FINAL REPORT on NIAC Phase I Contract 07600-011 with NASA Institute for Advanced Concepts, Universities Space Research Association CISLUNAR TETHER TRANSPORT SYSTEM Report submitted by: TETHERS UNLIMITED, INC. 8114 Pebble Ct., Clinton WA 98236-9240 Phone: (206) 306-0400 Fax: -0537 email: [email protected] www.tethers.com Report dated: May 30, 1999 Period of Performance: November 1, 1998 to April 30, 1999 PROJECT SUMMARY PHASE I CONTRACT NUMBER NIAC-07600-011 TITLE OF PROJECT CISLUNAR TETHER TRANSPORT SYSTEM NAME AND ADDRESS OF PERFORMING ORGANIZATION (Firm Name, Mail Address, City/State/Zip Tethers Unlimited, Inc. 8114 Pebble Ct., Clinton WA 98236-9240 [email protected] PRINCIPAL INVESTIGATOR Robert P. Hoyt, Ph.D. ABSTRACT The Phase I effort developed a design for a space systems architecture for repeatedly transporting payloads between low Earth orbit and the surface of the moon without significant use of propellant. This architecture consists of one rotating tether in elliptical, equatorial Earth orbit and a second rotating tether in a circular low lunar orbit. The Earth-orbit tether picks up a payload from a circular low Earth orbit and tosses it into a minimal-energy lunar transfer orbit. When the payload arrives at the Moon, the lunar tether catches it and deposits it on the surface of the Moon. Simultaneously, the lunar tether picks up a lunar payload to be sent down to the Earth orbit tether. By transporting equal masses to and from the Moon, the orbital energy and momentum of the system can be conserved, eliminating the need for transfer propellant. Using currently available high-strength tether materials, this system could be built with a total mass of less than 28 times the mass of the payloads it can transport. Using numerical simulations that incorporate the full three-dimensional orbital mechanics and tether dynamics, we have verified the feasibility of this system architecture and developed scenarios for transferring a payload from a low Earth orbit to the surface of the Moon that require less than 25 m/s of thrust for trajectory targeting corrections. In addition, the Phase I effort investigated the feasibility of using a similar tether system to provide rapid round-trip travel between low Earth orbit and low Mars orbit. A key technology required for both tether systems is hardware and techniques for rendezvous between the payloads and the rotating tethers. Automated rendezvous and capture systems currently under testing by NASA should, with further development, be capable of facilitating the tether-payload dockings. By providing a fully reusable infrastructure and by minimizing the need for propellant expenditure, tether transport systems can significantly reduce the cost of frequent travel to and from the Moon and Mars. NIAC Phase I Report TABLE OF CONTENTS I. INTRODUCTION...................................................................................................................................................1 II. RESEARCH OBJECTIVES..................................................................................................................................2 III. PHASE I RESULTS................................................................................................................................................2 III.A. DESIGN OF THE CISLUNAR TETHER TRANSPORT SYSTEM ...........................................................................3 III.A.1. Cislunar System Architecture ..............................................................................................................3 III.A.2. LEO-to-LTO Tether Boost Facility Design ........................................................................................5 III.A.3. Lunavator™ Design...............................................................................................................................6 III.A.4. Cislunar System Dynamics Verification Through Simulation ...........................................................7 III.A.5. Analyses of Lunar Transfer Targeting.................................................................................................7 III.A.6. Stability Analyses of Lunavator™ Orbits.............................................................................................8 III.A.7. Maintenance of Rotating Tether Orbits by Tether Reeling ................................................................8 III.B. LEO HEFT FACILITY ANALYSIS AND DESIGN ............................................................................................9 III.C. TETHER SYSTEMS FOR EARTH ⇔ MARS TRANSPORT................................................................................11 III.C.1. Mars-Earth Rapid Interplanetary Tether Transport System .............................................................11 III.C.2. Tether Boost Facility Design for the Human Mars Mission.............................................................12 III.D. COMPARISON TO COMPETING TECHNOLOGIES...........................................................................................13 III.E. HIGH-STRENGTH TETHER MATERIALS.......................................................................................................14 III.F. HIGH-SURVIVABILITY TETHER STRUCTURES ............................................................................................16 III.G. KEY FEASIBILITY ISSUE: PAYLOAD RENDEZVOUS & CAPTURE WITH A ROTATING TETHER..................16 III.H. INCREMENTAL SYSTEM DESIGN .................................................................................................................18 III.H.1. Tether Transport System Technology Design Effort........................................................................19 III.H.2. STOTS: Spinning Tether Orbital Transfer System Experiment......................................................19 III.H.3. TORQUE: Tether Orbit-Raising Qualification Experiment............................................................19 III.H.4. Earth-orbit Tether Boost Facility.......................................................................................................20 III.H.5. Lunavator™ Facility............................................................................................................................20 III.H.6. MarsWhip Tether Facility..................................................................................................................20 IV. SUMMARY............................................................................................................................................................20 APPENDICES A. Design of Earth-Orbit Tether Facilities for Lunar Transfer Orbit Injection B. Lunavatorª Tether and Orbital Design for the Cislunar Transport System C. Cislunar System Dynamics Verification Through Simulation D. Analyses of Lunar Transfer Options E. Stability of Lunavatorª Orbits F. Maintenance of Rotating Tether Orbits by Tether Reeling G. LEO HEFT Facility Design H. Mars-Earth Rapid Interplanetary Tether Transport (MERITT) System, AIAA Paper 99-2151 I. MarsHEFT J. The Hoytetherª K. Momentum-Exchange Tether White Paper L. Cislunar Tether Transport System: AIAA Paper 99-2690 NIAC Phase I Report Tethers Unlimited, Inc. Final Report Cislunar Tether Transport System I. INTRODUCTION Motivation If mankind is to move beyond its current tenuous foothold in low Earth orbit and develop a sustained and prosperous presence on the Moon, Mars, and elsewhere in the solar system, the cost of transporting supplies, equipment, and personnel to these locations must be reduced by several orders of magnitude. The US space program is currently seeking to achieve such cost reductions for Earth-to-orbit transport by developing reusable launch vehicles. To achieve these cost reductions for the in-space propulsive needs of an interplanetary civilization, it will be necessary to develop a highly reusable transportation architecture that minimize the amount of mass that must be launched into orbit to provide in-space propulsion. Background: Momentum-Exchange Tethers Momentum-exchange tethers can provide a means for transporting many payloads without utilizing propellant, and thus can provide the infrastructure of a low-cost in-space transportation system. A momentum-exchange tether is essentially a long, high-strength cable rotating in orbit. This cable can provide a mechanical connection between two objects in orbit, enabling one object to transfer momentum and energy to the other object, much like a hunter can cast a stone with a sling. A momentum-exchange tether facility will consist of a central station, a long, tapered, high-strength cable, and a grapple vehicle at the tether tip. The tether will be deployed from the station, and the system will be induced to spin using tether reeling maneuvers or electrodynamic forces. The direction of tether spin is chosen so that the tether tip is moving behind the tether facilityÕs center-of-mass on its downswing, and moving ahead of it on its upswing, as illustrated in Figure 1. With proper choice of tether orbit and rotation, the tether tip can then rendezvous with a payload when the tether is at the bottom of its swing and later release the payload at the top of its swing, tossing the payload into a higher orbit. The orbital energy and momentum given to the payload comes out of the energy and momentum of the tether facility. The tetherÕs orbit can be restored by reboosting with propellantless electrodynamic tether propulsion or with high specific impulse electric propulsion; alternatively, the