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AIAA 98–2445 Prediction and Validation of Mars Pathfinder

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AIAA 98–2445 Prediction and Validation of Mars Pathfinder AIAA 98–2445 Prediction and Validation of Mars Pathfinder Hypersonic Aerodynamic Data Base Peter A. Gnoffo, Robert D. Braun, K. James Weilmuenster, Robert A. Mitcheltree, Walter C. Engelund, Richard W. Powell NASA Langley Research Center Hampton, VA 23681-0001 7th AIAA/ASME Joint Thermophysics and Heat Transfer Conference June 15–18, 1998 / Albuquerque, NM For permission to copy or republish, contact the American Institute of Aeronautics and Astronautics 1801 Alexander Bell Drive, Suite 500, Reston, VA 20191 Prediction and Validation of Mars Path nder Hyp ersonic Aero dynamic Data Base Peter A. Gno o, Rob ert D. Braun, K. James Weilmuenster, Rob ert A. Mitcheltree, Walter C. Engelund, Richard W. Powell NASA Langley Research Center Hampton, VA 23681-0001 Post ight analysis of the Mars Path nder hyp ersonic, continuum aero dynamic data base is presented. Measured data include accelerations along the body axis and axis normal directions. Comparisons of pre ight simulation and measurements show good agreement. The prediction of two static instabilities asso ciated with movementof the sonic line from the shoulder to the nose and back was con rmed by measured normal accelerations. Reconstruction of atmospheric density during entry has an uncertainty directly prop ortional to the uncertainty in the predicted axial co ecient. The sensitivity of the moment co ecient to freestream density, kinetic mo dels and center-of-gravity lo cation are examined to provide additional consistency checks of the simulation with ight data. The atmospheric density as derived from axial co ecient and measured axial accelerations falls within the range required for sonic line shift and static stability transition as indep endently determined from normal accelerations. Intro duction Nomenclature A axial acceleration, earth g A A normal acceleration, earth g N HE hyp ersonic aero dynamic data base used in C axial force co ef. A Tthe Mars Path nder entry, descent, and landing C drag co ef. D EDL was predominantly generated using computa- C pitching moment co ef. m tional uid dynamics. While ground-based and ight C pitching moment co ef. m; data from the Viking pro ject were used to check p or- derivative, p er radian tions of the aero dynamic data for Path nder, di er- C normal force co ef. N ences in entry velo city and angle-of-attack between D prob e diameter, m Path nder and Viking required a more critical eval- h altitude, km uation of high temp erature e ects on hyp ersonic aero- M Machnumber dynamics. A systematic evaluation of Path nder aero- m entry mass, kg 2 dynamics as a function of velo city, altitude, and angle- p pressure, N/m of-attack in a continuum regime revealed twointervals T freestream temp erature, K 1 2 during a nominal entry in which static aero dynamic in- S reference area, m stabilities would induce vehicle \wobble". The static V freestream velo city, m/s 1 aero dynamic instabilitywas asso ciated with changing x; y ; z Cartesian co ordinates, m sonic line lo cation and asso ciated pressure distribu- angle-of-attack, degrees tions as a function of chemical kinetics and ow energy. total angle-of-attack T 2 2 1=2 These predictions and explanations were published in + , degrees 1, 2 journal articles in 1996 and the \wobble" was con- angle of yaw, degrees rmed by Path nder ight data accelerometers on e ective ratio of sp eci c heats 3 July 4, 1997. The physical phenomena leading to freestream density, kg/m 1 the static aero dynamic instability are reviewed, the measured accelerations con rming the instability are discussed, and additional p ost- ight analyses are pre- sented. The delity of comparison b etween pre- ight prediction and ight data signi cantly reduces uncer- c Copyright 1998 by the American Institute of Aeronautics and Astronautics, Inc. No copyright is asserted in the United taintyin the atmospheric reconstruction pro cess and States under Title 17, U.S. Co de. The U.S. Government has provides enhanced con dence in the application of aroyalty-free license to exercise all rights under the copyright CFD to future planetary missions with reduced design claimed herein for governmental purp oses. All other rights are margin. reserved by the copyrightowner. 