The paper deals with the aerodynamic analysis of a manned braking system entering the Mars atmosphere with the aim to support planetary entry system design studies. The exploration vehicle is an axisymmetric blunt body close to the Apollo capsule. Several fully three-dimensional computational fluid dynamics analyses have been performed to address the capsule aerodynamic performance. To this end, a wide range of flow conditions including reacting and nonreacting flow, different angles of attack, and Mach numbers have been investigated and compared. Moreover, nonequilibrium effects on the flow field around the entry vehicle have also been investigated. Results show that real-gas effects, for all the angles of attack considered, increase both the aerodynamic drag and pitching moment whereas the lift is only slighted affected. Finally, results comparisons highlight that experimental and CFD aerodynamic findings available for the Apollo capsule in air adequately represent the static coefficients of the capsule in the Mars atmosphere.
The paper deals with the aerodynamic analysis of a manned braking system (MBS) entering the Mars atmosphere with the aim to support planetary entry system design studies.
The human exploration of Mars will be a complex undertaking. It is an enterprise that will confirm the potential for humans to leave our home planet and make our way outward into the cosmos. Though just a small step on a cosmic scale, it will be a significant one for humans, because it will require leaving Earth with very limited return capability. The commitment to launch is a commitment to several years away from Earth, and there is a very narrow window within which return is possible. This is the most radical difference between Mars exploration and previous lunar explorations [
The paper reports on some aerodynamic analysis of an Apollo-shaped vehicle performed for flight conditions compatible for a manned mission entering the Mars atmosphere. With this in mind, those results may be used to provide numerical data for understanding requirements for human exploration of Mars. To this end, aerodynamic analysis has been made at several levels. For instance, vehicle aerodynamic assessment has been extensively addressed through an engineering-based design approach as hypersonic panel methods. Then, a number of computational fluid dynamics (CFD) simulations of the hypersonic flow field past the entry capsule have been performed, and results were provided in the paper.
The reasons that suggest getting the Mars manned exploration ready are several. Mars is the most accessible planet beyond the Earth-Moon system where sustained human presence is believed to be possible. The technical objectives of Mars exploration should be to understand what would be required to sustain a permanent human presence beyond Earth. Moreover, the scientific objectives of Mars exploration should be to investigate the planet and its history to better understand Earth. The human exploration of Mars currently lies at the ragged edge of achievability. The necessary technical capabilities are either just available or on the horizon. Commitment to the program will both effectively exploit previous investments and contribute to advances in technology. Finally, the goals of Mars exploration are grand; they will motivate our youth, benefit technical education goals, and excite the people and nations of the world.
The crew will travel to and from Mars on relatively fast transits (4 to 6 months) and will spend long periods of time (18 to 20 months: days nominal) on the surface, rather than alternative approaches which require longer time in space and reduced time on the surface [
Typical fast-transit interplanetary trajectory to Mars and return [
In the paper, however, neither mission architecture needed to reach Mars from Earth or neighbour Earth space, nor surface exploration have been addressed. Only capsule aerodynamics in Mars atmosphere have been focused on. In this framework, fully three-dimensional CFD analyses, both Euler and Navier-Stokes, have been performed to address the aerodynamic performance of the exploration vehicle, considering an entry approach scenario to the red planet compliant with the spacecraft released from circular parking orbit [
The Martian atmosphere has been considered as a mixture of 95.7% carbon-dioxide, 1.6% argon, and 2.7% nitrogen. The flow has been modelled as a reacting gas mixture of 9 species (Ar, CO2, N2, O2, CO, NO, N, O). The fluent code together with user-defined functions, developed in order to simulate mixtures of gas in thermochemical nonequilibrium, has been used for CFD computations with a nonequilibrium chemical model suitable for Martian atmosphere [
The MBS configuration, under investigation in this work, is shown in Figure
The manned braking system.
Such a system design choice has been addressed in order to reduce overall development cost and design risk. In fact, capsule technology is still the safest and cheapest way to get an exploration crew into orbit and then entry to planetary atmosphere as in the case of Mars. Moreover, even if the configuration is essentially ballistic, the vehicle is able to exhibit lifting capabilities by offsetting the centre of gravity (CoG). Note that the aerodynamic lift capability is fundamental for range extension and manoeuvrability in the descent and landing phases, since lift permits the correction of errors occurring in the guidance, navigation, and control systems, thus attaining the desired landing site in spite of such errors. In addition, aerodynamic lift gives desirable advantages in the form of operational flexibility in the positioning of the line of nodes of the parking orbit and in maximizing the time available for performing the deorbit manoeuvre.
