ANALYSIS OF TOUCH-DOWN DYNAMICS AND SAMPLING SEQUENCE OF MUSES-C

Size: px

Start display at page:

Download "ANALYSIS OF TOUCH-DOWN DYNAMICS AND SAMPLING SEQUENCE OF MUSES-C"

Cora Spencer
5 years ago
Views:

1 ANALYSIS OF TOUCH-DOWN DYNAMICS AND SAMPLING SEQUENCE OF MUSES-C Kazuya Yoshida 1 Yoichi Nishimaki 1 Hiroshi Kawabe 1 Takashi Kubota 2 1 Dept. of Aeronautics and Space Engineering, Tohoku University, Aoba 01, Sendai, , Japan 2 The Institute of Space and Astronautical Science, 3-1-1, Yoshinodai, Sagamihara, , Japan ABSTRACT The Institute of Space and Astronautical Science, Japan (ISAS) is developing a spacecraft that obtains samples from the surface of an asteroid, then return to Earth. The spacecraft, named MUSES-C targets 1998SF36, one of near- Earth asteroids. Since the gravity of the asteroid is considerably small, the spacecraft will not be able to stand on its surface, and thus will have to acquire samples in a dynamic sequence. The touch-down behavior of the MUSES-C is studied based on the dynamics model of a multibody system with frictional contact. To verify the mathematical model, experiments with a miniature model have been carried out under the micro-gravity environment in a dropshaft facility, MGLAB. Full-scale experiments with the hardware components of the proto-flight model have been also carried out using the ISAS s robot simulator. Nominal and critical cases of the touch-down sampling are examined by comparing the results of experiment and numerical simulation. INTRODUCTION The Institute of Space and Astronautical Science, Japan (ISAS) launches an exploration robotic probe named MUSES-C toward 1998SF36, one of near earth objects, with [m] across. The mission MUSES-C is the world s first attempt of sample and return from an asteroid. Considering versatility to the micro-gravity of the asteroid s surface, and to unknown surface conditions such as flatness and hardness, the crush sampling and touch-and-go strategy is taken among various candidate strategies. The accepted strategy minimizes the physical contact with the surface of uncertainty, yet ensures the sample acquisition. The physical contact will be made at the endtip of a conical probe supported by a deployable compliant structure. During the contact, a projectile is projected inside the probe to crash the surface. The ejected fragments of the surface will be concentrated in the conical probe and collected in a sample chamber located at the top corner of the cone [1][2]. This touch-down sampling is one of the most critical events in the mission. If the strength of the probe structure is not enough, or the spacecraft tumbles over the surface, the mission will fail. Therefore it is very important to properly assess the impact forces at the contact and tumbling motion after the contact. For the understanding of the touch-down dynamics, including the impact and tumbling, we have carried out various experiments and numerical analyses. One of the experiments was with a drop-shaft facility to have physical micro-gravity environment, and the contact behavior was studied using a miniature model of the spacecraft. Another experiment was hardware verification with a full-scale proto-flight model of the sampling probe using a mechanical motion simulator to represent the relative motion between the probe and the surface [3]. This paper summarizes the study on this unique strategy of touch-and-go sampling, from initial discussion to final pre-flight assessments. The topics cover (1) a general discussion of possible strategies for sampling from a minor body, (2) modeling of the dynamics of the spacecraft including structural compliance and frictional contact, (3) experimental verification to understand the contact dynamics, and (4) detailed assessments on critical cases. SMPLING FROM A MINOR BODY Candidate Strategies Key consideration in the sampling on a minor body is versatility to micro-gravity and unknown hardness of the surface. As a general discussion, the following strategies are considered possible candidates (see Figure 1): (a) Anchor and Drill: Drilling is a common idea to obtain core samples from surface to interior. However to achieve the drilling, the spacecraft must be anchored firmly on the surface to accommodate the reaction (see Figure 1 (a)). Both drilling and anchoring will be easier on soft surface, such as the surface of a comet. ROSETTA, an European comet probe takes this strategy [4]. (b) Harpoon and Penetrator: Figure 1 (b) describes an idea to penetrate a sampling probe into the target 1

