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Why Not Space Tethers?

Nobie H. Stone SRS Technologies, Inc. Huntsville, AL 35806

ABSTRACT

The Tethered System Space Shuttle missions, TSS-1 in 1993 and TSS-1R in 1996, were the height of 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 . 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

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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 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

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• 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

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Characteristics & Capabilities

• Device Masses < 20/0 of host mass· • Dormant during satellite operation • Deploys tether when sat~lIite dies • Tether drags against geomagnetic field, de- satellite in weeks • No propellant required • Self-powered - needs no input oJ power • Can deorbit a dead satellite

Rational for ED Boost of HST TECHNOLOGilE8

Raising HST to a Permanent Parking Orbit:

Removes HST from active space w/o its destruction and at potentially a lower cost to NASA,

Circumvents any possibility of impacting populated regions on earth.

Provides flexibility--allows extended orbit maintenance and future recovery from parking orbit. SRS ISS Tether Reboost System (TRS) TECHNOLOGIES

~ • Twin 2.5 km electrodynamic tethers Power Supply Twin 2.5 km Satellite • Enables orbit maintenance and conductive, (will use space­ reboost long-life qualified DS1 technology) • Only mechanical interface with ISS tethers • Uses no Station power • Acceptable Station eM shift (<3 m) Tether break mitigation ~ • Naturally stabilizes station attitude (roll axis) • No expellant re-supply over design life (5 yr) • Total system mass <1000 kg • Tether monitoring & break protection • Instantaneously jettisonable SRS Exchange ED Reboost (MXER) TECHNOLOGIES

The'Momentum-eXchangel Electrodynamic Reboost (MXER) Tether Facility is a reusable, propellantless, in-space upper stage for sending payloads from LEO to GTO and beyond.

• Tether is 90-120 km long and operates in an elliptical, equatorial orbit • Rapid rotation of the tether allows its tip to match position and velocity with

• oJ the payload instantaneously. • Orbital energy given by the tether to the payload is restored over 30-45 .. days using electrodynamic tether •. propulsion...... I