Experimental Research on Roll-up Storage Method for a Large Solar Sail

Transcription

1 Experimental esearch on oll-up Storage Method for a Large Solar Sail By Kazuya SAITO 1), Nobukatsu OKUIZUMI 2), Hiraku SAKAMOTO 3), Junji KIKUCHI 2), Jun MATSUMOTO 2), Hiroshi FUUYA 3), Osamu MOI 2) 1) Institute of Industrial Science, The University of Tokyo, Tokyo, Japan 2) The Institute of Space and Astronautical Science, JAXA, Sagamihara, Japan 3) Tokyo Institute of Technology, School of Engineering, Tokyo, Japan (eceived 1st Dec, 216) Large membranes used as solar sails should be stored compactly to reduce launch volume. In addition, their stored configuration should be sufficiently predictable to guarantee reliable deployment in orbit. The roll-up method is a workable option for storage of large solar sails, as demonstrated in IKAOS. However, with this method, it is difficult to predict the roll-up position because the membrane s thickness causes deviation from the ideal position. In this presentation, these deviations are evaluated in an experiment involving storage of a 5-m solar sail, and a position control method for roll-up storage of a large solar sail is proposed. These techniques contribute strongly to the repeatability of storage configurations of large solar sails. Key Words: Deployable Structure, Storage Method, Solar Sail 1. Introduction An efficient method for storing a large membrane in a compact space is essential for the solar sail systems 1-4. Therefore, researchers have proposed various types of solar sail folding methods. In the case of IKAOS 3, 4, one of the successful solar sail missions operated by JAXA, the roll-up storage method and centrifugal-force deployment were demonstrated (Fig. 1). These storage and development systems are also planned to be used in JAXA s next-generation solar sail missions 5. Figure 1 shows a schematic of the roll-up storage method discussed in this paper. A square-shaped solar sail is divided into four trapezoidal membranes (Fig. 2(a)), and each membrane is folded into a belt-shaped petal by z-folding (Fig. 2(b), (c)). Each petal is fixed to the center of the satellite body (dram) (Fig. 2(d)). As indicated by the red circle in Fig. 2, each petal is connected half-and-half to the next petal by the harness and reeled on the dram together. Z-folding and the roll-up process were executed by hand in IKAOS, and similar manual techniques are planned to be used in next-generation solar sail projects. However, with the manual roll-up method, it is difficult to ensure repeatability of storage shape because it is difficult for workers to maintain constant tension during the roll-up process. This inconsistency in roll-up forces causes misalignment in the fixed position of the petals each time the storage process is executed. To cope with this difficulty, we analyzed the roll-up position control method. Before roll-up, equally spaced markers are placed on each petal. By using these markers and the scale on the dram, workers can roll up the petals to predetermined positions on the dram without having to maintain constant tension. If the target positions of all markers are given properly, that is, the rollupped petals have no biased tension that may slide the rolledup shape, so it is possible to maintain a consistent storage configuration. The purpose of this research is to develop a procedure for determining the target positions in the above-mentioned manual roll-up process. In a previous study, a few of the authors of the present study performed a roll-up experiment by using a dram with a radius of 15 mm and 1-m class solar sail membranes, and investigated the relation between the Fig. 1 Deployment sequence and mechanism of IKAOS 8. (a) (b) (c) (d) Fig. 2 Schematic of the roll-up storage method. 1

