ADVANCED PROPELLANTIADDITIVE DEVELOPMENT FOR 

FIRE SUPPRESSING GAS GENERATORS: HYBRID SYSTEMS 

Stephen Fallis and Russell Reed 

Naval Air Warfare Center Weapons Division 

Jennifer L. McCormick, Kim A. Wilson, and Gary F. Holland 

General Dynamics OTS-Aerospace 

ABSTRACT 

The “Advanced Propellant/Additive Development for Gas Generators’’ project is a collaborative effort between 

General Dynamics OTS-Aerospace (GD), Redmond, WA and the Naval Air Warfare Center-Weapons Division 

(NAWCWD), China Lake, CA. The project objective is to develop highly efficient, environmentally acceptable, 

chemically active fire suppressant capabilities based upon solid propellant gas generators, and to improve the understanding of propellant and additive effectiveness in fire suppression through testing. This paper reports on two 

segments: (I) development ofcool burning propellant formulations and (2) fire testing of chemically active agents 

within the Solid Propellant Gas Generator (SPGG) and Hybrid Fire Extinguisher (HFE) configurations. Propellant 

formulations in development contain 5-aminotetrazole and the new high nitrogen compound BTATZ (C,H,N,,). 

These fuels are being refined to reduce overall combustion temperatures while maintaining ballistic robustness. 

Fire testing conducted using chemically active agents (potassium iodide and potassium carbonate) demonstrated a 

>50% improvement in effectiveness when tested parallel to inerting systems. Similar performances were also observed between SPGG and HFE gas generator devices on a total inerting agent load basis. Fire testing was conducted at the GD facility in the Fire Test Fixture (FTF) against a controlled JP-8 fire. This paper summarizes the results 

to date of propellant formulation development, chemically active agent performance in a tire scenario, and HFE 

versus SPGG effectiveness. 

INTRODUCTION 

A proven approach to nonhalon fire suppression is based upon technology used in automobile airbag 

devices, whereby the fire suppression agent is a mixture of inert gases stored not in pressurized bottles 

but in the form of solid propellants [I, 2,3]. Upon combustion in an SPGG, the solid propellant rapidly 

produces large quantities of inert gases such as nitrogen, carbon dioxide, and water vapor; these gases can 

be directed into a fire-containing volume at quantities sufficient to suppress combustion. The compact 

nature of the SPGG device makes it a volumetrically efficient means for chemically “storing” gaseous 

agents in a solid form. 

SPGGs can also be used in conjunction with alternative fire suppression fluids such as fluorocarbons or 

water, in what is generally called an HFE. In an HFE configuration, the gas produced by the SPGG 

(a) pressurizes and (b) mixes with the “hybrid fluid.” The SPGG therefore eliminates the need to store a 

pressurized fluid, thereby decreasing volumetric requirements, and also provides a means for more rapid 

distribution of the agent. As an added benefit, the hybrid fluid cools the exhaust of the SPGG. 

The radical scavenging capability of Halon 1301 makes it a much more efficient fire suppressant than 

most of its replacement candidates, which tend to be chemically inactive, physically acting suppressants. 

Recent work has characterized the nature of different chemically active species, including halogen, and 

metallic and non-metallic species, in the fire suppression process [4]. Several workers point to the synergy that derives from combining chemical activity with physical (cooling, dilution) suppression processes [5]. Earlier work by GDMAWC [l] has demonstrated the improvements possible by incorporating chemically active precursors into the SPGG event. This present work describes the benefits achievable by incorporating the chemically active additive directly into the propellant and/or hybrid fluid. 

SPGG and HFE approaches to fire suppression have been developed and demonstrated in full-scale testing by GD 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 [I]. 

