16.522, Space Propulsion Lecture 5 Prof. Manuel Martinez-Sanchez Page 1 of 12 16.522, Space Propulsion Prof. Manuel Martinez-Sanchez Lecture 5: Chemical Thrusters for In-Space Propulsion HYDRAZINE Hydrazine was first isolated by Curtius in 1887, and in 1907 a suitable synthetic method was developed by Raschig. Anhydrous hydrazine is a clear, colorless, hygroscopic liquid with an odor similar to that of ammonia. Anhydrous hydrazine is a strong reducing agent and a weak chemical base. Aqueous hydrazine shows both oxidizing and reducing properties. Although potential data show hydrazine to be a powerful oxidizing agent in acidic solutions, reactions with many reducing agents are so slow that only the most powerful ones reduce it quantitatively to ammonium ion. Hydrazine will react with carbon dioxide and oxygen in air. When hydrazine is exposed on a large surface to air, such as on rags, it may ignite spontaneously due to the evolution of heat caused by oxidation with atmospheric oxygen. A film of hydrazine in contact with metallic oxides and other oxidizing agents may ignite. Hydrazine is an endothermic compound and will decompose spontaneously in a similar way to hydrogen peroxide. The reaction of hydrazine with the oxides of copper, manganese, iron, silver, mercury, molybdenum, lead or chromium may be particularly violent. The spontaneous or artificially induced decomposition of hydrazine does not follow the reaction N 2 H 4 = N 2 +2H 2 , but a more exothermic one such as 2N 2 H 4 = 2NH 3 +N 2 +H 2 . SUMMARY AND CONTENTS PHYSICO-CHEMICAL PROPERTIES METRIC VALUE ENGLISH REFERENCE FIGURE Molecular Formula N 2 H 4 (1) Molecular Weight 32.04 (1) Freezing Point 1.5 C null 34.7 F null (6) Freezing Point Diagram with Additives 1 Boiling Point 113.5 C null 236.3 F null (10) 4 Critical Properties C P = 145 atm. c T =380C (716F) nullnull d c = 0.231 g/cc (5) (5) (5) 3 3 3 Density, liquid 1.0045 g cc @ 25 C null 8.482 lb gal @ 77 F null 2, 3 Density, vapor and liquid (5) 3 Vapor Pressure 14.38 mm @ 25 C null 0.0189 @ 77 Fatm null (6) 4 Surface Tension 66.67 dynes/cm @ 25 C null 62.32 dynes/cm @ 35 C null .004568 lb ft @ 77 F null .004270 lb ft @ 95 F null (7) (9) Viscosity, liquid 0.90 centipoise @ 25 C null .000605 lb ft -sec @ 77 F null (11, 12) 5 Heat Flux at q ul / Pressure (8) 6 Heat Flux at q ul / Temperature (8 7 Heat Flux at q ul Velocity (8) 8 16.522, Space Propulsion Lecture 5 Prof. Manuel Martinez-Sanchez Page 2 of 12 Heat of Fusion 3.025 kcal/mole @ 1.5 C null 37.51 Btu lb @ 34.7 null (6) Heat of Vaporization 9.600 kcal/mole @ 113.5 C null 54.0 Btu lb @ 236 F null (3) Heat Capacity (liquid) Temp. 23.62 cal/mole- C @ 25 C nullnull .737 Btu lb- F @ 77 F nullnull (6) 9 Heat of Combustion N 2 H 4 (l)+O 2 =N 2 +2H 2 O (l) 148.6 kcal/mole @ 25 C null 8.346 Btu lb @ 77 F null (20) Heats of Formation at 25 C (77 F) nullnull N 2 +2H 2 = N 2 H 4 (g) N 2 +2H 2 = N 2 H 4 (liq) N 2 +2H 2 +H 2 O = N 2 H 4 .H 2 O N 2 +2H 2 + aq = N 2 H 4 . aq 22.750 kcal/mole 11.999 kcal/mole 10.300 kcal/mole 8.140 kcal/mole 1278 Btu/lb 675 Btu/lb 579 Btu/lb 457 Btu/lb (20) (20) (20) (20) Index of Refraction, D 1.4644 @ 25 C (77 F) nullnull (9) Dielectric Constant 51.7 @ 25 C (77 F) nullnull (19) Electrical Conductivity -6 -1 310 ohm @ 25C (77F)× nullnull 11) Flash Point (open cup) 52 C null 126 F null (18) Explosive Limits (in air, 1 atm.) 4.7% lower 100% upper (18) (18) B. Materials: The following table gives an evaluation of the compatibility data that are available for numerous metals, plastics, elastomers, and miscellaneous material: COMPARATIVE COMPATIBILITY OF VARIOUS MATERIALS WITH HYDRAZINE AND HYDRAZINE MIXTURES (Ref. 28) A-Material is acceptable for general