2 Figure 1: Variety of sampling strategies Figure 2: Sampling sequence of MUSES-C using its kinetic energy. If properly designed, samples will be packed in the penetrator, and if tethered they can be retrieved. In this strategy, the spacecraft needs hovering over the sampling site, but landing or touchdown is not needed. Hovering may be less critical than touch-down when without tether. But with tether, its deploy and retrieval becomes a challenging issue. (c)(d) Crash Sampling: If a bullet-like projectile is projected with certain velocity, the surface will be crashed and fragments are ejected. Then, one idea is to collect such fragments in an orbit (see Figure 1 (c)). The dust collection technology used in STARDUST mission [5] will be applied here. But as the distance between the crash and sampling sites is far away, the sample acquisition becomes uncertain and, even if obtained, it is difficult to distinguish the point where each fragment comes from. Another idea is to collect the crushed fragments on or at close vicinity of the surface, as shown in Figure 1 (d). In this option, the spacecraft is required to make physical contact with the surface although, if the projectile is projected inside a probe that has a conical shape, the ejected fragments will be deflected along the cone and concentrated at the top corner. With this strategy, samples are efficiently collected from a specific point of the surface. The strategy is applicable for a wide range of surface hardness from basalt, for example, to regolith. Also, since the sampling will be completed instantaneously, the time of the physical contact with the asteroid s surface can be short, then the sampling sequence will be like touch-and-go. Sampling Sequence of MUSES-C and Critical Issues From the above four candidates, the last strategy was chosen for MUSES-C and a number of tests have been carried out to make a detailed design. In the flight model, the projectile of 5 [g] is projected at 300 [m/s]. The test results show that several hundred milligrams to several grams of fragmented target materials can be collected by a single sampling action [6]. Figure 2 illustrates the sampling sequence. The spacecraft descends toward a specific point of the asteroid using a vision-based autonomous guidance system, so that the contact velocity is controlled within 0.1 [m/s] vertically and 0.08 [m/s] horizontally. The conical probe is supported by Double-reverse Helical Spring (DHS), a dedicated deployable structure with compliance. The length of this sampling device, termed Sampler Horn hereafter, is 1 [m]. Then the maximum clearance beneath the bottom of the spacecraft becomes less than 1 [m] during the sampling. Local obstacles that may interfere this clearance will be detected and avoided using optical sensors. Local inclination at the contact point of the sampler horn is assumed less than ± 30 [deg] as a design criteria. As soon as the contact is detected, the projectile is projected. The momentum of the projectile reaction is about 1.5 [Nms], which is negligible comparing to the momentum of the spacecraft itself that is about 40 [Nms]. However, the tumbling motion caused by the contact reaction through the sampler horn is much more critical, because the horn is mounted on the spacecraft with about 0.7 [m] of offset from its centroid. The DHS deforms during the contact. The magnitude of the deformation depends on stiffness of the spring and touch-down conditions. We need to check if the deformation is within an acceptable range. For example, if the projectile is projected after the horn tip is largely deformed, it may hit the horn itself but not the asteroid. After the sampling, gas-jet thrusters will be used to lift the spacecraft off the surface. As the spacecraft starts tumbling immediate after the contact, the delay of firing the lift-off thrusters will result in a critical situation. In order to be aware of such potential risks, the motion of the spacecraft for various touch-down and liftoff conditions is carefully examined in this paper. MATHEMATICAL MODEL Equation of Motion A schematic model of MUSES-C is illustrated in Figure 3. The spacecraft is modeled by a multibody system including compliant elements. The sampler horn is discretized into multiple rigid segments, so that each joint represents compliant characteristics in bending angle θ, axial and lateral deformation ɛ. The endtip of the horn receives the external force from the ground contact. The equation of motion of this system is given by the following equation. F e is a contact force applied at the endtip of the sampler horn. The deformation of the horn, that is made of a compliant structure, is described by ɛ and θ. where H v 0 ω 0 ɛ θ + c = F 0 N 0 f τ H : inertia tensor of the entire system + J T e F e (1) 2

Figure 3: A schematic model of MUSES-C (a) Double-reverse Helical Spring (b) axial deformation of DHS Figure 4: Model of the sampler horn (c) lateral deformation of DHS c : velocity dependent terms v

bending angle of DHS f : axial and lateral force on DHS τ : bending moment on DHS J e : Jacobian matrix (a) experimental setup in the drop-shaft capsule (b) simulation model of the spacecraft with an

divided into a component perpendicular to the contact surface, F n, and a component tangent to it, F t.

