The Search for Safer Combat How Close Are We Getting?

nte airtlior's consulting j h i, D. R. K C I I W V & Associates, of Los Altos, Cal., Itas done mtensive work in the area of conibat wliicle ninivabilit?, for the Defense Depart-rtierit. Much of the threat to armored fighting vehicles comes from within - the propellants, explosive warheads, and fuel carried inside. When hit by a penetrating shot, they explode or burn, causing injury or death to the crew and destruction of the vehicle. Iehicles: P This M151 Sheridan AR/AAV, seen in a salvage yard in Vietnam, was destroyed by an antitank mine and the secondary explosion of its own ammunition. Current research is directed at preventing secondary fuel and ammunition fires. While designers have made some progress in recent years to limit vulnerability through better vehicle design, propellants and on-board explosives - by their very nature - continue to challenge designers. Fuel fires are less of a problem than 'they were in WWII. The widespread use of diesel engines, rather than gasoline-fueled power plants, lowered the fire risk. More recently, the addition of automatic fire-extinguishing systems, like the Halon@ systems in U. S. tanks and APCs, greatly reduced vulnerability to Fuel fires. These systems work almost instantly to snuff out a developing fuel fire by depriving it of oxygen. This approach doesn't work with propellants, explosives, and pyrotechnics: each contains its own oxidizer. Once the explosive reaction begins, usually from heat or impact, the materials will burn or detonate, even in the absence of atmospheric oxygen. Materials wl-nerahle to this sort of catastrophe include large-caliber tank gun ammunition, small arms cartridges, antitank guided missiles, shoulder-fired rockets, mines, and pyrotechnic signal and smoke devices. At high pressure or high temperature, propellants and explosives rapidly produce large volumes of gas. If this chemical transformation happens rapidly enough, a shock wave develops, creating an explosion. The grain size of the propellants and the degree to which the material is confined help determine how rapidly the transformation occurs. Nearly 50 percent of the vehicles lost in combat succumb to weapons-induced fuel or ammunition fires. If the ammunition burns, there is a high probability of crew deaths and loss of the machine. Propellant Fires Of all the materials stored on board combat vehicles, the propellant in gun ammunition and rocket motors is the most vulnerable, not onlv because thcre is so much of it, but also because it is the most vulnerable to KE penetrators, spall fragments, and the jets of HEAT charges. Designers have been able to limit propellant-fueled explosions by using two techniques. One is to limit the confinement of the material so that pressure cannot build up. For example, a caseload of propellant will continue to burn, but not explode, if the case is breached, relieving pressure. An- other approach is to deluge the developing fire with water, which cools the fire, preventing further heat buildup. In light of bitter, hard-won experience during WWII, this approach increased the survivability of the British Sherman Firefly (but not the U. S. version of the M4). Main gun ammunition was stored in water jackets. When the jackets were penetrated, the water escaped and quenched the fire. Some other approaches include: 0 Arranging the vehicle stowage to minimize the possibility of penetrators hitting ammunition. Ammunition in ready racks above the turret ring was particularly vulnerable, judging by the battle experience ol T-62 crews in the Arab-Israeli wars.. Adding local protection, such as ballistic blankets, to keep hot frag-ARMOR - September-October 7 988 39

I ments from reaching heat-sensitive propellant materials. 0 Cartridge cases that readily fail when subjected to high temperatures or pressures when unconfined. One way of doing this would be to pre-fragment the cases - perhaps by grooving them longitudinally - so that they break up into strips when the charge is ignited outside the gun breech. Explosives Vulnerability Explosives present a paradox: compared to propellants, they are less likely to explode or bum when exposed to hot fragments or heat, but once the detonation process begins, vehicle and crew loss is virtually certain. Like propellants, the degree of confinement makes explosives more or less vulnerable. In addition, some explosives are more sensitive than others. Finally, vulnerability depends on the amount of energy acting on the explosive. There is substantial research