EDL analysis for "HAYABUSA" reentry and recovery operation はやぶさ カプセル帰還回収運用における EDL 解析

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1 EDL analysis for "HAYABUSA" reentry and recovery operation Kazuhiko Yamada, Tetsuya Yamada (JAXA), Masatoshi Matsuoka (NEC Aerospace Systems, Ltd) Abstract HAYABUSA sample return capsule returned to the Earth with success in 13th June, The instrument module which has the asteroid ITOKAWA sample and two heat shields were completely recovered as scheduled at Woomera Prohibit Area in Australia. The prediction of the reentry trajectory and the landing point by EDL (Entry, Descent and Landing) analysis contributed to smooth recovery operation. When planning a trajectory correction maneuver, the target landing point was determined based on EDL analysis. In the on-site recovery operation, EDL analysis delivered the accurate information about the reentry trajectory and the landing position by cooperating with the orbital determination. In this paper, the contribution and the achievement of EDL analysis in HAYABUSA reentry and recovery operation are reported. はやぶさ カプセル帰還回収運用における EDL 解析 山田和彦, 山田哲哉 ( 宇宙航空研究開発機構 ), 松岡正敏 (NEC 航空宇宙システム ) 摘要 2010 年 6 月 13 日に小惑星探査機 はやぶさ の再突入カプセルが地球に帰還し カプセル本体 及び ヒートシールドの回収は成功裏に完了した 本稿では, その帰還回収運用においての EDL (Entry, Descent and Landing) 解析の役割と成果について報告する EDL 解析による再突入軌道と着地点の予測は, 軌道計画段階において着地点目標の策定に必須であり また 回収運用の現場において 軌道決定と連携し 迅速かつ正確な着地点予測を行うことにより スムーズな回収作業に貢献した 1. Introduction The Japanese asteroid explorer HAYABUSA returned to the Earth after 7-year round trip to asteroid ITOKAWA in 13 th June, ). JAXA recovery team deployed in the vast Woomera Prohibit Area (WPA 2) ) in Australia and recovered the instrument module (I/M) which has dust samples of the asteroid and two heat shields (H/S) which protected its sample container from severe aerodynamic heating during the reentry smoothly and completely in only three days after reentry. EDL (Entry Descent and Landing) analysis played an important role in the reentry and recovery operation of HAYABUSA sample return capsule (SRC). EDL analysis is to predict the reentry trajectory and the landing position by solving the motion equation of the SRC in atmosphere from the given initial condition, that is the time, position and the velocity of the SRC at altitude 200km. When planning a trajectory correction maneuver, the target landing point was determined based on the EDL analysis. In the on-site recovery operation, EDL analysis delivered the accurate prediction about the reentry trajectory and the landing point by cooperating with the orbital determination. In this paper, the function and the contribution of the EDL analysis for the HAYABUSA SRC reentry and recovery operation are reported. 2. Overview of HAYABUSA return and recovery operation The Figure 1 shows the outline of the trajectory correction maneuver (TCM) strategy when HAYABUSA returned to the Earth. Four TCMs were carried out. At first, HAYABUSA was guided to toward the earth rim at more than 200km altitude in perigee before TCM-3. At TCM-3, HAYABUSA change the target to WPA. In TCM-3, the predicted landing point appeared on the Earth surface for the first time. TCM-4 is final adjustment of the landing point to the center of the recovery team. After TCM-4, the HAYABUSA went toward the target in the WPA in ballistic flight without any trajectory correction. Fig.1: Schematic of the trajectory correction maneuver strategy when HAYBUSA returned.