364 Halon Options Technical Working Conference 24-26 April 2001 
In this project, the GD/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 

PROPELLANT DEVELOPMENT: COOLER FORMULATIONS 

Initial efforts on the GDMAWC-WD, NGP effort have considered propellant modifications that can be 

readily compared with the GD FS01-40 chemically inert solid propellant formulation. The formulations 

studied on the NGP program have thus far attempted to both reduce combustion temperatures-by 

increased levels of nitrogen generation and coolant addition-and incorporate additives to provide some 

amount of chemical activity to the propellant exhaust. Propellant formulations incorporating the new 

high nitrogen compound BTATZ (C,H,N,,), while structurally similar to the 5-aminotetrazole (SAT) fuel 

used in the GD FS01-40 propellant (Figure I), appear to provide increased means for reducing propellant 

combustion temperatures. 

Figure 1. GD Fire Test Fixture (FTF). 

The preparation of BTATZ has progressed to the 1 Ib scale. On this scale the material is more difficult to 

purify in that extensive solvent extraction is required to remove the small amount of impurities present. 

This may not be significant in the long run as > 97% purity is easily achieved without extensive purification (Le., 97% pure material maybe sufficient). The final purity of BTATZ is generally > 99% on smaller 

scale experiments and is also achievable on the 1 Ib scale with extra purification. Our synthesis of 

BTATZ so far has used intermediates provided by Los Alamos National Laboratory (LANL). These 

materials have been exhausted and will now have to be synthesized in-house. Work on the 3 gal. scale 

has proven successful. The recent acquisition of a 50 gal. reactor will facilitate project scale-up to such 

an extent that the synthesis of intermediates need only be carried out periodically. 

Further safety data on BTATZ itself has been obtained. BTATZ shows acceptable friction and impact 

sensitivity but is somewhat sensitive to electrostatic initiation. When formulated into a molding powder 

with poly(ethy1 acrylate) (3% as a binder), electrostatic sensitivity is still a concern, even when 0.5% 

carbon black is added; however, when pressed into pellets or deposited as a thin layer, the material meets 

the criteria set for routine handling of energetics. 

Initial ballistic characterization of material prepared at NAWCWD indicates similar performance to the 

BTATZ prepared at LANL. A number of formulations are currently under ballistic evaluation. NAWCWD has made progress on bum rate measurements for formulations based on the new high nitrogen compound BTATZ(C,H,N,,). The results are presented in Table 1. 

Halon Options Technical Working Conference 24-26 April 2001 365 
TABLE I. BTATZ-PROPELLANT PROPERTIES. 

Formulation 5-AT BTATZ' Oxidizer 0 psi 500 psi 1000 psi 1750 psi 2500 psi n 

1 97 0.25 1.75 2.14 2.64 3.62 0.51 

2 48 48 0.18 0.55 1.18 1.66 I .4 

3 24 72 1.57 1.78 2.63 3.44 0.7Ib 

4 43 43 IO(KCI0,) - 0.34 1.31 2.07 2.46 

5 43 43 IO(KN0,) - 0.14 0.61 1.03 2.00 1.6 

6d - 86 IO(KN0,) 0.18 1.29 2.15 2.4612.62 3.03 0.55 

7d - 86 lO(KCI0,) 0.16 1.12 1.80 2.3812.66 3.11 0.57 

a The balance remaining in each formulation is made up of binder, opacifier, and process aid. 

E Non-linear: approximately 0.5 (1750-2500 psi), decreasing at lower pressures. 

Linear over 1000 - 2750 psi; n is lower below IOOOpsi. 

Measurements at I500 and 2000 psi. 

Over the range 1750-2500 psi, the pressure exponent for formulation 4 is 0.5 and decreasing. This formulation, a promising candidate as the bum rate, is relatively high at low pressures and has a decreasing 

pressure dependence of the bum rate. Formulations 6 and 7 also exhibit ideal ballistics for gas generator 

applications with pressure exponents around 0.5 and with relatively high bum rates. Future work 

includes suppression testing with Formulations 4-6 and modifications thereof. 

