ADVANCED PROPELLANTiADDITIVE DEVELOPMENT 

FOR FIRE SUPPRESSING GAS GENERATORS 

Stephen Fallis and Russell Reed 

Naval Air Warfare Center 

Yeu-Cherng (Frank) Lu, Paul H. Wierenga, and Gary F. Holland 

Primex Aerospace Company 

INTRODUCTION 

Most replacement candidates for Halon 1301 are tluorocarbon agents like HFC-I25 (C~HFS) and 

HFC-227 (or FM-200, C3HF7) [I]. The absence of heavy halogen atoms (e.& CI, Br, or I) make 

these agents primarily physically acting suppression agents, functioning through cooling and 

dilution effects [2]. As such, these replacement agents are much less efficient that Halon 1301 

and so require greater concentrations to attain the same effectiveness. 

A proven approach to nonlialon fire suppression is based upon technology similar to that used in 

automobile airbag devices. In this approach, the fire suppression agent is a mixture of inert 

gases, which are stored not in pressure bottles but in the form of solid propellants 13, 41. On 

combustion in a solid propellant gas generator (SPGG), the solid propellant produces large 

quantities of inert gases such as nitrogen, carbon dioxide, and water vapor. The compact nature 

of the SPGG device makes it a remarkably efficient means for chemically storing gaseous agents 

in a solid form. Early developments of this approach were pioneered at NAWC-China Lake in 

1986 IS] and later refined [6] and demonstrated in full-scale testing by PAC in a variety of 

vehicle platforms, including military aircraft drybays and engine bays, military land vehicle 

engine and crew compartments, and commercial automobile engine compartments [4]. 

The current state ofthe art SPGG fire suppression system-PAC FS01-4Gis competitive in 

size with Halon 1301 systems on both mass and volume bases. It is clean, effective, and fast 

acting, environmentally rugged and reliable, nontoxic, and SNAP approved. Current products 

are based on commercial automotive airbag technology, thereby providing for a smooth transition 

from military to commercial applications. 

A class of “superagents” has long been known, which exhibit fire suppression effectiveness 

much greater than Halon 1301 [7]. Although the foremost example of these agents is iron 

pentacarbonyl, Fe(CO)5, other examples include chromyl chloride, Cr02C12, and tetraethyl lead, 

Pb(CzWs)d, as well as powdered materials such as K3Fe(CN)h. While effective, these superagents 

are highly toxic, and so are not used in general suppression applications. However, much insight 

into the mechanism of chemically active fire suppression has been gleaned by a study of tlic 

chemical kinetics associated with the action of these superagents upon hydrocarbon combustion 

[8, 91. For instance, chemical kinetic modeling of “superagent” suppressants indicates that the 

Fe(C0)s-based scavengers. or iron oxide species, such as FeO and FeOH, are more effective than 

halogens (e.g., I., Br*) in flame suppression, bccause they are niorc easily regenerated. The key 

to the effectiveness of iron pentacarbonyl is the gas phase iron-oxo chemistry. 

Next-generation SPGG fire suppression systems will capitalize upon the current technology and 

further enhance performance by using chemically active additives such as found in “superagent” 

sysems. SPGG technology provides an extremely flexible agent platform whereby one can 

readily incorporate additives that can be converted into the high-performance metal-oxo species 
of the superagents-without the associated toxicity risk [IO]. These next generation systems are 

expected to provide a means to reduce agent loads by 2-5x compared to current baselines. 

In this NGP project, the PAC/NAWC team has developed propellant compositions that provide 

significant improvements in fire protection performance. These developments have resulted in 

cooler, more efficient and more compact fire suppression agent systems. These accomplishments 

are made possible by taking advantage of the great flexibility of solid propellant technology. 

RESULTS 

Initial efforts on the PAChVAWCWD, China Lake NGP effort have considered propellant modifications that can be readily compared with the PAC FS01-40 chemically inert solid propellant 

formulation. The formulations studied on the NGP program have thus far attempted to reduce 

combustion temperatures by (I) increasing levels of nitrogen generation and coolant addition, 

and (2) incorporating additives to provide some amount of chemical activity to the propellant 

exhaust. 

COOLED PROPELLANT FORMULATIONS 

Improved cooling and more efficient agent generation have been accomplished via variations in 

the overall solid propellant composition. Different combinations of fuels and oxidizers have 

been modeled and tested; these results are summarized in Table 1. Several high-nitrogen fuels 

were considered (Figure I). These additives increased the production of the inerting agent 

nitrogen. Several compositions have been designed to deliver agent at a very rapid rate; this rate 

can be tailored by the incorporation of endothermically acting chemical coolants. 