2 petal thickness and misalignment in a roll-up position 6, 7. On the basis these results, we conducted additional roll-up experiments using a dram with a radius of 15 mm and an 18-m-class solar sail, which is intended for use as the next-generation large-sized solar sail. First, we present the basic idea of the proposed rollup position control method and explain how to calculate the target position. Second, we describe the roll-up experiments and discuss the validity of the proposed method. This method enables us to guarantee repeatability of the storage configuration, even in the case of manual roll-up storage. 2. oll-up Position Control Method Consider an L-long petal reeled on a dram of radius, as shown in Fig. 3. The petal has marks (A1, A2,.) at intervals of l. Phase angles (1,2, ) express the rolled-up position of each mark on the dram. For a zero-thickness petal, the phase angle of An can be calculated as n = nl/. (1) In a real roll-up process, the marks position slide from above the ideal phase angles. Owing to the thickness of the petal, the actual roll-up radius is larger than the dram radius. To express this slide value quantitatively, we use deviation dn defined as follows. For a petal with uniform thickness tc, assuming that the actual roll-up radius is (+ tc) (see Fig. 4(b), deviation dn is given as dn {nl/ nl/ ( + tc)} n tcl/ = tcn. (2) Here, we assume that the dram radius is much greater than the petal thickness tc. In fact, the actual petals are not uniform in thickness but have a complex profile because of the shape of the original (unfolded) membrane and owing to the presence of many equipped devices such as solar panels and harnesses. Expressing this thickness profile by the function t(), deviation d() at point in the petal is given by the following equation: d(φ) = φ t(θ)dθ For simplicity of calculation, petal length is expressed by the form of the phase angle. (Using x, the distance from the roll-up starting point, = x / [rad]). From eq. (3), deviation d equals the area of thickness profile t(). In the case of multiple roll-ups with more than one petal, overlaps on other petals occur, accompanied by an additional increase in the actual roll-up radius, leading to further deviation. If the petal length exceeds the dram perimeter, a self-overlap would occur as well. For calculating deviation, these overlap effects can be considered in the thickness profile s function. Figure 5 shows an overlap case; petal-a with a thickness profile t1() overlaps petal-b with a thickness profile t2() at the point In this situation, the deviation can be calculated from the practical profile T(), which is obtained by summing the two profile functions as T() = t1() + t2() (4) In the case of three or more petals self-overlapping, deviation d() can be calculated in a similar manner using the practical profile (3) T(). The phase angle n of rolled-up position An can be predicted as follows: n = n dn/ (4) dn = (θ n ) T(θ)dθ To determine n, the practical thickness profile T() is required. This function must include not only the thickness of the membrane but also that of the equipped devices. In addition, the roll-up tension force in the outer petal imposes a pressing force on the inner petal. Therefore, the roll-upped thickness is thinner than that of an un-rolled petal. The previous study also reported bulging between inner and outer petals 6,7. For these reasons, it is difficult to measure the practical thickness profile T() directly. As discussed in the following sections, we therefore use an indirect method to determine T(). Before determining the target phase angle, the first part of petal was rolled up preliminarily. By investigating the trend in deviation on each mark, the practical profile T() can be approximately calculated. Fig. 3 elationship between dram radius and phase angle Fig. 4 Ideal (zero-thickness) petal model and uniform thickness petal model. Fig. 5 Schematic of calculation of the practical profile T() in overlap case. T t1 n An t2 2 nl A + An A A2 A1 1 l l A2 L A1 A A n t2 d nl tc (5) tc t1 2

3 d [mm] d [mm] 3. Single oll-up Experiment 3.1. Setup To investigate the practical thickness profile T, the petal was reeled one round on the dram and the deviation of each mark was measured. Figure 6 shows the schematics of the dram and the petal used. The dram radius is 1,328 mm with petal holders at 1/8 th point of each round. The petal is made from trapezoidal solar sail membranes having dummy solar cell devices. The membrane is folded into a belt-shaped petal with a length of 17,472 mm and width of 45 mm through Z-folding and is held by clips to maintain the folded shape. The petal has marks at intervals of 1,48 mm as targets of the roll-up positions. At the start of the experiment, the petal was housed in another small dram for easy handling. During the roll-up process, a worker tried to maintain constant tension on the petal. Two types of roll-up configurations, one-fold roll up and twofold layer roll-up, are examined, as shown in Fig esult The experimental results are summarized in Figs. 8 and 9. The points in Fig. 8 show the deviation of each marker in the one-fold roll-up experiment. The deviation increases approximately linearly, which indicates a uniform-thickness model, as shown in Fig. 4(b). Eq. (2) can be used for calculation. The gradient of the approximate line that corresponds to the petal thickness tc is 38 mm. Figure 9 shows the results of the two-fold roll-up experiment. As indicated by the dotted line in Fig. 9 (a), the change in the deviation can be approximated by a parabolic curve. This means that the thickness profile T() can be considered to follow the tapered-thickness model, in which thickness decreases linearly. The practical thickness profile T() can be calculated from the approximate curve in Fig. 9(a), and it is shown in Fig. 9(b). 17,472mm Fig. 7 Schematics of one-fold roll-up (upper) and two-fold roll-up experiments (lower) y = x [rad] Fig. 8 Deviation of each marker in one-fold roll-up experiment. (a) y = x x [rad] (b) 5 4 T [mm] Fig. 6 Schematics of dram and petal used in the experiment. Fig. 9 Deviation of each marker in two-fold roll-up experiment (a) and calculated thickness profile T() (b). 3

