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形態: カラー図版あり, Physical characteristics: Original contains color illustrations, Accepted: 2020-06-24, 資料番号: PA2110026000

形態: カラー図版あり, Physical characteristics: Original contains color illustrations, Accepted: 2019-02-17, 資料番号: PA1910065000

This paper proposes a guidance approach for detumbling space debris using thruster plume impingement. It is difficult to capture rapidly rotating space debris. Previous studies have revealed the feasibility of reducing the rotational speed of such large debris as a rocket body or satellite. However, no study has presented concrete guidance during thruster plume impingement. This paper proposes an approach to rate damping from a e/i vector separated ellipsoidal trajectory. This trajectory is a safe trajectory with a low risk of collision in contingency cases. Moreover, it offers higher efficiency in reducing the rotational motion of a debris because the debris removal satellite flies around the debris and can inject a thruster plume from various directions. Conversely, if the debris removal satellite tries to damp the rotation from V-bar, the torque applied to the debris has strong directional sensitivity, thereby making it difficult in the worst case to damp the rotation from V-bar. Both situations were simulated and compared to verify the validity of the proposed approach. The torque and force applied to the debris were obtained by interpolating a predefined torque/force database. This database was obtained by conducting more than 900 cases of high-fidelity Computational Fluid Dynamics (CFD) simulations, changing the direction, distance, and point of thruster plume impingement. The simulation results revealed that rotation of the debris was reduced using the proposed approach. Rate damping from the e/i vector separated trajectory applied larger torque on the debris than those from V-bar. Hence, the rotation was reduced with less fuel and a shorter time. In addition, the e/i vector separated trajectory has less risk of collision with space debris even in a contingency case as compared to the V-bar hold. Thus, a safe and efficient rate damping approach to debris rotation was proposed, and will contribute to the probability of success for active debris removal missions.

This paper aimed to investigate the feasibility of a contactless method to detumble space debris by using thruster plume impingement during proximity operation. To detumble the rotational debris, t...

形態: カラー図版あり, Physical characteristics: Original contains color illustrations, Accepted: 2017-11-03, 資料番号: PA1810069000

形態: カラー図版あり, Physical characteristics: Original contains color illustrations, Accepted: 2017-07-06, 資料番号: PA1810059000

2016 AIAA/AAS Astrodynamics Specialist Conference (September 13-16, 2016. Long Beach Convention Center), Long Beach, California, USA, 形態: カラー図版あり, Physical characteristics: Original contains color illustrations, 資料番号: PA1710040000, レポート番号: AIAA 2016-5660

In order to improve the accuracy of the expected casualty risk prediction for the destructive re-entry, the related uncertainty factors are identified and the uncertainty quantification approach are proposed. Based on our studies and experience, the identified dominant uncertainty factors are the model accuracy, the attitude stability mode, the shape complexity and shape change, and the initial conditions at the entry start and the break-up. High-fidelity numerical simulations play the important role to model complex multi-disciplinary physics and to cover the wide range of environmental conditions with small numbers of model validation data. The real shape and the deformation effects are initialy modeled by the high-fidelity numerical simulations and finally modeled by the reduced empirical physics-based models. Flight data acquisition also plays important the role to quantify the uncertainties of the attitude stability mode, the break-up altitude, and the temperature distributions and to validate the integrated risk analysis model. Some of the key findings obtained by the high-fidelity simulation are discussed. It is also shown that various types parameter dependencies such as Mach number, wall temperature, surface curvature, and the effect of the turbulent flows on the aerodynamic characteristics and the heat flux distributions should be considered in the empirical models.

The reactions of NO2 with cis-/trans-CH3NHNH, CH3NNH2 and CH2NHNH2 have been studied theoretically by quantum chemical calculations and steady-state unimolecular master equation analysis based on RRKM theory. The barrier heights for the roaming transition states between nitro (RNO2) and nitrite (RONO) isomerization reactions and those for the concerted HONO and HNO2 elimination reactions from RNO2 and RONO, affect the pressure dependences of the product-specific rate coefficients. At ambient temperature and pressure, the dominant product of the reactions of NO2 with cis-/trans-CH3NHNH and CH2NHNH2 would be expected to be HONO with trans-CH3NNH and CH2NNH2, respectively, whereas it is CH3N(NH2)NO2 for CH3NNH2 + NO2. The product-specific rate coefficients for the titled and related reactions on the same potential energy surfaces were proposed for kinetics modeling., 形態: カラー図版あり, Physical characteristics: Original contains color illustrations, 資料番号: PA1610065000

