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Mars Microprobe Entry Analysis Mars Microprob e Entry Analysis y Rob ert D. Braun Rob ert A. Mitcheltree F. McNeil Cheatwo o d NASA Langley Research Center NASA Langley Research Center Vigyan Inc. Hampton, VA 23681-0001 Hampton, VA 23681-0001 Hampton, VA 23666-1325 (757) 864-4507 (757) 864-4382 (757) 864-2984 [email protected] [email protected] f.m.cheatwo o [email protected] Abstract{The Mars Microprob e mission will 1 Introduction provide the rst opp ortunity for subsurface The ob jective of NASA's New Millennium pro- measurements, including water detection, near gram is to demonstrate and ight qualify tech- the south p ole of Mars. In this pap er, p er- nology elements required for the science mis- formance of the Microprob e aeroshell design is sions of the next century [1 ]. The program's evaluated through development of a six-degree- second ight pro ject, Deep Space Two (DS{ of-freedom (6-DOF) aero dynamic database and 2) is fo cused on the design of two small Mars ight dynamics simulation. Numerous mission entry prob es. As a result, DS{2 is often re- uncertainties are quanti ed and a Monte-Carlo ferred to as the Mars Microprob e mission. This analysis is p erformed to statistically assess mis- pro of-of-concept system is intended to demon- sion p erformance. Results from this 6-DOF strate key elements of future network science Monte-Carlo simulation demonstrate that, in a missions [2 , 3]. Attached to the cruise stage ma jority of the cases (approximately 2{ ), the of the Mars 98 Surveyor Lander, these two p enetrator impact conditions are within current Microprob e vehicles will b e launched to Mars design tolerances. Several tra jectories are iden- by a Delta II ro cket in January 1999, arriving ti ed in which the current set of impact re- in Decemb er 1999. Each of these Microprob e quirements are not satis ed. From these cases, capsules houses instrumented p enetration de- critical design parameters are highlighted and vices designed to analyze the subsurface layers additional system requirements are suggested. by p erforming soil sampling and water detec- In particular, a relatively large angle-of-attack tion. On impact, the p enetrators are designed range near p eak heating is identi ed. to pierce their protective aeroshells, driving this subsurface instrumentation 0.3-2.0 m b elow the Table Of Contents surface. Subsurface data will b e relayed back to Earth through a link with the Mars Global Sur- 1. Introduction veyor orbiter (Septemb er 1997 Mars arrival). 2. Nomenclature 3. Impact Requirements The entry, descent, and impact (EDI) phase of 4.1 Aeroshell Selection the DS{2 mission b egins as the two capsules are 4.2 Aerodynamics mechanically separated from the cruise stage [4]. 4.3 Atmos. Flight Dynamics This event is preceded by separation of the Mars 5.1 Impact Sizing 98 Lander from the cruise stage (approximately 5.2 Monte-Carlo Simulation 1.5 s earlier). As a result of (1) the brief p erio d 6. Conclusions b etween these two separation events, (2) the lack of control of the cruise-stage after the 98 Space Systems and Concepts Division, Mail Stop 365 y Aero and Gas Dynamics Division, Mail Stop 408A Lander separation, and (3) geometric mounting 2 Q dynamic pressure, N/m constraints which do not allow the Microprob e V velo city, m/s vehicles to b e aligned with the ight path, the angle-of-attack, deg capsules will separate in an unknown angular m 2 orientation with non-zero angular rates. Sta- kg/m ballistic co ecient C A D ble ight of the Microprob e vehicles must b e ight-path or incidence angle, deg achieved passively, and maintained until surface impact. Subscripts a relative to the atmosphere Design of the DS{2 entry prob es is compli- A axial force cated by several unique aero dynamic challenges. D drag force The vehicles must p ossess enough aero dynamic l static rolling moment stability to achieve passive re-orientation from m static pitching moment an arbitrary initial motion prior to p eak heat- mq dynamic pitching moment ing. Since stable ight at impact is required, n static yawing moment the sup ersonic and transonic dynamic stability N normal force problems which have plagued other entry mis- nr dynamic yawing moment sions [5, 6 , 7 ] must also b e mitigated. Addi- p p enetrator tionally, the p enetrators must b e protected from r relative to the horizon the intense aerothermo dynamic environment of t total a 7.0 km/s Mars entry and satisfy a stringent Y side force set of surface impact constraints. 