Showing papers in "Journal of Spacecraft and Rockets in 2003"

Turbulent Boundary Layer in Compressible Fluids

Abstract: The continuity, momentum, and energy differential equations for turbulent flow of a compressible fluid are derived, and the apparent turbulent stresses and dissipation function are identified. A general formula for skin friction, including heat transfer to a flat plate, is developed for a thin turbulent boundary layer in compressible fluids with zero pressure gradient. Curves are presented giving skin-friction coefficients and heat-transfer coefficients for air for various wall-to-free-stream temperature ratios and free-stream Mach Numbers. In the special case when the boundary layer is insulated, this general formula yields skin-friction coefficients higher than those given by the von Karman wall-property compressible-fluid formula but lower than those given by the von Karman incompressible-fluid formula. Heat transfer from the boundary layer to the plate generally increases the friction and heat-transfer coefficients.

The buckling of thin cylindrical shells under axial compression

Abstract: In two previous papers [1,2] the authors have discussed in detail the inadequacy of the classical theory of thin shells in explaining the buckling phenomenon of cylindrical and spherical shells. It was shown that not only the calculated buckling load is 3 to 5 times higher than that found by experiments, but the observed wave pattern of the buckled shell is also different from that predicted. Furthermore, it was pointed out that the different explanations for this discrepancy advanced by L. H. Donnell [3] and W. Flugge [4] are untenable when certain conclusions drawn from these explanations are compared with the experimental facts. By a theoretical investigation on spherical shells [1] the authors were led to the belief that in general the buckling phenomenon of curved shells can only be explained by means of a non-linear large deflection theory. This point of view was substantiated by model experiments on slender columns with non-linear elastic support [2] . The non-linear characteristics of such structures cause the load necessary to keep the shell in equilibrium to drop very rapidly with increase in wave amplitude once the structure started to buckle. Thus, first of all, a part of the elastic energy stored in the shell is released once the buckling has started; this explains the observed rapidity of the buckling process. Furthermore, as it was shown in one of the previous papers [2] the buckling load itself can be materially reduced by slight imperfections in the test specimen and vibrations during the testing process.

Two-Dimensional Subsonic Flow of Compressible Fluids

Abstract: The basic concept of the present paper is to use a tangent line to the adiabatic pressure-volume curve as an approximation to the curve itself. First, the general characteristics of such a fluid are shown. Then in Section I a theory is developed which can be applied to flows with velocities approaching that of sound, whereas the theory of Demtchenko and Busemann only give an approximation for flows with velocities smaller than one-half of the sound velocity. This is done by a generalization of the method of approximation to the adiabatic relation by a tangent line, conceived jointly by Th. von Karman and the author. The theory is put into a form by which, knowing the incompressible flow over a body, the compressible flow over a similar body can be calculated. The theory is then applied to calculate the flow over elliptic cylinders. In Section II the work of H. Bateman is applied to this approximate adiabatic fluid and the results obtained are essentially the same as those obtained in Section I.

Reentry Flight Controller Design Using Nonlinear Dynamic Inversion

Abstract: The application of nonlinear dynamic inversion to the design of a e ight controller for atmospheric reentry is discussed. Nonlineardynamic inversion is used due to thelarge e ight envelopethat characterizes thereentry of the small wingless lifting body vehicle. Moreover, the increased computational capability of modern e ight computers, as well as the mission requirements of crew return vehicles, supports the application of a nonlinear control law. The control design is based on nonlinear/linear inversion of the vehicle model and is divided in three phases: nonlinear inversion of the vehicle’ s dynamics, linearization and linear inversion of the aerodynamic database, and actuator allocation. The vehicle’ s dynamics were inverted assuming timescale separation between attitude and attitude rates. The assignment of the vehicle’ s actuators was scheduled against dynamic pressure, accounting for the efe ciency of each actuator on the different phases of e ight. Input ‐output linearization was performed for the phase of e ight where only two actuators were available. The correctness of the controller and its performance are evaluated with numerical simulations of two different entry trajectories.

