Why Not Space Tethers? Nobie H. Stone SRS Technologies, Inc. Huntsville, AL 35806 ABSTRACT The Tethered Satellite System Space Shuttle missions, TSS-1 in 1993 and TSS-1R in 1996, were the height of space tether technology development. Since NASA's investment of some $200M and two Shuttle missions in those two pioneering missions, there have been several smaller tether flight experiments, but interest in this promising technology has waned within NASA as well as the DOD agencies. This is curious in view of the unique capabilities of space tether systems and the fact that they have been flight validated and shown to perform as, or better than, expected in earth orbit. While it is true that the TSS-1, TSS-1R and SEDS-2 missions experienced technical difficulties, the causes of these early developmental problems are now _mown to be design or materials flaws that are (1) unrelated to the basic viability of space tether technology, and (2) they are readily corrected. The purpose of this paper is to review the dynamic and electrodynamic fundamentals of space tethers and the unique capabilities they afford (that are enabling to certain types of space missions); to elucidate the nature, cause, and solution of the early developmental problems; and to provide an update on progress made in development of the technology. Finally, it is shown that (1) all problems experienced during early development of the technology now have solutions; and (2) the technology has been matured by advances made in strength and robustness of tether materials, high voltage engineering in the space environment, tether health and status monitoring, and the elimination of t]_e broken tether hazard. In view of this, it is inexplicable why this flight-validated technology has not been utilized in the past decade, considering the powerful and unique capabilities that space tethers can afford that are, not only required to carryout, otherwise, unobtainable missions, but can also greatly reduce the cost of certain on-going space operations. TEC)H N OI=O(_IES Why Not Space Tethers? Nobie H. Stone SRS Technologies, Inc. Huntsville, Alabama 54 th JANNAF Propulsion Meeting Denver, Colorado May 14-18, 2007 This document is approved for public release; distribution is unlimited. Developmental History of Space Tethers 1970's Dynamics of long gravity-gradient Giuseppe Colombo stabilized tethers shown feasiblel & Mario Grossi, NASA presented TSS concept SAO 1979-80 NASA/OSSA Facilities Peter Banks, chair Requirements Definition (U of Mich.) Team (FRDT) 1980's NASAOSF Technological interests formulated Ivan Becky - electrical power generation - orbital transfer - de-orbit, etc. August 1992 TSS-1 Electrodynamic: 20-km conducting tether w/Reel-type deployer (NASA/ASI) 1993,1994 SEDS-1 and 2 Dynamic: 20-km Non-Conducting Tether wi Spindle Type Deployer (NASA) June "1993 PMG Bi-polar operation; i.e., generator and motor modes (NASA) February 1996 TSS-1R Electrodynamic: 20-km conducting tether w/Reel-type deployer (NASA/ASI) June 1996 TiPS Dynamic: 4 km long x 2 mm diameter tether (NRL) Tethered Sate lite Orbita Dynamics TEI3HNOLOGIIE8 d(mV o) mVo2 Vo I Fc dt r V3 V2 Vl L1 V 1 As altitude increases L1 < L2 < L L, E (total energy, V1 > V 2 > V 3 T (period) increase T1 < T2 < T3 V, Fg, Fc decrease Tethered Sate lite Orbita Dynamics TEI3HNOLOOIE8 Fg + T = Fc Fg + F£ = Fc Fc_ V o Fg Fe F, F F c +F.L Ft. Fc +T Fg / / F c TEOHNOL.OG IE8 L Generation Mode Motor Mode F=iOLxB) f / •i J* s \ s \ / \ / \ / , //- B , t ! t ! J .--_Vo , I / I | % I B I ! t \ / i % \ \ i/ _\\ #i ii \,, t # \ / / / / i up-I_ Negative Acceleration i Down-b, Positive Acceleration 011109Ae0003.ai End-body Stab lization Rec uirement TECHNOLOGIES F 2 s End-Body Stabilizer and Contactor m 2 M 1 = (L/2)F Tcos o_- LF2sin o_ = 0 I F T = dfed= IB COS o_(L) fed- IB cos _ dr 0 C_ X F T 0 c F eq Fz = m2 [( Vl 2/r2)-(MG/r22) ] M2 = (F Tcos o:)/2 2[( V12/r2)-(MG/r22)] f V o Positive Results from Tether Missions Dynamics • Dynamic Stability (TSS-l) Gravity-gradient stabilization achieved at < 300 m. • Ease ofDeployment and Control (SEDS-l/2 & TSS-l) Deployment to 20 Ian, station keeping for more than 20 hrs, and satellite retrieval have been demonstrated. • Recovery from Dynamic Upsets & Slack Tether TSS recovered from severer dynamic perturbations, slack tether and satellite pendulous motions. • Retrieval (TSS-l) Near retrieval (most critical aspect) from 276 m was nominal (shown at