3 Surface Impact Requirements In this pap er, the criteria used to select the aeroshell geometry are presented. After re- The p enetrators have b een designed to op er- view of the aeroshell shap e and mass prop- ate prop erly under a range of impact condi- erties, compilation of the Microprob e aero dy- tions. Mission success demands that the EDI namic database is discussed. This database is system meet several surface impact constraints. compiled from past studies, computational uid Three{ requirements on surface impact velo c- dynamic calculations, and ground-based test ity (140 V 200 m/s), p enetration angle- r data. Development of a six-degree-of-freedom of-attack (0 10 degrees), and p en- p (6-DOF) Monte-Carlo tra jectory simulation for etration incidence angle (j j 20 degrees) p Microprob e EDI is also presented. Results from have b een sp eci ed [4 ]. These requirements this 6-DOF Monte-Carlo simulation are used to are currently b eing validated through a rigor- statistically assess the e ect of combinatorial ous ground-testing program. variations in the signi cant EDI parameters. The dynamics of the surface impact event are illustrated in Fig. 1. The Microprob e aeroshell, 2 Nomenclature lo cal horizon, and lo cal surface slop e are shown, 2 A reference aero dynamic surface area, m along with the velo cities with resp ect to the b yaw/roll reference length, m ground (V ) and atmosphere (V ). By conven- r a c pitch reference length, m tion, the ight-path angles shown are negative. C aero dynamic force or moment co ecient Atmospheric winds cause the di erence b etween g Mars surface slop e, deg V and V . Total angle-of-attack ( in Fig. 1) r a t Kn Knudsen numb er is de ned as the angle b etween the vehicle's axis m mass, kg of symmetry and V . a M Mach numb er 2 I b o dy{axis moments of inertia, kg{m During ight, the forces on the aeroshell are a 2 q_ stagnation{p oint heat rate, W/cm function of the relationship b etween the vehicle tolerances until impact. Finally, it must Vehicle small symmetry the payload from intense aero dynamic axis protect heating. To meet these ob jectives a 45-degree cone with rounded nose and shoul- Horizon half-angle γ . The afterb o dy r g ders is selected for the foreb o dy α γ t a is hemispherical with its center at the vehicle's Vr ter-of-gravity lo cation. Va cen Surface Blunted 45-degree sphere-cones were used for the successful Pioneer-Venus and Galileo mis- sions [8 ]. Both of these missions entered at- Figure 1. De nition of Mars Microprob e sur- mospheres much denser than Mars. For the face impact angles. Mars entries of Viking and Mars Path nder, 70-degree sphere-cones with a zero angle-of- and the atmospheric velo city vector ( and V ). t a attack drag co ecient near 1.7 (versus the 45- However, at impact, the orientation of the Mi- degree cone value of 1.05) were selected [9]. croprob e payload relative to the surface is of sig- Choice of cone angle calls for a compromise of ni cance. The p enetration angle-of-attack and drag, stability and packaging. Blunter cones ex- incidence angle are de ned as: hibit more drag p er surface area; sharp er cones = + ( ) (1) p t a r p ossess more stability. Viking and Path nder = + 90 g (2) make use of high drag aeroshells since b oth p r of these entries required deceleration of much As discussed b elow, the surface impact velo city heavier spacecraft at suciently high altitudes and incidence angle constraints may b e achieved for parachute deployment. In contrast, the through the selection of the appropriate ballis- Mars Microprob e vehicles are more than two tic co ecient (see Section 5); whereas, satis- orders of magnitude lighter and must impact faction of the impact angle-of-attack constraint the surface at a high velo city (140-200 m/s). is a function of the vehicle's aero dynamic sta- Additionally, each Microprob e capsule requires bility (geometry and center-of-gravity lo cation). the highest p ossible aero dynamic stability to re- Aero dynamic design of the Microprob e capsules cover quickly from any initial tumbling motion. is presented in Section 4. The variance in each of the signi cant impact parameters as well as The degree of nose bluntness has little e ect statistical data regarding the aeroshell heating on the drag co ecient for a 45-degree half- environment is presented in Section 5. angle cone, although increased bluntness do es slightly decrease static stability. On the other 4 Analysis hand, increased bluntness decreases the stagna- tion p oint heat rate during the hyp ersonic p or- Aeroshel l Selection tion of the tra jectory.
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