Hypersonic Flight Transition Data Analysis Using Parabolized Stability Equations with Chemistry Effects

Abstract: Analysis of boundary-layer transition data from supersonic quiet tunnels, as well as flight experiments has indicated that, in the absence of surface roughness and high levels of freestream disturbances, linear stability theory can be used as a guide for estimation of the onset of transition. Transition data from two different hypersonic flight experiments are analyzed using parabolized stability equations, including chemistry effects associated with high-temperature boundary layers. The results suggest that transition in both of these cases is caused by the amplification of second mode disturbances. The analysis shows that, consistent with previous findings for supersonic flows where first mode disturbances induce laminar-turbulent transition, N factors of about 9.5 and 11.2 correlate the transition onset locations from these two high-Mach-number experiments. Therefore, the e N method can be used for smooth body transition prediction in hypersonic vehicle design. The effect of chemistry on boundary-layer stability is also studied and is shown to be destabilizing.

Attitude Stability Criteria for Dual Spin Spacecraft

Abstract: Attitude stability eriteria are developed for "dual spin" spacecraft, which consist of two primary bodies capable of unlimited relative rotation about a common axis. Such vehieles, typified in existing hardware by the Orbiting Solar Observatory (OSO) satellites, have applicability to missions for which the simplicity, reliability, and longevity of spin stabilization combine with a requirement for unidirectional pointing of a component, such as an antenna or a solar panel. The utility of dual spin systems is substantially increased by the results of attitude stability analysis, which reveals the possibility of obtaining passive stable spin-axis attitude by spinning a vehicle about its axis of greatest or least inertia, providing only that an effective energy dissipation device is attaced to a counter-rotating platform which has greatly reduced "spin." This result is coutrary to common interpretation of the familiar "major axis spin" requirement for stability, according to which energy dissipation in a vchicle rotating about its axis of least inertia must produce instability. Stability eriteria are developed first by Routhian analysis of a system with energy dissipation capability in only one of the two primary bodies, and subsequently in more general (but less rigorous) terms for a fully dissipative vehiele. Numerical integration and model studies provide corroboration.

Flexibility and the value of on-orbit servicing: New customer-centric perspective

Abstract: A new customer-centric perspective on on-orbit servicing, where the value of on-orbit servicing is studied independently from its cost, is proposed. A framework is developed that captures the value of e exibility provided by on-orbit servicing to space systems. Several options are made available to space missions through on-orbit servicing, such as the option to service for life extension or to upgrade, that need not be set before launch; they can be exercised after the spacecraft has been deployed, depending on how events unfold (market changes, new military contingency, etc. ). It is argued that only by accounting for this e exibility that the true value of on-orbit servicing can beevaluated. Applications of this framework to both nonproe t and commercial systems areprovided that demonstrate the usefulness of this new perspective on on-orbit servicing.

Design and Optimization of Low-Thrust Trajectories with Gravity Assists

Abstract: Missions such as Mariner 10, Voyager 1, Galileo, and Stardust all used gravity-assist flybys to achieve their mission goals efficiently. Methods to design such gravity-assist missions are fairly well developed and generally assume all major maneuvers are performed impulsively by chemical rockets. The recent success of the low-thrust Deep Space 1 mission demonstrates that low-thrust (high-efficiency) propulsion is ready to be used on future missions, potentially reducing the required propellant mass or the total time of flight. By combining both gravity-assist flybys and low-thrust propulsion, future missions could enjoy the benefits of both. To realize such missions, an effective design methodology is needed. A two-step approach to the design and optimization of low-thrust gravity-assist trajectories is described. The first step is a search through a broad range of potential trajectories. To speed up this search, a simplified shape-based trajectory model is used. The best trajectories are chosen using a heuristic cost function. The second step optimizes the most promising trajectories using an efficient parameter optimization. method. Examples of missions designed using this approach are presented, including voyages to Vesta, Tempel 1, Ceres, Jupiter, and Pluto.

Scalable Solar-Sail Subsystem Design Concept

Abstract: A scalable solar-sail concept, which integrates recently developed gossamer coilable longeron mast technology, has been developed, providing simple reliable deployment and structural robustness with minimum weight. This sail system is also unique in that it is composed of tensioned membranes without the incorporation of catenaries. This simplification is made possible through a mathematical demonstration of the insignificance of structural wrinkles on propulsive effectiveness. The sail package is a mass-optimized propulsion subsystem that can be mounted to a general heritage spacecraft to provide continuous low-level thrust. The design baseline is a three-axis-stabilized four-quadrant 40-m-square sail with attitude controlled by gimbaling the spacecraft on an extended boom. Considerations for the baseline design definition and the resulting performance vs size are reviewed.