concerning the vulnerability of both cased and uncased explosives and propellant materials to fragment, bullet. and both KE and CE (Le., shaped charge) impact. Researchers have developed several tests to determine explosive sensitivity, including both large- and small-scale gap tests, drop hammer tests (five difference types), 'Susan tests," and bullet impact tests. These, and other considerations, help rank potentially usable military explosives from "most hazardous'' to "safest." Recently, one of the U. S. Navy's several explosive development facilities ranked 69 explosives. Most of the explosives in widespread use today, rank as "hazardous" and "very hazardous." Lower power explosives such as TNT, DNT, and Explosive D (Picric acid) rank among the "safer" explosives, yet these are rarely employed in modern anti-armor munitions because of their comparative lack of power, and such physical problems as too low melting point, exudation, poor strength, poor long-term storage, etc. Thus, we presently face either having very high performance but very hazardous explosives, or safer and much lower-performance explosives that are not suitable for modern high-performance explosive munitions operating in today's more severe environments. The Army continues to study low-vulnerability explosives and propellants in the Low Vulnerability Ammunition (LOVA) program. A 1985 report indicates good results with a formula known as NOS-365, a liquid propellant. Typical 105-mm cartridge cases filled with this material did not detonate even when both shaped-charge jets and hot fragments penetrated. Certain other newly-developed explosives show a high tolerance to both heat and ballistic impact, but they lend to be very expensive, difficult lo ignite, and have a poor energy output compared to common existing formulas. Although LOVA solid and liquid propellants show promise, it will be years before such materials are in general use. For the near term, we must still be concerned with the wl-nerability of the present large inventory of M-30 and other high-vulnerability propellants. Over the past 10 years, armored vehicle experts have made many suggestions to improve survivability. Some of their conclusions include: "Tank Innovations," by Joe Wil- 0 Non-explosive, liquid bi-propel- 0 Non-burning, heat-resistant fuel. 0 Compartmentalization of ammunition. 0 Heavily protected, encapsulated crew compartments. "Closing the Survivability Gap," by Brigadier Richard Simpkin (November-December 1951 ARMOR). liams (May-June 1975 ARMOR). lants.. Carry fuel in jettisonable tanks within externally vented compartments, except for a small emergency reserve. Try to incorporate the fuel into the tank's protective system. 0 Replace metal-cased main gun rounds with semi-combustible cases. Design extinguishing containers for individual rounds or small groups of rounds. Vent the magazines and make them jettisonable. 0 As a "far-out alternative," leave the ammunition partially exposed, but reasonably well spaced, in skeleton ready racks that are partially shielded by other elements, like the gun or running gear. "Israel's Chariot of Fire," by Peter Hellman (March 1985 Atlarzfic Motthl). 0The Israeli Merkava uses every possible component as a buffer for the crew. Fuel tanks incorporated in the hull armor help break up shaped-charge penetrations. Machine gun ammo belts are part of the armor protcction of the hull. olsolate main gun ammunition in a heat-resistant container set low in the hull. This system protects ammunition for as long as 40 minutes in a fire. Richard M. Ogorkiewicz, analyzing the Merkava in an article in 111- iernational Defense Review, notes that locating the engine in the front of the hull acts to protect the crew, as does the fuel cell in the hull floor, which helps limit mine damage to crew and ammunition. Another tank in the roof, used for drinking water, adds a layer of protection against top attack. The glass-fiber reinforced plastics used in the fire-resistant main gun ammunition magazine also act as a spall shield. Joseph E. Backofen's article in the January-February 1984 issue of ARA40R calls for all main gun ammunition stowage below the turret ring and as low as possible in the hull. Blast doors should separate 40 ARMOR - September-October 7988 crew from ammunition. Ammunition racks should provide wet stowage for main gun ammunition. Backofen also noted that haged charges, as used by the British, did not explode immediately when hot projectiles or fragments hit them. In the few seconds before ignition, water jackets could