service. B-Material is acceptable for limited service. C-Material which must be avoided. MATERIAL ANHYDROUS HYDRAZINE HYDRAZINE HYDRATE HYDRAZINE- HYDRAZINE NITRATE- WATER MIXTURES Metals Aluminum 2S A A B 2SO A 2SH A A B 3S A 3SH A A B 24ST A A 40E B B B 43 B 52ST A A A 16.522, Space Propulsion Lecture 5 Prof. Manuel Martinez-Sanchez Page 3 of 12 61ST A A A 75ST A MATERIAL ANHYDROUS HYDRAZINE HYDRAZINE HYDRATE HYDRAZINE- HYDRAZINE NITRATE- WATER MIXTURES XA-545 B B B 716 B Brass B B B Cobalt C C C Copper C Inconel B B B Inconel X B B B Iron C C C Lead C Magnesium C C C Manganese Molybdenum C C C Monel B B C Nickel B C Nickel-chrome alloys (Chromel-A, Nichrome) B B C Silver B B Steel Mild C C C Stainless 302 B B B 303 C C C 304 A A A 315 C C C 316 C 317 C C C 321 B B B 329 C C C 347 A A A 410 B B B 416 C C C 420F C 430 B B B 430F C C C 440A C 440C C C C W B B B Stellite B Tantalum A A A Tin C C C Titanium A A A Zinc C Plastics and Elastomers 16.522, Space Propulsion Lecture 5 Prof. Manuel Martinez-Sanchez Page 4 of 12 Cellulose acetate C C C Diallyl phthalate C C C MATERIAL ANHYDROUS HYDRAZINE HYDRAZINE HYDRATE HYDRAZINE- HYDRAZINE NITRATE- WATER MIXTURES Epon B B B Ethyl cellulose B B B Furane resin B B B Hycar B Kel-F B B B Lactopreme C C C Lucite B B B Melamine formaldehyde B B B Nylon B Phenolic B B B Polyester C C C Polyethylene A A A Polystyrene and polydichlorostyrene C C B Polyvinyl alcohol C C C Polyvinyl chloride (Koroseal, Vinylite, etc.) B B B Rubber Natural gum C C C Synthetic B B B Saran C C C Silastic B B B Teflon A A A Tygon B B B U.S. Rubber Plastic L7825 B B B M20995 B Veloform C C C Miscellaneous Materials Asbestos B B B Glass Soft A A A Pyrex A Graphite B B B Graphitar B Pipe-joint compounds AN-C-53 B B B Oxyseal B Thread-Tite B B B Rags C C C 16.522, Space Propulsion Lecture 5 Prof. Manuel Martinez-Sanchez Page 5 of 12 Silicone lubricants DC-200 series B B B MATERIAL ANHYDROUS HYDRAZINE HYDRAZINE HYDRATE HYDRAZINE- HYDRAZINE NITRATE- WATER MIXTURES DC-550 B B B DC-710 B Plug-cock grease B B B Solder Lead-tin B B B Silver B Varnish B B B Wood C C C Wool C C. Equipment: The selection of materials of construction of equipment for use with hydrazine should be limited to those previously listed as acceptable for general service. Hydrazine and hydrazine rockets - (1) Properties. Hydrazine monopropellant rockets have become standard for NSSK and other on- board maneuvering needs. This is because of the simplicity of a monopropellant rocket, storability of hydrazine over many years, and relatively good performance achievable. Also, the technology of hydrazine engines is well developed. There had been other monopropellants used or considered before (H 2 O 2 , nitromethane) but they are inferior either in safety or in performance. Formula: N 2 H 4 Clear, very similar to water in physical properties ( 3 =1g cmρ , freezes at 1.5 C null , boils at 113.5 C null , surface tension = 66.7 dyn/cm at25 C null , viscosity = 0.9 cp at 25 C null , specific heat 0.72 cal/g C null at 25 C null ). Other properties listed in the given table. Chemically, however, N 2 H 4 is very different from H 2 O. It will react with CO 2 and O 2 in air; if exposed to a large surface of air, as in wet rags, it may ignite; it may also ignite in contact with metallic oxides. Its combustion is very exothermic: 24 2 2 2 Kcal NH+O N 2HO+149 Mole →+ and so hydrazine is a good fuel for a bipropellant rocket (Hydrazine + O 2 ). Variations are the monomethyl hydrazine (MMH) NH 3 CH 3 and the unsymmetric dimetyl hydrazine (UDMH, NH 2 (CH 3 ) 2 ), which are very similar but somewhat 16.522, Space Propulsion Lecture 5 Prof. Manuel Martinez-Sanchez Page 6 of 12 inferior thermochemically, but