The US Army implemented a research and development programme which resulted in two specifications being issued, ATPD-2070 Edition 6A and ATPD-2071 Edition 6A, for key components of an advanced state-of-the-art automatic fire detection and suppression system.

HTL/Kin-Tech Division of Pacific Scientific (HTL) subsequently developed a complete crew and engine compartment protection system which has proven itself through extensive live fire testing on armoured vehicles against a large variety of anti-armour weapons. HTL has qualified this system for use with the US Army and is now supplying complete systems for such vehicles as the United Defense M992 Field Artillery Ammunition Support Vehicle (FAASV). This system is also in use internationally on such programmes as the South Korean Hyundai Precision & Industry Co Ltd K1 MBT.

In 1981, as the US Army was developing requirements for an advanced state-of-the-art automatic fire detection and suppression system for its fleet of armoured fighting vehicles, HTL began development of a system to meet the new specifications.

In 1984, HTL won the first production contract for systems meeting these latest specifications (ATPD-2070 Edition 6A and ATPD-2071 Edition 6A) for use on the M992 FAASV. The system was qualified through the most extensive live fire testing programme ever conducted on this type of system by the US Army at Aberdeen Proving Grounds. The US Army has purchased 675 FAASVs from United Defense to date, all with HTL systems in both the engine compartment and the crew/ammunition compartment.

HTL also provides earlier technology AFES components to the US Army M1 MBT and M2/M3 Bradley programmes. HTL has also supplied components for qualification testing, to the Bradley AFES Upgrade programme. The US Army has decided to upgrade the automatic fire extinguishing system in the crew compartment of the Bradley with components meeting the latest specifications such as those used on the M992 FAASV. ATPD-2070 Edition 6A and ATPD-2071 Edition 6A have now been converted to MIL-M-62545A, MIL-M-62546A and MIL-V-62547A.

The HTL AFES is actually made up of two subsystems, one subsystem to protect the crew compartment and a separate subsystem to protect the engine compartment, plus a manual activation system.

There are two principle reasons the AFES is divided into two subsystems. Firstly, the fire threats are very different in each compartment. The primary threat in the crew compartment is exploding fuel or hydraulic fluid initiated by an armour penetration. Even a small amount of fuel or hydraulic fluid, such as from a severed hydraulic line, when atomised and ignited bya penetrating munition, will produce an explosion that, il not suppressed, will cause overpressure high enough to injure or kill the vehicle crew and will be followed bys fire that can destroy the vehicle. The secondary threat in the crew compartment is of small fires which result from electrical short-circuits! and ipoor vehicle maintenance.

To counter these threats, HTL has designed a system that can react to the start of an explosive fire and disperse fire suppressing agent so that overpressures are controlled before they reach levels likely to-cause injury and the fire is extinguished before it can cause serious injury to the crew and damage to the vehicle.

Since speed of detection is critical, HTL selected an array of three optical sensors for each detector assembly. Utilising a patented logic concept, the start of explosive fires can be detected within a few milliseconds. Employing three sensors has allowed HTL to achieve much greater false alarm immunity than earlier two-sensor detectors. This was verified by the US Army testing against 28 potential false alarm sources.

The patented detection logic and the use of three sensors has allowed HTL to advance the state-of-the-art by achieving the first truly discriminating detector, that is a detector which can distinguish between an armour penetration that results in an explosion and/or fire and one that does not.

This has become an important requirement as the accuracy and firing rates of anti-armour weapons have improved. Statistics show that if a vehicle has been hit once, a second hit is very likely if the first does not destroy the vehicle. Many times a penetrator or the plasma jet will simply pass through the vehicle not striking fuel, hydraulic fluid or ammunition. If the AFES can distinguish between this event and one where an explosion and/or fire occurs, the fire extinguishing agent can be saved for when it is really needed.

Anotherfeatureof the HTL system and a requirement of the latest specification is a second shot capability. This feature is also to reduce the threat of a second hit. There is a second set of fire extinguishers which are discharged automatically if a second explosion and/or fire is detected.