One reduced combustion temperature formulation is based upon 5-amino tetrazole (5-AT) and 

potassium perchlorate (KP), with an elastomeric binder added to facilitate pressing of pellets and 

increase abrasion resistance. This formulation bums well at 1000 psi (BRIO00 = 0.2 ids), but is 

somewhat sensitive to combustion pressure (n=0.8); this sensitivity limits commercial use of the 

formulation. 

A new high nitrogen compound BTATZ (C~H~NM) (courtesy of Mike Hiskey, Los Alamos 

National Laboratory) is currently under evaluation. Initial results indicate that this compound 

burns well at low pressures, and the bum rate has a low dependence on pressure (slope = 0.2, to 

be confirmed). Impact, friction, and electrostatic sensitivity data have been obtained. We have 

included BTATZ in several formulations in combination with 5-AT in an attempt to increase 

efficiency of combustion while keeping oxidizer content to a minimum. A composition (with 

10% KP has a very low dependence of bum rate on pressure, with calculated combustion temperature approximately 1700 K (2600 OF), and gas production efficiencies yield over 4 mol gas/ 

1 OOg, nearly 60% of which is N2. 

Other high nitrogen compounds [l 11, e.g., GAZT, BT, GBT and TAGN, were tested in formulations and in thermochemical calculations. At this level of added oxidizer and with no coolants, 

all of the formulations exhibited high combustion temperatures and very high bum rates. The 

fact that the bum rates were so high indicates that at a lower level of oxidizer (or by addition of 

362 Halon Option.; Technical Working Conference 2-4 May Zoo0 
TABLE I. COOL, HIGH NITROGEN DEVELOPMENTAL PROPELLANT 

COMPOSITIONS. 

Composition Current SOA 5AT SAT/BTATZ( I j 5AT/BTATZ(2) 

Exhaust species Nz 45 N2 57. I N2 66.4 N2 61.4 

(VIA%, @ T,)) eo> 35 HZ 35.4 HZ 32.1 H2 30.2 

HzO 20 co 7.5 co x.5 

KCl(s) 5.3g KCl(s) 5.3g 

Tex, K (OF) IO00 ( 1400) x74 ( I I 13) 9OX ( I 175) 967 (12x1) 

Gas Output, mol/ 2 (5) 4.41 (7.41) 4.14 (6.X7) 4.01 (6.X6) 

1OOg (mo1/100cc) 

Pressure exponent, ti 0.5 0.80 I .6 (500-1OOOpsi) 0.7 (10()0-2750 psi) 

BRiow. ids 0.5 0.17 0.55 1.31 

GAZT BT GBT TAGN 

Exhaust species Nz 37.6 N2 44.6 Nz 42.9 N2 28.3 

(~01% @ T,) HrO 28.8 HrO 15.0 H20 28.8 H20 40.6 

COz 15.5 eo* 25.8 coz 20. I CO2 16.4 

co 11.4 co 6.6 co 12.8 02 11.5 

KCl(s) 32.6g KCl(s) 33.6g KCl(s) 33.1s KCl(s) 5.3g 

Tex, K (OF) 1436(2125) 1761 (2711) I457 (2163) 1421 (2098) 

Gas Outpul, mol/ 2.5 I (4.99) 2.13 (4.28) 2.19 (4.40) 2.44 (4.86) 

100g (mol/lOOcc) 

Pressure exponent ND ND ND ND 

BRiu~n~, ink >2.5 >2.5 >2.5 >2.5 

an oxidizer that promotes cooler burning) combustion will be efficient in terms of nitrogen 

generation as well as burning cooler. 

CHEMICALLY ACTIVE ADDITIVES 

Fire suppression effectiveness of SPGG systems was also increased by incorporation of additives 

capable of interfering with and terminating radical processes associated with hydrocarbon combustion. Because the operational conditions inside a SPGG are similar to those in a fire, radicaltrapping intermediates can be generated inside the GG and delivered with the inert gas coolant to 

the fire zone. Some additives tested in the course of this project include various alkali halides 

(e.g., KBr, KI). alkali carbonates (e.g., K2COI) and polyhalogenated aromatics (pentabromophenyl ether). The physical properties of several chemically active suppressant coinpositions are 

summarized in Table 2. 

Two different approaches were evaluated for incorporation of potassium into propellant exhaust: 

one begins with potassium nitrate, the other with potassium carbonate. Both formulations resulted in slightly reduced gaseous agent output when compared to an inert baseline composition, but 

had comparable ballistic characteristics. Likewise, iron oxide containing formulations exhibited 

viable rates of agent generalion. 

