ABSTRACT. Keywords: flat electrodynamic tether, grazing impact, ballistic limit 1. INTRODUCTION

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1 SURVVIBILITY TO HYPERVELOCITY IMPCTS OF ELECTRODYNMIC TPE TETHERS FOR DEORBITING SPCECRFT IN LEO. Francesconi*, C. Giacomuzzo*, F. Branz*, E.C. Lorenzini* *University of Padova CISS G. Colombo, Padova, Italy, University of Padova Deartment of Industrial Engineering BSTRCT This aer reorts the results of 16 hyerveity imact exeriments on a comosite flat electrodynamic tether for LEO sacecraft end-of-life deorbiting. The system is being develoed within the EU FP7 BETs rogram. Imact tests were carried out at CISS imact facility, with the aim of deriving failure equations that include the imact angle deendence u to grazing incidence. Exeriments were realised with 1.5 and 2.3 mm aluminium sheres, at veities between 3 and 5 km/s and imact angle from 0 to 90 from the tae normal. fter a reliminary ost-imact insection of the target, the damage extension on the tae was evaluated using an automatic image rocessing technique. Ballistic limit equations were develoed in the exerimental range using a rocedure that allows to estimate the uncertainty in the failure redictions starting from the measurement of the damage area. Exeriments showed that the imact damage is very close to the rojectile size in case of normal imact, while it increases significantly at highly oblique imact angles. the system mission life. The l-1100-h19 and PEEK LITE tether samles that were tested in the framework of this activity were both 2.54-cm wide and 0.05-mm thick. In articular, the highly directional ballistic resonse of the flat-tae tether was taken into account by deriving ballistic limit equations (BLE) which exlicitly consider the imact angle deendence u to grazing incidence. To date, only few exerimental data have been ublished on the imact survivability of tether structures, and to the knowledge of the authors of this aer no secific work on tae tethers was done before. Rather, referring to tethers with circular cross section, it is believed that every imact with an object whose size is between 20% and 50% of the tether diameter is ical [2]. more sohisticated erion for the assessment of the lethality of the single imact was roosed by [3], that reorted tests on olymeric tethers (Dyneema, Kevlar, Sectra) and defined an exerimental correlation between the damage extension on the cable cross section and the kinetic energy of the rojectile. Keywords: flat electrodynamic tether, grazing imact, ballistic limit 1. INTRODUCTION BETs (Bare Electrodynamic Tethers) is a research roject funded by the Euroean Commission in the FP7 framework which aims at studying and develoing an innovative technology that could be used in the future by every LEO satellite for end-of-life deorbiting [1]. The BETs system emloys electrodynamic drag on a current-carrying conductive tether, without the need for roellant while at the same time generating ower for on-board use (Fig.1). The BETs tether consists of two different flat taes connected in series: the first one is the electrodynamic tether (EDT) made of aluminium (l-1100-h19) to carry the electric current while the second one is an inert tether made of non-conductive material (PEEK LITE) to increase the dynamic stability of the system during deorbiting. In this context, considering the large area otentially exosed to the micrometeoroid and sace debris flux, articular care was given to the imact survivability of the tether, that is related to the robability of ical failure (cut-off) as consequence of hyerveity imacts of micrometeoroid and/or sace debris through Figure 1. Schematic of BETs system In this scenario, this aer resents the results of sixteen hyerveity imact (HVI) exeriments on both the l-1100-h19 and PEEK LITE taes. fter this introduction, section 2 (Exerimental methods) describes the test setu and the rocedures emloyed for evaluating the damage on the targets; section 3 Proc. 6th Euroean Conference on Sace Debris Darmstadt, Germany, ril 2013 (ES SP-723, ugust 2013)

