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Development of a Gas-Generator Propellant for a Rescue System for Submarines based on the Energetic Binder GAP
Dr. Peter Jacob BAYERN-CHEMIE / PROTAC P. O. Box 1131 D-84544 Aschau/Inn GERMANY
SUMMARY Rescue systems for submarines currently use hydrazine which catalytically decomposes into hydrogen, nitrogen, ammoniac and water. These gases displace water out of the submarine’s bow and aft ballast tanks in order to surface the submarine by the generated buoyant force.
The use of hydrazine got under criticism because of its toxic and carcinogenic properties. An additional risk is induced by the potential formation of an explosive gas-mixture due to the presence of hydrogen. Both led to the requirement for an alternative gas generating system to be installed in new submarines and eventually to replace existing hydrazine systems.
The consequent objective was the development of a new solid propellant which generates a high amount of non flammable gases, being insoluble or slightly soluble in water and environmentally clean combustion products only. As a matter of principle only the gases nitrogen and carbon dioxide came into consideration. The energetic binder GAP was a candidate fuel for development because of its high amount of nitrogen (app. 42 %), oxygen and also its availability. The oxidizer strontium nitrate was selected because of its high density, good oxygen balance and the ability to form a stable slag which remains more or less in the gas generator. Additionally, because of the size of the gas generator, a castable propellant was preferred.
The development of the propellant started with thermodynamic calculations in order to obtain an oxygen- balanced formulation and to determine the combustion temperature and combustion products. Based on these results, the propellant was developed on a laboratory scale in order to study the processibility and the influence of particle-size distribution of the oxidizer on the burning rate and pressure exponent. The influence of the curing agent and the equivalence ratio on the mechanical properties were studied. The propellant was also investigated in terms of compatibility and aging behavior.
Today, the patented propellant and the gas generator are fully qualified by the German authorities WIWEB and WTD 91. The system passed all examinations successfully. The industrialisation took place in 2000 and 2001. The start of serial production for German customer (BWB) started in 2001. The new rescue system will be integrated into the U 212 submarines, currently built for the German and other European Navies.
INTRODUCTION Rescue Systems for Submarines currently use hydrazine which catalytically decomposes into hydrogen, nitrogen, ammoniac and vaporous water. Besides the toxic properties of hydrazine, there is the theoretical possibility of an ignition of the exhaust gases due to the potential formation of oxyhydrogen gas. This resulted in the requirement to replace the existing “RESUS” system by an Inert Gas-generator
Paper presented at the RTO AVT Specialists’ Meeting on “Advances in Rocket Performance Life and Disposal”, held in Aalborg, Denmark, 23-26 September 2002, and published in RTO-MP-091.
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(INGA) with equivalent properties in respect of yield of gas, volume, weight, service life, operating reliability and shock resistance against water bombs. Additionally the combustion products must not be toxic or flammable.
The study of an alternative system using a solid propellant started with a consideration of all possible gases which can be created chemically during gas generator burning. In order to get a high efficiency of the system, the generated gaseous products should be insoluble or only slightly soluble in water and exhibit a small molecular weight. Nitrogen, carbon monoxide, hydrogen, methane and oxygen are possible candidates. The noble gases helium, neon and argon can not produced chemically and were not considered further.
Due to their small molecular weight carbon monoxide, hydrogen and methane yield in high volume even at small weight percentage. In order to avoid those combustible gases, the propellant formulation has to be oxygen-balanced. Oxygen is produced only by use of excess oxidizer. Herewith a decrease of burning temperature is achievable but oxygen also had to be avoided because of its corrosive effects on the boat structure.
Each combustion of organic material unavoidably generates the components carbon dioxide and water. In fact those gases contribute only little to the displacement of water out of the ballast tanks. According to thermodynamic calculations carbon dioxide contributes due to slow rate of dissolving and diffusion into the water significantly (about 50 %).
One original design goal of cool burning (1500 K) could not be fulfilled because the gas generator propellant has to be stoichiometrically balanced. In order to cool down the hot gases after the nozzle, water is mixed into the gas stream by an ejector.