IMon Option\ Technical Worhinf Cmlerencc ?-4 May 20lHl 363 
Guanidinium 

Bitetrazole 

(GBT) 

Triaminoguanidinim 

Nitrate(TAGN) 

Bisguanidinium 

Bitetrazole(BT) 

Bis(aminotetrazoly1)tetrazine 

(BTATZ) 

Figure 1. High-nitrogen fuels used in CLPAC propellant development. 

Propellant compositions were formulated both with and without pentalbromophenyl ether, added 

as a chemically active flame inhibitor, and underwent preliminary ballistic characterization. 

Larger pellets of the chemically active formulation were burned in a motor and the temperature 

of the gaseous products measured. The outlet (nozzle) temperature was approximately 675 TC, 

which was in good agreement with the temperature predicted from thermochemical calculations. 

The chemically active formulation was also found to be impact, friction, and electrostatically 

insensitive. Suppression effectiveness was demonstrated by directing the exhaust from propellant combusted in an Adam Motor into a 2 ft3 vented box, where it successfully extinguished a 

JP-8 pool fire. UV camera data indicated that no HZ burned during the discharge event. 

SPGG AND FIRE EFFECTIVENESS TESTING 

Fire suppression effectiveness testing was undertaken using a mid-scale Fire Test Fixture (FTF), 

developed by PAC and representative of typical military aircraft fire scenarios (e.g., [12]). 

Operational parameters for the PAC FTF are given in Table 3, with a drawing of the fixture in 

Figure 3. 

364 Halon Options Technical Working Conference 2-4 May 2000 
TABLE 2. PROPERTIES OF CHEMICALLY ACTIVE DEVELOPMENTAL 

PROPELLANT COMPOSITIONS. 

Composition 

Exhaust 

species 

(~015% @ T,, 

or g/ I OOg 

propellant) 

T,,, K (OF) 

Gas output, 

mol/ I OOg 

(moI/l00cc) 

Pressure 

exponent 

BRI(HIO, inls 

Current SOA 

NZ 45 

co2 35 

H2O 20 

0.5 

0.5 

0807-Mod KN 

N2 44. I 

cor 35.2 

HrO 20.7 

(KzCOj 9,Og) 

(0. I3 mol K) 

I 102 ( 1524) 

1.61 

0807-Mod K-Carh 0807-ModD SATIDECABr 

N2 42.6 Nz 44 

eo2 37.6 COr 36 

H2O l9.Y H2O 20 

(KzCO? 9.Og) Fez03 5: 

(0.13 mol K) (0.06 mol Fe) 

1098(1517) 

I .8X (4.87) 

1206 ( I7 1 I ) 

I .78 (4.09 

N2 56.1 

H2 34.0 

co 7.8 

KCI(\) 2.0: 

KBr(5) 5.4g 

874 ( I I 13) 

4.24 (7.29) 

0.66 0.59 0.62 I .o 

0.4 I 0.52 0.64 0. I 

TABLE 3. SUMMARY OF TEST FIXTURE PARAMETERS, PAC FIRE TEST FIXTURE. 

Airflow Rate 

Mass flow rate, ma, g/s (Ihmis) 

Volumetric Flow Rate, L/s (ft3/s) 

Linear Flow Rate (in pipe), cm/s (ft/s) 

Fuel Flow Rate 

Fuel 

Mass flow rate, ma, g/s (Ibmis) 

Volumetric flow rate, mL/s (gal/s) 

Stoichiometry 

m*,,dm*,,,,i 

Equivalence Ratio 

Fire Zone Dimensions 

T, K (OF) 

Intensity, kW 

Length, cm 

Cross-Sectional Area, anZ 

Volume, L (ft') 

Residence time, s 

lniection Interval. ins 

384 (0.85) 

316(11) 

670 (22) 

JP-X 

20.23 (0.0446) 

25 (0.0661) 

19 

I200 ( 1700) 

IO00 

I80 

3700 

670 (24) 

0.27 

up to 8000 
Fuel Spray, 

Figure 3. PAC Fire Test Fixture. 

The fixture is located in a concrete reinforced test cell. The floor is painted with fuel-resistant, 

non-absorbing material. The test facility consists of several major subsystems including: 

Main Test Chamber 

Air Supply 

Fuel Supply 

Ignition 

Suppressant Discharge 

C02 Emergency Extinguishing 

Control and Data Acquisition 

NGP FIRE TEST CONFIGURATION 

For tests conducted under this program, the test fixture was usually operated at an air to fuel 

mass ratio of about 19. The fuel flow rate was about 0.04 Ibm/sec (or 20 gm/sec). Using a fuel 

heating value of 46.4 MJ/kg, the fire intensity was about 1.0 MW. 

