Abstract:
Disclosed is a group of tertiary amine azides useful as hypergolic fuels for hypergolic bipropellant mixtures. The fuels provide higher density impulses than monomethyl hydrazine (MMH) but are less toxic and have lower vapor pressures that MMH. In addition, the fuels have shorter ignition delay times than dimethylaminoethylazide (DMAZ) and other potential reduced toxicity replacements for MMH.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     The U.S. Government may have certain rights in this invention pursuant to Contract No. W31PQ06C0167 awarded by the U.S. Army 
    
    
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     INCORPORATED-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a hypergolic rocket fuels that simultaneously possess high-performance propellant characteristics and low toxicity relative to Monomethylhydrazine (MMH). The fuels provide propellant performance as high as or higher than MMH, but have lower toxicity. 
     2. Description of Related Art 
     Monomethylhydrazine (MMH) is a widely employed fuel in hypergolic, bipropellant systems. MMH possesses desirable propellant properties but it is highly toxic, carcinogenic, and corrosive. Although gelling has dramatically improved the safety of handling and storing the propellant, its toxicity and carcinogenicity are still of major concern. Therefore, there is a need for alternative liquid hypergolic fuels that are less carcinogenic and less toxic than MMH but also have equal or higher energy densities, lower vapor pressures and ignition delays than MMH. These fuels, like MMH, may be used in the form of gels to further improve safety. 
     Although DMAZ is hypergolic, its ignition delay with IRFNA is significantly longer than MMH. A longer ignition delay requires a larger combustion chamber to avoid pressure spikes that can damage the engine. 
     U.S. Pat. No. 6,013,143, incorporated by reference herein in its entirety, discloses three chemicals, each comprising a tertiary nitrogen and an azide functional group that are hypergolic when mixed with an oxidizer such as IRFNA, hydrogen peroxide, nitrogen tetroxide, and hydroxyl ammonium nitrate. The chemicals are dimethylaminoethylazide (DMAZ), pyrollidineylethylazide (PYAZ), and bis (ethyl azide)methylamine (BAZ). Inhibited Red Fuming Nitric Acid (IRFNA) type IIIB and monomethyl hydrazine (MMH) deliver a specific impulse of 284 Ib f  sec/Ib m  and a density impulse of 13.36 Ib f  sec/cubic inch in a rocket engine operating a pressure of 2000 psi. DMAZ, PYAZ, and BAZ are proposed as potential replacements for MMH. DMAZ, under the same conditions as MMH, delivers a specific impulse of 287 Ib f  sec/Ib m  and a density impulse of 13.8 Ib f  sec/cubic inch. The patent discloses the mixing of the hypergolic fuel chemicals with gellants and additives such as aluminum and boron to increase specific impulse and density impulse values. 
     U.S. Pat. No. 6,926,633, incorporated by reference herein in its entirety, discloses a family of amine azides having cyclic structures and for use as hypergolic rocket propellants. The amine azide compounds comprise at least one amine, including tertiary amines, and an azide functional group pendant from a cyclic structure. The propellants are disclosed as being used with oxidizers and, optionally with catalysts present in fuel or oxidizer. Fuel properties for the amine azides are provided based on computational quantum chemistry calculations. 
     U.S. Pat. No. 6,949,152, incorporated by reference herein in its entirety, discloses hypergolic propulsion systems comprising a fuel composition and an oxidizer composition. The fuel composition contains an azide compound having at least one tertiary nitrogen and at least one azide functional group. The oxidizer contains hydrogen peroxide in water. The hypergolic reaction between oxidizer and fuel is catalyzed by a transition metal, preferably compounds of cobalt and manganese. 
     Unlike hypergolic fuels disclosed previously, the present fuels exhibit lower toxicity and higher performance than MMH. The fuels require no catalyst to achieve high performance and are hypergolic with commonly used oxidizers. The fuels of the present invention may be used alone, in combination with each other, or in combination with other fuels in blends. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is a group of tertiary amine azide chemicals useful as hypergolic fuels for hypergolic bipropellant mixtures. The fuels provide higher density impulses than MMH but are less toxic and have lower vapor pressures that MMH. In addition, the fuels have shorter ignition delay times than DMAZ and other potential reduced toxicity replacements for MMH. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  illustrates the structures of chemicals (I)-(VIII). 
         FIG. 2  illustrates the structures of chemicals (IX)-(XI). 
         FIG. 3  illustrates the structures of chemicals (XII) and (XIII). 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is a rocket fuel composition comprising one or more of the molecules shown in  FIGS. 1-3 . The fuel is hypergolic when combined with a strong oxidizer such as IRFNA, hydrogen peroxide, nitrogen tetroxide, or hydroxyl ammonium nitrate. Relevant chemical and physical properties of the molecules have been calculated using validated molecular modeling techniques, including quantum chemistry and Conductor-like Screening MOdel for Real Solvent (COSMO-RS) methods. The fuel molecules have been selected for one or more desirable propellant properties including:
         High heat of formation: Heat of formation is directly related to the specific impulse which is a measure of energy content in the molecule.   Density: Density of the liquid fuel is important because density impulse, a measure of fuel performance, is the product of density and specific impulse.   Vapor Pressure: Low vapor pressure improves safety during handling and storage.   Absence of N—N single bonds: Absence of N—N single bonds reduces toxicity.   Short Ignition Delay: Short ignition delay time with the oxidizer is desirable to minimize engine size and damage resulting from pressure spikes during combustion.
 
