guidance electronics capable of being jammed/tricked/or spoofed; wherever the RPG has been aimed, it is going to hit.  Thus, a kinetic approach is one of the defense mechanisms being considered to counter RPG engagements against aircraft.  One such method is to use an APS to detect, track, and deploy a defensive missile (aka a kill vehicle [KV]) and destroy the incoming RPG before it can make contact with the aircraft.

The focus of this project is to deter-mine what types of KV payloads are capable of destroying an RPG once the KV has successfully guided to its intercept.  Our design objective was constricted to fit the KV within the standard Mobile Jettison Unit (MJU) flare size for the ALE-47 flare dispenser (shown in Figure 2).  In the event that the objective could not meet lethality requirements, however, the ALE-47 tubes could be modified to fit a larger warhead.

Figure 2. ALE-47 Flare Dispenser.

Another critical performance requirement was to minimize collateral damage, whether it be to troops on the ground, a wingman, or the launching aircraft itself.  Strict timelines of intercepting close-range RPGs, near-by friendly troops during landing or take-off, and wingman aircraft could cause some engagements to occur extremely close to friendly aircraft or troops.  Thus, lethal fragments from either the KV or RPG itself must be minimized.

NOTABLE WORK

Destruction of RPGs has been studied in the past by many Government laboratories and contractors.  Most related to the work for this project is the research that was conducted by Lovelace et al. [1] at the U.S. Army Aviation and Missile Research, Development, and Engineering Center (AMRDEC) and Suarez et al. [2] at the U.S. Army Armament Research, Development and Engineering Center (ARDEC).  In addition, based on the similarities in their goals, the U.S. Naval Air Warfare Center – Weapons Division (NAWCWD) teamed up with ARDEC to further the research in  this area.

The new warhead designs were based on work performed by Suarez and this team in 2014, where they statically tested both pure blast warheads and controlled fragmentation warheads against RPGs at specific standoff distances.  The team concluded that pure blast warheads were not reliable at breaking up or “dudding” an RPG at tested distances.  Their controlled fragmentation warheads, however, were highly effective, many times sympathetically detonating the explosive within the RPG.  Nonetheless, the controlled fragmentation warheads would not meet the low collateral damage criteria for this project and could also produce jetting of the shaped charge liner, which could be lethal to nearby friendly aircraft or troops.

NAWCWD’s approach to reduce collateral damage uses an optimized liner designed to produce high-velocity fragments in the near field of the engagement but to slow down rapidly shortly after the engagement.  The material, shape, and size of the fragments have been specifically chosen to provide effectiveness within the miss distance of the APS projectile and then slow down rapidly from air-induced drag.

Additionally, Lovelace et al. were able to demonstrate improved performance against RPGs over pure blast explosives by adding additional impulse to the engagement.  However, there was some concern if this approach would be viable, when combined with the smaller amount of explosive, and ultimately be able to damage the RPG.

As such, a related approach was further investigated.  Mike Jenkins from the U.S. Air Force Research Laboratory (AFRL) has performed multiple experiments on the efficiency of driving liners by explosive [3].  Using his research, a warhead configuration was optimized for the destruction of RPGs.  Figure 3 provides an example of estimating liner ejection velocity and pattern as a function of distance from explosive detonation.

Figure 3. AFRL Hydrocode Modeling.

NAWCWD formulated two different types of experimental explosives:  one impulse-loaded explosive and one highly brisant explosive.  The impulse-loaded explosive is designed to produce a longer lasting pushing force on the warhead liner, where the brisant explosive is designed to produce a sharp high-amplitude force on the liner.  These warhead designs were then both fabricated in two different diameters for a total of four warhead designs (shown in Table 1).

Table 1. NAWCWD Warhead Designs

To test the effectiveness of the experimental warheads, two RPGs were placed at the same distance from the center of the warhead prior to detonation (as shown in Figure 4).  One RPG was placed on its side to represent perpendicular intercept geometry, while the other RPG was placed nose down, representing parallel intercept geometry.  To test the low collateral nature of the warheads, plywood boards and foam panels were also set up at selected distances.