2 (Results) rovides a summary of all the tests with the selected imact conditions (rojectile diameter, seed and imact angle) and the major outcomes of the exeriments. Conclusions are finally given in section EXPERIMENTL METHODS s described in section 1, the objective of the exerimental activity was to derive directional BLEs for the BETs tether, i.e. suitable equations roviding the minimum article diameter d, which roduce a tether ical damage (cut-off) at given seed v and imact angle α, measured in the tae reference frame, see Fig. 2 (the debris relative veity v is suosed to be in the x-y lane). Figure 3. Tether suort structure mounted in the LGG imact chamber 2.2. Damage evaluation For BLE derivation, a new emirical aroach was emloyed [6, 7], which makes it ossible to estimate the uncertainty in the target s failure rediction. The new method consists of four stes (for each tye of tether, l1100-h19 and PEEK): Figure 2. Tae tether geometry and reference frame. and B are the axis of the ellitic imact damage For this urose, BLEs were derived in the form of Eq. 1 and, considering the tether flat shae, secial attention was given to the equations accuracy for highly oblique imact angles (close to 90 ). d ( v, α ), = f (1) Since the selected tae tether design is comosed by two different taes connected in series (luminum alloy 1100-H19 and PEEK LITE), the imact damage was investigated for both of them Test setu The imact tests reorted in this aer were conducted at CISS Hyerveity Imact Facility, using a twostage light-gas gun (LGG) caable of accelerating articles in the range mm at seed u to 6 km/s [4, 5]. secial tether suort structure were designed and realized to hold multile samles and maximize the test success rate even at high imact obliquity (close to 90 ), see Fig. 3. a) utomatic analysis of the imact damage on highresolution images of samles after imact. The damage s shae is assumed to be ellitical, and its size is therefore secified by the values of the ellise s major and minor axes and B, resectively along the y and x directions (see Fig.2). b) Derivation of emirical co-relations (damage equations) between the damage s major axis and the imact arameters (article size, seed and imact angle): D ( v, d, α ) = f (2) c) Emirical determination (and/or assumtion based uon available data or theoretical modeling) of the damage s major axis ical value (, ). By definition, if,, the tether is cut-off, i.e. the tether is severed when the damage extension in the y direction reaches a certain ical ercentage of the tae width. d) BLE derivation by introducing the ical value in the damage equation and inverting the formula: d, 1 D (, v, α ) = f (3) The key advantage of this method is that BLEs are given with uncertainty bands, thanks to the fact that both the damage equations f D and the ical damage value are derived from exeriments. On the contrary, as ointed out by [8], following traditional aroaches BLEs are simle demarcation lines

3 between fail and no-fail conditions, with no statistical significance. Differently, the method here described is based uon the definition of a damage arameter () that is hysically related to the tether cut-off henomenon. Such arameter varies monotonically across the failure threshold, assuming a articular ical value, (that can be redicted from the exeriments) at the ballistic limit. ll the available data, even well away from the ballistic limit, can be therefore used to statistically follow the ical arameter evolution. In this way, it is ossible to rovide an estimation of the test conditions at the ballistic limit, even inside the bounds defined by the two closest non-ical and ical exeriments. 3. RESULTS s regards the two above oints, l1100-h19 and PEEK LITE show a very similar behavior. Figure 4. Test no (l-1100-h19, α =0 ): setu (left); detail of the tae damage 3.1. Tests summary To date, 16 HVI exeriments have been comleted. Both the two tae tethers (l1100-h19 and PEEK) have been subjected to imact at different angle and seed. Test conditions as well as damage s major axis values are reorted for each test in Tab.1. The right column of Tab.1 was filled after comleting ste a) of the rocedure outlined in section 2.2. The uncertainty values are below 0.1 mm for the damage features and below 1% for the rojectile seed. Test id Tae tye α [ ] d [mm] v [km/s] [mm] 8855 l , l , l , l , l , l , l , l , PEEK 90 1, PEEK 90 1, PEEK 0 1, PEEK 0 2, PEEK 30 1, PEEK 60 1, PEEK 80 1, PEEK 80 1, Table 1. Test conditions and results Figure 5. Test no (l-1100-h19, α =90 ): setu (left); detail of the tae damage Figure 6. Test no (PEEK LITE, α =0 ): setu (left); detail of the tae damage Some of the results are resented in the following figures. From a raw visual insection, it aears that: The tethers damage after normal imacts is not much significant, since the hole s major axis is very close to the rojectile diameter. The imact damage increases considerably at high oblique angles. Figure 7. Test no (PEEK LITE, α =90 ): setu (left); detail of the tae damage

4 3.2. Ballistic limit equations Ballistic limit equations for the two taes were derived with the rocedure described in sub-section 2.2. s reliminary considerations, it is worth to highlight two issues: No one of the imact tests resulted in a tether cutoff. This means that the ical value of the damage major axis cannot be determined emirically from the available data. The only ossibility in this case is to assume a reasonable value for, based uon literature data and/or other consistent theoretical hyotheses. Most of the imact tests were conducted with 1.5 mm rojectiles. This means that the available data are not enough diversified to emirically infer the influence of d on the tether s damage. For this reason, it was assumed that is always roortional to the debris diameter. This hyothesis is in excellent agreement with the results of tests no. 8855, 8856, 8873, 8874 (see Tab.1), where the ratio /d is constant for both taes (considering the measurement uncertainty). The remainder of this section refers to last three stes of the rocedure outlined in section 2.2 above. b) n emirical co-relation between the damage s major axis and the imact arameters was develoed from all the exeriments (results for both the l H19 and PEEK taes are well fitted by the same equation, i.e. Eq. 4). Unfortunately, the data available for α =90 are affected by a relevant uncertainty, that is related to the imossibility of redicting the exact imact oint on the tae s edge (see Fig. 8) and hence the damage major axis has no statistical significance for α =90. For this reason, Eq. 4 is sensible from α =0 to α =80 only. To extend the damage equation s validity u to α =90 requires accurate data at such imact obliquity; this could be achieved e.g. by hydrocodes simulations. = 0.45 d v cosα 0.65 (4) For Eq. 4, the correlation arameter r 2 is equal to 0.77, and the standard deviation of the estimation of is σ fit =0.18. Units are as secified in Tab.1. Eq. 4 is lotted in Fig. 9 for all the exeriments (excluding those for α =90 ). It aears that: Eq. 4 well reresents the exerimental data for both tae materials. The imact damage increases significantly for highly oblique imact angles. α Figure 9. Normalized damage major axis in function of the imact angle c) ccording to the second consideration reorted at the beginning of this subsection, the ical value which defines the failure threshold was not derived from exerimental data. Rather, it was assumed that the tether is cutoff when the residual cross section of the tae is just able to withstand the maximum redicted tensile load on the system, i.e. 10 N. Hence, failure occurs when equals or exceeds the following ical values, that were comuted with reference to the materials ultimate tensile strength at 150 C., l 1100 H19 = [ mm] (5a), PEEK = [ mm] (5b) The uncertainty in Eq. 5 results from an assumed 10% uncertainty in the knowledge of the taes materials ultimate tensile strength. Eq. 5a shows that the l H19 tae is severed when the damage s major axis equals the tether s width. Figure 8. debris (red) could strike the tae centrally (left) or off-axis (right): the uncertainty in the imact oint makes the exerimental data useless for α =90 d) s a final ste, ballistic limit equations are develoed by introducing in Eq. 4 the ical values reorted in Eq. 5 and solving for d, :