1of10 American Institute of Aeronautics and Astronautics Paper 98{2445 corner .06625 m bubble leeside 70o x 2.65 m .6638 m .59 m nose V∞ α windside 43.4o V∞ 1.506 m α = 2o Fig. 1 Diagram of the Mars Path nder prob e. Pre ight Analysis Fig. 2 Sonic line lo cation over the Mars Path nder The Mars Path nder con guration is shown in Fig. at 2 deg. angle-of-attack and Machnumb ers equal o 1. The foreb o dy is a 70 half angle cone with base to 22.3 left, 16.0 center, and 9.4 right showing diameter 2.65 m, nose radius 0.6638m and shoulder the e ect of gas chemistry. radius 0.06625 m. The on-axis reference p oint for aero- dynamic moment co ecients is lo cated 0.662 m b ehind the nose. The actual center-of-gravity CG lo cation 1.0 is x ;y ;z = 0.000107, -0.000435, 0.71541 CG CG CG where z is distance b ehind the outer mold line of the nose and x and y are orthogonal to the b o dy axis. Predictions of a static aero dynamic instability for ath nder vehicle asso ciated with shifting of the Mars P 0.9 the sonic line from the shoulder to the nose and back 2 ∞ 1, 2 have b een describ ed in the literature. Sp eci c con- V ∞ ρ / clusions of the rst citation relating to the o ccurrence p y on the 70 degree, of the static aero dynamic instabilit M 0.8 ∞ Path nder con guration are sum- spherically capp ed 22.3 marized here. 16.0 ath nder prob e descends through 1 As the Mars P 9.4 the Mars atmosphere the minimum value of the p ost- k e ective rst decreases from frozen gas chem- sho c 0.7 -1.0 -0.5 0.0 0.5 1.0 alues to equilibrium values 1.094 corresp ond- istry v x, m ing to a velo city of 4.86 km/s. As the prob e continues to decelerate through a near equilibrium p ost-sho ck Fig. 3 Pressure distributions in the plane of sym- gas chemistry regime, the value of increases, until metry over the Mars Path nder at 2 deg. angle-of- reaching its p erfect gas value at parachute deployment attack and Machnumb ers equal to 22.3, 16.0, and 0.42 km/s. 9.4 showing the e ect of gas chemistry. 2 For the spherically blunted, 70 degree half-angle immediately b ehind the b ow sho ck just downstream of cone at small angles of attack the sonic line lo cation an in ection p oint in the bow sho ck. The sonic line shifts from the shoulder to the nose cap and back again from the nose skirts ab ove the edge of the b oundary on the leeside symmetry plane b ecause of the change layer. Still later, at Mach 9.4, the bubble expands to in . This transition in sonic line lo cation is shown in merge with the subsonic region at the b oundary layer Fig. 2 for a 2 degree angle-of-attack. The subsonic ow edge. The in ection p oint on the bow sho ck disap- region, as de ned by the frozen sound sp eed, app ears p ears. as the light gray region. The darker region represents 3 Pressure distributions on the cone fustrum ap- zones of sup ersonic ow b ehind the b ow sho ck. Ata proaching the shoulder tend to b e very at when the Machnumb er 22.3 on the pre ightPath nder simula- sonic line sits forward over the spherical nose, as shown tion, the sonic line on the leeside dips deep into the in Fig. 3 for Mach 22.3. E ects of the expansion b oundary layer over the spherical nose cap. Later in over the shoulder can only b e communicated upstream the tra jectory, at Mach 16.0, a subsonic bubble forms 2of10 American Institute of Aeronautics and Astronautics Paper 98{2445 through the subsonic p ortion of the b oundary layer. In trast, pressure distributions on the cone fustrum con 1.80 approaching the shoulder tend to be more rounded when the sonic line sits on the shoulder, exhibiting a more pronounced in uence of the expansion on the 1.75 upstream owasshown in Fig. 3 for Mach 9.4. In general, windside pressures exceed leeside 4 0deg. A pressures on the cone fustrum, pro ducing a stabi- C 2deg. moment that pitches the prob e back to zero lizing 1.70 angle-of-attack. However, the b ehavior of the pres- sure distribution in the vicinity of the shoulder sig- 5deg. tly in uences the pitching moment co ecient ni can 1.65 b ecause of the larger moment arms and surface area at the edge of the prob e as compared to the inb oard nose and fustrum regions. For the Mars Path nder angle-of-attack, the at, leeside pres- prob e at 2 deg. 2000400060008000 V∞,m/s sures approaching the shoulder when the sonic line sits over the nose can exceed the rounded windside a Axial co ecient pressures approaching the shoulder when the sonic line sits over the shoulder.
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