Generally speaking, the MBS design depends on mission flight scenario requirements, which define capsule entry corridor. For instance, the entry corridor envelopes all the flyable/admissible entry trajectories whose loading environment is tolerable by the capsule. It is bounded from one side by the peak heat flux and the maximum deceleration, from the other by the ablator thermal limitations (total heat load), if present, and the skip angle. The dispersion of the trajectory within the entry corridor depends on two main design parameters that are the entry flight path angle and velocity, which are characterized by the selected planetary approach trajectory. Indeed, the angle and the velocity at entry interface determine the time of permanence in the Martian atmosphere. The shallower the entry angle, the bigger the flight time and the dispersion due to the atmospheric model error, and, hence, the worse the landing accuracy. From the point of view of approach strategies, the different values of velocity at entry interface (given the entry angle) will characterize the MBS design by means of mechanical loads (i.e., pressure and acceleration), thermal loads (i.e., heat flux peak and integrated heat load), and landing dispersion. These parameters counterbalance with each other, in the sense that the higher the entry velocity (or the steeper the entry angle), the larger the deceleration during the descent path (higher structure solicitations), and the higher the heat flux peak (higher TPS solicitations). Moreover, the lower the entry velocity (or the shallower the entry angle), the bigger the total heat flux (thicker ablative materials layer), the longer the atmospheric flight time, hence, the higher the landing dispersion (bigger atmospheric model errors).
In this paper, the flight scenario refers to entry conditions compatible with a capsule released from Mars parking orbit that the overall expedition system to Mars achieves after the red planet capture through aerobraking maneuvers.
With this in mind, fully three-dimensional CFD simulations both for perfect and chemically reacting gas approximation have been computed, according to the space-based design approach, at the freestream conditions listed in Table
Freestream conditions of CFD computations.
Mach (−) | Pressure (Pa) | Temperature (k) | AoA (deg) |
---|---|---|---|
5 | 1400 | 560 | 10 |
10 | 1400 | 560 | 10 |
20 | 1400 | 560 | 20 |
20 | 1400 | 560 | 28 |
Moreover, also nonequilibrium computations have been performed, since, as it is well known, one of the most challenging problems facing the design of atmospheric entry vehicle is the phenomenon of “real gas behaviour”. For instance, the shock wave produced ahead of the vehicle travelling at hypersonic speeds suddenly elevates the temperature of the gas surrounding the vehicle, so that the thermal energy of the gas may be comparable with the energy associated with a whole range of gas phase chemical processes, such as the excitation of molecular modes of vibration; the dissociation of atmospheric molecules into their atomic forms; the formation of other chemical species through recombination reactions; the ionisation of both molecular and atomic species [
Since the ratio between the specific heats (
Further, the “real gas effects” play a relevant role in the thermodynamics of the flow around the vehicle. For example, thermodynamic equilibrium is not established instantaneously in the moving gas, but requires a finite time known as relaxation time. Departure from thermodynamic equilibrium can have significant effects on shock wave structure, thus affecting the flow field around the vehicle [
The chemical dissociation of the flow in the shock layer can result in a large density ratio
Further, both the shock shape and standoff distance are markedly influenced by
Moreover, the nondimensional distribution of surface pressure relative to stagnation point pressure is changed as highlighted by numerical results collected hereinafter.
The sonic line position shifts because of the change in
Body stability is a critical requirement for re-entry vehicle, because of static instability could lead to catastrophic failure if the thermal shield is not protecting the vehicle anymore. This is the explanation of the relation that exists between pitching moment coefficient (
In order to address the real gas effects, the Martian atmosphere has been considered as a reacting gas mixture of nine species (Ar, CO2, N2, O2, CO, NO, N, O) involved in 49 forward and backward chemical reactions [
Reactions mechanism and rate parameters.