ADDITIVE DEVELOPMENT: ACTIVE COMPOSITIONS 

SPGG technology enables chemically active agents to be stored and generated synergistically with inerting or cooling agents. Combustion of the solid propellant produces inert or cooled gases; the chemically 

active agent is liberated and entrained in the discharge. HFE technology introduces additional capabilities, whereby chemically active agents may be integrated into the gas generator solid propellant or hybrid 

liquid. Chemically active additives tested in the course of this project include various alkali halides (e.g., 

KBr, KI), alkali carbonates (e&, K,CO,) and polyhalogenated aromatics (pentabromophenyl ether). 

Three different approaches were evaluated for incorporation of chemically active additives into suppressant exhaust: (I) blend the additive (or a precursor) directly into the propellant for SPGG delivery; (2) use 

a chemically active propellant formulation with an inert hybrid fluid for HFE delivery; and (3) blend the 

additive (or a precursor) directly into the hybrid fluid for HFE delivery. Delivery approaches 1-3 were 

assessed by testing on a single common test platform using as hybrid fluid HFC-227. 

FIRE EFFECTIVENESS TESTING: TEST FIXTURE 

Fire suppression effectiveness testing was undertaken using a mid-scale FTF, developed by GD to simulate typical military aircraft fire scenarios (e.g., 141). Gas generator fire suppression devices were used 

to deliver both inert and active agents into the fire. Operational parameters for the GD FTF are given in 

Table 2; the fixture is illustrated in Figure 2. The test fixture operates with an air to fuel mass ratio of 

about 31, giving the fire an intensity of about 1 MW. This was derived by using a fuel heating value of 

46.4 MJkg for P-8 fuel and a fuel flow rate of about 0.03 Ibm/sec (15 gkec). 

Instrumentation was installed in the test fixture to monitor and control test variables, and to make sure 

they stay in similar ranges from test to test. The gas generator devices were fired and the agent discharged after a steady-state fire was established. A waiting period of 30 sec between JP-8 fire ignition and gas 

generator device firing was kept to ensure steady-state, repeatable test conditions. The gas generator 

device was located in the forward chamber of the FTF (upstream of the fire). The gas generator device 

was mounted on an arm in the middle ofthe airflow, but shielded from the fire zone by a baffle. Figure 2 

illustrates the gas generator device placement in the FTF. All GPGG and HFE discharge times were 

maintained at -200 ms for ease of comparison. 

366 Halon Options Technical Working Conference 24-26 April 2001 
TABLE 2. FIRE TEST FIXTURE PARAMETERS. 

Airflow 

454 gls 1.0 Ibmls 

385 LIS 13.6 ft'ls 

762 cmls 

Mass flow rate 

Volumetric flow rate 

Linear Fflow rate (in pipe) 

Fuel JP-8 

25 tils 

Fuel Flow 

Mass flow rate 15 gls 0.033 lbmls 

Volumetric flow rate I9 mlls 0.005 gal/s Stoichiometrv 

Air-fuel ratio (m-a,,/m-fu,, ) 31 

Equivalence ratio 0.50 

Fire Zone Dimensions 

Flame temperature 1000 K 1300 "F 

Intensity 700 kW 700 kW 

Length 122 cm 4ii 

Cross-sectional area 3700 cm2 4 ft' 

Residence time 1.2 s 1.2 s 

Volume 450 L I6 A' 

Injection interval -100-200 ms -100-200 ms 

tY1 4 \, 

I:, i 

,..~~~ ......... ~ ..................... ~~ ...... Rsmols co2 chi ; ,E",E4 fiffgy 

I ssg 

. ~ .......... ~ ......... ~ ............ ....... 

J- 

-i: 

Figure 2. FTF test facility schematic. 

