No abstract

The low-pressure gas in the vacuum plume produced by the chemical thrusters contaminates the spacecraft when adsorbed on the low-temperature surface. To provide theoretical support for further research on gaseous plume pollutants, the adsorption isotherms of low-pressure H 2 O were measured by a quartz crystal microbalance (QCM) at temperatures ranging from 233 to 273 K. The measured isotherms are similar to the type-I and type-II isotherms and have been correlated by various models (e.g., the Langmuir, Dubinin–Radushkevich, Brunauer–Emmett–Teller (BET), and universal models). It shows that the universal model has a great advantage in predicting the adsorption at a specific temperature point in our study. To estimate the adsorption at the continuous temperature range, the critical parameters of the multi-Langmuir model were expressed in semiempirical formulas. Since the normalized isotherms of H 2 O at different temperatures converge well, a simplified multi-Langmuir (SML) model was proposed. The experimental results at the temperature and pressure ranges we explored are consistent with the results predicted by the SML model, suggesting that the SML model is more suitable and convenient to predict the low-pressure adsorption of H 2 O for a continuous low-temperature range. Moreover, the low-pressure adsorption behaviors of H 2 O and CO 2 on the low-temperature surface are compared and discussed.

Chemical reactions significantly influence aerodynamic performance during spacecraft entry into the Martian atmosphere. Several chemical reaction models have been proposed in the direct simulation Monte Carlo simulation. The quantum-kinetics (Q-K) model has been applied in the case of diatomic molecules. Given that the Martian atmosphere consists primarily of CO2, it is crucial to find ways of implementing the Q-K model for polyatomic molecules. Although the chemical reaction rates involving CO2 have been investigated using the Q-K model, the problem of achieving detailed balance remains. Multiple vibrational modes exist for polyatomic molecules. Under the Q-K distribution, the average vibrational level of each mode is higher than that under the equilibrium distribution, and the total energy may be insufficient. Hence, its applicability to polyatomic molecules needs to be revealed. In this study, a comparison is made of the respective results obtained using the Larsen-Borgnakke (L-B) and Q-K distribution methods for the energy distribution of the reaction CO2+O⇋CO+O2, and detailed balance is achieved with the Q-K method but not the L-B method. Under the conditions assumed in this study, the vibrational energy distribution of CO consumed by the reverse reaction is not in good agreement with that generated by the forward reaction, leading to the failure of the L-B method. Finally, the results indicate that detailed balance is reached only when the collision temperature, based on the translational and vibrational energy, is employed to adjust the activation energy rather than the translational temperature generally adopted in the literature.