Coolant trucks, better known as "coolers" among MechWarriors, station themselves close to the battlefield, where they can be hooked up to heated 'Mechs for the purpose of flushing away the excess heat with super-cold liquid nitrogen.
         The coolant truck was first developed during the Reunification War, when the Star League sought to extend its control over worlds in the Periphery.   The need for a battlefield device able to provide quick cooling for overheating 'Mechs became especially apparent during this period because so many of the Periphery worlds were water-poor.   Though the Star League 'Mechs were superior to the Periphery troops, Star League lost several battles before the development of coolant trucks simply because the Periphery armies were able to exhaust the Star League 'Mechs with unrelenting attacks.
         The first coolant trucks appeared on the battlefield in 2588, participating in the siege of Cerberus, a world in the Outworlds Alliance.   As the world was a desert world, the coolant trucks more than doubled the range of the League 'Mechs.   Since that time, coolant trucks have been an important part of many 'Mech armies.

         The Class 135-K coolant Truck was built for the Draconis Combine by Buda Imperial Vehicles, a small company on the planet Buda.   It differs from the coolant trucks used by other Houses only in its choice of weapons and its coolant capacity.
         Code-named "The Lifesaver", the 135-K carries nine tons of coolant, (seven tons of liquid nitrogen, and two tons of liquid oxygen) in armored tanks kept cold with small recirculation motors.   The motors also stir the tank's contents to prevent dangerous temperature gradients that might rupture the tank.
         A 'Mech has two uses for liquid nitrogen.   The first is as a coolant for its reactors.   In most 'Mechs, a twin-jacketed fusion core is insulated by a surrounding reservoir of liquid nitrogen. This nitrogen is kept in circulation to keep the heat of the reactor within performance norms.   As the 'Mech exerts more energy, its heat rises, which causes the liquid nitrogen to expand to dangerous pressures.   Sensing this rise in pressure, the control system will vent a portion of the nitrogen into the air until the pressure falls.   If a pilot is forced to do this several times, his 'Mech is also losing precious coolant in the process.   To prevent a shutdown or explosion the MechWarrior of a severely overheating 'Mech will retire from the battle and move to the coolers standing by. Once attached to the truck the lukewarm nitrogen is flushed from the 'Mechs reservoir and replaced with cold nitrogen.  
         A BattleMech also uses nitrogen to cool other of its parts.   By diverting a portion of the liquid through tubes called "arteries", the liquid is spread throughout the 'Mechs structure.   In this way, it cools other hot spots that may occur at the various actuators in the missile system, and so forth.
         Coolant Trucks also carry liquid oxygen, which is used to replenish the small jump jet tanks.   The pure oxygen is used to supplement the normal air passing over the hot reactor, creating the jet propulsion.   Liquid oxygen can also be pumped through the 'Mechs artery system to cool burning joints.   However as oxygen is more volatile than nitrogen it is used for coolant only if the coolers nitrogen supply is exhausted.
         The Lifesaver coolant truck has the added feature of two turret-mounted flamers.   Though seldom used, they are functional weapons.   Because of a coolers limitless supply of coolant and liquid oxygen to make the fuel burn more furiously, the gunner on a coolant truck can fire at will without the normal worries about heat buildup.   If approached by a 'Mech either in flames or on the verge of exploding, the trucks two flamers are quickly converted.   Now, instead of liquid oxygen igniting fuel, the flamers spew liquid nitrogen onto the wounded 'Mech, dousing the flames and cooling the 'Mech off in a cloud of super-cool mist.   Noting this ingenious use of flamers the Federated Suns has removed the usual lasers from the turrets of their coolant trucks and replaced them with similar convertible flamers.
         Most coolers have slightly more armor than might be expected for a non-combat vehicle. The protection has been added because a single laser bolt or hot piece of flying metal would explode the trucks volatile cargo.
         The Lifesaver has five receptacles for the long rubber/plastic hoses used for attachment to overheating 'Mechs.   The first four dispense liquid nitrogen while the fifth contains liquid oxygen.   Most 'Mechs have hookup points for just one hose, but heavy 'Mechs require two for cooling.

         As coolant trucks are attached to regimental maintenance, they are under the control of the Master Tech.   A single, well-maintained regiment will usually have about six coolers.   Unless the Master Tech feels one battalion is going to face a particularly rough time, the trucks are usually divided into groups of two for each battalion.   During battle, one will be actively cooling while the other is refilling its tanks at a depot to the rear.   In this way an overheating 'Mech will almost always have access to a ready cooler.
         Slowly following an advance, coolant trucks and their six man crews sometimes face enemy fire.   Teams consist of a driver, a gunner, and four hose-men.   Assignment to a hose team is not particularly popular, yet the better teams do take a certain pride in their work.   Often MechWarriors call them "ice men" for their stoical behavior in carrying out their work as the battle swirls around them.
         Coolant trucks are valued as prizes.   If a cooler and its team are captured intact, they will usually be offered a chance to work for their captors.   Mercenary units are always on the lookout for seasoned cooler teams, and have been known to offer them a share of the units booty equal to what two Techs would earn. Failing to win over a team, the capturing side might then offer them up in exchange for some of their own captured personnel.
         'Mechs are especially vulnerable when hooked up to a cooler.   After the collapse of the Star League, however, a new battlefield etiquette evolved, which extended a special 'courtesy ' to a cooler-hooked 'Mech.   The enemy will usually ignore a 'Mech that is hooked to a cooler, as long as it does not shoot.   Once the 'Mech has disconnected and moved a few meters away from the trucks, it becomes fair game once more.

         Many types of coolant trucks exist.   Some are hovercraft and others are tracked vehicles. Most carry the two super-cold liquids, though some may carry only liquid nitrogen and leave it to other coolers to bring the liquid oxygen.
         Coolant trucks are usually armed, sometimes with a combination of weapons. Others have no weaponry. Every house is currently studying the installation of dual-purpose flamers that the Lifesaver uses so effectively.