Insensitive Reduced-Smoke Propellant with Low-Cost Binder*

Transcription

1 ABSTRACT * May Lee Chan, Tri Bui and Alan D. Turner 1 Administration Circle (Code 4T4310D) Naval Air Warfare Center, Weapons Division China Lake, CA USA A family of reduced-smoke propellants, IMAD-116, was developed based on low-cost binder technology. The propellant formulation consists mainly of HTCE as the binder, and AP (70%) and AN (10%) as the oxidizers. The candidate propellants have undergone small-scale IM tests. IMAD-116 tested to be a zero card propellant according to the Naval Ordnance Laboratory card gap test results. The propellant also showed mild cookoff reaction in the variable confinement cookoff test. INTRODUCTION The development of the hydroxyl-terminated polyether (HTPE) propellant is one of the success stories of the Navy-funded Insensitive Munitions Advanced Development (IMAD) Program. The comparatively low sensitivity of this propellant to typical insensitive munitions (IM) hazardous stimuli such as shock, heat, and impact has been demonstrated in several propulsion configurations. Unfortunately, the propellant community anticipates potential problems associated with the availability and cost of the HTPE polymer. Thus, the authors have been evaluating other commercially available polymers that could possibly replace the HTPE polymer (which is a copolymer of polytetrahydrofuran and polyethylene oxide) without adverse effects to the IM qualities of the propellant. The replacement polymer under study is HTCE polymer, which is a copolymer of polytetrahydrofuran and polycaprolactone. (When HTCE is properly plasticized and incorporated in the propellants, it behaves in a way very similar to that of the HTPE polymer.) The results attained at the Naval Air Warfare Center Weapons Division (NAWCWD), China Lake, Calif., indicate that this polymer, which are similar to the HTPE polymer, can be equally effective as binder materials for IM propellants. These polymers present the dual advantage of being available commercially and at a low cost (less than $2 per pound). BACKGROUND High-performance, reduced-smoke propellants have always been important for Navy tactical applications. Conventional Class 1.3 reduced-smoke propellants, which are typically composed of ammonium perchlorate * This effort was performed under the sponsorship of the Naval Sea Systems Command Insensitive Munitions Advanced Development Program (Don Porada, NOSSA Code N-6, Cognizant Technology Manager). Paper presented at the RTO AVT Specialists Meeting on Advances in Rocket Performance Life and Disposal, held in Aalborg, Denmark, September 2002, and published in RTO-MP-091. RTO-MP

2 (AP) as the oxidizer and hydroxyl-terminated polybutadiene (HTPB) as the binder material, have reached a performance plateau. This insufficiency springs from the inherent combustion characteristics of AP, which often impart the propellant a high-pressure slope break between 2500 to 3500 psi. [1,2] To avoid this slope break region, propulsion systems designers generally devise motors that operate below psi, even though it is known that much higher performance can be achieved if the motor can operate at pressures higher than 2000 psi. Because of the recent successes in the manufacture of new composite motor cases that can sustain much higher chamber pressures, it is possible to develop a motor that can deliver increased specific impulse (Isp). The slope break experienced by the conventional booster propellants has hindered the achievement of this objective. Thus, it is highly desirable to search for a high-energy propellant material that does not exhibit this pressure slope break. Part of solving this dilemma is in understanding it. While the exact cause is unknown, Cohen [3] postulates that it is due to a change in the mechanism from diffusion flame control to solid AP flame control at the higher pressures that exist in burning propellants. [4,5] For example, burning solid AP is much more sensitive to pressure fluctuation (i.e., high slope) than materials experiencing diffusion-control-type burning. This phenomenon is especially obvious for those propellants that contain high fractions of largeparticle-size AP (over 2000 µm). As a consequence of the aforementioned evidence, in formulating propellants, developers try to minimize the use of large-size AP particles. In addition, they avoid using exotic burn rate catalysts, because some of these compounds are toxic and hazardous to the environment and exhibit poor IM qualities. Thus, the investigators have used ammonium nitrate (AN) to obtain desirable IM responses in their HTPE propellant work. [6] EXPERIMENTAL RESULTS The following sections provide information about the new IMAD-116 propellant formulations, their calculated performances, and their safety, thermal, burning, mechanical, and hazard characteristics. Propellant Formulations NAWCWD investigators have formulated new propellants, which they have designated as IMAD-116. The compositions of these propellants are provided in Table 1. Table 1: Chemical Composition of High-Energy, Reduced-Smoke Propellants Propellant Ingredient Wt % HTCE/BuNENA or TMETN (1:2) AN AP Curative and catalyst BuNENA = n-butyl-2-nitratoethyl nitramine TMETN = trimethylolethane trinitrate These propellants incorporate a low-cost HTCE polymer (a block copolymer of polycaprolactone and polytetrahydrofuran in a 1 to 2 ratio). [7] HTCE polymer, a hydroxyl-terminated diol from Solvay Interox, Inc., has an equivalent weight of 1000 and can be cured with isocyanates to form tough and rubbery compositions RTO-MP-091