Selected chemical agents were tested in the Fire Test Fixture to evaluate their effectiveness in 

fire suppression for the controlled JP-8 fire. The agents were loaded inside the discharge section 

of a neutral-burning, solid-propellant gas generator (SPGG) (Figure 4). The exhaust of SPGG 

consisted mainly of COz, Nz, and HzO. During SPGG discharge, the chemical agents were supposed to be vaporized and delivered into the fire zone by the high-pressure and high-temperature 

exhaust gases. Fine-grid metallic meshes and porous plates were used downstream of the agents 

to enhance the mixing between SPGG exhaust and agents prior to entering the distribution tube. 

The distribution tube was split into two lines, one enters the test fixture, and the other discharges 

directly to ambient (Figure 4). By varying the split ratio between these two lines, the amount of 

agent entering the fire zone can be controlled. In the tests, the split ratio was fixed to be 50%, 

and the initial agent quantity in the SPGG was varied. Thus, the objective of the tests was to 

366 Halon Options Technical Working Conference 2-4 May 2000 
Agent exhaust 

-e, il 

Figure 4. PAC Solid Propellant Gas Generator Test Unit. 

determine the amount of initial agent required to put the JP-8 fire out. The relative efficiency of 

different agents was then ranked based upon their threshold quantity. 

Various instrumentation was installed in the SPGG and test fixture to monitor and control test 

variables, and to make sure they are in the similar ranges from test to test. The SPGG was fired 

and the agent discharged after a steady-state fire was established. A waiting period of 45 sec 

between JP-8 fire on and SPGG firing was kept to assure steady-state, repeatable test condilions. 

Some of the identified chemical agents, including KI, KBr, KzCO?, FeZ03. Fe (oxalate), ferrocene and pcntabromophenyl ether. werc tested in a fire test fixture to evaluate their effectiveness 

in fire suppression for a controlled JP-8 fire. Table 4 summarizes the results of this testing. The 

agents were vaporized and delivered into the fire zone by high-temperature exhaust gases produced by a neutral-burning solid-propellant gas generator producing a blend of COz, Nz, and 

HzO. 

TABLE 4. SUMMARY OF NGP FTF DATA. 

Neat wt. Test Result 

Agent (g) No. 

KI 20 027-06 Fire not out 

40 027-07 Fire not out 

40 027-05 Fire out 

60 027-04 Fire out 

KBr 40 032-02 Fire not out 

60 032-03 Fire not out 

60 032-0 I Fire not out 

KKOI 20 03.5-03 Fire not out 

40 035-02r Fire out 

60 03.5-0 I Fire mtt 

Neut Wt. Te\t Rewlt 

Agent (g) No. 

Fez03 40 03X-01 Fire not out 

80 03X-02 Fire not out 

Ferrocene 40 O3Y-(I I Fire not out 

80 03Y-02r Fire not out 

Iron Oxalate 40 040-01 Fire not out 

PBPE 60 041-01 Fire not out 

Hillon Uptiom Technical Working Contcrcnce 2-4 May 2000 367 
The test results indicated that K2C03 and KI are more effective than KBr in suppressing JP-8 fire 

in the PAC Fire Test Fixture. The threshold amount for successful fire extinguishment seemed 

to be around 40 g of neat K2CO3 or KI powder used in each SPGG. As evidenced by some tests 

using agents with more distinctive colors, the powder seemed to be ejected out of the SPGG 

throughout the entire test duration of about 7.5 to 8 sec. Testing with ferrocene resulted in 

growth of needles of ferrocene crystals on the exit port of the delivery tube. 

DISCUSSION 

The cooler, high gas output propellant compositions described in Table 1 represent an important 

step towards increased efficiency SPGG fire suppression devices. Gas temperatures were reduced in some cases by 1G20% when compared to the current baseline. The findings of increased 

burn rate compositions, while maintaining relatively low gas temperatures, provide a means for 

further increases in agent cooling when these compositions are further modified with endothermic chemical coolants. Compositions incorporating combustion inhibitors were demonstrated to 

bum efficiently and were shown to be effective in fire suppression. It is interesting to note that 

the incorporation of species to suppress fires does not prevent combustion of the propellant. 