Dot-Projection Photogrammetry and Videogrammetry of Gossamer Space Structures

Abstract: The technique of using hundreds or thousands of projected dots of light as targets for photogrammetry and videogrammetry of gossamer space structures is documented. Photogrammetry calculates the three-dimensional coordinates of each target on the structure, and videogrammetry tracks the coordinates vs time. Gossamer structures characteristically contain large areas of delicate, thin-film membranes. Examples include solar sails, large antennas, inflatable solar arrays, solar-power concentrators and transmitters, sun shields, and planetary balloons and habitats. Using projected-dot targets avoids the unwanted mass, stiffness, and installation costs of traditional retroreflective adhesive targets. Four laboratory applications are covered that demonstrate the practical effectiveness of white-light dot projection for both static-shape and dynamic measurement of reflective and diffuse surfaces, respectively. Comparisons are made between dot-projection videogrammetry and traditional laser vibrometry for membrane vibration measurements. A promising extension of existing techniques using a novel laser-induced fluorescence approach is introduced.

Governing equations for weakly ionized plasma flowfields of aerospace vehicles

Abstract: A generalized set of conservative equations for simulating the flowfield in a hypersonic weakly ionized gas flows is presented. Additional numerical and physical complexities associated with the plasma state are identified and discussed, including the influence of external and space charge fields and the nonequilibrium coupling of reactive and nonreactive collisions. A restricted set of equations is then employed to simulate and analyze the flowfield of an air plasma generated by associative ionization. With use of a seven-species air model, details of the plasma flowfields are presented and compared with available experiments. Refined estimates of dissociation products, which are the precursors to the ionization reaction, are obtained. Specific attention is given to limiting forms of the electron diffusion coefficient and the influence of vibration-dissociation coupling. Details of the chemical reactions in the flowfield influencing the elastic and inelastic collisional energy transfer are reported to highlight the more important species and reaction mechanisms.

Optimal Two- and Three-Impulse Fixed-Time Rendezvous in the Vicinity of a Circular Orbit

Abstract: Minimum-fuel, multiple-impulse orbital rendezvous is investigated for the case in which the transfer time is specified (time-fixed case). A method for obtaining optimal solutions is employed which is applicable to rendezvous or orbit transfer between elliptical orbits of low eccentricity. In this method optimal solutions are constructed by satisfying the necessary conditions for the primer vector. It is assumed that the terminal orbits lie close enough to an intermediate circular reference orbit that the linearized equations of motion can be used to describe the transfer. The linear boundary value problem for the impulse magnitudes for rendezvous is then solved analytically. As an application of the method, optimal two-and three-impulse fixed-time rendezvous transfers between coplanar circular orbits are obtained for a range of transfer times. These linearized solutions combined with previously obtained four-impulse solutions provide a complete solution for fixed-time coplanar circle-to-circle rendezvous between close orbits for transfer times up to nearly two terminal orbit periods.

Total Densities Derived from Accelerometer Data

Abstract: The German satellite Challenging Minisatellite Payload (CHAMP), carrying the STAR accelerometer onboard, was launched in July 2000. The CHAMP mission profile is compatible with studies of the thermosphere: it will providegood geographical and altitude coverage over a period of five years. The preprocessing of the accelerometer data consists of correcting them for maneuvers, specific events, and instrumental bias. The total density can then be reconstituted, employing a model for the aerodynamic coefficient. We have employed a theory based on diffuse reemission applied to a 15-plate macromodel. The accuracy of the observed densities depends mainly on the uncertainties of the estimated accelerometer calibration parameters and of the aerodynamic coefficient, as well as on geomagnetic activity. The six weeks of data analyzed in this preliminary study showed their high precision and model shortcomings. Assimilation of at least one year of data in a thermosphere model will significantly increase its accuracy, which, in turn, will improve satellite drag modeling.

Turbulence Modeling for Computational Aerodynamics

Abstract: The status of turbulence modeling for external aerodynamic flows is reviewed, and closure concepts for the compressible form of the Reynolds-averaged Navier-Stokes equations are briefly outlined to establish a framework for comparison. The importance of experimental requirements for developing and verifying turbulence models is emphasized. Attention is then given to three important flow categories: attached flows, separating and reattaching flows, and trailing-edge flows. Examples of comparisons between experiments and computations for twoand three-dimensional flows are presented to illustrate the status of modeling. It is shown that, for most two-dimensional attached flows, eddy viscosity concepts will probably be adequate. For attached three-dimensional flows, however, eddy-viscosity and Reynolds-stress models are both deficient, failing to predict the proper surface shear-stress in rapidly skewing boundary layers, regardless of whether they are pressure driven or strain driven.

