U.S. naval vessels responding to the over-the-horizon exigencies of the Cold War have evolved into champions with long reaches—and glass jaws.
Their lightweight aluminum-steel construction renders them vulnerable to small arms and other weapons in key areas. This vulnerability is magnified because the Cold War weapons are now in the hands of those who might attempt to employ them using hit-and-run tactics. Applying composite and reactive armor is the logical response.
Several factors make naval vessels likely targets of either a single smart weapon surprise attack or an in-close small arms attack during low-intensity, littoral conflict. First, U.S. warships will likely be on-scene because of their inherent flexibility as instruments of U.S. policy. Second, such conflicts since the end of the Cold War have involved many competing interests, any one of which might lash out at a U.S. vessel—even one on station for peacekeeping. Third, the news media may transform even a simple single-weapon attack into a political victory.
The 1987 Iraqi attack on the USS Stark (FFG-31) serves as a grave reminder of the potency of missiles against modern aluminum and steel ship construction. Composite and smart armors could minimize damage and might allow an active defense by enabling enough systems to survive a weapon’s impact.
Composite armor is a sandwich of various materials based on the expected conditions that the layer will be subjected to. Figure l’s ballistic sequence illustrates why composite armors are effective. A bullet could penetrate a thin layer of ceramic and a bullet could penetrate a thin layer of Kevlar; layering the two, however, prevents penetration.
As a bullet strikes the ceramic layer, it begins to shatter because it is harder than the bullet and the shock wave from the impact is transmitted through the ceramic layer and then reflected from the second layer of armor. Prior to hitting the Kevlar layer, the bullet loses its pointed shape and the focus of energy is expanded. Given the partial destruction and dissipation of energy in the ceramic layer, the Kevlar layer is strong enough to prevent penetration. Figure 1 does not show the spall layer, which surrounds the entire armor and prevents fragments of the armor from becoming destructive.
Many different types of materials are used in composite armor. Table 1 shows the classifications of various advanced materials. Three of the most common are Spectra, Kevlar, and S-2 Glass.
Several factors determine the optimal design for a composite armor system. Table 2 lists some key design considerations. Weight gives composite armor an advantage because it offers more protection per pound than traditional metal armors. Backface deformation is relevant to composite armors because the layers of material are much thinner than traditional metal plate armor and composite armors deform to a greater extent. Structural considerations are important because, as Figure 1 shows, the outer ceramic layer may not be able to protect against a second round in the same location.
Because multiple layers are used in composite armor, partial delamination is possible, either from the impact of projectiles or environmental factors. The expected threat and conditions unique to the platform on which it is applied determine the shock, vibration, and flammability requirements. Finally, the operating conditions of the platform will determine which environmental fluids that the armor must be able to withstand.
Reactive armor is a “smart” armor that responds to impact by firing a shaped charge outward. Its key components are the detonation mechanism and the shaped charge. Because it is used in conjunction with other armor, it does not have to completely destroy the incoming projectile; it damages, deflects, or dissipates the incoming projectile’s energy enough to permit the standard armor to defeat it. Gulf War lessons suggest that reactive armor on fighting vehicles was effective.
Various features have been incorporated into projectiles in an attempt to defeat reactive armor. Projectile speed has been increased in an effort to defeat the armor by penetrating before the shaped charge fires. Causing the armor to fire prematurely during the impact sequence by incorporating an initial charge in the projectile warhead is another approach.
The defense, however, is still ahead. An improved mesh sensing system that senses the speed and direction of the incoming projectile and fires shaped charges in the optimum sequence for defeating projectiles as large as 120-mm is being incorporated into the new Smart Armor System.
Composite and reactive armors are currently used in many different applications—of particular interest are area protection and component protection. Area protection is used to shield ship compartments, aircraft cabins, and the interior of armored vehicles.
In 1993, composite armor made of boron carbide particles in an aluminum matrix—lighter than aluminum yet six times stiffer, was used on U.S., British, and NATO C-130s flying into Sarajevo during the Bosnian airlift. In May 1994, armor consisting of a ceramic/metal matrix bonded to Kevlar was fitted to 13 U.S. Air Force C-141s.
In addition to providing protection against the penetration of projectiles and shrapnel, area protection can minimize damage from heat and fire. Mindful of the lessons learned in the Falklands Conflict, the Royal Navy is evaluating the use of barrier material to protect all high-risk and high-value compartments, zone boundaries, and the damage-control deck. Refractory ceramic fiber may be used.
Although modern composite armors tend to be lighter and cheaper than traditional armors, they do bring significant weight penalties and they are not cheap; C-130 armor weighs 5.4 pounds per square foot. Protecting only selected components is one way to reduce the weight. The composite material is either molded to fit the component or the materials that make up composite armors are incorporated into the material used to make the component. Silicon carbide armor, for example, has been machined to fit snugly around aircraft hydraulic systems. Applications to phased-array panels are being investigated.
Naval composite armor designers should remember the following:
- Area protection composite armor plating should be modular to allow for flexibility. Composite armors tend to be expensive; providing a set of the most modem, tailor-made armor to every important ship space or aircraft cabin may not be possible. Standard, interchangeable armor is the answer to meet unforeseen contingencies and facilitate upgrades—and retrofit other ships.
- Component protection should be designed in from the start. Integrating composite material into the component saves weight because the armor replaces a part of the component’s structure and eliminates re-engineering problems. Spending research funds on material integration makes sense—this may be the next century’s top dual-use technology.
Composite materials are widely used in civilian aircraft and automobiles, yet these uses continue to be limited by concerns over damage tolerance. Since this would be a key focus of defense research, there should be valuable spin-offs for the private sector. Composite jet engine turbine blades and nacelles are good examples of dual-use composite material application. The Navy may be interested in turbine blades and nacelles that can survive small-arms fire and shrapnel; these designs also protect against foreign object damage and bird strikes, however— dangers shared by civil aviation.
Reactive armors may have a naval application farther in the future. It would be unrealistic to cover the exterior of a ship completely with Smart Armor System panels, but it may make sense to investigate whether this type of system could protect key areas of a ship. Just as armor belts protected large warships against torpedoes during World War II— placing additional armor at the most likely points of impact—smart armor may be able to protect ships against several types of current weapons.
An alternate approach would place a reactive armor panel on a trolley system as part of a larger system that would calculate the impact point of an incoming weapon and position the panel prior to impact. This might not completely cancel the explosive force, but redirection or even partial energy dissipation might allow the vessel to survive and conduct an active defense. The Smart Armor System’s potential to neutralize incoming rounds of 120-mm or larger makes a good case for considering it for future ships.
Composite and smart armors could decrease the risk posed by the combination of Cold-War ship design, America’s sustained presence in the littoral zones of regional conflict, and the ongoing proliferation of smart weapons around the world.
Lieutenant Hart is a lawyer in Seattle, Washington. While on active duty, he was the missile fire control officer in the USS Arkansas (CGN-41).