REQUIREMENTS FOR THE PROPELLANT
Combustion Products • High amount of gases insoluble or slightly soluble in water • The amount of flammable gases must be out of the explosion limits at all time • All combustion products have to be environmentally clean and non-toxic
Ballistic Properties • The rate of burning shall be between 5 and 7 mm/s at 100 bar • The operational pressure shall be between 70 and 130 bar • The pressure exponent shall be below 0.7 • The gas generator must burn stable even at high pressures
Mechanical Properties • The mechanical properties between –10 °C and +40 °C should guarantee a safe burning of the gas generator even after an water-bomb attack at the submarine (400 g / < 5 msec)
Service Life • The service life shall at least 10 years (without additional life extension programs)
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Environment • The gas generator should not produce any environmentally harmful or toxic combustion products. • The propellant formulation should not contain toxic components.
Safety • The storage classification must not be 1.1 • The grain must not have detonative properties.
Refurbishment • The refurbishment after the service life should be possible and easy to perform.
ASSESSMENT OF NITROGEN RICH FUELS The following table summarizes the main properties of nitrogen rich fuels. The main focus lies on the nitrogen-content, processibility, burning temperature, burning rate, environmental properties, availability and price.
Table1: Candidate Fuels (Nitrogen Rich)
Formula Name N2-Content Properties Manufacturing [%] Process toxic raw material, NaN3 Sodiumazid 64,6 high burning rate, press only N2, cold burning, solid
NH2 NIGU 53,8 cheap raw material, press HN C low burning rate, NHNO cold burning, solid 2 H expensive raw material, N 5-ATZ 82,3 moderate burning, press H N 2 C N moderate temperature, hygroscopic, N N solid teratogene NH NH2 TAGN 58,7 Produces much H2, expensive, press H2N N C * HNO3
NH NH not in large scale production, 2 explosive, solid N N N N C N N C N N expensive, N N − − GZT 78,8 press ⊕⊕ H H H H not in large scale production, N N
C C solid H2N NH2 H2N NH2 toxic, produces much H2, – H2N-NH2 Hydrazene 87,4 liquid
H expensive raw material, CH2 C O GAP 42,0 cast produces much N2, CH N 2 3 in serial production, liquid n
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All the listed solid fuels (sodium azid, Nigu, 5-ATZ, TAGN and GZT) show an excellent amount of nitrogen and a relatively low burning temperature. But in order to manufacture a grain of about 150 kg the pressing technology appears inadequate. Nevertheless first tests with the systems Nigu/strontium nitrate, Nigu / strontium nitrate /ammonium nitrate, Nigu / ammonium nitrate, Nigu/ strontium nitrate / titanium dioxide and 5-ATZ/ammonium nitrate were performed. The burning behavior was tested by strands/Crawford. As a result, all those mixtures show a burning rate between 5 and 12 mm/sec at 100 bar with a pressure exponent between 0.75 and 1.15. Furthermore the ignition is very difficult. Therefore it became very quickly obvious that the energetic binder GAP was the only promising alternative. In terms of processibility, GAP allows to manufacture a castable propellant with the perspective to fulfil the requirements described above.
ASSESSMENT OF OXIDIZERS In order to get an oxygen balanced propellant it is necessary to manufacture a mixture with a high content of solid oxidizer. It is also useful to select an oxidizer with a high nitrogen content and without chlorine for environmental aspects. It has further to be considered that different to HTBP-binders, GAP can not be filled with a high amount of solid oxidizer.
Ammonium nitrate has both a high nitrogen and oxygen content. Drawbacks are low performance, low burning rates and phase transitions that influence the propellant properties. Furthermore this material is very hygroscopic.
AP generates hydrochloride and exhibits a very high flame temperature while KPC has no nitrogen at all.
Since the N2-content and the O2-content of potassium nitrate and strontium nitrate are similar, it was decided to use strontium nitrate, because of its higher density allowing to achieve a processibility during the mixing process. According to thermodynamic calculations strontium nitrate (SrO) forms a stable slag with a very high melting point which remains to a large extent in the gas generator.