right). Electrodynamics • Current collection in space ten times more efficient than predicted (TSS-IR) Even greater efficiency obtained w/gas emissions. Pre-TSS theoretical models much too conservative. • Energy conversion from spacecraft orbit into electrical power demonstrated (TSS-IR) A peak power of> 3.5 kW was generated. • Bi-polar operations (PMG) Polarity and current flow reversal performed, demonstrating power and propulsive thrust generation. Hardware Flight Heritage • Tether Survivability Demonstrated In-Space (TiPS) The TiPS tether (2 nun x 4 Ian) remains intact on orbit for 10 years. • Deployer In-Space Validation (6 missions) Successful deployment with simple spool deployer (SEDS-l & 2, PMG and TiPS), and with real type (TSS). TSS Data -vs- Standard Theory TECHNOLOGIES r .J '20 -' 0 "'""" ""-...... -' . 10 o o 200 400 500 000 1,000 1,200 sa I Ile Vol (Volts) L sab e Power: TSS-1 R Data vs Theory TEOHNOLOI_IES I I. 1,500 I t t t TSS | t | 1,000 - -4 I ! 1 ! 1 ! ! l | t I | I ! I I I 5O0 I I I I I !1 I PM 0 0 0.1 0.2 0.3 0.4 0.5 Tether Current (Amps) Open Issues • Tether long-term survivability in meteorite, debris, and atomic oxygen environments • Broken Tether Hazard entanglement of mother sic by slack tether following rebound • Stability long-term electrodynamic-dynamic coupling (with tether current) • Deployer development simple, robust, low mass, multi­ purpose • Plasma contactor development simple, low consumables, mass and power-or passive Grid-S 3here Low-Drag E ectrode TEOHNOLOGIIE8 Alenia Tether Deployer Hardware TECHNOLOGIES Long-Life Tether Designs TECHNOLOGIES I a , 1 , Y , Y , , V I Y I Ii , 1 , If'lf\,/\.", 11\11\"\11\1 \1 \, " \1 \,' I,' 'I " I.' I,' First Level of Secondary Lines Redistributes Load to Adjacent Nodes Secondary Lines Second Level of Secondary (initially Severed Primary -4-I~'" unstressed) Uliles Line Redistri butes Load Back to Undamaged 0.2 to Portion of 10's of Primary Line meters " ",1\\ ,I, 1" I\ I 1\1' \1 I \I ' 'r l l'I.'II' I 0.1- 1 meter I Prototype Hoytether Tether Tether Optical Fiber Impact Monitor TECHNOLOGIES Solar-Powered T55-1 R Tether, Break Relaxation End-Body 80.0 3.0 Tether With Optical 64.0 0 Impact Monitor ,. ..... ..... -- -3.0 48.0 Tether Tension . I Newtons "Speed (VR) MIIII-g 32.0 Zeros At Orbiter -6.0 -3 km/sec 16.0 -9.0 0 -12.0 GMT(sec) 29:45 29:15 29:20 29:25 29:30 Tension Zeros Lat(deg) 1 2 2 2 2 ~~ At End Body Lon(deg) -98 ~ Rebound (-) -100 -100 -100 -99 Reflector Single Fiber Guillotine Tether With International MUltiple Optical Fibers Space Station Woven Into Outer Sheath TEOHNOI..OOIE8 S-875-92-46-01 Cha_ , // Tether From Spool Lavet W'_nd D_kfe Chain Ball Sorew "'" Wedge Block Inse_t Pol_ Tether FR_rn INTERFERENCE AREA SIDE VIEW Bal Nut Collar Nut Collar Enlargement Modification _ I Insert ,,:.. _,,_-----_ ..... .. Poirm o_ Ir_erlerence Dynamic Up-Set and Relaxation TECHNOLOGIES TSS-1R 'elher-Break Data TEOHNOI..OI_ IE8 STS-75, TSS-IR Mission, 19961057101:29:09:997 i i i ( i i I i i i i i i i i i I i ; i i I ( € = i 1 i i i i i i i O -1000.0 D 0.0_ > -2000:0_]1ooo.o_ asured at Orbiter) i -3000.0] _4000.01 ,, ,, ,,, i, ,,, i, I,, _ _,, , _ , _ , _ ,, , , , , , ORB EGA ORB EGA2 1200.0 900.0 < 600.0 E 300.0 0.0 -300.0 800.0# ................................... > 48°°1 _ Sat Potential 160.0_ , -480.0_-160.0-_ L,_ U Orbiter Potential _800.0 _ , ,,,, ,,, ,, ,,,, ,, ,, ,,,, I,,, _, ,, ,, , , SAT Z ACOEL Or) Z O LIJ 48.0_64.0-_........32.0-_Tether800_116.0]Tension.......................... "_' S_'_at Z A ccelerometer I Z 0.01 I I I I I I I I I I I I I I I ' ' ' f I I I I I I I I I I I I I QMT (sec) 29:45 29:15 29:20 29:25 29:30 29:35 29:40 Lat (deg) 1 2 2 2 2 2 2 Lon (deg) -98 -100 -100 -100 -99 -99 -99 051128Ae0800,ppt TECHNOLOGII=8 OA OV + ,,.it ¥ 3soo,v %: 1 1!_+ V OY' 0_ OA • + 7 TECHNOLOGIEE TSS Tether (,onstruction Nome× Core 7 Cu -_J Conduclor _ati _Kelv . .,ON_ . a,r -!Nomex Braid Strength, Member Diameter 2.54 mm (0.I in) Max Mass 8,2 kg/km (5,5 _b/kft) Breakstrength 1780 N (400 ib) Temp Range -! 00 ° 1o +i 25 ° C (- 1 4,8° to +257 ° F) Eqec_ Characteristics Carry 1-A Current at 10 kV 5 mA (Max) Leakage Max Etong ation_ 5% at 1'780 N Classical ED (;onf'c uration TECHNOLOQIE8 PlaLsma _ Sat _ l= JAsheath "__ = € noA I(kTe/me) 1/2 _ B if] "/.i Fvo 4 */' _ Electron Emission Orbta ED orce Var'ations TEE_IN, I_OJ,_O_ES' Resulting Force Variation 2 Tether I ,, _ Average /I / I 1 • Type: Insulated 1.6 ........