3 Figure 3: A schematic model of MUSES-C (a) Double-reverse Helical Spring (b) axial deformation of DHS Figure 4: Model of the sampler horn (c) lateral deformation of DHS c : velocity dependent terms v 0 : velocity of the spacecraft base ω 0 : angular velocity of the spacecraft base F 0 : thruster force on the base N 0 : thruster moment on the base ɛ : axial and lateral deformation of DHS θ : bending angle of DHS f : axial and lateral force on DHS τ : bending moment on DHS J e : Jacobian matrix (a) experimental setup in the drop-shaft capsule (b) simulation model of the spacecraft with an offset probe, touching over an inclined surface Figure 5: MGLAB experiment for the study of contact dynamics in micro-g environment F e : ground contact force Contact Model The contact force F e is divided into a component perpendicular to the contact surface, F n, and a component tangent to it, F t. The magnitude of F n is modeled using a penetration depth into the surface, d: F n = K w (d) r + D w ( d) s (2) where we assume r = s = 1 for simplicity. F t is switched between static friction and kinetic friction using a threshold value µ 0 : For static friction (F t /F n <µ 0 ) F t = K t (d t ) r + D t ( d t ) s (3) For kinetic friction (F t /F n >µ 0 ) F t = µf n (4) Sampler Horn As introduced previously, double-reverse helical spring (DHS, see Figure 4 (a)) is used for the sampler horn. DHS shows high stiffness when it is fully stretched, but it shows compliance otherwise. For the sampler horn, the length of the DHS is constrained not to become fullstretch. The axial and lateral compliance of the sampler horn thus identified as shown in Figure 4 (b) and (c). The characteristics are nonlinear. EXPERIMENTAL VERIFICATION MGLAB Experiments The experiments using a miniature model were conducted in Micro-Gravity Laboratory of Japan (MGLAB), a drop-shaft facility providing 4.5 [s] duration of microgravity environment. In this series of experiments, the basic characteristics of the mathematical model with frictional contact and compliant probe were verified. Figure 5 (a) shows the experimental setup in the drop-shaft capsule. In this setup, four sets of a miniature spacecraft model as shown in Figure 5 (b) are installed, so that they are simultaneously projected with a certain velocity to hit a test surface by the compliant probe, during the 4.5 [s] of free-fall. The contact force is monitored by a force/torque sensor mounted behind the contact surface, and the motion of the spacecraft model is recorded by a video camera. From careful analysis, it became clear that the compliance of the probe has a dominant effect on the normal (perpendicular) contact force, and the frictio does on the tangential contact force. And that, by tuning those parameters on the compliance and friction, the motion profile obtained from the video can coincide with the motion obtained from numerical simulation, using Equations (1)- (4) [7]. 3

Table 1 Touch-down conditions mass of the spacecraft, m 430 [kg] m.o.i. (pitch), I y 150 [kg/m 2 ] vertical velocity, v z - 0.1 [m/s] horizontal velocity, v x 0.08, - 0.

4 Table 1 Touch-down conditions mass of the spacecraft, m 430 [kg] m.o.i. (pitch), I y 150 [kg/m 2 ] vertical velocity, v z [m/s] horizontal velocity, v x 0.08, [m/s] local inclination, ψ [deg] global inclination, φ ± 5 [deg] friction coefficient, µ Figure 6: The proto-flight model of the Double-reverse Helical Spring (DHS) mounted on the 3-D Hardware Simulator (HWS) ζ ζ ISAS HWS Experiments Another series of experiments were carried out with the 3-D Hardware Simulator (HWS) in ISAS, using a proto-flight model of DHS, see Figure 6. The HWS can demonstrate relative motion between the DHS probe and the contact surface by using a 9 axis motion table. The motion is computed based on a numerical dynamics model using the input from the force/torque sensor that measures the contact reaction force on the spacecraft. With this facility, the touch-down and lift-off sequence is verified using a real hardware setup. The touch-down is detected by a laser range finder (LRF) to sense the endtip deformation of the sampler horn. Its sensing resolution is ±1 [cm]. The touch-down experiments were carried out for various combination of vertical and horizontal approaching velocities, and inclination and friction of the contact surface. Detailed analysis will be made in the following section though, regarding the contact detection, it was confirmed that the LRF successfully detected the contact within 0.3 [s] in average, and the cases with longer delay are the cases with smaller deformation during the contact. CRITIAL ANALYSIS Touch-Down Conditions In this section, the touch-down and lift-off sequence is analyzed to check critical conditions. The touch-down conditions used in the HWS experiments and numerical simulations are listed in Table 1. For the analysis of critical cases, the ground clearance is evaluated. As shown in Figure 7, local inclination of the surface in the scale of the horn diameter is denoted by ψ, whereas global inclination of the sur- Figure 7: Touch-down parameters for critical analysis Figure 8: Result of parametric survey on touch-down clearance (v x =0.08,v z = 1.0 [m/s], t = 3 [s] after the contact) Figure 9: Result of parametric survey on horn displacement (v x = 0.08,v z = 1.0 [m/s], t =0.5 [s] after the contact) 4

