Monday, January 29, 2007

TETHER UPLINK: Surface to GEO

 Thought Experiment (TE) defines the "Uplink" portion of the Space Elevator as the Tether segment from Earth's Marine Anchor to the GEO Node as well as the climber(s) which ascend this tether portion.
SPACE ELEVATOR CABLE (commonly called "tether") must carry its own weight plus climbers and payloads. Throughout its entire length from the Marine Anchor on Earth's Equator to the GEO Node (35,786 km), cable must carry the enormous weight of the cable below. Tether's maximum tension would be at geosynchronous altitude; so, the cable must be widest and thickest there and carefully tapered as it approaches Earth. 
Such a cable needs a material with a large tensile strength to density ratioEXAMPLE: The Edwards space elevator design assumes a cable material with a specific strength of at least 100,000 kN/(kg/m). This value considers the entire weight of the space elevator. An untapered space elevator cable would need a material capable of sustaining a length of 4,960 kilometers (3,080 mi) of its own weight at sea level to reach geostationary altitude. Thus, cable material requires enormous strength with an absurdly, tiny weight, an extreme challenge. 
Consider metals like titanium, steel or aluminium alloys, they have much less breaking lengths of only 20–30 km. Modern fibre materials such as kevlarfibreglass and carbon/graphite fibre have slightly greater breaking lengths of 100–400 km, but they still fall far short of stated requirement of almost 5,000 km. Recent developments such as carbon nanotubesdiamond nanothreads and graphene ribbons (uniform two-dimensional sheets of carbon) might someday attain breaking lengths of 5000–6000 km at sea level, and they can also conduct electricity, a useful bonus. HOWEVER, carbon nano-tube (CNT) technology still has a long way to go to meet requirements for space elevator cables. EXAMPLE:As of 2014, CNT technology grew tubes only up to a few tenths of a meter.
Dr Bradley C. Edwards received his PhD  in Physics from the University of Wisconsin–Madison in 1990 with a thesis in astrophysics. As a staff scientist at Los Alamos National Laboratory, he worked on several important projects. In 1998, Dr Edwards began work on the space elevator concept. Edwards received funding from the NASA Institute for Advanced Concepts to examine the idea and published two papers in 2000 and 2003He proposed methods for deploying a space elevator and overcoming perceived obstacles such as orbital debris, anchoring, climber design, and power delivery and examined construction costs and scheduling, laying the groundwork for current discussions.
Edwards also published two relevant books: The Space Elevator: A Revolutionary Earth-to-Space Transportation in 2003 and Leaving the Planet by Space Elevator in 2006 which gained coverage on major news media. In interviews, Edwards proposed cost of launching into low-earth orbit could shrink by 99%. Edwards spent eleven years working at the Los Alamos National Laboratory in New Mexico, researching advanced space technologies.  He pursued a number of ventures associated with the space elevator concept before spending six years as a senior engineer at Sea-Bird Electronics, an oceanographic company. He recently started a new company to further develop CNT technology.
Uplink's Tether Description
Assessed by recent AIAA report as "excellent feasibility" assuming successful progress of certain technologies. Of course, some scientists are much less optimistic (see later text).
Specific Strength:  IAA feasibility study provides detailed analysis of required strength throughout entire length of tether. From layman's point of view, tether must taper with widest point just below GEO node and slimmest point just above surface of Earth.
Widest point must be at GEO node because it holds tether's entire uplink length (35,782 km) and needs max strength at that point. 

Fortunately, feasibility gets an assist from following factors:
....a) Earth's gravity is weakest at GEO node; thus, max mass there has less weight than max mass closer to Earth.
....b) Tapered tether contains much less mass than nontapered; thus, much less weight than nontapered.
Uplink's Climber Description
Dimensions are spacious; the cylindrical climber is 20 meters in diameter and 15 m high.

Climber drive train is electric driven, "linear motor drive" which enables an average climb speed of 60 m/sec = 216 km/hr. Total climb time is about seven days.

Assume Solar powered from 40 km above surface to GEO. Since solar panels are large and fragile, they might be carried in separate container for first 40 km of ascent (see right side figure).

Assume tether can service seven simultaneous climbers; then, climber frequency would be about one per day.