Table 2: Candidate Oxidizers
Formula Name N2-Content O2-Content Density [%] [%] [kg/m³]
NH4NO3 AN 35,0 60,0 1725
NH4ClO4 AP 11,9 54,5 1950
KClO4 KPC 0,0 46,2 2520
KNO3 Potassium nitrate 13,9 47,5 2109
Sr(NO3)2 Strontium nitrate 13,2 45,4 2986
THERMODYNAMIC ASSESSMENT With the propellant formulation according to Table 3 thermodynamic calculations have been performed. The calculations were based on the assumption that the investigated system is in thermodynamic equilibrium. Kinetic factors like rate of burning, catalytic effects etc. had to be investigated experimentally. In order to avoid the presence of oxygen, carbon monoxide, hydrogen and methane the mixture ration oxidizer/fuel has to be 1.0.
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The theoretical properties and the reaction products can be seen in Table 4 and Figure 1 respectively. The gas generator propellant theoretically produces 38.1 % slag in form of strontium oxide. The chemical analysis of the slag remaining in the gas generator after firing shows a considerable amount of strontium carbonate which lowers the practical total amount of gaseous reaction products because of the consumption of carbon dioxide. Downstream the nozzle or in contact with water strontium oxide reacts completely to strontium hydroxide. The gas yield (without condensed water) is 315 l gas/kg propellant or 725 l gas/l propellant. Figure 1 shows a small amount of carbon monoxide. But there is no risk of afterburning because the concentration of this gas is far below the explosion limit between 12.5 and 75.0 vol-%.
Table 3: Propellant Formulation
GAP / Isocyanate 21.7 % Strontium nitrate 77.9 % Stabilizer / Additives 0.4 %
Table 4: Theoretical Properties
Flame Temperature (Chamber) TC = 2780 K
Specific Impulse (100 : 1; Equilibrium) Isp = 1850 Ns/kg Characteristic Velocity c* = 1063 m/s Mean Molecular Weight MW = 49.7 g/Mol
Reaction Products (100 :1)
Chamber Nozzle 40
35
30
25
20
Weight % 15
10
5
0 CO2 (g) H2O (g) N2 (g) CO (g) O2 (g) Sr(OH)2 (g) SrO (s) Chamber 28,849 10,267 18,687 2,174 0,895 1,824 36,563 Nozzle 31,927 10,846 18,863 0,215 0,001 0,002 38,140
Figure 1: Reaction Products (Chamber and Nozzle).
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BALLISTIC PROPERTIES The rates of burning at various pressures and temperatures were tested by means of small scale motors (2 inch) with a graphite nozzle. Most of those tests were performed between 50 and 120 bar.
As a result - according to Figure 2 – the burning rate could not significantly be influenced by the particle size distribution of the oxidizer. With a particle size variation between 15 µm and 200 µm there was only a variation in burning rate of approximately 10 %. Therefore the particle size distribution could be optimized in terms of processibility and mechanical properties. In order to test the behavior of the gas generator at high pressures (> 200 bar) for safety reasons small scale motors and prototypes have been fired. The pressure exponent rises but as a self-regulating effect the nozzle erosion becomes dominant. In other words: the higher the pressure rises, the higher is the erosion of the graphite nozzle. The internal- ballistic modelling of the gas-generator namely the configuration of the bore had to take this result into account.
Small Scale Motors at Ambient Temperature
10.0
GAN 018 Strontiumnitrate 15 µ / 200 µ = 50:50 9 GAN 024 Strontiumnitrate 15 µ / 200 µ = 70:30 GAN 025 Strontiumnitrate 15 µ / 200 µ = 80:20 GAN 026 Strontiumnitrate 15 µ / 200 µ = 60:40 GAN 017 Strontiumnitrate 15 µ = 100 % 8 GAN 017 Strontiumnitrate 30 µ = 100 %
7 ) s
r (mm/ 6
5
4 50.0 60.0 70.0 80.0 90.0 100.0 120.0 140.0 pressure [bar]
Figure 2: Results of the Small Scale Motor Firing.