suppress the progress of a fire or explosion. A study of U. S. armored vehicle losses in Vietnam by the Ballistic Research Laboratories came to these conclusions: 0 Diesel and gasoline-powered APCs burned with about the same frequency. In vchicles lost to shapedcharge attack, two-thirds involved fires, and of those that burned, two-thirds involved fires that reached the ammunition.. Mines more often led to diesel fuel fires, prohahly because of the location of the fuel tanks.. When a loss was accompanied by fire, personnel casualties were 50 percent higher. The late Brigadier Simpkin and other experts have been showing an increased concern about the vulnerability of lighter armored vehicles, especially against top attack by helicopters and cannon-firing aircraft. Protection of thc turret is essential because of the large quantities of automatic cannon ammunition present, but too much armor high on the vehicle leads to stability problems. Simpkin suggested the possibility of storing cannon ammunition in turret bustles to isolate it from the crew and to act as a sort of reactive armor. This is similar to M1 tank practice. Pulling Ideas Together From these experts and others, we can come to a concensus of techniques that will limit or eliminate catastrophic ammunition fires: 0 Provide external, jcttisonable ammunition containers.. Limit internal stowage, and locate it low in the hull, hut not too low to be vulnerable to mines. Water-jacket rounds stowed inside the crew compartment. 0 Use small caliber ammunition to protect larger caliber rounds. Use drinking water stores to protect internally-stored ammunition. 0 Employ external fuel tanks as armor. 0 Isolate crew from ammunition with blast-proof doors. Employ water jacketed magazines and deluge cooling when this is not possible. Several of these suggestions require additional research. In attempting to use small caliber ani-munition to protect larger rounds, some sources report detonation of the small arms ammo. Additional tests, including HEAT penetrations, would verify or disprove this. The first recommendation - that ammunition be stored in jct-tisonable external containers - dovetails with another requirement long delayed, the need for a rapid rearm capability for main gun ammunition. Perhaps pods of ammunition, representing a daily basic load, could be delivered to a tank or APC‘s armored, external magazine cornpartmcnt, allowing rearming during NBC conditions. Crew access to the ammunition would be through a sealed, blast-proof door. The pod could be water-jacketed, so fresh water would be delivered to the tank along with the ammo, and it might even be possible to include crew rations with the package, or additional fuel. The pod would act as spaced armor and would separate from the vehicle if penetrated. Under normal conditions, the pod would be used to collect used brass, crew waste, and NBC-contaminated material. The pod-rearming concept would also dovetail with any external-gun/autoloader concept in a future tank. Both gun and magazine pod would be isolated from the crew. Finally, the crew should he clothed in garments that protect from flash fires, smoke, small fragments, and NBC effects, especially the facial area and other exposed skin. The uniform should also provide breathing air, cooling and heating, and communications. In this way, even if the ammunition caught fire, the crewmen would have enough time to evacuate, while deluge systems and other improvements worked to delay the fire’s progress. While low vulnerability explosives and propellants are still being dcveloped in the laboratories, we have the necessary technologies to greatly reduce catastrophic losses from fire, and recognized experts in the armor field have told us how to use them. The time has come for us to heed their advice. Donald R. Kennedy is a 1948 graduate of San Diego State University with Distinction in Engineering and in 1978 established D. R. Kennedy & Associates, Inc., an internationally recognized consultant firm in the field of nonnuclear ordnance, particularly in the field of armor/antiarmor technology. A veteran of Pearl Harbor, Kennedy’s armor experience began in 1941 with tests of 50-caliber machinegun ammunition against M2A3 armor. Since then, he -has investigated the problems and properties of shaped charges, behind armor effects of explosives, spall protection for armor crews, etc. to name a few of his areas of expertise. He has contributed to ARMOR on such subjects as shaped charges and spall liner protection for APCs. I ARMOR - Septernber-October 1988 41 I