Aeronautics and astronautics – Spacecraft – Spacecraft formation – orbit – or interplanetary path
Reexamination Certificate
2000-08-30
2002-07-16
Eldred, J. Woodrow (Department: 3644)
Aeronautics and astronautics
Spacecraft
Spacecraft formation, orbit, or interplanetary path
C244S166000, C244S172200
Reexamination Certificate
active
06419191
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to apparatus and methods useful for changing the state vector of a space object when the space object is moving relative to a magnetic field. More specifically, the present invention relates to an apparatus and method of using a conducting tether to produce an electrodynamic force to change the orbit of a satellite around a celestial body, such as the Earth, which has an associated magnetic field, and still more specificly to deorbit a satellite from its orbit.
BACKGROUND ART
The present invention has its principal utility in outer space, primarily for changing the state vector of a space object, for example deorbiting satellites at the end of their useful life to mitigate the harm and reduce the liability created by the proliferation of space debris. In order to obtain a better understanding of the present invention it is helpful to understand the prior art of space tethers, especially tether dynamics and tether electrodynamics. The present invention may be more readily understood through a review of the experimental prior art and a mathematical analysis of electrodynamic space tethers.
Prior Art Tethers
A tether was originally a rope or chain used to fasten an animal so that it grazed only within certain limits. Tethers have been used for decades in space to attach astronauts to their spacecraft.
In 1974 Professor Guiseppe Colombo, holder of the Galileo chair of astronomy at the University of Padua in Italy, proposed using a long tether to support a satellite from an orbiting platform. U. S. Pat. No. 4,097,010, which issued to Professor Colombo and Mario Grossi on June 27, 1978, teaches a satellite connected by means of a long tether to a powered spacecraft. Colombo actively pursued the design of a tethered satellite system.
Several NASA experiments, such as the two Small Expendable Deployer System (SEDS
1
&
2
) experiments and the Plasma Motor Generator (PMG) experiment used tethers in space. SEDS used a nonconducting tether. The PMG used a 500-meter conducting tether. The Tethered Satellite System flights in 1992 and 1996 (TSS-1 & 1R) used a 20,000-meter conducting tether.
On the TSS-1 mission the tether deployed only 260 meters (853 feet) before the deployer failed. On the TSS-1R the tether was deployed 19,500 meters. In the SEDS-2 flight, a 0.8-mm diameter, 20,000-meter long braided single-line tether was deployed to study tether dynamics and lifetime. Orbital debris or a meteoroid severed this tether in less than four days.
In the TSS-1R flight, the conducting single-line tether was severed after five hours of deployment. This failure was caused by an electric arc produced by the 3,500 volts of electric potential generated by the conductive tether's movement through the Earth's magnetic field.
The Tether Physics and Survivability (TiPS) satellite consists of two end masses connected by a 4,000-meter long non-conducting tether. This satellite was deployed on Jun. 20, 1996 at an altitude of 1,022 kilometers (552 nautical miles). Its tether is an outer layer of Spectra™ 1000 braid over a core of acrylic yarn. The yarn will “puff” its outer braid to two millimeters to “give it a larger cross section to improve its resistance to debris and small micrometeoroids”, according to the National Reconnaissance Office (NRO), which is a sponsor of the TiPS mission. As of Jun. 21, 2000 the TiPS tether had survived four years.
REFERENCES
1. Joseph A. Carroll, “SEDS Deployer Design and Flight Performance”, paper WSEDSA-1 at the 4
th
International Conference on Tethers in Space, Washington, DC, April 1995.
2. James E. McCoy, et. al. “Plasma Motor-Generator (PMG) Flight Experiment Results”, pp.57-84, Proceedings of the 4
th
International conference on Tethers in Space, Washington, DC, April 1995.
3. W. John Raitt, et.al. “The NASA.ASI-TSS-1 Mission, Summary of Results and Reflight Plans, pp. 107-118, Proceedings of the 4
th
International conference on Tethers in Space, Washington, DC, April 1995.