have a wider liquid range. A popular bipropellant combination is A50 (50% hydrazine, 50% UDMH). These cannot be used as monoprops, because they poison the catalyst. Hydrazine evolves NH 3 , and smells like it. Its vapors damage the eyes and the pulmonary tract. The liquid, if pure, is fairly inert, but vapors form flammable mixtures in air for vapor pressures corresponding to T >40C null . The liquid decomposes exothermically if catalyzed by iron oxide, copper oxide, or oxides of Pb, Mn, Mb, Ag, Hg or Cr. It is not sensitive to friction or impact. Materials compatibility is an important consideration, especially for long term storage and for parts of thrusters with long life exposed to it. Materials which are OK are aluminum, stainless 304 or 347, titanium, tantalum, rhenium and platinum. Also glass, Teflon and polyethylene plastics. To be avoided are copper, cobalt, iron, lead, magnesium, manganese, molibdenum, mild steel, high Mo stainless (416, 303), most plastics (except as noted), wood, rags, paper. See table handed out. Oils are OK for lubrication, but not if there is a catalytic bed, since it gets poisoned by the oil. Hydrazine is expensive ($ 50-60/lb for MMH, as of 1994). (2) Thermochemistry. When catalyzed either by an oxide or by a hot platinum surface, N 2 H 4 decomposes. Since at low temperatures ammonia, NH 3 , is stable, the preferred end products would be NH 3 +N 2 : 24 3 2 3N H 4NH N→+ (a) However, this is a very exothermic reaction, and the equilibration T would be 1650 K, null at which temperature NH 3 is not stable anymore. Hence, the final equilibrium composition would contain very little NH 3 , due to 322 2NH N +3H→ (b) and would be at intermediate T, since the latter reaction is endothermic. In practice reaction (a) is very fast (less than 1 msec) if catalyzed, while (b) is slow. Hence, for small decomposition chambers and high flow rates, when the residence time gas ch ρ Vm i is short, reaction (b) proceeds only partially, the extent being controllable by the design conditions. The final composition and overall reaction assuming a fraction x of NH 3 decomposes is 24 3 2 3 322 41 N H NH + N (1) 33 Form (1)+ x(2) ; x = fraction of NH that decomposes 42 NH N +2H (2) 33 ? → ? ? ? ? → ? ? () () 24 3 2 2 41 N H 1- x NH + 1+2x N +2xH 33 → 16.522, Space Propulsion Lecture 5 Prof. Manuel Martinez-Sanchez Page 7 of 12 For an adiabatic combustion (no heat loss, no heating), we must have (starting from liquid N 2 H 4 at 298 K null ) ()() () ()() () 24 3 2 2 NH NH N H 41 h ,298 1-x h T 1+2x h T 2xh T Equation for T 33 =+ +→ null null The respective molar enthalpies can be fitted by () 3 2 cal NH K T h T = -16.83+12.35 +0.983 = mole 1000 K ?? θθθ ?? ?? null () 2 2 N h T = -2.83+7.75 +0.183 θθ () 2 2 H h T = -1.967+6.6 +0.367 θθ ( ) 300 T 4000 K≤≤ null (Notice () p ccalmoleC null ) is dh dθ , with h in Kcal/mole) From the table given, () 24 NH h ,298 K 12 Kcal/mole= null null . We can now solve for T at various arbitrary values of x: () 2 139.3 - 20.75x +8.45x - 9.525+0.95x = 1.372 - 0.455x θ x (fraction of NH 3 decomposed) 0 0.2 0.4 0.6 0.8 1 T( K), null adiabatic temperature 1659 1502 1343 1182 1023 863 16.522, Space Propulsion Lecture 5 Prof. Manuel Martinez-Sanchez Page 8 of 12 Equilibrium Hydrazine Decomposition If we allowed “infinite” time for the reaction, hydrazine products would reach an equilibrium with little ammonia left (depending on pressure). The product, N 2 , H 2 , NH 3 , must then satisfy () 3 22 NH p3 1 22 NH P =K T P P ; () 6289 -1 T -6 p K1.089×10e atmnull (P’s in atm) and the pressure is 322 NH N H P=P +P +P To conserve moles of H and N, starting from (arbitrarily) 3 moles of N 2 H 4 , we must have × ? ? ? × ? ? 