Assume climber's max gross weight is 20 metric Tonnes (mTs) with max payload of 14 mTs; then, max climber capability could be:
14 mT/day = 98 mT/week = 5,096 mT/year.
 
When in EARTH's SHADOW, 
climber "sleeps" or hibernates.
Climber normally starts its climb from Earth's surface at "first light" (assume 0600) to maximize first day's exposure to sunlight.  Thus, first "night" might be about only 6 hours, because climber was able to enjoy 12 straight hours of sunlight and traveled about 216 km/hr × 12 hr = 2,592 km "straight up" on the Uplink tether toward GEO node.  Normal night on Earth's surface at Equator would entail about 12 hours of night time; however, an altitude over 2,500 km enables climber  to avoid some of Earth's shadow.
Furthemore, successive "night times" get ever shorter as climber moves away from Earth to GEO node.  So short, as to be inconsequential.

Climber VulnerabilitiesRespective Mitigations
Lower Atmosphere Weather. For first 40 km of ascent, climbers must survive atmospheric effects, such as high winds and lightning strikes, etc.Lower Atmosphere Weather. Int'l Space Elevator Consortium (ISEC) proposes High Stage One as innovative way to avoid lower atmospheric effects.
Need Efficient Energy:  Climber vehicles will require large quantities of energy to ascend over 35,000 km from surface to GEO. Unfortunately, energy storage (i.e., large batteries or fuel reservoir) requires considerable mass (i.e., very heavy) and is very inefficient. Need Efficient Energy: Large, light (unfortunately, fragile; thus, climber doesn't deploy them till above 40 km) solar panels can efficiently power climbers throughout most of the ascent. (Climbers must hibernate during short "night times"; see Earth Shadow, above).
Human Based Threats:  High Powered Lasers can easily target large solar panels; even worse,  misses will likely go undetected, and bad guys can keep firing til they achieve a hit. Guided Missiles are persistent; even when hit with defensive rounds,  unguided missile momentum keeps mass coming as a deadly dirge of shrapnel. Human Based Threat:  
Numerous armed drones can form protective swarms around climbers.
Even better, they can attack incoming missiles as well as deployed laser banks.
Radiation:
Earth's radiation belts might harm passengers and cargo.
Radiation:  Deploy tethers such that climbers can avoid dangerous belts; use limited shielding because max shields impact payload capability.
Tether Dependency.  
Perhaps the climber's worse vulnerability is the tether itself.
If the tether snaps; then, attached climbers must  fall with it.
Tether Dependency:  Climbers might need to detach then serve as lifeboats. Thus, they might need ablative heat shield, aerodynamic wings and/or parachutes to land safely. Recovered climbers could be refurbished, reattached to tether and returned to operation.
Tether Vulnerabilities 
  • 1) Inevitable Space Debris, 2) Terrorist Threats and Defensive Tactics, 3) Disaster Recovery Methods
Space debris includes both natural (meteoroid) and artificial (man-made) particles. Orbital debris is any man-made object orbiting the Earth which is no longer useful. More than 20,000 debris larger than a softball now orbit the Earth. At speeds up to 17,500 mph, a small orbital debris can damage a climber or even the tether. There are 500,000 debris items the size of a marble or larger. Many millions are too small to track.
Tether has a design requirement to absorb objects as large as 10 cm in diameter. Perhaps tether will include numerous nanobots which can reconfigure as required to meet this requirement.  Thus, tether's best use might be to clear space debris from orbit.

Collect All Debris. Imagine a strong, strand of fly paper stretching from any position on Earth's equator (nadir) straight up to its corresponding zenith in the GEO.   Such a strand would eventually collect all space debris orbiting Earth.  In addition, this device would likely collect many small meteorites (i.e., less than 10 cm diameter).  If the tether acted like this sticky, fly paper; then, climbers could collect this debris on their way up to GEO Node for further processing. (NOTE: Either return to Earth for salvage, or send up to Apex Anchor to add more much needed mass.)
Nanobots. To prevent space debris (or incoming missile shrapnel) from shredding the tether, specialized swarms of nanobots could surround a blast zone to shield the surrounding area much like heavy, magnetized armor plates. However, these "armor plates" could quickly dissolve when no longer needed and redistribute throughout the tether in an optimal fashion.