For shear coaxial injectors, which are widely used in liquid oxygen–gaseous hydrogen rocket engines, the key parameters that characterize injector geometry are the recess length, taper angle, and wall thickness of the liquid oxygen post, which is the inner tube of the coaxial injector. In the present study, the effects of the recessed length and taper angle of the liquid oxygen post on cryogenic coaxial jets at supercritical pressures were experimentally investigated. The large-scale structure and mixing enhancement of cryogenic nitrogen and gaseous helium coaxial jets were evaluated using a high-speed backlighting technique. Instantaneous backlit images revealed that the dark core, which is believed to be dense nitrogen, exhibited sinusoidal forms induced by the presence of the outer helium jets. However, the sinusoidal oscillations were less influenced by the injector geometry. The mixing enhancement of coaxial jets was evaluated with respect to the dark-core length and the jet spread angle. The recessed liquid oxygen post enhanced the mixing of cryogenic nitrogen when the recess length was sufficiently large, which suggests that a recess of a certain length is required to improve the mixing of cryogenic coaxial jets at supercritical pressures. Because the decrease in the dark-core length with the recessed length was consistent with the increase in the jet spread angle at the injector exit, the mixing enhancement caused by the recess can be regarded as a result of an improvement in the mixing between the inner and outer jets in the recessed region. The effects of the liquid oxygen post exit taper on the mixing enhancement were more significant than those of the recess. The decrease in the dark-core length with the inner-to-outer momentum flux ratio (J number) is consistent with the relevant empirical correlation at supercritical pressures. Therefore, the mixing enhancement caused by the liquid oxygen post exit taper can be considered the result of an increase in the J number, rather than the result of giving azimuthal velocity to the cryogenic nitrogen jet at the liquid oxygen post exit., 資料番号: PA1510017000

Hydrazine (N2H4)/nitrogen tetroxide (NTO) co-flowing jets were simulated to explore the hypergolic ignition processes in N2H4/NTO bipropellant thrusters. The Navier−Stokes equations with a detailed chemical kinetics mechanism were solved by direct numerical simulation to reveal the influence of the distinct chemical reaction; i.e., hydrogen abstraction by nitrogen dioxide (NO2). In the ignition processes, the hydrogen abstraction sequence, especially the reaction of N2H3 and NO2, played a significant role in preheating the mixture gases and it was significant in the NTO-rich conditions rather than in the specifically stoichiometric conditions. Thus, in the N2H4/NTO co-flowing jets, the preheated regions correspond to the regions where the NTO-rich flows existed and contacted the N2H4 jet. The ignition eventually occurred in the region where NTO has sufficient concentration. Hence, it was found that the ignition timing and position strongly depend on whether the unsteady behavior of the NTO jet provides appropriate environments for the hydrogen abstraction reactions., 資料番号: PA1510040000

Hydrazine (N2H4)/nitrogen tetroxide (NTO) co-flowing plane jets were simulated to explore the hypergolic ignition processes and flame structures in N2H4/NTO bipropellant thrusters. The Navier–Stokes equations with a detailed chemical kinetics mechanism were solved in a manner of direct numerical simulation to reveal the influence of the distinct chemical reaction, i.e., hydrogen abstraction by nitrogen dioxide (NO2) and the thermal decomposition of N2H4. In the ignition processes, the hydrogen abstraction sequence played a significant role in preheating the mixture gases. Further, the ignition eventually occurred in the region where both N2H4 and NTO were sufficiently supplied for preheating. Hence, the ignition position and delay strongly depended on the fluid-mixing conditions. After the flames reached a steady state, the combustion flames uniquely comprised two types of flames, the outer diffusion flame and the inner decomposition flame. The outer diffusion flame came from the oxidization by NTO. The inner decomposition flame was caused and maintained by the heat transfer from the outer diffusion flame and a high rate of heat release from the thermal decomposition of N2H4. Because of the decomposition flame, the decomposition products such as NH3 and H2 were the major constituents of the downstream combustion gases., 資料番号: PA1510062000

Direct numerical simulation (DNS) of a cryogenic nitrogen/nitrogen gas temporally developing mixing layer in transcritical conditions was conducted to explore the influence of the pseudo vaporization at the pseudo-critical temperature. For comparison, a cryogenic nitrogen/nitrogen gas mixing layer in ideal-gas conditions was also simulated with the same initial profiles of density and velocity. Flow visualization and statistical analysis revealed that dominant eddies in development of turbulence were not directly affected by the pseudo vaporization, and were formed in the upper stream region of the mixing layer in the same manner as in the ideal-gas condition. Thus, the Reynolds stress and turbulent heat flux were similar between the two conditions. Accordingly, the mean and fluctuation profiles of streamwise velocity and specific enthalpy were also similar between the two conditions. Therefore, it is noted that the turbulent transport mechanisms of momentum and heat are little affected by the pseudo vaporization. In contrast, the density profiles were influenced by the pseudo vaporization. The cryogenic dense nitrogen was entrained deeply into the middle part of the mixing layer under the transcritical condition. This is because the nitrogen maintains a relatively high density until heated up to the pseudo-critical temperature. As a result, the mean profile of density in the transcritical condition was much higher than that in the ideal-gas condition through the mixing layers. Consequently, the transport mechanism of mass under the transcritical condition is considered to be influenced by the pseudo vaporization.