A third system feature' to reduce the threat of a second hit in the system sends a signal which turns on the vehicle ventilator in the exhaust mode to vent the smoke and fire extinguishing agent after the fire is extinguished. This means the crew can stay in the vehicle, continuing to fight and manoeuvre the vehicle to avoid or defeat the threat.

HTL has developed a fast-reacting solenoid-actuated valve which opens in a few milliseconds and rapidly discharges extinguishing agent. The US Army, through extensive live fire testing, has found Halon 1301 to be the most effective non-toxic extinguishing agent available. A distribution nozzle, which has been developed and proven through live fire testing is utilised with each valve to achieve adequate extinguishing agent concentration throughout the protected compartment. The extinguishing agent capacity of the valve and bottle is also tailored to the unique requirements of each vehicle.

Other key features of the HTL Crew Compartment AFES are extensive Built-in Test Equipment (BITE) and minimum maintenance requirements. Each time vehicle power is switched on, a complete test sequence is initiated. The test includes an optical check of each detector. This is done by simulating a fire. Each detector has an externally mounted Light Emitting Diode (LED) that simulates a fire to the sensors which determines if the detector can see a fire (or whether the sensors are dirty), can process the input and issue a fire signal to control electronics. The control electronics are tested to assure a fire signal can be processed and each of the valve solenoid driver circuits can actuate a valve. Thus, before the vehicle is driven off, the crew is assured they have a properly functioning AFES. The system also continually monitors the pressure in the fire extinguishers and the continuity of all the electrical cables. If a system problem occurs, a fault light is illuminated and an LED illuminates, identifying the malfunctioning module(s) on the crew interface panel. No separate test equipment is required to verify system performance.

To deal with small secondary threat fires typically resulting from electrical shorts or poor maintenance, the HTL system has a small fire warning. A fire warning light flashes and an audible warning signal is sent to the crew. This gives the crew the option of using hand-held extinguishers to suppress the fire thus saving the system extinguishers for combat. The crew can also discharge the system by a guarded switch on the crew interface panel.

Explosion is not a threat in the engine compartment since it is vented and separated from the crew compartment. Therefore speed of detection becomes secondary to ruggedness. Optical detection is not successful in engine compartments as the optical sensors quickly become dirty and viewing angles are very limited due to the cramped nature of the compartment. The HTL system therefore utilises a loop of thermal sensing wire. The flexible wire allows routeing to all the potential fire hazards (for example fuel lines, disc brakes, fuel cell and floor of engine compartment). The wire is not affected by dirt, fuel, hydraulic fluid or water. Again, two levels of warning are provided. If a small slow growth fire occurs, the crew is warned with AFES allowing them the options of extinguishing the, fire with hand-held extinguishers or discharging the system extinguishers by guarded switch on the crew interface panel. If a large vehicle-threatening fire occurs, the system automatically extinguishes it.

The same fast-reacting rapid discharge Valve and Bottle Assemblies that are used in the crew compartment are also used for the engine compartment to simplify logistics. They are piped into the engine compartment into a distribution system which discharges the Halon 1301 extinguishing agents so that the fire threat areas are covered and fires are extinguished. Extensive testing by the US Army has demonstrated this system's ability to detect and extinguish fires, even with the engine running at maximum speed and the cooling fans operating, with no damage to the engine. The engine does not stall as a result of the fire being extinguished, thus assuring mobility during and after a fire.

The HTL Engine Compartment AFES also has extensive BITE and requires no separate test equipment to verify its performance. As with the Crew Compartment AFES, each time vehicle power is switched on, a complete BITE test sequence is initiated. The control electronics are tested to verify fire signals and can be received and processed. The solenoid drive circuit is tested to verify it can actuate a valve. The thermal wire monitors the pressure in the fire extinguishers, the continuity of the electrical cable and the thermal wire loop for electrical short circuits. Again the crew knows if they have an Engine Compartment AFES before they start their mission. If the system has a problem, a fault light and an LED are illuminated isolating the malfunctioning module(s). The system also has a second shot feature to provide protection in the event of a second fire.