Applying Jet Interaction Technology

Abstract: The state of the art in aerodynamics engineering associated with the design and/or evaluation of reaction controls on flight vehicles operating in the atmosphere is reviewed. Various configurations of the aerodynamics interference problem are described to partition domains of the problem. To maintain an applications frame of reference, those descriptions are in the context of the dominant phenomenology observed under differing flight environment and vehicle geometry combinations. Following that, approaches to predicting or evaluating interference effects are reviewed. Approaches to the design of subscale wind-tunnel tests are discussed with a view toward relating appropriate scaling law approximations to the different domains of dominant phenomenology. Then results, found in the literature, describing the evolution of analytical and computational modeling are reviewed. Finally, some conclusions and observations on the state of the art are offered.

Direct Simulation of Transitional Flow for Hypersonic Reentry Conditions

Abstract: This paper presents results of flowfield calculations for typical hypersonic reentry conditions encountered by the nose region of the Space Shuttle Orbiter. Most of the transitional flow regime is covered by the altitude range of 150 to 92 km. Calculations were made with the Direct Simulation Monte Carlo (DSMC) method that accounts for translational, rotational, vibrational, and chemical nonequilibrium effects. Comparison of the DSMC heating results with both Shuttle flight data and continuum predictions showed good agreement at the lowest altitude considered. However, as the altitude increased, the continuum predictions, which did not include slip effects, departed rapidly from the DSMC results by overpredicting both heating and drag. The results demonstrate the effects of rarefaction on the shock and the shock layer, along with the extent of the slip and temperature jump at the surface. Also, the sensitivity of the flow structure to the gas-surface interaction model, thermal accommodation, and surface catalysis are studied.

Flowfield Around Spike-Tipped Bodies for High Attack Angles at Mach 4.5

Abstract: The requirements for the design of a new short-range high-velocity missile are both the drag reduction and the correct information acquisition for the optoelectronic sensors embedded in the hemispherical nose. High anglesof attack must be studied to fulfill the maneuverability requirements of present and future missiles. A supersonic missile generates a bow shock around its blunt nose, which causes rather high surface pressure and temperature and, as a result, the development of high drag and damage of embedded sensors. The pressure and the temperature on the hemispherical nose surface can be substantially reduced if an oblique shock is generated by a forward-facing spike. Both the experiments and the computations are carried out to study the flowfield around three-dimensional blunt bodies equipped with forward-facing spikes for a large range of attack angles at a Mach number of 4.5. A blunt body, a classical disk-tip spike, a sphere-tip spike, and a biconical-tip spike are studied. The experiments involve high-pressure shock tunnel investigations using a shock tube facility. The differential interferometry technique is applied to visualize the flowfield around the different missile spike geometries. The differential interferogram pictures as well as surface pressure measurements are compared with numerical results. Numerical simulations based on steady-state three-dimensional Navier-Stokes computations are performed to predict the drag, the lift, and the pitching moment for the blunt body and for each spike-tipped missile. The computations allow one to bring out the advantages of each spike geometry in comparison to the blunt body.

Magnetohydrodynamic Control on Hypersonic Aircraft Under "Ajax" Concept

Abstract: The scramjet with magnetohydrodynamic control being developed in the framework of the "Ajax" concept is considered. Model of magnetohydrodynamic generator with nonequilibrium conductivity is discussed. Simple relations for calculation of electron concentration in air plasma sustained by electron beam are proposed. A model of scramjet with magnetohydrodynamic control is considered in the one-dimensional approach. A model of inlet with magnetohydrodynamic control is considered in two-dimensional approach. It is shown that magnetohydrodynamic interaction allows one to increase specific impulse of scramjet and modify flowfield in inlet.

Some Recent Advances in the Investigation of Shell Buckling

Abstract: A survey is presented which includes the buckling of shells under loads for which the shell is sensitive to initial imperfections. Results for such cases show that improvements in experiment and theory have produced previously unobtainable agreement. The necessity of the correct (and consistent) theoretical specification of boundary conditions is then demonstrated. Recent stiffened cylinder results are surveyed to expose the large effects on the buckling strength of internal or external stiffening, axial load applied eccentric to the wall neutral surface, and the addition of small meridional curvature.