Figure 4. Experimental Warhead Arena Depiction.

RESULTS

All four experimental warheads exhibited effectiveness against the perpendicular RPG in a similar manner; the RPG warhead section was pitted and ripped apart, much of the explosive remained inside the RPG, and the shaped charge jet (SCJ) liner was jettisoned from the body.  In addition, the RPG section facing the warhead showed evidence of being blasted with high-velocity liner material (as shown in Figure 5).  The nose cone on the front of the RPG was also blown off of the body, removing the impact sensor.  The potential damage by the RPG debris from a successful KV interception is being studied simultaneously in a separate JASP-funded project.

Figure 5. RPG Warhead Section With Evidence of High-Velocity Liner Impact.

The parallel RPG was affected most by Warhead B (see Figure 6).  As shown, the RPG was severed at the base of the nose cone, and the SCJ liner was jettisoned along with some of the explosive material.

Interestingly, Warhead D was the same warhead design as Warhead B, albeit scaled to a larger diameter, yet it produced less damage against the parallel RPG.  It was later assessed that the increase in the amount of explosive in Warhead D shocked the liner in the warhead hard enough to spall it into smaller fragments before they made contact with the RPG.  This phenomenon has been studied by Ripley et al. [4].  The smaller fragments had much less mass and were unable to impart as much kinetic energy onto the RPG as Warhead B.

Figure 6. RPG Warhead Section Severed From Nose Cone (Top), and SCJ Liner Folded and Jettisoned From RPG (Bottom).

Warhead B showed the most effectiveness against both parallel and perpendicular engagement geometries, but the collateral damage metric still needed to be evaluated.  The foam and plywood panels were examined after each test and revealed no perforations through the material from any of the tests.  The foam material was much closer than the defined nonlethal distance and still remained undamaged.  High-speed video evidence showed rapidly decelerating fragments ricocheting off the foam barrier, unable to impact with enough energy to penetrate into the foam (as shown in Figure 7).  The only blemishes on the foam were blast marks created from panels set up in the arena to protect instrumentation (aka “testism”, or artifacts resulting from the test setup itself), indents from where the parallel RPG would make contact after being heaved, and bits of plastic foam from the test stand.  All of the warhead designs were extremely low collateral, with one of the designs showing improvements in effectiveness over the others.

Figure 7. Foam Panel Exhibited No Lethal Indications Within the Minimum Engagement Distance.

FUTURE PLANS

For the future, we will focus on increasing the effectiveness against RPGs using another unique lethality mechanism:  self-consuming fragments.  These fragments start reacting upon impact with the detonation wave and start ejecting mass until they become nonlethal.  The idea behind these fragments is that they will be potentially more lethal within the expected miss distance of the engagement but will consume themselves shortly after and become nonlethal.

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 5 years, he has developed warheads for both aircraft defeat and protection.  Mr. Schuetz is also an annual instructor in warhead effects for the Joint Combat Assessment Team (JCAT).  He holds a B.S. in mechanical engineering from Gonzaga University.

References

  1. Lovelace, Donald E., Scott Howard, Robin P. Wallace, Jillian L. Quillen, and Joel S. Williams.  “Rocket-Propelled Grenade (RPG) Debris Report.”  TR-RDMR-WD-14-47, Redstone Arsenal, AL, 2015.
  2. Suarez, Daniel, Tony Wei, Henry Hsieh, and Richard Fong.  “Helicopter Active Protection System (HAPS) Warhead Development.” ARMET-TR-14061, U.S. Army Armament Research, Development and Engineering Center, Picatinny Arsenal, NJ, 2015.
  3. Jenkins, Charles M., and Robert Ripley.
    “Blast Characteristics of MBX and Other Heavy Metalized High Explosives.”  AFRL-RW-
    EG-TR-2013-020, U.S. Air Force Research Laboratory, Eglin AFB, FL, 2013.
  4. Ripley, R. C., L. Donahue, and F. Zhang.
    “Fragmentation of Metal Particles During Heterogeneous Explosion.”  Journal of Shock Waves, vol. 25, no. 1, January 2015.