Heats of Formation
       

     First-principle ab initio quantum chemistry methods are the most accurate and suitable technique for calculations of molecular geometries, heats of formations, and activation barriers. Quantum chemistry techniques are based on the principles of quantum mechanics. The procedure numerically solves a many-electron Schrödinger equation to obtain a molecular wave function and energy. The molecular energies can be used to calculate heats of formation. 
     CBS-QB3 [Ochterski, 1995; Montgomery, 2000] and PBEPBE/6-311++G(d,p) [Perdew, 1996] combined with isodesmonic reaction methods were used to calculate the heats of formations, and activation barriers for the molecules in  FIGS. 1-3 . Heat of vaporization was calculated using a COSMO-RS technique [Klamt, 1995 and 2000]. Table 1 shows the computed heats of formation for hydrazine, MMH, DMAZ, and the compounds of the present invention Numbers in parentheses are National Institutes of Standards and Technology (NIST) experimental data. The molecules of the present invention possess higher heats of formation than MMH, and are therefore expected to possess specific impulse values that exceed those for MMH. 
     Densities 
     Wong et al. [Wong, 1995] have developed a procedure for calculating molecular volume, defined as the volume occupied by 0.001 au (1 au=6.748 e/Angstrom) electron density envelope. Once the molecular volume is known, the density can be computed using molecular weight. Calculated and known densities were compared for a number of amines and amine azides to validate density calculations. 
     Calculations were performed at the PBEPBE/6-311++G(d,p) level. Table 2 compares experimentally measured densities with calculated densities with and without corrective correlation. Error! Reference source not found, tabulates the predicted densities of molecules shown in  FIGS. 1-3 . 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Computed Heats of Formation 
               
             
          
           
               
                   
                   
                 Gas Phase 
                 Gas Phase 
               
               
                   
                 Molecule 
                 ΔH f   298 K  kcal/mol 
                 ΔH f   298 K  cal/gm 
               
               
                   
                   
               
               
                   
                 Hydrazine 
                 23.8 (22.8) 
                 744.9 (712.5) 
               
               
                   
                 MMH 
                 23.0 (22.6) 
                 500.9 (492.2) 
               
               
                   
                 DMAZ 
                 73.4 
                 643.6 
               
               
                   
                 I 
                 96.2 
                 858.9 
               
               
                   
                 II 
                 149.8 
                 1361.9 
               
               
                   
                 III 
                 110.1 
                 781.0 
               
               
                   
                 IV 
                 134.8 
                 1078.2 
               
               
                   
                 V 
                 112.2 
                 738.3 
               
               
                   
                 VI 
                 90.0 
                 489.0 
               
               
                   
                 VII 
                 112.2 
                 679.7 
               
               
                   
                 VIII 
                 110.0 
                 516.3 
               
               
                   
                 IX 
                 114.3 
                 747.2 
               
               
                   
                 X 
                 89.6 
                 577.8 
               
               
                   
                 XI 
                 128.5 
                 537.6 
               
               
                   
                 XII 
                 106.5 
                 578.9 
               
               
                   
                 XIII 
                 144.6 
                 510.9 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Calculated and Measured Densities 
               
             
          
           
               
                   
                 Computed Density 
                 Experimental 
                 Density after 
               
               
                 Molecule 
                 (raw data) 
                 density 
                 correlation 
               
               
                   
               
               
                 (CH 3 ) 2 NH 
                 0.9307 
                 0.671 
                 0.7038 
               
               
                 CH 3 NH 2   
                 0.9140 
                 0.694 
                 0.6849 
               
               
                 CH 3 N 3   
                 1.1225 
                 0.869 
                 0.9212 
               
               
                 C 2 H 5 N 3   
                 1.1187 
                 0.876 
                 0.9170 
               
               
                 2-azido-N- 
                 1.1649 
                 0.990 
                 0.9693 
               
               
                 cyclopropylethanamine 
               
               
                 H 2 NCH 2 CH 2 N 3   
                 1.1791 
                 1.040 
                 0.9855 
               
               
                 I, DMAZ 
                 1.1100 
                 0.933 
                 0.9096 
               
               
                 HN 3   
                 1.3116 
                 1.090 
                 1.1356 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Predicted Densities 
               