Reaction | Third body M | |||
---|---|---|---|---|
CO2 + M→CO + O + M | CO2, CO, N2, O2, NO | 6.9 × 1021 | −1.5 | 63275 |
Ar | 6.9 × 1020 | |||
C, N, O | 1.4 × 1022 | |||
CO + M→C + O + M | CO2, CO, N2, O2, NO | 2.3 × 1020 | −1.0 | 129000 |
Ar | 2.3 × 1019 | |||
C, N, O | 3.4 × 1020 | |||
N2 + M→N + N + M | CO2, CO, N2, O2, NO | 7.0 × 1021 | −1.6 | 113200 |
Ar | 7.0 × 1021 | |||
C, N, O | 3.0 × 1022 | |||
O2 + M→O + O + M | CO2, CO, N2, O2, NO | 2.0 × 1021 | −1.5 | 59750 |
Ar | 3.0 × 1021 | |||
C, N, O | 3.0 × 1022 | |||
NO + M→N + O + M | CO2, C, N, O, NO | 1.1 × 1017 | 0.0 | 75500 |
Ar | 5.0 × 1015 | |||
CO, N2, O2 | 5.0 × 1015 | |||
C2 + M→C + C + M | All | 2.0 × 1021 | −1.5 | 59750 |
NCO + M→CO + N + M | All | 6.3 × 1016 | −0.5 | 24000 |
NO + O→N + O2 | 8.4 × 1012 | 0.0 | 19450 | |
N2 + O→NO + N | 6.4 × 1017 | −1.0 | 38370 | |
CO + O→C + O2 | 3.9 × 1013 | −0.18 | 69200 | |
CO2 + O→CO + O2 | 2.1 × 1013 | 0.00 | 27800 |
Note that this reaction scheme neglects ionic reactions since the degree of ionization is expected to be low in the environment of interest (e.g., entry below 9 km/s), as suggested by Park et al. in [
The aerodynamic analysis of MBS is shown in terms of lift (
The reference parameters that have been chosen for the definition of the aerodynamic forces and moment nondimensional coefficients are the longitudinal reference length (
The evaluations of the vehicle aerodynamic database (AEDB) has been performed by means of engineering tools and CFD computations to focus on some critical design aspects not predictable with simplified tools as, for example, real gas effects.
Engineering-based aerodynamic and aerothermodynamic analyses have been extensively performed by using a 3D Panel Methods code, namely, SIM (surface impact method) developed by CIRA in the frame of its research activities on preliminary design of re-entry vehicles. This tool, at high supersonic and hypersonic speeds, is able to accomplish the aerodynamic and aerothermodynamic analyses of a complex vehicle configuration by using simplified approaches as local surface inclination methods and approximate boundary-layer methods, respectively. The SIM typical of hypersonics are Newtonian and Modified Newtonian theories. In Figure
The MBS panel mesh.
MBS aerodynamic results provided by engineering-based analysis cover
As an example of SIM results, Figure
Pressure coefficient contours on MBS surface at
The curves of lift, drag, and aerodynamic efficiency are shown in Figure
Lift, drag, and
Computational fluid dynamics analyses are performed to simulate the flow field past the entering vehicle to assess MBS aerodynamic performance. Both perfect gas and reacting gas with finite rate chemistry models (see Table
The first term is the conductive heat-flux from fluid to the wall due to the temperature gradient. The second one is the diffusion term due to the species gradient. The latter contribution depends strongly on the surface catalytic properties of vehicle heat-shield [
Transport coefficient for pure species are derived from kinetic theory of gases; while the global transport properties of the gas mixture, semiempirical rules have been applied, such as the Wilke mixing rule for viscosity
The CFD analysis of the MBS has been preceded by a code validation phase performed considering the available numerical and experimental data for the Mars Pathfinder probe [
As an example of the results provided by the validation phase, Figure
Mars Pathfinder. Mach number contours at trajectory peak heating conditions. Comparison between perfect gas (upper side) and equilibrium flow. Axisymmetric computation.
Mars Pathfinder. Temperature contours at trajectory peak heating conditions. Comparison between perfect gas (upper side) and equilibrium flow. Axisymmetric computation.
As one can see, both comparisons between perfect gas and equilibrium flow numerical computation underline that real gas effects markedly affect the flow field around the capsule and, hence, its aerodynamic performance. The effects of chemical dissociation can be recognized in Figure
Mars Pathfinder. Contours of CO2 and N2 mass fractions at trajectory peak heating conditions. Axisymmetric computation.