3 Calculated Performances The investigators estimated the performance of the new formulations with the Propellant Evaluation Program (PEP), and the calculations indicated an Isp of 248 seconds under standard conditions (1000 exit to 14.7 psi). Figure 1 shows that increased performance can be achieved if the chamber pressure is increased from a standard operating pressure of 1000 to 8000 psi. For example, if a motor loaded with one of these new propellants is fired at a 4000-psi chamber pressure, the calculated Isp performance can be as high as 273 seconds, which is substantially greater than that for state-of-the-art propulsion systems. Notwithstanding these calculated performance results, it is crucial to understand that the expansion conditions are important when comparing Isp values. This point is necessary to comprehend the potential benefits of high-pressure operation while avoiding its pitfalls Isp (O pt imum) Isp ( Epsilo n = 10 ) BASELINE Isp ( Epsilon = 10) 0 Isp, lbf-s/lbm Pressure, psia Figure 1: Calculated Performance of IMAD-116 Propellant as a Function of Chamber Pressure. To that end, the optimum Isp increases with pressure primarily because the expansion ratio (ε) increases to fulfill the constraint of ambient pressure at the nozzle exit. For most tactical and many strategic missiles, there is a geometric envelope requirement that limits the nozzle exit diameter and, therefore, ε. Consider Figure 1 where the theoretical performance as a function of chamber pressure is presented for the IMAD-116 propellant. At 1000 psia, the optimum ε is 10.4:1 and at 5000 psia it is 35.2:1. Depending upon the missile envelope, the propellant ballistics, and the throat diameter, the relatively large ε seen in Table 2 may not be physically allowed. For example, ε greater than 25:1 may not be possible due to launcher constraints. The practical value for Isp at high pressure falls somewhere between an upper and lower bound, and this value is dependent upon various engineering factors that include propellant grain design, burning rate, motor throat diameter, and missile configuration. RTO-MP

4 Table 2: Relationship between Optimum Expansion Ratio and Chamber Pressure Chamber Pressure, psia Optimum ε 6.08:1 10.4:1 17.5:1 23.8:1.7:1 35.2:1 40.5:1 45.6:1 50.6:1 Safety and Thermal Characteristics The safety properties of IMAD-116 propellants (Table 3) are well within requirements. In addition, no known processing or handling problems exist. Table 3: Safety Properties of IMAD-116 Propellant Impact at 50% point, cm ABL lb Electrostatic Discharge, 0.25 J 17 10/10 NF 10/10 NF The IMAD-116 propellants underwent differential scanning calorimetry (DSC) and vacuum thermal stability (VTS) screening; the results indicated no thermal stability or thermal compatibility problems. The thermal properties are provided in Table 4. Two exotherm peaks were observed in DSC analyses; they represent the decomposition of AN and AP. Table 4: Thermal Properties of IMAD-116 Propellant 80 C DSC (onset/peak), C 0.14 cm 3 /g/48 hr 172/180 (AN), 304/384 (AP) Burning Rate IMAD-116 exhibited desirable combustion properties. In fact, the burning rate of this propellant seems to respond well to changes in AP particle size and to varying levels of Al 2 O 3 (Figure 2). Burning rates from 0.23 to 0.6 in/s at 1000 psia with slopes ranging from 0.57 to 0.61 can be obtained. The addition of Al 2 O RTO-MP-091