4. Joseph C. Anselmo, “NRO Orbiting Spacecraft Studies Tether Survivability”, Aviation Week, page 24, Jul. 1, 1996.
These experiments all used single line tethers.
The following reference is illustrative of the current state of the art in space tethers: Paul A. Penzo and Paul W. Ammann. Tethers in Space Handbook—Second Edition. NASA Office of Space Flight, NASA Headquarters, Washington, DC 20546. See also the hundreds of references in the 33 page bibliography at the end of the handbook.
The “Hoytether”™, an Improved, High-Reliability Tether
In 1991, one of the present inventors, Robert Hoyt, invented a lightweight net-like structure that provides many redundant load-bearing paths. A number of primary load bearing lines running the length of the structure are connected periodically by diagonal secondary lines. The disclosed embodiment of this invention has the secondary lines firmly connected by knots to the primary lines. The secondary lines are connected only to the primary lines. At either end of the disclosed structure, a support ring enforces the cylindrical spacing between the primary lines. The secondary lines are designed with a small amount of slack. These secondary lines are only put under load if a primary line fails. This specific tether structure was disclosed to the public in 1992 (Forward, R. L., “Failsafe Multistrand Tether Structures for Space Propulsion”, AIAA paper 92-3214, 28
th
Joint Propulsion Conference, Nashville, Tenn., 1992 (hereinafter “1992 Hoytether structure”). This structure was named a “Hoytether”. The term “Hoytether” is used throughout the remainder of this specification for this type of tether structure.
The present invention uses an improved Hoytether, which was invented by the same inventors as the present invention. This improved Hoytether is the subject of a copending PCT application. The Hoytether is discussed briefly in this specification to aid understanding of the present invention.
The 1992 Hoytether design teaches that the normally slack secondary lines have half the cross-section (0.707 the diameter) of the primary lines. There are twice as many secondary lines as primary lines, thus the mass of the secondary lines is equal to the mass of the primary lines. In an undamaged Hoytether, the primary lines carry the entire load, while none of the secondary lines are under load.
While the survival probability of a single-line tether decreases exponentially with time, the Hoytether can maintain a high, i.e. greater than 99 percent, survival probability for periods of months or years (Forward and Hoyt, “Failsafe Multiline Hoytether Lifetimes”, Paper AIAA 95-2890, 31
st
Joint Propulsion Conference, July 1995).
REFERENCES
1. Robert L. Forward, Failsafe Multistrand Tethers for Space Propulsion, Forward Unlimited, P.O. Box 2783, Malibu, Calif. 90265, July 1992, Final Report on NASA Contract NAS8-39318 SBIR 91-1 Phase I.
2. Robert L. Forward and Robert P. Hoyt, Failsafe Multistrand Tether SEDS Technology Demonstration, Final Report on NAS8-40545 with NASA/MSFC (Jun. 14, 1995).
3. Robert L. Forward and Robert P. Hoyt, “High Strength-to-Weight Tapered Hoytether for LEO to GEO Payload Transfer” Final Report on contract number NAS8-40690 with NASA/MSFC (Jul. 10, 1996).
The Hoytether is essentially a tri-axial net structure, with ‘primary’ lines running along the length of the tether and two sets of ‘secondary’ lines connecting these primaries diagonally. They can be made by hand and connected with knots as is taught by the 1992 Hoytether structure. Because knotted connections severely limit the strength of a structure, it is desirable to use a knotless fabrication technique to achieve interconnections that have strengths approaching the limits of the constituent material. As these tethers may be many kilometers long; fast and inexpensive mechanical methods are required for their practical fabrication.
Hoytethers may be made by mechanical braiding, i.e. three-dimensional braiding, such as 3-D rotation braiding using braiding machines such as those developed by the Herzog Company in Germany (August Herzo
Forward Robert L.
Hoyt Robert P.
Dula Arthur M.
Eldred J. Woodrow
LandOfFree
Electrodynamic tether control does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Electrodynamic tether control, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Electrodynamic tether control will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2909556