Figure 3 shows the result of small scale motor firings of the selected propellant at –10, +20 and +40 °C between 60 and 160 bar.
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Small Scale Motors
11 - 10 °C 10.0 + 20 °C + 40 °C
9
8 ] s / m [m 7 te a R g n i n
r 6 u B
5
4 50.0 60.0 70.0 80.0 90.0 100.0 120.0 140.0 160.0 180.0 200.0 Pressure [bar]
Figure 3: Small Scale Motor Firings of the Selected Propellant.
THERMAL AND MECHANICAL PROPERTIES The mechanical properties between –10 °C and +40 °C should guarantee a safe burning of the gas generator even after an water-bomb attack at the submarine (400 g / < 5 msec). Therefore the design-goal was to develop a propellant with good mechanical properties within the operational temperature range. In order to be able to absorb the shock wave a high elongation of the propellant is necessary.
With additional use of IPDI the highest elongation at –20 °C (up to 80 %) was achieved for the propellant. Surprisingly the elongation decreases down to 30 % at elevated temperatures. The strength (uniaxial measuring) of the propellant was not affected negatively in a significant way. As mentioned before the bimodal particle size distribution of the oxidizer contributes as well to the good elongation.
The first technical approach was to cure the propellant with the commercially available hardener Desmodur N 100. The result was a very hard grain with poor elongation (Figure 4) at all temperatures.
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Elongation
with Desmodur N 100 with Desmodur N 100 / IPDI
60 ) % ( n o i t 40 nga o l E
20
0 -10 °C +20 °C +50 °C Temperatur
Figure 4: Elongation of Different Propellants.
Strentgth
2.5 with Desmodur N 100 with Desmodur N 100 / IPDI
2.0 m²] m / 1.5 (N t h g t n re t
S 1.0
0.5
0.0 -10 °C +20 °C +50 °C Temperature
Figure 5: Strength of the Propellant.
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Table 5: Thermal and Mechanical Properties
Density at 20 °C g/cm³ 2.29 Hardness Shore A 54 Heat of Explosion J/g 3700 –10 °C 20 °C 50 °C Young’s Modulus N/mm² 9 5.4 4.6 Max. Strength N/mm² 2.4 1.2 0.9 Elongation at max. strength % 72 40 28 Elongation at break % 78 41 28 Thermal expansion ppm/K 50 60 75 Heat conductivity W/mK – 0.53 –
Table 6: Safety Aspects
Drop hammer test 2 kpm with 5 kg weight Friction sensitivity > 36 kg Ignition temperature > 250 °C Electrostatic sensitivity 0,8 J ≡ 40 kV (no ignition)
CHEMICAL COMPATIBILITY WITH INSULATION The gas generator consists of an insulated (EPDM) steel motor with a case bonded grain. Therefore a compatibility test between the propellant and the insulation had to be performed.
Test Method The determination of the chemical compatibility is described in TL 1376-600 (Arbeitsvorschrift für die chemische und die physikalische Untersuchung an Treibladungspulvern) and was modified by ICT in order to avoid the handling with mercury. In contrast to the original method the weight loss is measured instead of the volume of the separated gas.
The goal of this test method is to determine the weight loss of the two materials in direct contact and without contact after 40 h at 100 °C. Three testing tubes were used to perform the whole test procedure. The first one has been filled with 2.5 g propellant, the second with 2.5 g contact material (EPDM) and the third one with a mixture (2.5 + 2.5 g) of propellant and EPDM.
Calculation of the Reactivity G + G G + G R = G − 1,1 1,2 + 2,1 2,2 mg M 2 2
R Reactivity [mg] of a mixture of 2.5 g propellant and 2.5 g contact material G1,2 Weight loss [mg] of propellant sample 1 G1,2 Weight loss [mg] of propellant sample 2 G2,1 Weight loss [mg] of contact material sample 1 G2,1 Weight loss [mg] of contact material sample 2 GM Highest weight loss [mg] of three parallel samples of the mixture of 2.5 g propellant and 2.5 g contact material
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Acceptance Criterion and Result Two materials are chemically compatible if the reactivity R is smaller or equal 5 mg. With a result of R = 0.4 mg the combination propellant and insulation is judged to be compatible.