5 definitely enhances the burn rates. The investigators typically did not observe any slope break up to 8000 psi. However, a slope break at 2000 psi was observed when no Al 2 O 3 was used as the burn rate additive Burning Rate, in/s (0.60% Al2O3) IM (0.50% Al2O3) IM (0.20% Al2O3) IM (0.00% Al2O3) IM Pressure, psia Figure 2: Effect of Al 2 o 3 on the Burning Rates of IMAD-116. Mechanical Properties In early work conducted on IMAD-116, when the authors used HTCE as the polymer, they found that the strain values were drastically reduced when the propellant material was subjected to arctic cycling. The conditions for arctic cycling are as follows: 1. Cool down to F, hold for 2 days. 2. Warm to 0 F, hold for 2 days. 3. Repeat the same cycle 5 to 7 times. 4. Test the mechanical properties of the aged propellant at, -20, and 0 F. After a great deal of effort, the formulators solved the problem by replacing portions of HTCE with a polyether polymer. The optimized mechanical properties of the IMAD-116 propellants are presented in Tables 5 and 6 at two different isocyanate/hydroxyl (NCO/OH) ratios. With a proper combination of HTCE and polyether binders, the resulting propellants maintained excellent strain capabilities in a wide range of temperatures (-65 to 145 F). Strain remained ~30% after 7 arctic cycles. These results lead the authors to conclude that the possibility of binder crystallization at low temperatures can be ruled out. RTO-MP

6 Table 5: Mechanical Properties of IMAD-116* (HTCE/polyether (1:1)/TEGDN/BuNENA, NCO:OH= 1.00) Temperature, F E 0, psi σ m, psi ε m, % ε b, % σ tm, psi Initial Arctic Cycles (5 Times) 0-20 Arctic Cycles (7 Times) * Standard JANNAF specimens were used at a strain rate of 2 in/min. TEGDN = triethylene glycol dinitrate Table 6: Mechanical Properties of IMAD-116 (HTCE/polyether(1:1)/TEGDN/BuNENA, NCO:OH= 1.05) Temperature, F E 0, psi σ m, psi ε m, % ε b, % σ tm, psi Initial Arctic Cycles (5 Times) 0-20 Arctic Cycles (7 Times) * Standard JANNAF specimens were used at a strain rate of 2 in/min RTO-MP-091

7 Hazard Properties The investigators subjected the IMAD-116 formulations to the Naval Ordnance Laboratory (NOL) card gap test, [8] and the test results indicated a zero card propellant. IMAD-116 was subjected to the variable confinement cookoff test (VCCT), [9,10] at the slow cookoff heating rate of 6 F per hour. The results for the VCCT indicated a mild deflagration reaction with a material temperature at the time of cookoff of 7 F. Figure 3 shows the hardware recovered after the test the sample tube and outer sleeve were recovered and no dent was found on the witness plate. Figure 3: Results of IMAD-116 VCCT. SUMMARY AND CONCLUSIONS A new family of reduced-smoke propellants was formulated to take advantage of low-cost binder material. These propellants exhibited excellent mechanical properties to meet the stringent temperature requirements of the tactical environment. In addition, they have highly desirable combustion characteristics and mild IM response. Higher delivered specific impulse, fast response, and greater maneuverability are compelling reasons for applying these propellants to higher-pressure operations. Engineering constraints still exist in firing motors at high pressure (i.e., 4000 to 5000 psi); however, the recent advances in composite case manufacture, which increase case strength and reduce weight, make high-pressure operation of these propellants much closer to being a reality than ever before. Additionally, nozzle erosion was not detected during these firings (up to 3000 psi); an outcome that indicates that high-pressure operation could be feasible, since nozzle erosion at high pressure is one of the major engineering concerns with reducedsmoke propellant. RTO-MP