5 face in the scale of the solar array s span is denoted by φ. Then the clearances under the corner of the spacecraft ζ 1 and under the solar panel ζ 2 are evaluated. The results show ζ 1 is always smaller than ζ 2. Critical Cases Figure 8 depicts the result of parametric survey obtained by numerical simulation for critical cases regarding the clearance ζ 1 at 3 [s] after the contact. The cases of positive v x are shown because those of negative v x are less critical, due to asymmetric design of the spacecraft. The results show that the friction coefficient does not give significant influence, but the local ground angle ψ makes major influence. Particularly, in the cases of positive large ψ, the clearance becomes less than 0.3 [m], and if any actions are not taken, the spacecraft will hit the surface. For this reason, the spacecraft must start thruster propulsion within a few seconds after the contact. Figure 9 depicts the result of parametric survey obtained by numerical simulation for critical cases regarding the lateral deformation of DHS, ɛ at 0.5 [s] after the contact. In this evaluation, the cases of negative v x are always critical. The results show that the friction coefficient does not give significant influence, but the local ground angle ψ makes major influence, again. Particularly, in the cases of negative large ψ, the deformation becomes more than the radius of the sampler horn. If the projectile is projected in such a situation, the projectile will hit the horn, not crush the asteroid. Motion Sequence with Lift-Off Thrusters The touch-down motion sequences are illustrated in Figure 10, for six different cases. The conditions are classified into nominal and critical as listed in Table 2. In each set of figure, the top raw is the motion obtained from the HWS experiment. In the HWS experiments, the sampler horn is always vertical and the relative motion is represented by the motion of the ground plane, i.e. the pictures are the observation w.r.t. the spacecraft fixed coordinate. The middle raw shows the graphical representation of the above experiment. Here the pictures are rendered w.r.t. the ground fixed coordinate, so that the tumbling motion of the spacecraft can be clearly seen. Note that the deformation of the sampler horn is not expressed here. The bottom raw depicts the result of the corresponding simulation including the deformation of the horn. For the cases with thrusters, four of 20 [N] thrusters are fired at 0.15 [s] after the contact detection, according to the design baseline of the flight model. In the cases of A1 and B1, the tumbling motion after the contact is in a non-negligible order. Particularly, case B1 is critical. But with the powered lift-off by the thrusters, the spacecraft s crush on the ground can be avoided (cases A2 and B2.) In the case of C1, the tumbling is relatively small although, the lateral deformation of the horn becomes large. The thruster lift-off will help to terminate the Table 2: Nomical and critical cases Nominal Critical 1 Critical 2 v x [m/s] v z [m/s] ψ [deg] φ [deg] deformation (case C2.) CONCLUSIONS In this paper, the touch-down sampling sequences of MUSES-C are examined by experiment and simulation. The MUSES-C uses a novel sampling strategy of impact sampling and touch-and-go, versatile to unknown surface hardness and micro-gravity environment on a small planetary body. The sampling technology is very promising though, the spacecraft can start tumbling after the touch-down with the surface of the asteroid. Through the experimental and numerical assessments, it is highly recommended that the projection for impact sampling and thrusters for lift-off shall be started immediately after the contact detection. The projectile projection later than 0.5 [s] or starting the thruster propulsion later than 3 [s], may cause critical situation. REFERENCES [1] A Proposal of the Asteroid Sample Return Mission: MUSES-C, The Institute of Space and Astronautical Science, (in Japanese) [2] J. Kawaguchi, K. Uesugi and A. Fijiwara, Readiness of the MUSES-C Project and the Spacecraft Flight Model Status, 22nd Int. Symp. on Space Technology and Science, ISTS 2000-o-3-06v, Morioka, Japan, [3] K. Yoshida, T. Kubota, S. Sawai, A. Fujiwara, M. Uo, MUSES-C Touch-down Simulation on the Ground, AAS/AIAA Space Flight Mechanics Meeting, Paper AAS , Santa Barbara, California, pp. 1-10, February [4] (as of Nov. 2002) [5] (as of Nov. 2002) [6] H. Yano, S. Hasegawa, M. Abe, A. Fujiwara, MUSES-C Impact Sampling Strategy for Microgravity Asteroids, Paper B , World Space Congress, Houston, Oct [7] K. Yoshida, A. Noguchi, H. Katoh, Frictional Contact Dynamics in Micro Gravity, 23rd International Symposium on Space Technology and Science, ISTS 2002-d-10, Matsue, Japan, 1 6,

6 A1: Nominal case (No Thrusters) A2: Nominal case (With Thrusters) B1:Critical case 1 (No Thrusters) B2: Critical case 1 (With Thrusters) C1:Critical case 2 (No Thrusters) C2: Critical case 2 (With Thrusters) Figure 10: Comparison of HWS experiment and corresponding simulation for 6 different cases 6

Sampling and Surface Exploration Strategies in MUSES-C and Future Asteroid Missions

Sampling and Surface Exploration Strategies in MUSES-C and Future Asteroid Missions Kazuya Yoshida*, Yoichi Nishimaki*, Takeshi Maruki*, Takashi Kubota**, and Hajime Yano** * Department of Aeronautics