5 0.65 v l 1100 H19 d, = 56.4 cosα PEEK v d, = 50.9 U cosα d U d (6a) (6b) The uncertainty U d is equal to 35% of d,, and was calculated using the well-known Kline- McClintock method [9] for roagating to the final result the uncertainty on the value of and on the fit model used in Eq. 4. Indeed, the uncertainty on ballistic limit redictions is mainly related to the accuracy of Eq.4 and hence to the scattering of exerimental data. This is a common conditions for HVI exeriments. Fig.10 and Fig.11 resent some redictions of the ballistic limit equations for the l-1100-h19 and PEEK LITE tae, resectively.! "!# "!$ " Figure 10. Ballistic limit curve for l-1100-h /. -, ) * + (% '& '% & %?@B?C%D?@B?CE%D?@B?CF%D & '% '& (% 78 9 : ; < => Figure 11. Ballistic limit curve for PEEK LITE 4. CONCLUSION This aer reorted the results of 16 hyerveity imact exeriments on a comosite flat electrodynamic tether for LEO sacecraft end-of-life deorbiting. The system is being develoed within the EU FP7 BETs rogram. The damage extension on the tae was evaluated using an automatic image rocessing technique and exeriments showed that the imact damage is very close to the rojectile size in case of normal imact, while it increases significantly at highly oblique imact angles for both target materials. Ballistic limit equations were develoed in the exerimental range and the uncertainty on their rediction was calculated using a statistical aroach which makes it ossible to directly relate the ballistic limit to the extension of the major axis of the imact damage on tether samles. CKNOWLEDGEMENTS The authors wish to thank Mr. Gabriele Masiero, Mr. Francesco Babolin and Mr. Luca Tasinato for their excellent suort to the execution of the imact test activity. Project (BETs) is funded by the Euroean Commission under the FP7 Sace Program. REFERENCES 1. Sanmartin J.R. at al. Universal System to Deorbit Satellites at End of Life. Journal of Sace Technology and Science 26 (1), 21-32, Gittins G. L., Swinerd G. G., Lewis H. G., et al. study of debris imact collision robabilities to sace tethers. dvances in Sace Research, 34, , Sabath D., Paul K. G. Hyerveity imact exeriments on tether materials. dvances in Sace Research, 20, , ngrilli, F., Pavarin, D., De Cecco, M., Francesconi,. Imact facility based uon high frequency two stage light-gas gun. cta stronautica 53 (3), , Pavarin, D., Francesconi,. Imrovement of the CISS high-shotfrequency light-gas gun. Int. J. Imact Eng. 29 (1 10), , Francesconi, Giacomuzzo C, Kibe S, Nagao Y, Higashide M. Effects of high seed imacts on CFRP lates for sace alications. dv. Sace Research 50 (5), , Francesconi, Giacomuzzo C, Grande.M, Mudric T, Zaccariotto, M, Etemadi, E, Di Landro, L, Galvanetto, U. Comarison of selfhealing ionomer to aluminium-alloy bumers for rotecting sacecraft equiment from sace debris imacts. dv. Sace Research 51 (5), , Schonberg WP, Evans HJ, Williamsen JE, Boyer RL, Nakayama GS. Uncertainty considerations for Ballistic Limit Equations. Proc. 4th Euro. Conf. on Sace Debris, aril 2005, ESOC, Germany. ES SP-587, Kline, S.J., and McClintock, F.., Describing Uncertainties in Single-Samle Exeriments, Mech. Eng., 75 (1), 3-8, 1953.