"The past is never fully gone. It is absorbed into the present and the future. It stays to shape what we are and what we do."
Sir William Deane, Governor-General of Australia, Inaugural Vincent Lingiari Memorial Lecture, August 1996.

Fire Retardant Mixing Systems

Barry Marsden (bio)

This article is still being developed.

In 1939, Victoria was one of the first locations in the world to trial the ‘bombing’ of bush-fires using aircraft, and in 1967 the FCV made the first operational use of aerial-firebombing in Australia.

Overseas experience, and earlier local trials, had suggested that mixing water with chemicals, to create a fire retardant, made operations far more efficective. Retardants were subsequently found to be particularly useful in slowing the spread of lightning-caused fires in inaccessible terrain, thereby improving the likely success, and safety of follow-up ground crews.

Manual Mixing Operation
Early Firebombing Base
Late 1960's, location unknown
Source: B Marsden
Venturi retardant mixing on site
Source: B Marsden
Venturi retardant mixing on site
Source: B Marsden

The earliest form of mixing fire retardants by FCV personnel involved pouring the powder into a tank of water, and then mixing it by hand with a rakehoe, to create a slurry which was loaded into the aircraft. During the late 1960s fire retardants were being trialled and tested on various forest fuel types using different aircraft.

Inline Funnel/Venturi Effect (Mobile) - 1970

As a means of introducing and blending the retardant powder into the water, as it was being pumped to the aircraft storage tank, a venturi action was developed utilising an inline funnel.

The inline funnel was installed into the water delivery line from the pump to a storage tank. The funnel was installed higher than the water level in the tank to ensure that there was no flow back to the dry powder being introduced. The throat of the inline funnel would clog easily if the powder was introduced at too fast a rate; the powder had to match the water flow while being introduced.

To facilitate mixing at field locations a number of former Bedford Fire Tanker, 2700 litre tanks were equipped with an inline funnel and a 75 mm filler-pump mounted on the tank lid. As such, a ‘slip-on’ mixing unit was created that could be transferred by a tip-truck to any designated airfield. The 25 kg bags of fire retardant would be transported separately by tray-truck. Once the required number of bags of retardant had been introduced, further recirculation of the slurry, via the water-pump, assisted with the mixing process.

Obviously a water source was required at an airfield or remote mixing site.

Issues that then emerged included:

  • The manual handling of 25 kg bags of retardant
  • Retardant dust emissions; and
  • Clogging issues

Meanwhile, an alternative mixing method was developed using cement-mixing trucks. These could be used to support large-scale operations.

Internal Paddle Batch-Mixer (Fixed Base) - 1978

Retardant mixing issues led to the development of a 1,700 litre internal paddle batch-mixing unit.

Prototype 1700 litre Internal Paddle Mixer
Source: B Marsden

A water tank from a former FCV International tanker provided the means for developing an improved mixing tank. A 200mm diameter four-blade propeller, attached to an enclosed shaft, and driven by a second-hand Holden engine, provided the basis of a trial of an improved unit. An elevated hopper, with a course mesh screen installed internally (a safety requirement), was bolted to the tank lid opening, directly above the four blade paddle.

The hopper included a fold-down table to support a bag of retardant, while the powder was being poured into the tank directly above the rotating paddle, thus creating the mixing action.

The internal paddle technique proved to be a success, resulting in several more mixers being developed and installed at FCV fire-bombing bases.

FCV 1500 Litre Internal Paddle Mixer (Mobile) - 1984

A mobile unit was then developed at the Fire Protection Workshop at North Altona in 1984. The mixer could be operated by a crew of two and could prepare a 1500 litre batch of retardant in approximately 15 minutes. It was designed to be towed by a truck which carried the bags of retardant, two filler pumps, hoses and relay tanks. An independent water supply was obviously required.

1500 litre Internal Paddle Mobile Mixer
Source: B Marsden

This unit was powered by a Volkswagen industrial engine through a lever operated clutch and 1:2 reduction unit.


Capacity-1500 litre/8 bag mix
Gross weight (unladen) - 1.1 tonne
Overall length - 3,625 mm
Overall width - 1,830 mm
Overall height - 2,700 mm
Tandem 14”Holden wheels with brakes.

3000 Litre  Mixer (Fixed and Mobile) - 1998
3000 litre Fixed-Base Mixer
with Volkswagen Engine
Photo: B Marsden
3000 litre Mobile Mixer
with Isuzu Diesel Engine and Support Trailer
Photo: B Marsden

A system which utilised a dimensioned water-tank, fitted with an internal shaft-driven paddle, powered by petrol, or diesel-engine, through a lever operated clutch was then developed. The tank was filled with water to a given level to allow for expansion of the retardant slurry. Phos-Chek D75-R retardant in 25kg bags was then added to the water at the recommended ratio of 14.4 kg per 100 litres.