Methodology for Turbulence Model Validation: Application to Hypersonic Flows

Abstract: Hypersonic transitional e ows over a e at plate and a sharp cone are studied using four turbulence models: the one-equation eddy viscosity transport model of Spalart ‐Allmaras, a low-Reynolds-number k‐" model, the Menter k‐! model, and the Wilcox k‐! model. A framework is presented for the assessment of turbulence models that includes documentation procedures, numerical accuracy, model sensitivity, and model validation. The accuracy of thesimulationsisaddressed,and thesensitivitiesofthemodelstogridree nement,freestream turbulencelevels,and wally + spacing are presented. The e at-plate skin-friction resultsare compared to the well-established laminar and turbulent correlations of Van Driest. Correlations for the sharp cone are discussed in detail. These correlations, along with recent experimental data, are used to judge the validity of the simulation results for skin friction and surface heating on the sharp cone. The Spalart ‐Allmaras model performs the best with regards to model sensitivity and model accuracy, whereasthe Menter k‐! model also performs well for these zero pressuregradient boundary-layer e ows.

First- and second-order theory of supersonic flow past bodies of revolution

Abstract: Methods are studied for improving the existing perturbation theories of supersonic flow past bodies of revolution. Applicability of the theory at high Mach Numbers is emphasized. For axial flow, a second-order solution isfound which represents a considerable improvement over the first-order result. For inclined flow, a second-order solution is not feasible except for a cone. Comparison with the exact solutions for cones shows that the slender-body series expansion causes large inaccuracies in both axial and inclined flow. The conclusion that first-order theory predicts the flow no better than slender-body theory is shown to be erroneous. When first-order theory is properly used,making no unnecessaryapproximations, greatly improved agreement is found with exact solutions and with experiment. The order estimates used to justify the approximations are shown to be invalid in most practical cases. A "hybrid" theory, combining first-order cross flow and second-order axial flow, gives further improvement. A physical explanation is advanced for the marked superiority of first-order theory over the true "linearized" theory. Nonlinearity in lift is shown to result primarily from viscous separation of the cross flow along the after portions of a long body. The magnitude of the resulting normal force can be estimated with reasonable accuracy using two-dimensional viscous sweep-back theory.

Prediction of damping coefficients using the unsteady Euler equations

Abstract: A prediction method for dynamic damping coefficients using the unsteady Euler equations is presented. Direct unsteady simulation can be used to compute the pitch-damping moment without any geometric approximations when compared to the steady methods using the coning motions. A forced harmonic pitching motion is employed to generate the pitch-damping moments. To compute the pitch- and the roll-damping moments for the basic finner, a dual-time stepping algorithm combined with an implicit multigrid method is applied. The computed coefficients show good agreement with the experimental data. Grid refinement and parametric studies are performed to assess the accuracy of the numerical method. The linearity of the angular rates and the variation with Mach numbers are examined for both pitch- and roll-damping moment coefficients. Through analysis of the pressure distributions at various Mach numbers, the large variations of roll-damping moment coefficient in the transonic region are explained in detail.

Laboratory Tests on 110-Volt Solar Arrays in Simulated Geosynchronous Orbit Environment

Abstract: Engineering Test Satellite VIII will be the first Japanese geosynchronous orbit satellite to have 110-V satellite bus voltage, and its solar arrays will generate the electricity at 110 V once it is launched in 2004. Laboratory tests on charging and arcing of the Engineering Test Satellite VIII solar arrays are carried out in a simulated geosynchronous orbit environment irradiating a solar array test coupon with an electron beam in a vacuum chamber. The concept of inverted potential gradient between the coverglass and the interconnector is verified experimentally. Arcs are observed once the potential difference exceeds approximately 400 V. It is confirmed that the sustained arc does not occur between the adjacent array strings as long as silicon rubber grouts the gap between the strings. Another type of sustained arc is observed between the array strings and the aluminum honeycomb substrate through defect on the Kapton ® sheet. Based on the test results, modification of the array design is possible.

