Hypervelocity impact experiments on tether materials

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HYPERVELOCITY IMPACT EXPERIMENTS ON TETHER MATERIALS D. Sabath and K. G. Paul Fachgebiet Raun@hrttechnik,TUMunchen,Richard-Wagner-Strape18, 80333 Munchen,Germany

ABSTRACT Tethered systems are new and exciting means for various applications, such as the re-entry of small payloads from the space station. Due to payload mass constraints of the launch vehicle, the mass of the tethered system should be minimised. Therefore, fibres are the choice for tether materials. The probability of a severe impact into the tether is very high due its large surface area despite its small diameter. Hence, the risk of an impact of a micrometeoroid or a space debris particle cutting the tether should be investigated prior to flight. This work reports first observations of hypervelocity impact experiments on three different braided materials used for tether applications. The tether samples - Dyneema, Kevlar and Spectra were tested using the plasma drag accelerator (PDA) facility of the Fachgebiet Raumfahrttechnik (LRT), Technische Universitiit Mtinchen (TUM). An overview of the morphology of such impacts is presented. The extent of damage is compared to other materials commonly found on spacecraft. A risk assessment of an impact cutting the tether with current meteoroid and debris models and data from LDEF, Eureca and HST solar arrays, is also given. ~1997COSPAR.Publishedby ELeevierScia~~ Ltd. INTRODUCTION For many years, tethers are discussed as very promising means of future space infrastructure because the connection of two spacecraft with a long rope offers many advantages. Tethers can be divided in two classes, conducting and non-conducting tethers. Conducting tethers interact with the magnetic field of the Earth and provide electrical power to the end masses. These types of tethers are much more complex than the non-conductive types. Not only the tether dynamics, such as deployment, station keeping - stabilising the tether system in an orientation along the radius vector - and retrieval, but also the disturbances from the electro-dynamic interaction have to be considered. The work presented here focuses on non-conductive tethers. This decision is also based on the experience of the already flown tether missions Tethered Satellite System I (TSS-I), TSS-IR. Small Expendable-tether Deployment System I and 2 (SEDS-I and 2). The TSS missions which were to investigate dynamic and electro-dynamic effects on tethers simultaneously, were not successful. On the other hand, the SEDS missions which focused only on the dynamic effects, demonstrated the feasibility of the deployment and stabili1433

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D. S&.&i and K. G. Paul

sation of the tethered system. The SEDS missions’ only flaw was the cutting of the tether six days after the deployment probably due to a particle impact. This and proposed long term missions with tethers have become the motivation for this investigation

on risk assessment of tether being cut by small particle impacts,

TEST CAMPAIGN AND CRATERS RESULTED The hypervelccity

impact testing was performed in the laboratory of the LRT-TUM

employing its PDA

(Igenbergs er al., 1987). which is capable to accelerate small glass beads with diameters up to 100 urn to a velocity of I5 km/s and more (Sabath, 1995). Three different tether materials were tested during the campaign: l

Dyneema SK60 with a diameter dr = 1 mm (proposed for Rapunzel mission on a Russian RESURS),

.

Kevlar 49 with a diameterdr

.

Spectra 1000 with a diameterdr

= 1 mm (designed for FIESTA mission on a Russian PHOTON), and = 0.75 mm (as used in the SEDS-1 and SEDS-2 missions).

All three materials are braided of fiber yams, which offer some advantages for the handling of the ropes. In the course of the campaign, a total of 16 hits on the target materials could be achieved. An impact of a 90 urn glass bead with a velocity of 7.5 km/s on the Spectra Tether in Fig. I shows the typical crater morphology tested. Due to the braided

of the materials tether, the impact

produces many cut fibers. In addition, the crater is not circular although the particle hit the target at normal incidence. This morphology could be observed

on all impacts

on all materials

tested. We suggest that the individual fibers be treated as independent from each other as the transport

of energy inside one fiber is much

higher than between adjacent fibers. Unfortunately,

the accelerator

Fig. 1: Impact on Spectra Tether

is not capable of launching particles with an energy high enough to cut a

tether completely. The biggest impact seen in the campaign destroyed about 10% of the cross section of the tether. The comparison

of the impacts of all three types of materials showed that there is no significant

difference between Dyneema, Kevlar and Spectra. Therefore, a single empirical model for all materials is developed in the next chapter. MODEL AND RESULTS Using the experimental results described in the previous chapters a model for the risk probability for tether materials is developed. Assuming that the energy of the impacting particle Eki. is proportional

to the re-

moved crater volume, an empirical relationship for the damaged cross section AK of the tether by hypervelocity impacts can be developed. Due to the behaviour of the fibers of the braided material during impacts we assume that the model for small impacts as achieved during the campaigns can be extrapolated

to the

dimension of the whole tether. We also assume a power-law relationship for the model with the exponent K and the proportionality

factor ‘7:

1435

(1) Minimising

the errors by the least square fitting, the following parameters 17= 0.0498;

K =

are calculated:

0.727

(2)

Figure 2 plots the damage cross section AK of the craters

periments

generated

in the ex-

and the model for the tether

damage (Eq. 1). The fracture

strengths

$

of the tether materials

used are much

.-

higher than the actual

in-orbit

i

ment loads. Assuming

that the tether

experi-

fails if 90% of the cross section of the tether AQ is damaged by an impact, the model leads to an energy of 17 J for a

a1

0,Ol

B %O.cQl !i

failure for the Spectra tether and 38 J for

O.aml 0,001

Dyneema and Kevlar.

-----

0,Ol

031

0.9A,_,,

1

10

100

Kineticenergy [.tl The flux, the velocity and the density of micrometeoroids

Fig. 2: Laboratory impact data and damage model for various

and space debris were

tether materials.

taken from the models from Grim et al. (1985) and Kessler et al. (1989)

respectively.

The average collision velocity of micrometeoroids

17 km/s and the density 2.5 g/cm.‘, the velocity of space debris is about 10 MS

is about

and the density 2.8 g/cm’.

Using these values, the following critical diameter for both particle types dp Dcb.cri, and dp ,w.~,;, can be calculated (Table 1). Tab. I : Critical diametersd

for particles impacting tethers.

The risk calculation for Dyneema and Kevlar uses the 1990 flux (the highest observed flux ever by LDEF experiments)

because there is no date for the Rapunzel mission set yet. For the Spectra tether, the 1994

flux is used, the year the mission took place. This leads to the following cumulative micrometeoroids

QMr,and space debris particles

fluxes of “critical”

@&, for the respective materials in an altitude of 260 km

for Rapunzel and 350 km for SEDS: Tab. 2: Cumulative flux of critical particles. Micrometeoroids

@,u~,

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D.SabathmdK.G.Pud

According to Eichler and Rex (1988), the face dependent flux of space debris Koch on a cylindrical tether that is gravity-gradient stabilised in orbit can be calculated as: K Deb -[2.27+[+2(157.dJ)]@o,

(3)

For micrometeoroids, the whole surface of the tether has an uniform impact probability for all directions, therefore the flux for micrometeoroids K,,, is:

These assumptions and the nominal length of the tether 1of 52 km for Rapunzel and of 20 km for SEDS-2, respectively, lead to an average time span between two impacts of particles with sizes according to Table 1 for the various tether materials and mission dates as seen in Table 3: Tab. 3: Impact risk of tethers

The average distances between two impacts are about 9 months for the Dyneema/Kevlar material in the Rapunzel mission and about 15 months for the Spectra tether in the SEDS missions, respectively. Therefore, the impact risk for short-term tether mission is very low and the cutting of the tether in the SEDS-2 mission appears to be a singular event. CONCLUSION The hypervelocity impact experiments on three different tether materials such as Dyneema, Kevlar and Spectra show that the risk probability for a cutting the tether during a short term mission up to a few days is very small. Nevertheless, new types of tethers, i.e. consisting of three connected ropes, have to lower the risk in a long-term mission to an acceptable level. REFERENCES Eichler, P.. and D. Rex, The risk of collision between manned space vehicles and orbital debris - Analysis and basic conclusions, Zeitschrifrfiir Flugwissenschqfienund Weltraumforschung, 14,5, pp. 145-154 (1990). Griin E., H. A. Zook, H. Fechtig, and R. H. Giese, Collisional Balance of the Meteoritic Complexlcarus, 62, pp 244-272, (1985). Igenbergs E., S. Aigner, A. Htidepohl, H. Kuczera, M. Rott, and U. Weishaupt, Launcher Technology and Impact Diagnostics at the TUM/LRT, ht. J. hnpuct Engng., 5, pp. 37 l-380 (1987). Kessler. D. J., R. C. Reynolds, and P. D. Anz-Meador, Orbital Debris Environmentfor Spacecrafr Designed to Operate in Low Earth Orbit, NASA TM 100 471, Houston, TX, USA, (1989). Sabath. D., Untersuchung von Kratern in Flechtseilen fur Fesselseilanwendungen, RT-TB 95/15, Fachgebiet Raumfahrttechnik, Technische Universitiit MUnchen, Munchen, Germany, (1995).