             
          
           
               
                 Molecule 
                 Density (raw data) 
                 Density after using the correlation 
               
               
                   
               
               
                 I 
                 1.1320 
                 0.9346 
               
               
                 II 
                 1.1334 
                 0.9362 
               
               
                 III 
                 1.2114 
                 1.0246 
               
               
                 IV 
                 1.3325 
                 1.1619 
               
               
                 V 
                 1.4048 
                 1.2438 
               
               
                 VI 
                 1.2153 
                 1.0290 
               
               
                 VII 
                 1.3801 
                 1.2158 
               
               
                 VIII 
                 1.2347 
                 1.0510 
               
               
                 IX 
                 1.2449 
                 1.0626 
               
               
                 X 
                 1.1381 
                 0.9415 
               
               
                 XI 
                 1.3249 
                 1.1532 
               
               
                 XII 
                 1.2433 
                 1.0608 
               
               
                 XIII 
                 1.2539 
                 1.0728 
               
               
                   
               
             
          
         
       
     
     Specific and density impulse are the two most important parameters describing the performance of a fuel. Density impulse is a measure of the performance per volume of the fuel. Table 4 shows the computed specific and density impulse for each of the molecules shown in  FIGS. 1-3  with IRFNA as the oxidizer. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Computed Specific and Density Impulse 
               
             
          
           
               
                   
                   
                 Density 
                 % 
               
               
                   
                   
                 Impulse = density*I sp *10 −3   
                 Improvement 
               
               
                 Molecule 
                 I sp  (lb f -sec/lb m ) 
                 (lb f -sec/ft 3 ) 
                 over MMH 
               
               
                   
               
             
          
           
               
                 I 
                 280.0 
                 16.3 
                 4.1 
               
               
                 II 
                 286.4 
                 16.7 
                 6.6 
               
               
                 III 
                 280.2 
                 17.9 
                 14.2 
               
               
                 IV 
                 280.7 
                 20.4 
                 29.7 
               
               
                 V 
                 272.4 
                 21.2 
                 34.7 
               
               
                 VI 
                 276.8 
                 17.8 
                 13.3 
               
               
                 VII 
                 267.8 
                 20.3 
                 29.5 
               
               
                 VIII 
                 278.0 
                 18.2 
                 16.2 
               
               
                 IX 
                 283.4 
                 18.8 
                 19.7 
               
               
                 X 
                 277.5 
                 16.3 
                 3.9 
               
               
                 XI 
                 277.6 
                 20.0 
                 27.3 
               
               
                 XII 
                 279.0 
                 18.5 
                 17.7 
               
               
                 XIII 
                 278.4 
                 18.6 
                 18.8 
               
               
                   
               
             
          
         
       
     
     The Isp values were calculated using the PROPER thermochemical code and correspond to the optimum fuel/IRFNA ratio. Table 4 shows that there is a substantial improvement of density impulse over MMH. 
     Synthesis of Hypergolic Fuels 
     The molecules of the present invention may be synthesized by those skilled in the art using known chemical synthetic reactions. For example, the synthesis of compound V can be accomplished by the using the known condensation of guanidines with haloacetates [Webb, 2003] followed by reaction with PCl 5  and treatment with NaN 3 . Compound VII can be prepared from 2,4-dichlorotriazine by sequential substitution of the chlorine atoms. The dichloride 5 can be prepared by condensation [Harris, 1981] of iminyl chloride. The preparation of compound XII can be accomplished, for example, by transamination [Flores-Parra, 1999] between two symmetric triazinanes. 
     REFERENCES 
     The following references are incorporated by reference in their entirety.
     Flores-Parra, A.; Sanchez-Ruiz, S. A.  Heterocycles  (1999) 51: 2079-2092.   Godbout, N, Salahub, D. R., Andzelm, J., and Wimmer, E.;  Can. J. Chem . (1992) 70: 560   Harris, R. L. N. “The synthesis of Triazines from N-Cyanocarbamimidates”  Synthesis  (1981) 1981:907-908   Klamt, A; J. Phys. Chem., (1995) 99: 222   Klamt A.;  Fluid Phase Equil ., (2000) 172: 43   McQuaid, M. J.; Stevenson, W. H., and Thompson; D. M. (2004) 24th Army Science Conference, Orlando, Fla.   Montgomery Jr. J. A., Frisch, M. J. Ochterski, J. W., and Petersson, G. A. (2000)  J. Chem. Phys.  112: 6532.   Ochterski, J. W., Petersson, G. A., and Wiberg, K. B. (1995)  J. Am. Chem. Soc.  117: 11299   Perdew, J. P., Burke, K., and Ernzerhof, M. (1996)  Phys. Rev. Lett.  77, 3865   Wong, M. W., Wiberg, K. B., Frisch, M. J. (1995)  J. Camp. Chem.  16:385