Genetic algorithm and calculus of variations-based trajectory optimization technique

Abstract: A genetic algorithm is used cooperatively with the Davidon‐Fletcher‐Powell penalty function method and the calculus of variations to optimize low-thrust, Mars-to-Earth trajectories for the Mars Sample Return Mission. The return trajectory is chosen thrust-coast-thrust a priori, has a fixed time of flight, and is subject to initial and final position and velocity equality constraints. The global search properties of the genetic algorithm combine with the local search capabilities of the calculus of variations to produce solutions that are superior to those generated with the calculus of variations alone, and these solutions are obtained more quickly and require less user interaction than previously possible. The genetic algorithm is not hampered by ill-behaved gradients and is relatively insensitive to problems with a small radius of convergence, allowing it to optimize trajectories for which solutions had not yet been obtained. The use of the calculus of variations within the genetic algorithm optimization routine increased the precision of the final solutions to levels uncommon for a genetic algorithm.

Optimum Low-Thrust Rendezvous and Station Keeping

Abstract: Analytic solutions are determined for the optimum correction of all six elements of elliptic satellite orbits with low-thrust, power-limited propulsion systems. The optimum direction and magnitude of thrust are determinedas functions of time so as to minimize the fuel required to rendezvous in a given time or to minimize the fuel required for station keeping in the presence of known perturbations. The motion of the vehicle under the action of the optimum thrust program is also determined analytically. The solutions obtained are divided into two classes, depending on whether rendezvous requires few or many revolutions. In the latter case, simple analytic solutions are obtained explicitly by neglecting short-period perturbations.

Compressibility Correction for the Spalart-Allmaras Model in Free-Shear Flows

Abstract: A correction of the Spalart-Allmaras turbulence model to account for the compressibility effects in mixing-layer flows is presented. Unlike other corrections proposed for the K-e model, the present correction does not need the knowledge of the turbulent Mach number and, therefore, can be applied to those turbulence models, like the Spalart-Allmaras one, which do not integrate directly the turbulent kinetic energy equation. To explore the validity of the proposed correction, four mixing-layer flows and four supersonic backward-facing step flows, covering a wide range of flow conditions, were selected and computed using both the standard and the corrected Spalart-Allmaras model. The analysis of the numerical results and their comparison with the experimental data show that the proposed correction produces a significant improvement of the numerical predictions.

Simulation of dual one-way ranging measurements

Abstract: One of the main error sources for the microwave ranging method is the frequency instability of the oscillator that generates the carrier phase signal. A dual one-way ranging method is used to minimize the oscillator noiseeffect by combining two one-way ranging measurements. This study analyzed the dual one-way ranging system by simulation of the phase measurements with comprehensive error models. This simulation analysis was applied to the Gravity Recovery and Climate Experiment mission, which is a dedicated spaceborne mission with the objective of mapping the gravity field. The simulation results demonstrate that a high level of accuracy can be achieved by use of the dual one-way ranging system.

Atmospheric Flight on Venus: A Conceptual Design

Abstract: A solar-powered aircraft system is proposed for the exploration of Venus. The atmosphere of Venus provides several advantages for flying a solar-powered aircraft. At the top of the cloud level, the solar intensity is comparable to or greater than terrestrial solar intensities. The atmospheric pressure makes flight much easier than on planets such as Mars. Also, the slow rotation of Venus allows an airplane to be designed for flight within continuous sunlight, eliminating the need for energy storage for nighttime flight. These factors make Venus a prime choice for a long-duration solar-powered aircraft. Fleets of solar-powered aircraft could provide an architecture for efficient and low-cost comprehensive coverage for a variety of scientific missions.

Computational Aerothermodynamics in Aeroassist Applications

Abstract: Aeroassisted planetary entry uses atmospheric drag to decelerate spacecraft from super-orbital to orbital or sub- orbital velocities. Numerical simulation of flow fields surrounding these spacecraft during hypersonic atmospheric entry is required to define aerothermal loads. The severe compression in the shock layer in front of the vehicle and subsequent, rapid expansion into the wake are characterized by high temperature, thermo-chemical nonequilibrium processes. Implicit algorithms required for efficient, stable computation of the governing equations involving disparate time scales of convection, diffusion, chemical reactions, and thermal relaxation are discussed. Robust point-implicit strategies are utilized in the initialization phase; less robust but more efficient line-implicit strategies are applied in the endgame. Applications to ballutes (balloon-like decelerators) in the atmospheres of Venus, Mars, Titan, Saturn, and Neptune and a Mars Sample Return Orbiter (MSRO) are featured. Examples are discussed where time-accurate simulation is required to achieve a steady-state solution.