4 d [mm] d [mm] 4. Multi oll-up Experiment In the next-generation solar sail system, the satellite is equipped with petals measuring 25 m in length, and these petals would be rolled-up more than three times on the dram. To validate the effects of multi roll-up and self-overlapping, we conducted two- and three-lap roll-up experiments after the one-fold roll-up experiment described in the previous section. Figure 1 shows the experimental result of the first roll-up. In each lap, if the deviation increases linearly, it can be considered that the process follows the uniform thickness model. In the second and third laps, overlap occurs on the previously rolledup parts. As discussed in the section 2, these overlaps are predicted to increase the practical thickness profile T; therefore, the gradients of the deviation should increase with increasing number of laps. However, such an increase in the gradient because of the overlap effect was not observed in the second and third laps, as shown in Fig. 1. The gradient of the second lap is.5 times that of the first lap. The deviation in the third lap shows the same gradient as that in the second lap. 5. Discussion The petal used in this study is made from a Z-folded trapezoidal membrane (Fig. 1), so the cross-section of the petal is trapezoidal. However, the practical profile, T, calculated from the single roll-up results is different from the predicted shape. These differences are thought to be caused by Fig. 1 T Experimental result of 3-Lap roll-up experiment. [rad] Fig. 11 Experimentally determined thickness profile T and calculated target position. 1-Lap 2-Lap 3-Lap y = x [rad] 1-Lap 2-Lap 3-Lap tc1= 32.68mm tc2= 16.34mm tc3=.mm deformation because of the roll-up tension force and additional thicknesses of the harnesses and equipped devices. As a result, simple models such as the uniform-thickness model or linearly tapering model are effective for predicting the deviation. In the multi roll-up experiments, we confirmed that the practical thickness profile decreases as the number of roll-up laps increases. This decrease is considered to be caused by deformation of the rolled-up part. In the cases of second and third roll-ups, the petal is pressed on the previously rolled-up part. Then, the roll-up tension force may cause the actual rolledup radius to decrease. In Fig. 1, the gradient of the third lap shows no increase from that in the second lap; therefore, the decrease in roll-up radius is larger than the increase in petal overlap. The results indicate that the following method is effective for predicting the rolled-up target position. Firstly, an approximate model of the practical thickness profile T is determined from the result of the single roll-up experiment. The uniform thickness model or linearly tapering model can be selected according to the tendency of deviation. The target positions of the second and third laps are calculated by these results in the first lap. For simplifying discussion, we employed the uniform thickness model. If deformation of the pressed petal is not considered, the practical petal thickness in n-laps is given as t cn = nt c1. Considering the decrease in the roll-up radius, the proposed method uses an adjusted value for calculating (t c2, t c3). t c2 = 2 t c1 (6) t c3 = 3t c1 (7) In the case of the experiment in the previous section, 2 =.5 and 3 =.. Using the proposed method, the multi roll-up experiment was conducted again. In this experiment, the roll-up target positions of the second and the third laps were calculated from the results of the first lap. The results are summarized in Fig. 11. From the deviations of the first lap, the thickness profile can be approximated using the uniform thickness model and t c1 = 32.7 mm. Using the abovementioned adjusted value, t c2 and t c3 are calculated to be 16.4 mm and. mm, respectively, and the practical thickness profile T is shown in Fig. 11 (a). The red line in Fig. 11(b) indicates the target position of each marker calculated from Fig. 11(a) and Eqs. 4 and 5. The points in Fig. 11(b) show the results of the roll-up experiment conducted using these target positions. It is confirmed that each marker is positioned at the proper target phase in the storage shape. The marker positions did not change after the experiment; therefore, tension force is thought to be maintained properly in the stored shape. 6. Conclusion oll-up storage experiments were conducted on nextgeneration large-size solar sails, and deviation in the stored position of the petals was investigated. Using the experimental results, a method to calculate the proper target position for the roll-up process was proposed. These techniques strongly contribute to the repeatability of the storage configurations of large solar sails in manual roll-up storage. 4

5 Acknowledgments The experiments described in this study were conducted with the support of students at Kawaguchi Lab. At ISAS/JAXA, Sakamoto Lab. and Furuya Lab. at Tokyo Institute of Technology. The authors thank the Solar Sail Working Group members at ISAS/JAXA for meaningful discussions. eferences 1) Greschik, G. and Mikulas, M. M.: Design Study of a Square Solar Sail Architecture, Journal of Spacecraft and ockets, 39 No. 5, Sept.-Oct. (22), pp ) Murphy, D. M.: Validation of Scalable Solar Sailcraft System, Journal of Spacecraft and ockets, 44 No. 4, July-Aug. (27), pp ) Sawada, H., Mori, O., Okuizumi, N., Shirasawa, Y., Miyazaki, Y., Natori, M. C., Matunaga, S., Furuya, H. and Sakamoto, H.: Mission eport on The Solar Power Sail Deployment Demonstration of IKAOS, AIAA Paper , ) Tsuda, Y., Saiki, T., Funase,., Shirasawa, Y. and Mimasu, Y.: Shape Parameter Estimation of IKAOS Solar Sail Using In-Flight Attitude Determination Data, AIAA Paper , ) Funase,., Kawaguchi, J., Mori, O., Sawada, H. and Tsuda, Y.: IKAOS, A Solar Sail Demonstrator and Its Application to Trojan Asteroid Exploration, AIAA Paper , ) H. Sakamoto, S. Kadonishi, Y. Satou, H. Furuya, Y. Shirasawa, N. Okuizumi, O. Mori, H. Sawada, J. Matsumoto, Y. Miyazaki, M. Okuma: Development of epeatable Storage Method for a Large Solar Sail, Transactions of Japan Society for Aeronautical and Space Sciences, Aerospace Technology Japan, Vol. 12, No. ists29, 214, pp. Pc_75-Pc_82. 7) H. Sakamoto, S. Kadonishi, Y. Satou, H. Furuya, Y. Shirasawa, N. Okuizumi, O. Mori, H. Sawada, J. Matsumoto, Y. Miyazaki, M. Okuma: epeatability of Stored Configuration of a Large Solar Sail with Non-negligible Thickness, 54th AIAA/ASME/ASCE/AHS/ ASC Structures, Structural Dynamics, and Materials Conference, Structures, Structural Dynamics, and Materials and Co-located Conferences, (AIAA ) 8) 5