8 ACKNOWLEDGMENTS The authors wish to thank Ms. Antonella Thompson for her tireless support and invaluable assistance in the writing of this paper; Dr. Diana Woody for deriving the mechanical properties of the materials; and Ms. Nancy Carey for conducting the impact testing. REFERENCES [1] A.I. Atwood, P.O. Curran, C.F. Price, T.L. Boggs, and D. Booth. High-Pressure Burning Rate Studies of Ammonium Perchlorate (AP) Based Propellants, published in the Proceedings for Research and Technology Agency of North Atlantic Treaty Organization (NATO) 1999 Meeting on Small Rocket Motors and Gas Generators for Land, Sea, and Air Launched Weapon Systems, April 1999, Corfu, Greece. Paper UNCLASSIFIED. [2] T.L. Boggs, D. Netzer, and D.E. Zurn. Ammonium Perchlorate Combustion: Effects of Sample Preparation; Ingredient Type; and Pressure, Temperature and Acceleration Environments, J. Combustion Science and Technology, Vol. 7, (1973) pp Publication UNCLASSIFIED. [3] N. Cohen. A Review of Models and Mechanisms for Pressure Exponent Breaks in Composite Solid Propellants, published in the Proceedings of 23 rd JANNAF Combustion Meeting, October 1986, CPIA Publication 457, Vol. II. Paper UNCLASSIFIED. [4] Air Force Astronautics Laboratory. High-Pressure Characterization, by T.P. Rudy, H.H. Weyland, E.J. Shanabrook, R.S. Brown, L.S. Bain, and G.A. Flandro. Edwards Air Force Base, California, AFAL, July (AFAL-TR , Publication UNCLASSIFIED.) Work performed by United Technologies Corporation, Chemical Systems Division on Air Force contract F C-0034 with the Air Force Astronautics Laboratory. [5] T. Rudy, Proceedings of the 1988 Workshop on High-Pressure Combustion of Solid Rocket Propellants, published in the Proceedings of 26 th JANNAF Combustion Meeting, October 1989, CPIA Publication #5, pp. 9-8, Vol. IV. Paper UNCLASSIFIED. [6] T. Comfort, L.G. Dillman, M.G. Mangum, K.O. Hartman, and R.M. Steckman. Insensitive HTPE Propellants, presented at the Insensitive Munitions Technology Symposium, ADPA, San Diego, California, March Paper UNCLASSIFIED. [7] M.L. Jones and D.D. Tzeng. Low Cost Polymer for IM Application, published in the Proceedings of 1998 JANNAF Propulsion Meeting, July 1998, Cleveland, Ohio. Paper UNCLASSIFIED. [8] Naval Ordnance Laboratory. The NOL Large Scale Gap Test, III Compilation of Unclassified Data and Supplementary Information for Interpretation of Results, by Donna Price, A.R. Clairmont, Jr., and J.O. Erkman. White Oak Detachment, Silver Springs, Maryland, NOL, 8 March (NOLTR 74, Publication UNCLASSIFIED.) [9] Steve Collignon. Variable Confinement Cookoff Test of Metal Accelerating Explosives, presented at the Fifth Tri-Service Symposium on Explosive Testing, sponsored by the Explosive Safety Board, Department of Defense, hosted by the Naval Surface Warfare Center, Dahlgren Division, White Oak Detachment, Silver Spring, Maryland, April Paper UNCLASSIFIED. [10] Naval Surface Warfare Center. Variable Confinement Cookoff Test, by Kim Alexander and Kevin Gibson, Indian Head, Maryland, NSWC, 7 Nov Publication UNCLASSIFIED RTO-MP-091

9 NOMENCLATURE ABL Allegany Ballistics Laboratory AN ammonium nitrate AP ammonium perchlorate BuNENA n-butyl-2-nitratoethyl nitramine DSC differential scanning calorimetry E 0 initial modulus EVA small-scale evaluation motor HTCE hydroxyl-terminated polycaprolactone polyether block copolymer HTPB hydroxyl-terminated polybutadiene HTPE hydroxyl-terminated polyether IM insensitive munitions IMAD Insensitive Munitions Advanced Development Isp specific impulse NAWCWD Naval Air Warfare Center Weapons Division NCO/OH isocyanate/hydroxyl NF no fire NOL Naval Ordnance Laboratory PEP Propellant Evaluation Program SRM solid rocket motor TEGDN triethylene glycol dinitrate TMETN rimethylolethane trinitrate VCCT variable confinement cookoff test VTS vacuum thermal stability ε expansion ratio ε b elongation (strain) at break, % ε m strain at maximum tensile stress, % σ m maximum stress, psi RTO-MP

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