2 The reentry sequence from the separation from mother ship to the landing of HAYABUSA SRC is summarized in Fig.2. The SRC was separated from the mother ship in 3 hours before the reentry. Although the mother ship burns up due to the severe aerodynamic heating, the SRC withstands the aerodynamic heating with the ablator-type heat shields and becomes a fireball which can be observed from the ground stations. When the SRC reaches a predefined altitude (from 5km to 10km), the two heat shields are jettisoned and the parachute is deployed. After that, I/M transmitting the beacon signal descends slowly and makes a soft landing by the parachute. The two H/Ss free-fall after jettison. Fig. 2: Reentry sequence of HAYABUSA SRC 3. Outline of EDL analysis EDL analysis played mainly two important roles in HAYABUSA reentry and recovery operation. 1) To determine landing target in order to plan the trajectory correction maneuver and the recovery operation strategy. 2) To support the on-site recovery operation by delivering the accrete prediction about the reentry trajectory and the landing position. In the EDL analysis of HAYABUSA reentry operation, the point-mass 3-DOF motion equation of SRC was solved numerically to predict the reentry trajectory. The features of the simulation method are listed in the following. 1) The point-mass 3-DOF motion equation in geocentric Cartesian rotational coordinate system was solved by Runge-Kutta method. 2) In the analysis, the aerodynamic drag force and the gravity force was considered as the external force acting on the SRC. 3) WGS84 is used as the geodetic system. 4) The initial condition in J2000EQ coordinate system (geocentric inertial coordinate system) is acceptable. 5) Both GRAM-99 3) which is statistical data and NCEP 4) data which forecast data is acceptable as atmospheric model. GRAM99 model was used in the planning phase and NCEP data was used for the on-site analysis. 6)Monte Carlo simulation is available in order to estimate the landing position probability. The initial condition, the drag coefficient, the atmospheric density and the wind profile are selectable as the dispersion factors. If HAYABUSA SRC functions normally, three objects which are I/M, fore body H/S and Aft body H/S descends as shown in Fig.2. Besides its normal case, some malfunction cases were also analyzed, for example, in the case that the H/S does not jettison and the SRC is not separated from mother ship and so on. The specifications of these flying objects assumed in this analysis are listed in Table 1. Table 1: Specification of flying objects assumed in this analysis Mass Reference Area Drag Coefficient Whole capsule 16.27k g m 2 DB* 1 I/M with Parachute 6.2kg Drag Area = 2.88m 2 I/M 6.2kg m Aft body 2.5kg m H/S Fore body H/S 7.8kg m 2 DB* 1 DB* 1 : Refer to aerodynamic coefficient database of HAYABUSA Sample return capsule 5) 4. Determination of target landing position At TCM-3, HAYABUSA was guided from trajectory through the Earth rim to the trajectory to aim the inside of WPA. The requirement for the target landing position is to put the 4-sigma dispersion landing ellipse of the whole capsule after TCM-3 in inside of WPA completely. The figure 3 shows the 4-sigma dispersion landing ellipse of the whole capsule after the TCM-3 (All) and the boundary of WPA. The factor of the landing position dispersion included the initial condition variety, the atmospheric uncertainty and the drag coefficient error. The target landing position was determined to meet the requirement. Additionally in figure 3, other three 4-sigma dispersion landing ellipses are also shown. These

3 are the landing dispersion ellipse considering only the initial condition variety (Initial condition), the ellipse considering only the atmospheric uncertainty (Atmosphere) and the ellipse considering the drag coefficient error (Drag coefficient). This result indicates that the initial condition variety was dominant in these dispersion factors in 9 days before the reentry when the TCM-3 was carried out. west-east direction and 100km in north-south direction due to the wind profile uncertainty. Fig.4: 4-sigma dispersion landing ellipses of all objects in all assumed cases after TCM-4 in planning phase and the positions of the ground stations. Fig.3: 4-sigma dispersion landing ellipse of whole capsule after TCM-3 and the effects of the assumed dispersion factors. TCM-4 is the final adjustment of the landing position to the center of the recovery team. There are three requirements for the landing position target determination of TCM-4. The figure 5 shows the comparison between four 4-sigma dispersion landing ellipses of I/M with a parachute. These are the ellipse considering all dispersion factor (All), the ellipse considering only the initial condition variety (Initial condition), the ellipse considering only the atmospheric uncertainty (Atmosphere) and the ellipse considering the drag coefficient error (Drag coefficient). 1) 4-sigma dispersion landing ellipses of all objects are inside of WPA in any cases including the assumed malfunction cases. 2) The ground stations are not constructed in 4-sigma dispersion landing ellipses of the hard landing objects that are except I/M with a parachute. 3)More than two direction finding station can receive the beacon signal when the parachute deployed. The Figure 4 is shown the 4-sigma dispersion landing ellipses of all objects in any assumed cases and the parachute deployment dispersion ellipses in the normal case. The poisons of the direction finding stations (DFS) are also plotted in Fig.4. Altitude of the parachute deployment was set to 10km in this analysis. The final target landing point meets three requirements as shown in Fig. 4. In planning phase, the 4-sigma dispersion landing area of I/M with a parachute spread to 200km in Fig.5: 4-sigma dispersion landing ellipse of I/M with parachute after TCM-4 and the effects of the considered dispersion factors. These results indicate that the dominant factor of the landing position dispersion after TCM-4 is the atmospheric uncertainty besides the initial condition variety. The landing position of I/M with