The FTF test facility consists of the following major subsystems: 

Main test chamber 

Airsupply 

Fuel supply 

Ignition system 

Suppression device 

CO, emergency extinguishing 

Control and data acquisition 

Fuel Supply lowled in cen 

chamber in low ti on^ show 

attached atetch (nozzle 

assemblies are hidden lmm 

direct airflow by bame) 

Halon Options 'Technical Working Conference 24-26 April 2001 367 
FIRE EFFECTIVENESS TESTING: GAS GENERATOR AND 

HYBRID FIRE SUPPRESSION DEVICES 

The gas generator devices used in this testing consist of (1) a Solid Propellant Gas Generator (SPGG) 

(Figure 3), and (2) a Hybrid Fire Extinguisher (HFE) (Figure 4). SPGG technology rapidly produces 

large quantities of inert gases such as nitrogen, carbon dioxide, and water vapor to suppress combustion. 

HFE technology combines a solid propellant gas generator with a fire suppressing fluid, e.g., HFC-227, to 

provide fire suppression capabilities similar to the SPGG. Chemically active additives were incorporated 

into the solid propellant for both the SPGG and HFE devices and also into the hybrid fluid for the HFE. 

Figure 3. SPGG. Figure 4. HFE. 

Chemically inert and active agents were tested in the FTF to evaluate their effectiveness in fire suppression. The GD FSOI-40 propellant generates an inert blend of COz, N2, and H,O when functioned. Two 

active agents were evaluated, as incorporated into the FS01-40; these agents were KI and K,CO,. HFC227 was used as the hybrid fluid during all of the tests conducted with HFE devices. Chemically active 

agents were incorporated into the HFE devices by using either chemically active propellant formulations 

to pressurize and disperse the HFC-227, or inert FS01-40 to pressurize and disperse a mixture of active 

agent powder mixed in the hybrid fluid HFC-227. The two active agents mixed with HFC-227 during 

WE testing were KHCO, and NaHCO,. Agent effectiveness was evaluated in terms of several measures, 

including total agent mass required for flame extinction and a normalized FSN value (=agent threshold 

loading/(threshold load for GD’s FS01-40 SPGG)). The flow-rate adjusted mass fraction of agent, 6. was 

also used; this is defined [6] as 

where m-agent and mOair are the agent and air mass flow rates respectively. The critical mole fraction Xc 

was obtained from 6 by adjusting for (inert) agent molecular weight, 

FIRE EFFECTIVENESS TESTING: SPGG FIRE TEST RESULTS 

Chemically active agents were vaporized during propellant combustion and delivered into the fire zone 

by high-temperature exhaust gases from a solid propellant gas generator producing a blend of CO,, N,, 

and H,O. The results of SPGG testing are summarized in Table 3; the values are based on multiple tests 

368 Halon Options Technical Working Conference 24-26 April 2001 
TABLE 3. SPGG FTF DATA SUMMARY. 

Agent FS01-40 GD-02 GD-04 

- 

Active KI K*CO, additive - 

Gas fraction 50% SO% 50% 

MW, glmole 30 30 30 

Mole active (K)/IOOg 0 0.127 0.145 

GG Load 347 I57 I os 

Discharge time, ms 200 200 200 

Discharge mass, g 173.5 78.5 52.5 

Mole active (K) discharged 0 0.199 0.152 

m*(agent), g/s 868 393 263 

FSN(FS0 1-40) I 0.452 0.303 

Beta 0.656 0.464 0.366 

- Critical mole fraction 0.649 0.455 0.359 ~ 

and represent the threshold amount of agent needed to extinguish the fire. Typically, three tests were 

conducted at the threshold amount, and three additional tests were conducted at an agent load greater than 

the threshold amount. The threshold is defined as the amount of agent needed to extinguish the fire at 

least two out of three times. 

The test results indicate that K,CO, and KI yield significantly improved suppression effectiveness when 

compared to the inert propellant compositions. For a composition containing 0.1 moles KI per 100 grams 

propellant, the threshold propellant load was - 45% that of FS01-40, with similar discharge conditions 

and mass flow rates. A composition containing a similar amount of K,CO, (0.1 moles K per 100 grams 

propellant), resulted in threshold loads of 0.3~ FS01-40. 