AGEING CHARACTERISTICS During the development phase the stability and ageing behavior of the propellant was investigated on laboratory scale. The weight loss after 210 h at 80, 90 and 105 °C was determined as well as the energy content. At each temperature we found a weight loss between 0.04 and 0.07 % (Figure 6). The energy content decreased after 210 h at 105 °C by only 1 %. More interesting for composite propellants is the change in mechanical properties due to post-curing effects or binder degradation. After 3 months at 62 °C we observed only a slight decrease in elongation at –10 °C from 70 % to 60 % (Figure 7), from 30 % to 25 % at ambient temperature and from 20 % to 18 % at 50 °C. As expected Young’s modulus rose significantly due to post curing effects.
The main concern in the beginning of the program was the resistance to water bomb attacks at the end of the service life (10 years). Therefore two of the qualification motors underwent the following procedure: • Seven temperature cycles between 30 °C and 63 °C according to MIL-STD-810E 501.3 • Storage 72 h at –40 °C according to MIL-STD-810E 502.3 • Ageing 3.5 months at 62 °C • Vibration and transport according to MIL-STD-810E 514.4 • Water bomb shock according to BV 430 • Firing at ambient temperature
After each load and before firing, the motors have been x-rayed without findings due to cracks or debondings between insulation and propellant. After this program the performance of the motor has not changed in terms of burning time, maximum pressure and thrust.
Weight Loss at 80, 90 and 105 °C
0.10
0.08 ]
% 0.06 [ s s Lo ht g i e 0.04 W
0.02
0.00 0 24 48 72 96 120 144 168 192 216 240 Time [h]
Figure 6: Weight Loss during Ageing.
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Youngs Modulus Elongation 12 80
11 70 10
9 60
8 50 7 ] ² m 6 40 [%] /m [N 5 30 4
3 20
2 10 1
0 0 0123456 0123456 Ageing [month] Ageing [month]
Figure 7: Mechanical Properties after 3 Months at 62 °C.
Figure 8: Water Bomb Test with an INGA Gas-Generator.
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SUBMARINE U 212 AND GAS GENERATOR The rescue system INGA 212 using the propellant described in the preceding paragraphs, was designed for the new German (HDW) submarine U 212 by Astrium. The gas generators are located in a upright position in the front and aft section of the boat. The hot exhaust products are cooled down by an ejector which mixes water into the plume after the nozzle. This mixture blows directly into the water in the ballast tanks against a heat deflector. Because the inner ballistic properties depend on the firing position (vertical or horizontal) the motor has to be adapted when new requirements due to another application arise. Depending on the diving depth the gas generators are ignited according to a predefined program sequentially to surface the submarine.
Figure 9: U 212.
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The gas generator consists of a insulated steel motor case with a case-bonded propellant with a mass of about 160 kg and a graphite nozzle. The inner ballistic is controlled by a star configuration of the bore. The head ends are inhibited with a special polyurethane adapted to the Gap-binder. The propellant is ignited by a safety- and-arming unit. For the case of an unexpected pressure rise after a damage a second nozzle at the bottom opens in order to guarantee a controlled burning.
Figure 10: Gas Generator.
ACKNOWLEDGEMENT This effort was supported by the German Ministry of Defense (BWB). The system engineering was covered by Astrium Bremen and Trauen in cooperation with ARGE U212 Kiel. Bayern Chemie was in charge of the propellant and igniter development. The system was qualified by WIWEB Heimerzheim, WTD 91 Meppen and WTD 71 Eckernförde.
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SYMPOSIA DISCUSSION – PAPER NO: 8
Discusser's Name: Ahmet Gocmez Question: What kind of materials do you use as a liner and insulation?
Author’s Name: Peer Jacob Author’s Response: The propellant is bonded on the EPDM- insulation by a bond promoter. Between the substrates, there is no liner.
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