4 a parachute is sensitive to the wind profile because it descended slowly. On the other hand, the drag coefficient error does not have an impact on the dispersion in the planning phase. 5. Support of on site recovery operation After TCM-4, the orbit determination (OD) was carried out every half a day until reentry and the EDL analysis delivered the updated prediction of the landing position to the recovery team. The Figure 6 and 7 shows the transition of 4-sigma dispersion from the planning phase to the on-site prediction just before reentry. The Figure 6 is the parachute deployment position and Figure 7 is landing position of I/M with a parachute. In this analysis, the altitude of parachute deployment is set to 10km and GRAM99 is used as the atmospheric model. Fig.6: Transition of 4-sigma dispersion parachute deployment position from the planning phase to the on-site prediction just before reentry. The dispersion area of the parachute deployment position shrank as the accuracy of orbital determination was enhanced. Based on the OD data just before reentry (OD R#5), the 4 sigma dispersion area is 30km wide as shown in Fig. 6. On the other hand, the dispersion area of the landing position of I/M with a parachute did not shrink although the accuracy of orbital determination was enhanced as shown in Fig.7. The reason is that the descending trajectory of I/M with a parachute is very sensitive to the wind profile. Even if the OD data just before reentry (OD R#5) was used for the prediction, the 4-sigma dispersions landing position is 100klm-diameter circle as long as GRAM99 is used as the atmospheric model. In the on-site EDL analysis, NCEP forecast data is used as the atmospheric model in order to support the recovery team by more accurate prediction of the reentry trajectory and the landing position. The 4-sigma dispersion ellipse of the parachute deployment position and the landing position of I/M with a parachute estimated using NCEP forecast data is shown in Fig.8. The results was estimated with OD R#4 which delivered at 0:00(UT) in 6/13 and NCEP forecast data which delivered 0:00(UT) in 6/13. The altitude of the parachute deployment is set to 10km. In Figure 8, the 4-sigma dispersion ellipse calculated with GRAM99 is also plotted for the comparison. The result shows clearly that EDL analysis using NCEP data narrow down the landing position dispersion dramatically. The recovery team could search I/M and H/S efficiently according to this EDL results using NCEP data, although the NCEP data does not have dispersion information. Fig.7: Transition of 4-sigma dispersion landing position of I/M with parachute from the planning phase to the on-site prediction just before reentry. Fig.8: Comparison of 4-sigma dispersion area calculated using NCEP data and GRAM99 data.

5 The final prediction of the reentry trajectory and the landing position before the reentry is calculated with OD R#4 which delivered at 0:00(UT) in 6/13 and NCEP forecast data which delivered 0:00(UT) in 6/13. The altitude of the parachute deployment is 5km which was determined by G trigger and delay timer. The reentry trajectory and the landing position of I/M and two H/Ss predicted by the on-site EDL analysis were shown in Fig 9. According to the prediction, the SRC reenters from the west-northwest direction and changes the descending course to northerly direction due to the jet stream. After the parachute deployment at 5km altitude, I/M with a parachute moves to the west direction at about 3km due to the eastward wind near the ground surface. The two H/Ss drop near the parachute deployment position because the H/S is not swept into the wind compared with the parachute. The parachute deployment position was specified from actual discovery position of I/M using the wind profile based on NCEP data. The drop simulation of H/S was carried out using the parachute deployment position as the initial condition and the landing position of H/S was estimated. As shown in Fig 10, the discrepancy between the estimated maximum likelihood landing position and the actual discovery position of H/S is only 100m. The helicopter crew found the H/S immediately according to this information. Fig. 10 One of the results about H/S landing position estimation according to latest NCEP data and actual I/M landing position. Fig.9 Final prediction of the reentry trajectory and the landing position before reentry and actual discovery position of I/M and H/S. The recovery team searched I/M and H/S according to this prediction. All of the direction finding stations received the beacon signal as soon as the parachute deployed. Helicopter found I/M just one hour after the reentry in spite of the midnight searching. The actual recovery position of I/M was also plotted in Fig. 9. The discrepancy between the actual recovery position and the prediction of the final EDL analysis is less than 1km. Its result indicates that the initial condition, the drag coefficients of SRC and the atmospheric model of NCEP data is very accurate. The H/S landing position was predicted according to latest NCEP data which delivered 12:00(UT) in 6/13 and recovery position of I/M for the search of H/S in the next day. The figure 10 is one of the results of the estimation of H/S landing position. 6. Summary In the HAYABUSA reentry and recovery operation, the EDL (Entry, Descend and Landing) analysis contributed to the planning of trajectory correction maneuver and the smooth on-site recovery operation by predicting the reentry trajectory and the landing position. Especially, the quick and accurate prediction using NCEP atmosphere forecast data helped the smooth discovery of I/M and H/S at the on-site recovery operation. References 1) sa/index.shtml 2) 3)C.G Justus, D.L. Johnson: The NASA/MSFC Global Reference Atmoshperic Model 1999 Version (GRAM-99), NASA/TM , May )NCEP (National Centers for Enviromental Prediction) HP, 5)Koju Hiraki, Yoshifumi Inatani: The Aerodynamic Data Base for Asteroid Sample Return Capsule, ISAS Report SP, No.17, PP , March 2003.