FIRE EFFECTIVENESS TESTING: HFE FIRE TEST RESULTS 

The results of HFE testing are summarized in Table 4, which lists threshold amounts. The threshold is 

defined as the agent load needed to extinguish the fire two out of three times. FTF testing with inert and 

chemically active hybrid (HFE) configurations mirror the enhanced efficiency seen in the SPGGs. 

TABLE 4. HFE FTF DATA SUMMARY. 

Agent FS01-401 GD-021 GD-041 FS01-40/HFC-227 

HFC-227 HFC-227 HFC-227 KHCO, 

Active ~ KHCO, additive KI K&O, 

Gas fraction 95% 95% 95% 95% 

MW, glmol 170 I70 170 170 

Mole active (K)/100g 0 0.127 0.145 0.145 

GG Load 358 228 228 265 

Mole (K) discharged 0 0.040 0.046 0.052 

m-(agent), s/s 1701 1083 1083 1260 

Beta 0.789 0.705 0.705 0.735 

FSN(FSO1-40) I .960 1.248 1.248 1.452 

Critical mole fraction 0.430 0.324 0.324 0.321 

The test results again indicate that KI and K,CO, yield significantly improved suppression effectiveness 

when compared to the inert propellant compositions. Results also indicate that an active additive in the 

hybrid fluid yields significantly improved suppression effectiveness. 

Discharge mass, g 340.1 2 16.6 216.6 252 

Discharge time, ms 200 200 200 200 

m-(air), g/s 454 454 454 454 

~ - 

llalon Options Technical Working Conference 24-26 April 2001 369 
DISCUSSION 

Propellant formulations incorporating the new high nitrogen compound BTATZ (C,H,N,,), while structurally similar to the 5-aminotetrazole (5AT) fuel used in the GD FS01-40 propellant, appear to provide 

increased means for reducing propellant combustion temperatures. The preparation of BTATZ has progressed to the 1 Ib scale. This increased production capability plus the attractive ballistic (burn rate, 

pressure sensitivity) of BTATZ formulations make them ideal candidates for future work, including reformulation with additional chemical coolants as well as SPGG suppression testing. 

Figure 5 summarizes the test results and shows how the test configurations compare on a total agent 

weight basis and also on a K-molar basis. FTF testing with various inert and chemically active solid 

propellant compositions demonstrates that incorporation of 0.1 mole % additive into inert fire suppressants can have a dramatic effect upon suppression efficiency. The otherwise similar propellant compositions examined during this testing indicated a 50-70% reduction (by mass) of agent loading for suppression of identical fires when the propellant composition contained as little as 0.1 mole % (expressed as a 

fraction of the gaseous output). On an equimolar basis, potassium carbonate appears to be a more effective chemical additive than potassium iodide. 

FSOIAOSFGG IJo2SpGG UMSPGG FSOUOW 3302W 11Mmbne FSOIAOWCO3 

w 

Figure 5. Total agent weight needed to extinguish fire. 

The greater effectiveness of potassium carbonate is consistent with previous testing, both on the present 

test fixture as well as turbulent spray burner testing [l]. One possible explanation for this trend includes 

more fascile vaporization of the chemically relevant potassium species from the K,CO,. Although this is 

inconsistent with the higher melting point of K,CO, vs KI (891 "C vs 681 "C), respectively, it may be 

consistent at higher temperatures where K,CO, decomposes before boiling whereas KI melts without 

decomposition at 1330 "C. Another possibility is that there is an antagonistic interaction behveen the 

halogen and alkali metal species in the flame region. Similar findings have been reported by Linteris et 

al. [7] for premixed flames with ferrocenehon pentacarbonyl species in the presence of fluorocarbons. 