Finally, Figure
Mars Pathfinder. Comparison of surface pressure to stagnation pressure ratio between present computation and results of [
Present CFD computations of MBS have been carried out on a multiblock structured grid (shown in Figure
The computational domain.
The distribution of surface grid points has been dictated by the level of resolution desired in various areas of the vehicle, such as the stagnation region and the base fillet, according to the computational scopes. A close-up view of the 3D mesh on the vehicle surface can be seen on the right side of Figure
The results of CFD simulations are summarized hereinafter. For example, the flow field predicted around the MBS at
Mach number (a) and static pressure contours for
The same flow field features are reported in Figure
Mach number (a) and static pressure contours for
In order to have an idea of the three-dimensional flow field that takes place past the entry capsule at
The static temperature field on the capsule symmetry plane and on two flow field cross-sections at
In Figure
Static temperature contours for
For instance, as the flow turns around the capsule shoulder, it rapidly expands and can separate. The leeward side flow separates just after the shoulder whereas the windward side flow remains attached until the rear apex. The separated flow region is called the near wake. Further, a shear layer separates the outer flow from the recirculating inner core, which consists of multiple counter rotating vortices. The maximum flow field temperature in the case of perfect gas is close to about 40000 K. This means that thermochemical processes occur behind the bow shock as species vibrational excitation and dissociation. This is clearly shown by the maximum flow field temperature that, in the case of nonequilibrium computation, reaches only about 8000 K. The contour fields of carbon dioxide and nitrogen which arises at
Contours of CO2 and N2 mass fractions on the MBS pitch plane.
The curves of lift, drag, aerodynamic efficiency, and pitching moment coefficients are shown in Figures
Lift and drag coefficients versus
As shown, real gas effects increase both the aerodynamic drag and pitching moment coefficient, whereas the lift is only slighted influenced. Note that, Figures
The next set of comparisons, among CFD, experimental and numerical results, are reported in Figures
Pressure distribution in the capsule pitch plane for two AoAs (i.e., 0 and 10 deg). Comparison among MN, present CFD results, and WT data [
Pressure distribution in the capsule pitch plane for two AoAs (i.e., 20 and 28 deg). Comparison among MN, present CFD results, and WT data [
As one can see, numerical, experimental, and theoretical data compare well for all the AoAs, thus confirming reliability of the CFD simulations. Finally, it is worth noting that the differences existing between numerical and MN pressures, at the capsule corner (i.e.,
The paper deals with the aerodynamic analysis of a manned braking system for Mars exploration mission. A number of fully three-dimensional Navier-Stokes and Euler computational fluid dynamics simulations of the hypersonic flow field past an Apollo-shaped MBS have been performed in the framework of an entry loading environment compliant with the spacecraft released from a Mars circular parking orbit.
The range between Mach 2 and Mach 20 has been analyzed, with the goal to provide aerodynamic database at a Phase-A design level, for flight mechanics analyses. The aerodynamic coefficients have been provided as a function of Mach number and angle of attack (zero sideslip angle) according to the “space-based” design approach.
In the present analysis, only continuum regime (hypersonic speed ranges) with the flow modeled, both as perfect gas and reacting gas mixture, has been studied. Engineering-based analysis based on hypersonic panel methods has been extensively used in order to rapidly develop a very preliminary capsule aerodynamic database.
Finally, numerical results show that real gas effects increase both the aerodynamic drag and pitching moment coefficient whereas the lift is only slighted influenced. Moreover, several results comparisons highlight that experimental and CFD aerodynamic findings available for the Apollo capsule in air adequately represent the static coefficients of the MBS in the Mars atmosphere.
Drag coefficient
Lift coefficient
Pitching moment coefficient
Pressure coefficient
Aerodynamic drag, N
Aerodynamic force, N
Aerodynamic lift, N
Mach number/aerodynamic moment, Nm
Pressure, Pa
Dynamic pressure, Pa
Radius of curvature, m
Reynolds number
Reference area, m2.
Angle of attack, deg
Density, kg/m3
Specific heats ratio.
Base
Reference
Stagnation point downstream a normal shock
Wall
Pitching moment
Freestream conditions.