The second reason the crew and engine systems are discrete subsystems is, if one is damaged, the other is not affected and remainsioperational.

Lastly, the HTL AFES includes a manual actuator system. This is a mechanical leverage device which allows simultaneous discharge of the engine and crew compartment extinguishers from an external handle. In the event a fire is observed in an unattended vehicle and electrical power is off to the AFES, this handle can be pulled, extinguishing the fire. Connecting this handle to both subsystems eliminates the need to determine where the fire has started.

Key system components include:

This optical detector is a Type 1 Discriminating OFSA meeting MIL-S-62546A. The OFSA employs three optical sensors (two photodiodes and one thermopile) filtered to near and far Infra-Red (IR) and visible light wavelengths which allow the patented fire detection logic to sense the start of explosive fires quickly while discriminating against false alarm sources and penetration where no explosion and fire follow. The HTL OFSA provides two separate levels of fire warnings, small and large. It has optical BITE which verifies not only its electrical operation but also whether the sensor lenses are clean and can see a fire.

This unit, which is qualified to MIL-M-62546A, can monitor up to four OFSAs and discharge automatically up to four- fire extinguishers in a 2x2 two-shot configuration. It has a complete BITE capability which verifies the unit's ability to monitor OFSAs, process fire signals and perform the fire extinguisher's selection logic and the ability of its solenoid drive circuit to actuate the fire extinguisher valves. This unit also provides BITE monitoring and test sequence data and fault isolation data to the operator interface panel. Another model of this unit, which has been qualified on the M992 FAASV to the requirements of ATPD-2082, is also available which can monitor up to four OFSAs and discharge automatically up to six fire extinguishers in 3x2 two-shot configuration for larger compartmentalised vehicles.

Module, Standard Control Electronics (MSCE)

This unit, which is qualified to MIL-M-62546A, can monitor up to four OFSAs and discharge automatically up to four- fire extinguishers in a 2x2 two-shot configuration. It has a complete BITE capability which verifies the unit's ability to monitor OFSAs, process fire signals and perform the fire extinguisher's selection logic and the ability of its solenoid drive circuit to actuate the fire extinguisher valves. This unit also provides BITE monitoring and test sequence data and fault isolation data to the operator interface panel. Another model of this unit, which has been qualified on the M992 FAASV to the requirements of ATPD-2082, is also available which can monitor up to four OFSAs and discharge automatically up to six fire extinguishers in 3x2 two-shot configuration for larger compartmentalised vehicles.

This is the crew interface panel for the Crew Compartment AFES. Units are available for two shots, two extinguishers per shot systems and two shots, three extinguishers per shot systems. Both versions have been qualified on the M992 FAASV to the requirements of ATPD-2082. The panel provides indicators for fire, system fault, BITE test sequence pass and power status. Fault isolation indicators are provided for each major module. A BITE test initiate and lamp test switch is provided. A guarded manual discharge switch is included. There is also a screwdriver-actuated power switch for vehicle maintenance. The unit has a timer which senses the turn-off of vehicle power and keeps the system operational for 3 hours during this critical period.

The HTL valve and bottle assembly or fire extinguisher has been qualified to MIL-V-62547A. It is available in three capacities. It features a solenoid-actuated valve that opens in milliseconds quickly discharging its Halon 1301 fire extinguishing agent which has been superpressurised with dry nitrogen to speed discharge time.

This wire senses increases in heat caused by fires or engine overheating. Two levels of warning are provided. One for small fires or overheat which causes a fire warning indicator to flash on the crew interface panel or an audible warning signal can also be provided. The larger fire warning automatically discharges the Valve and Bottle Assembly extinguishing the fire.

This unit incorporates the features of the crew compartment MSCE and CCTAP into a single unit. The unit monitors the thermal detection loop and actuates the valve solenoid when a large fire occurs. It has the same type of BITE and fault isolations as the Crew Compartment AFES presenting an identical interface to the vehicle crew as that system. It has been qualified on the M992 FAASV to ATPD-2082. A similar version (with the mnnunciatonand control functions remoted to the driver panel) is qualified and in service on all South Korean K1 MBTs.