On a mass-to-mass basis, the inert HFE and SPGG systems appear to provide similar suppression protection, requiring - 360 grams to suppress the 1 MW GD FTF fire. The higher molar efficiency of the hybrid testing reflects the higher molecular weight of the HFC-227. As in the SPGG case, incorporation of 

chemically active species into the propellant improves the suppression efficiency for the HFE configura370 Halon Options Technical Working Conference 24-26 April 2001 
tion. Testing conducted with KHCO, added to the hybrid fluid ofthe HFE system produced results similar (based on moles of K) to those with active agents (KI, K,CO,) added to the propellant, which indicates 

that active additives in the hybrid fluid may be just as effective as active additives in the propellant. 

The improvement found for the HFE systems appears to be less than that found in the SPGGs, i.e., an 

approximate 30% reduction in suppressant mass. Note, however, that the propellant is 100% of the total 

agent weight for the SPGG configuration, but only 14% ofthe total agent weight for the HFE configuration, thus limiting the molar fraction of chemically active additive to - 2 mole %. It is likely that ifthe 

amount of active additive in the HFE systems were optimized, these systems would show a weight reduction similar to the SPGG systems. 

The flow-rate adjusted mass fraction of agent, !3, for these tests is higher than reported in other reports of 

baffle-stabilized flames [6, 81. However, those reports show that pas well as the critical mole fraction 

increases at shorter discharge times. This is consistent with the need for high agent concentrations in the 

free stream in order to achieve sufficient mixing of the agent into the flame recirculation zone and thereby achieve concentrations sufficient to effect extinction. Future work will address the correlation of discharge times and mass flow rate effects. 

SUMMARY 

Propellant formulations incorporating the new high nitrogen compound BTATZ (C,,H,N,,) appear to 

provide increased means for reducing propellant combustion temperatures. The preparation of BTATZ 

has progressed to the 1 Ib scale. This increased production capability plus the attractive ballistic (burn 

rate, pressure sensitivity) of BTATZ formulations make them ideal candidates for future work, including 

re-formulati011 with additional chemical coolants as well as SPGG suppression testing. Testing of propellant compositions containing potassium iodide and potassium carbonate as chemically active additives 

demonstrated enhanced effectiveness in fire scenarios as compared to chemically “inert” compositions. 

These new propellant formulations are lighter in weight thaii Halon 1301 for comparable fire suppression 

effectiveness, and the hazards associated with halon’s ozone depletion capability are not present. 

FTF testing with various inert and chemically active solid propellant compositions demonstrates that 

incorporation of 0.1 mole % additive into inert fire suppressants can have a dramatic effect upon suppression efficiency. The otherwise similar propellant compositions examined during this testing indicated 

a 50-70% reduction (by mass) of agent loading for suppression. On an equimolar basis, potassium carbonate appears to be a more effective chemical additive than potassium iodide. The greater effectiveness of 

potassium carbonate (vs. potassium iodide) is not yet well understood, but may be related to more fascile 

vaporization of the carbonate-based species after melting, or to an antagonistic interaction between the 

halogen and alkali metal species in the flame region. 

On a mass-to-mass basis, the inert HFE and SPGG systems appear to provide similar suppression protection. Incorporation of chemically active species into the HFE propellant improves the suppression efficiency for the HFE configuration. Testing conducted with additives incorporated into the hybrid fluid of 

the HFE system produced results similar to those achieved with active agents added into the propellant. 

This indicates that active additives in the hybrid fluid may be just as effective as active additives in the 

propellant. 

ACKNOWLEDGMENTS 

We thank our colleagues at General Dynamics (P.H. Wierenga, G. Gregg, C. Anderson, and R. Paxton) 

and China Lake (J. Twesme, Dr. B. Zentner, G. Meyers, Dr. T.P. Parr, R. Stalnaker, J. Hittner, P. Curran, 

A.1 Atwood, and T. Bui) for their technical support. We also gratefully acknowledge SERDP’s Next 

Halon Option5 Technical Working Conference 24-26 April 2001 311 
Generation Fire Suppression Technology Program (NGP) and Naval Air Combat Survivability Program 

for their financial support of this joint project. 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

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372 Halon Options Technical Working Conference 24-26 April 2001 

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