Once the correct quantity of retardant had been added, recirculation was necessary for approximately 10 minutes. The slurry was tested for viscosity and salt content, and then transferred direct to the aircraft, or to a storage tank.

Additional Equipment (Mobile Unit)

To support the 3,000 litre portable-mixer an ancillary, 6x4 trailer complete with 2 x 18 HP x 75 mm transfer pumps was built for situations when mixing and transfer of retardants may need to occur simultaneously.Two 9,000 litre floating collar tanks, coupling hoses, camlok couplings, valves, skates, tools and safety equipment were also deployable.

GPM Eductor Retardant Mixers (Mobile) - 2000

Due to OH&S manual handling, and dust-related issues the eductor system was introduced as an alternative to previous methods.

Prototype development occurred at North Altona nutilising a 200gpm eductor installed on a large tri-axle trailer, fitted with 2x4000 litre tanks.

Poly tanks - The tanks were fitted with a common manifold into which the eductor could equally deliver the mixed-retardant to both compartments simultaneously. The larger tank capacity allowed a Phos-bin, producing 6,700 litres of mixed retardant to be educted without interruption.

Equipment (Mobile)
Phos-bins,Tri-axle trailer c/w eductor; 2x75mm transfer pumps; ancillary hoses; 2x9,000 litre collar tanks; couplings, tools and safety equipment.

150 GPM Eductor Retardant-Mixer (Fixed Base) - 2001
Mansfield Fire Bombing Base
Source: B Marsden

In 2001 the Fire Equipment Development Centre, after reviewing the related technology in use in the United States, introduced the eductor mixer, described above, as the next generation of retardant-mixing technology. The system largely eliminated dust and manual handling issues.

The eductor-mixer was a patent-protected device capable of continuous high volume mixing. Powder was provided by the retardant manufacturer in a 907.2 kg ‘phos-bin’ pallet container. The base of the container was purpose- built to allow the powder to be aerated, and then withdrawn through openings designed into its structure.

The mixing operation took place in the eductor, where water flowing through a restriction is greatly accelerated, creating a low–pressure area (a vacuum). A separate air-blower fluidized the retardant powder within the ‘phos-bin’, which was then sucked through to the eductor-mixer.

The eductor delivered mixed-retardant via a delivery tube into a 10,000 litre poly-tank, which could then be transferred directly to an aircraft; or retained in storage for future use.
As the handling, moving and breaking of individual bags was eliminated, and the ‘in-place’ pallets were tapped by a probe without the need to move the unit during mixing. the unit not only became essentially a one-man operation, but one that was simple and clean to operate. Each ‘Phos-bin’, when blended with 6,300 litres of water, produced 6,770 litres of mixed-retardant.

Equipment (Fixed)

150 gpm Eductor-Mixer; Blower-pump; 9,000 litre floating collar tank, (optional); in-line flowmeter; hoses, couplings; 2x 18 HP x75mm transfer pumps (the latter again for when mixing and transfer of retardants may need to occur simultaneously).

All Departmental retardant bases were then gradually upgraded to the eductor-mixing system.

150 GPM Eductor Mixer (Mobile) - 2004
150 GPM Eductor Mobile Mixer
at North Altona Equipment Centre
Photo: B Marsden

A prototype mobile retardant-mixer utilising a 150 gpm eductor was developed in 2003. Following exhaustive trialling, additional mobile-mixers were assembled, culminating in an ongoing replacement program that commenced in 2004; disposing of the earlier, aging mobile internal paddle mixers.

Inline-eductors were selected as the preferred option for batch mixing long-term retardants as the technology provided a relatively dust-free environment for mixing crews.
Each unit trailer was provided fully equipped with couplings; pumps (complete with flow control technology); 2x 9000 litre collars; relay tanks; hose-skates; hoses; refractometer; and associated tools.

Retardant powder was provided via  ‘Phos-bins’ Phos-bin technology significantly reduced OH&S manual-handling and dust emission issues.


Operating water pressure 700 Kpa
Water flow 640 lpm
Operating vacuum-88

150 GPM Eductor Mixing Tanker (Mobile) - 200L
15000 litre ex. Milk Tanker
Bulk Mixing Unit with 150 GPM Eductor
Photo: B Marsden

A 15000 litre milk-tanker was converted to a retardant mixing tanker to provide bulk retardant transfer to airfields during fire emergencies. The tanker had a 150 GPM eductor mounted on top of the tank, and a larger 20 HP Honda pump to provide an increased flow of liquid. Air operated safety rails were installed above the tank, providing operator safety.

The advantage of the 15000 litre tank capacity was that it could transfer the load of slurry to an alternative site quickly, should an emergency arise, and immediately commence transferring the load to waiting aircraft.There was no lost time in waiting for the retardant to be mixed if the product was already available. Retardant could, in appropriate circumstances, be mixed prior to departure.

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