Since the key to superagent effectiveness, e.g., iron pentacarbonyl, is the gas phase iron-oxo 

chemistry [8,9], we considered developing alternative means to generating gas-phase or fine 

particulate iron oxide or other metallo-oxo species. This route then provides a means of delivering the chemical activity of Fe(CO)5 without the associated toxicity. The routes explored to 

date, involving different means for potassium-oxo and iron-oxo containing discharges, have 

yielded ballistic characteristics that are amenable to further study. 

With the test results available, it seemed that the iron-based agents and PBPE are less effective 

than KI and KzCO,, at least within the PAC Fire Test Fixture environment. The similarity in 

performance between the iodide and the carbonate is similar to that observed in turbulent spray 

burner testing at PAC; these data were interpreted as reflective of the greater role of the potassium ion in suppression effectiveness rather than the anion. 

Assuming a uniform agent discharge, the agent to propellant weight ratio is about 4.3% for 

threshold KI quantity. This is significantly lower than the 21 % of KI in one of PAC's active 

propellants. Since the PAC KI-based active propellant has already been proven to be very 

effective in suppressing fires in some large-scale testing, the SPGG and propellant design could 

be further optimized to reduce weight and volume. 

The lesser performance of the iron and pentdbromo species seems to conflict with observations in 

some of the earlier literature. Several possible causes might contribute to this discrepancy. The 

residence time-and/or time at temperature for the given particle size of agent-may not be long 

enough for the agents to break down. Inadequate time at temperature is supported by the observation of ferrocene crystal growth at the end of the delivery tube, which implies sufficient stability of ferrocene in air at - 1000 "C for 0.3 s. Together, this indicates that the adequate agent 

delivery mechanism needs to be carefully designed and implemented in the solid-propellant fire 

extinguisher to utilize fully the chemical capabilities of such agents. 

Although we have tried to keep all of the test variables as consistent as possible from test to test, 

the nature of this type of tests has shown certain degree of poor-repeatability in the test results. 

This non-repeatability is representative of the large-scale fire tests in which PAC has participated 

368 Halon Options Technical Working Conference 2-4 May 2ooO 
in the past. It is also possible that the poor repeatability in some of the tests was due to the fact 

that the agent quantity used was in the threshold range. The threshold quantity may not be a 

sharp line separating the regions between fire out and fire not out. It might be a band accommo 

dating variations in test variables. 

As a caution, ferrocene and its derivatives have been used in propellants as burn rate modifiers 

with great success; however. these compounds tend to migrate to the surface of the propellant 

where they are oxidized to ferrocinium compounds. Ferrocinium derivatives in combination 

with propellant ingredients give friction sensitive materials.“ This would tend to limit the use of 

I‘erroccne derivatives to hybrid systems where the propellant and additive were stored separately. 

SUMMARY 

Scveral key findings have been made to date, whereby cooler propellant compositions have been 

used in conjunction with chemically active additives or combustion radical scavengers. Adding 

“radical traps” to the exhaust provides means for cooling, dilution. and chemical termination of 

the combustion process, hence increasing the overall effectiveness of the fire suppre 

rent propellant formulations arc lighter in weight than Halon 1301 for comparable fire suppression effectiveness, but without the hazards associated with halon’s ozone depletion capability. 

The findings of this year’s NGP effort suggest a more in-depth examination of the role and 

livery requirements for highly active, radical-scavenging chemical additives as fire 

ts. 111 addition to “pure” solid propellant approllchcs, the use of solid propellant/liquid 

(or gaseous) agent “hybrid” technologies shall also be explored. 

Propellant development activities are planned in efl‘orts to further optimize cool-burning, highnitrogen content solid. Additionally, chemically active additives will be incorporated directly 

into propellant formulations and tested using the fire test fixture to assess effects of agent loading 

and combustion temperatures upon agent effectiveness. 

Further optimization testing and of these compositions-in terms of agent temperatures, additive 

identity and loading+an he expected to result in substantial reductions in agent requirements. 

A system utilizing high-output, cool gas and chemical activity can be expected to result in agent 

loads of 1/3 - I/S that of the current baseline. 

ACKNOWLEDGMENTS 

We thank our colleagues at Primex (Dr. E.J. Wucherer, C. Harvey, P. Wicks, C. Anderson and 

D. Halverson) and China Lake (G. Meyers. Dr. T.P. Parr, R. Stalnaker, J. Hittner, P. Curran, 

A.I. Atwood) for their technical support. We also gratefully acknowledge SERDP’s NextGeneration Fire Suppression Technology Program (NGP) and Naval Air Combat Survivability 

Program for their financial support of this joint project. 

~~~ ~ ~ 

‘k R. Reed, personal communication. 19Y9. 

Halon Options Technical Warking Conlerenuc 2-4 May 2000 369 
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