The vaporization and burning of the N2H4 and NTO droplets were simulated with the interface-tracking method to accurately explore the auto-ignition processes and the flame structures. The N2H4 vapor plume developed behind the N2H4 droplet and reacted with the ambient NO2 gas through the hydrogen abstraction reactions. Thus, the N2H4 vapor and NO2 gas mixtures behind the droplet were preheated and reached the auto-ignition at a few ms. The auto-ignition occurred in the multiple points almost at the same time. After the ignition, the premixed flame developed around the droplet. Thus, the vaporization of the liquid N2H4 near the surface became significant. Then, the double flame structures which comprise the inner decomposition flame and oxidation flame appeared around the droplet. The NTO droplet was not auto-ignited in the computational time of the present study because little N2O4 vapor near a saturated temperature decomposed to NO2 gas which is necessary for the hydrogen abstraction reactions. When the ignition was forced, the double flames developed. The outer decomposition flame propagated to the boundaries of the computational domain, while the inner oxidation flame appeared near the droplet. Except for the propagation of the decomposition flame, the NTO droplet combustion was similar to that of the industrial fuels.

5th European Conference for Aeronautics and Space Sciences, EUCASS 2013 (July 1-5, 2013.), Munich, Germany, "Many of liquid rocket engines used in first stage of launch vehicles operate at a pressure above the critical pressure of oxygen, 5.04 MPa. The cryogenic oxygen injected into a combustion chamber is heated up to several thousand kelvin, therefore the high-density cryogenic oxygen necessarily experiences sudden expansion called “pseudo-boiling”1 at the pseudo-critical temperature. The pseudo-boiling has large impact on the mixing characteristics, hence a jet injected from a temperature below the pseudo-critical temperature must be distinguished from that injected from a temperature above the pseudo-critical temperature. The former is referred to as “transcritical jet” while the latter is simply called supercritical jet. Quantitative measurement data of transcritical jets are indispensable both for the better understanding of mixing phenomena and for the validation of numerical methods, but there are only few quantitative experimental data made public.2 In this report, axial and radial temperature profiles of a single phase cryogenic nitrogen jet under supercritical pressure are measured, and the influence of pseudo-boiling upon the jet mixing will be discussed.", 資料番号: PA1510093000

Cryogenic single round and coaxial jets were observed under subcritical and transcritical conditions using a high-speed camera to explore the effects of the single-phase-like and two-phase-like properties of transcritical fluids on the unsteady behavior and flow structures of the jets. Cryogenic nitrogen and gaseous nitrogen were used as injectants. Cryogenic nitrogen was clearly visualized in all cases as the dark core. In the cases of single round jets, the large-scale characteristics of the jet behavior observed, such as the width of the dark core and the length scale of wave development in the mixing layers, were similar to those observed under subcritical conditions. Furthermore, dense nitrogen in the mixing layer entrained into the ambient environment and stretched in a manner similar to that observed under subcritical conditions. However, in contrast to the droplet formation observed under subcritical conditions, the entrained dense nitrogen diffused in a manner similar to turbulent mixing in a variable-density single-phase mixing layer. This means that the microscale behaviors that occur under transcritical conditions, which are comparable to turbulent eddies, are similar to those that occur in variable-density single-phase jets. In the cases of coaxial jets, the turbulent-eddy-scale phenomena observed were also similar to those observed for variable-density single-phase jets. However, in contrast to the phenomena observed for single round jets, the larger-scale behavior of the dark core was different from that observed under subcritical conditions. Two significant features were observed only under transcritical conditions. The first feature was the formation of shear-layer instability waves near the injector exit. The second feature was the shedding of dense nitrogen lumps from the end of the dark core at nearly uniform intervals. Because similar behavior was not observed under subcritical conditions, the single-phase-like properties of transcritical fluids are considered to have induced these features. The present correlation between the dark core length and the outer-to-inner jet momentum flux ratio was found to be in good agreement with the empirical correlation for variable-density single-phase coaxial jets., 形態: カラー図版あり, Physical characteristics: Original contains color illustrations, 資料番号: PA1610078000

The effects of LOX post recess length on the mixing characteristics of coaxial jets, such as the inner-jet-core length and jet spread angle, were investigated under supercritical pressures. In experiments, cryogenic nitrogen/gaseous helium coaxial jets were visualized using the shadowgraph technique. The LOX post recess enhanced the mixing of the inner nitrogen jet when its length was longer than one LOX post exit diameter. The outer helium jet also expanded as the recess length increased. In addition to the effects of LOX post recess, it is also noted that the inner nitrogen jet had sinusoidal structures only in coaxial injection cases. In numerical simulations, the numerical model for Reynolds-averaged Navier-Stokes (RANS) simulations was in the first place validated by comparing RANS results with Direct Numerical Simulation results of a temporally developing mixing layer at supercritical pressures. Unsteady RANS simulations of coaxial jets at supercritical pressures were conducted in the cases with and without the LOX post recess. As in experiments, the inner jet core in the case with the recess was shorter than that in the case without the recess. These effects of recess were most likely due to the enhancement of the inner mixing layer instability inside the recess region.