32 32 3232 NH H NH H NH NNH N H: 3n +2n =3 4=12 3P +2P dividing, = 2 P+2PN: n + 2n = 3 2 = 6 Define 3 NH P y= P (ammonia mole fraction). Then 2 2 H N 3y+2P P =2 y+2P P and 22 HN PP y+ + =1 PP Solving, 2 H P 25 =1-y P3 4 ?? ?? ?? 2 N P 11 =1-y P3 2 ?? ?? ?? 16.522, Space Propulsion Lecture 5 Prof. Manuel Martinez-Sanchez Page 9 of 12 and substituting into the equilibrium law, p12 32 y =KP 11 25 1- y 1- y 32 34 ???????? ???????? ???????? () 12 32 py12 32 y1 =PKTK 33 15 1- y 1- y 24 ???? ≡ ???? ???? ???? ???? ???? So, given P and T, we can solve this for 3 NH P y= P . But T itself must be consistent with energy conservation ()() () () 32224 NH N H N H 41+2x 1-x h T + h T +2xh T =h 33 where 24 NH h would be 12.0 Kcal/mol if we start from liquid hydrazine at 298K, or it could include the external heating energy due to an electrical heater, as in the case of an Electrothermally Augmented thruster (in fact, in this case the equilibrium assumption is realistic, given the long residence time in the heater, and the higher temperature). Now x is the fraction of NH 3 decomposed after the initial fast reaction 24 3 2 41 NH NH + N 33 → . To relate it to y, which is the mole fraction of NH 3 in the final products, we write () () () 4 1-x 41-x 3 y= = 41+2x 5+4x 1-x + +2x 33 or () 4-5y x= 41+y (notice for x=0 we have 4 y= 5 , i.e. 1 5 of the products is N 2 while for x=1, y=0). So, to complete the computation, we could iterate as follows: (1) Given P, guess T 16.522, Space Propulsion Lecture 5 Prof. Manuel Martinez-Sanchez Page 10 of 12 (2) Compute () ( ) ( ) ( ) ( ) 322 py NH N H K T , K T, P , h T , h T , h T (3) Solve y31 22 y =K y4 1- 1- y 25 ??? ? ??? ? ??? ? for y (4) Calculate () 4-5y x= 41+y (5) Calculate () 322 prods. NH N H 41+2x h=1-xh+ h+2xh 33 and the “error” 24 prods. N H h-h (6) Use this error to generate a new T guess, go back to (2) Some results (with no external heating) which, indeed, shows minimal NH 3 present For propulsion purposes, the important thing is not T, but I sp . This depends also on the molecular weight of the gas (and somewhat on γ ); since the gas gets lighter as NH 3 decomposes, this compensates for the lower T, and I sp is very insensitive to x for x<0.4. ～ Assuming frozen flow (constant γ , constant M), and a pressure ratio P e /P o , the exit velocity is -1 e e0 0 PR u= 2 T 1- -1M P γ γ ?? ?? γ ?? ?? γ ?? ?? ?? () () 41+2x 17x 1- x +28 +2x.2 33 M= 41+2x 1-x + +2x 33 96 M= 5+4x and the area ratio is P(atm) NH 3 P y= P T(K) 2 0.00098 864.8 5 0.0024 867.3 10 0.0046 871.3 20 0.0086 878.4 50 0.0185 895.5 100 0.0306 916.3 200 0.0479 945.0 16.522, Space Propulsion Lecture 5 Prof. Manuel Martinez-Sanchez Page 11 of 12 () +1 2-1 e * -1 1 ee 00 A -1 2 1 = 2+1A PP 1- γ γ γ γ γ ??γ ?? γ ?? ?? ?? ?? ?? ?? ?? and so, in vacuum * eee e ee e ee sp * 0 mu +P A u P A u P AFc I= = = + = + gggA mg mg mg i ii i with () +1 * 2-1 0*0 * RM TPA 2 m= ,c = ,T= T+c γ γ ?? γ ?? γ ?? i Take P e /P 0 =0.0012 (about A e /A * =50). Using chamber composition (pretty good), () () () ( ) () 41+2x 17 1- x +28 +4x 68 1- x +28 1+2x +12x 96 33 M= = = 41+2x 4 1- x +1+2x+6x 5+4x 1-x + +2x 33 1.986 ()( ) ()() ()( ) ()() θθθ γ 41+2x 1- x 12.35+2×0.983 + 7.75+2×0.183 +2x 6.6+2×0.367 = 1- x 10.36+1.966 + 5.76+0.366 +2x 4.61+0.734 33 x = 0.4 =1.343→θ ()()() ()()() θ γ 41.8 0.6 12.35+1.966×1.343 + 7.75+0.366×1.343 +0.8 6.6+0.734 33 = 0.8 10.36+ _ _ _ +0.6 5.76+ _ _ _ +0.8 4.61+ _ _ _ γ =1.234 (for comparison, at the throat T, γ * =1.238) As a practical matter, one wants T 0 as low as possible if the performance is acceptable. A very common choice is sp x = 0.4 I 250 sec.→ null (ideal) 16.522, Space Propulsion Lecture 5 Prof. Manuel Martinez-Sanchez Page 12 of 12