Session 12: Monopropellant Thrusters

Session 12 : Monopropellant Thrusters Electrothermal augmentation of chemical rockets was the first form of electric propulsion applied in space vehicles. In its original implementation, resistojets were used to improve the performance of regular hydrazine (N2 H4 ) thrusters in geostationary satellites. The mo­ tivation for this important application is relevant to the development of electric propulsion and will be discussed in detail in this lecture. We start by reviewing the fundamentals of hydrazine monopropellant propulsion. 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 N2 H4 → N2 + 2H2 , but a more exothermic one such as 2N2 H4 → 2NH3 + N2 + H2 . When catalyzed either by an oxide or by a hot platinum surface, N2 H4 decomposes. Since at low temperatures ammonia, NH3 , is stable, the preferred end products would be NH3 + N2 : N 2 H4 → 4 1 NH3 + N2 3 3 (1) However, this is a very exothermic reaction, and the equilibration temperature (T ) would be 1650 K, at which temperature NH3 is not stable anymore. Hence, the final equilibrium composition would contain very little NH3 , due to, 4 NH3 3 2 N2 + 2H2 3 → (2) and would be at an intermediate T , since the latter reaction is endothermic. In practice reaction (1) is very fast (less than 1 msec) if catalyzed, while (2) is slow. Hence, for small decomposition chambers and high flow rates, when the residence time ρv/ṁ is short, reaction (2) proceeds only partially, the extent being controllable by the design conditions. The final composition and overall reaction assuming a fraction x of NH3 decomposes is, N 2 H4 → 4 1 (1 − x)NH3 + (1 + 2x)N2 + 2xH2 3 3 (3) For an adiabatic combustion (no heat loss, no heating), we must have (starting from liquid N2 H4 at 298 K), 1 hN2 H4 (J, 298K) = 4 1 (1 − x)hNH3 + (1 + 2x)hN2 + 2xhH2 3 3 (4) The respective molar enthalpies (kcal/mol) can be fitted by, hNH3 (T ) = −16.83 + 12.35θ + 0.983θ2 hN2 (T ) = −2.83 + 7.75θ + 0.183θ2 hH2 (T ) = −1.967 + 6.6θ + 0.367θ2 (5) T . These expressions are valid in the range 0.3 < θ < 4. The specific heat 1000K dh . is cp (cal/mol K) = dθ The value of the enthalpy of liquid hydrazine at standard conditions can be found from tables, hN2 H4 (J, 298K) = 12 kcal/mol. We can now solve for the temperature at various arbitrary values of x, where θ = √ 139.3 − 20.75x + 8.45x2 − (9.525 + 0.95x) θ = 1.372 − 0.455x (6) Results from Eq. (6) are shown in the table below: x (fraction of NH3 decomposed) T (K) adiabatic temperature 0 0.2 0.4 0.6 0.8 1 1659 1502 1343 1182 1023 863 Up to this point, the ammonia decomposition fraction has been considered as a free parame­ ter. In practice, it will depend on residence time and operating conditions, in particular the chamber pressure. If we assume that residence time is long enough to allow for infinite time for the decomposition reaction, hydrazine products would reach an equilibrium with some ammonia left (depending on pressure). From the law of mass action, the products must satisfy, pNH3 1/2 3/2 pN2 pH2 = Kp (T ) (7) where the equilibrium constant is approximated by (pressures in atm), Kp (T ) ≈ 1.089 × 10−6 e 6289 T in atm−1 (8) and in terms of partial pressures we have, p = pNH3 + pN2 + pH2 Writing the reaction in terms of partial molar densities, 2 (9) nN2 H4 → nNH3 NH3 + nN2 N2 + nH2 H2 (10) we impose species conservation, for H → 4n = 3nNH3 + 2nH2 for N → 2n = nNH3 + 2nN2 (11) or in terms of the partial pressures, since p = nkT , 2 = 3pNH3 + 2pH2 pNH3 + 2pN2 (12) Eqs. (7-9) and (12) constitute a system of four equations and four unknowns, i.e., the partial pressures and temperature. We solve this system by defining the ammonia mole fraction, pNH3 p y = (13) After some manipulation, these equations can be written as, 1 y pNH3 = 1− p 3 2 pH2 2 5y = 1− p 3 4 y/p Kp (T ) = 1/2 (pN2 /p) (pH2 /p)3/2 (14) We also write the mol fraction Eq. (13) in terms of x, y = 4 (1 3 − x) 4 (1 − x) 3 + 13 (1 + 2x) + 2x = 4(1 − x) 5 + 4x (15) or, solving for x in terms of y, x = 1 − 5y/4 1+y (16) Given some pressure p and an initial guess for the temperature T , we find y and x. At this point we need to verify that the temperature used in this calculation is consistent with the energy balance in Eq. (4). In all likelihood the temperature from Eq. (4) will not coincide with the initial guess, so we need to perform a few iterations of this process. In practice, selecting the adiabatic temperature from the energy balance as initial guess for each iteration suffices to guarantee quick convergence. The following table shows some results using this procedure as a function of pressure. 3 pNH3 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 p (atm) y= In all these cases we observe that ammonia is near to complete decomposition if allowed to reach equilibrium conditions. If this result is not desirable in a thruster, then the only additional knob to control decomposition is residence time, so in many instances the geometry of the thruster is essential to determine the performance. Now, for propulsion purposes, the important thing is not the adiabatic temperature, but the Isp . This depends also on the molecular weight of the gas; since the gas gets lighter as NH3 decomposes, this compensates for the lower T , and Isp is very insensitive to x, for x < 0.4. F mu ˙ e + p e Ae ue pe Ae c∗ = = + mg ˙ mg ˙ g p 0 At g Isp = (17) where the characteristic velocity is defined as usual, p0 A t c∗ = = ṁ p R/M T0 Γ(γ) with Γ(γ) = √  γ 2 γ+1  2(γ+1 γ −1) Assuming frozen flow, and a pressure ratio pe /p0 , the exit velocity is, v " u   γ −γ 1 # u 2γ R pe ue = t T0 1 − γ −1M p0 (18) The molecular mass is obtained with a weighted average, M = MNH3 43 (1 − x) + MN2 13 (1 + 2x) + MH2 2x 96 = 4 1 5 + 4x (1 − x) + 3 (1 + 2x) + 2x 3 (19) A similar procedure can be used to compute the weighted average for the specific heat, and from here the ratio of specific heats γ = γ(x). The area ratio is, 4 Ae = At r γ−1 2  2 γ+1  2(γγ +1  −1) pe p0 − γ1 "  1− pe p0  γ−γ 1 #1/2 (20) The following page contains the results from these calculations for a pressure ratio pe /p0 = 0.0012, corresponding to an area ratio of about Ae /At = 50. As a practical matter, one wants T0 as low as possible if the performance is acceptable. A very common choice is x = 0.4 for a specific impulse of about 250 sec. 5 Figure removed due to copyright restrictions. Please see Figure 6.8 from Schmidt, Eckart W. Hydrazine and its Derivatives. Wiley-Interscience, 2001. 6