Move Tether.  To avoid operational satellites and larger space objects, tether could move.  Marine anchor can move entire tether in response to predicted collisions with satellites or large meteors.
TERRORIST THREATS and DEFENSIVE TACTICS
Threat: Determined missile attacks could certainly impact the tether (as well as climber, see above.)
Defense: An automatic Air Defense System (ADS) can detect and shoot down incoming missiles; however, shrapnel might still impact tether.
Additional layer of defense could use swarms of nanobots to shield the impact area much like armor plating; also, specialized nanobots would quickly repair any rips and tears.
Threat: Use 9/11 tactics to crash a commercial plane into the tether. 
Defense: Local Air Traffic Control (ATC) must be ever vigilant:
---a) Detect all intruders and quickly react to prevent all aircraft from approaching tether by accident or by intent.
---b) Reduce airborne threat with a few radar stations and well armed drones flying nearby Combat Air Patrols (CAPs). 
Threat: Transverse carbon nano-tube (CNT) cable could sever vertical tether along horizontal axis.  Given a "security diameter" of 20 miles radius, attackers could use two aircraft (or even boats) to drag a 40-mile horizontal CNT (thin and hard to detect) to and through the elevator. This horizontal CNT would likely severely damage the vertical CNT, i.e., the space elevator tether.
Defense: Local ATC must watch surrounding sea surface as well as nearby air; and keep those armed drones handy.
Threat: Suicide Bomber.  Walk a bomb into the space elevator's ground facility and detonate it. 
Defense: Carefully search everything and everyone going into and onto the space elevator. Carefully inspect all possessions from all involved personnel (crew, passengers, support staff) and quarantine all during a week long training session prior to each manned deployment (climber ascent).
BEST DEFENSE:  Make Friends Instead of Enemies.  Space Elevator Enterprise should publicize SE as the world's resource. They should be generous employers, benefactors to their surroundings and the whole world, and they should try to ensure the world's population has positive feelings about the elevator and its operators.  Thus, be very inclusive about the humans privileged to go up/down the tether and to live in the GEO Node Habitat.  Genetic and cultural diversity should be a hard and fast rule.
SUMMARY:  To minimize the risk of hostile acts against Space Elevator (SE), minimize hostile thoughts against it. Encourage all segments of world's population to enjoy riding and living in SE as well as reaping its multitudinous benefits.
DISASTER RECOVERY METHODS:
Well designed safety mechanisms should kick in.
For severed cable near bottom of tether:
---a) Wind up entire tether with high powered motors. 
---b) Clean up immediate damage near the base anchor 
---c) Lower backup cables from GEO Node to replace severed tether.
---d) Assess damaged cables to either repair or salvage for use on other tethers.
Cables cut near tether's top is even worse. 
---a) Configure Climbers as Lifeboats:  Control descent of climbers with emergency rocket motors; deploy parachutes when safe to do so.
---b) Nanobot Wings:  Use nanobots to form "wings" at carefully controlled spots along entire length of the tether. Once in the atmosphere, nanobot wings should aerodynamically control descent. OBJECTIVE:  Entire tether length (perhaps over 35,000 km) should spiral down into a tight area to facilitate rapid recovery.

Tether faces many lightning strikes 
during the inevitable thunderstorms.
Design the cable as an adequate lightning rod (after all, it anchors to Earth) to protect critical systems. Recall the tether contains carbon nanotubes (CNTs) which can be engineered to be highly electrically conductive, resistive or even semi-conductive. Thus, the tether includes some conductive strands to power the climbers. Thus, the tether ribbon must ground lightning strikes, and the insulated climber must withstand the power surge.
2
KEY RECOVERY METHOD:  Redundancy.
A spare tether is a necessary part of the Space Elevator System; fortunately, building the second one will be much easier than the first one.  After the initial space elevator deploys, core costs greatly reduce when you build the second one. First one can easily elevate many materials and components needed for second one.
R&D is mostly complete.  Most important, initial elevator clearly demonstrates a proof of concept; and SE team gains many lessons learned.
---a) Second tether could serve as a spare up link tether.
---b) Also, it can immediately fully function as a down link tether.
---c) It could also start collecting and disposing of space debris.
The most critical component in the space elevator is the tether, which requires material with very high strength and low density. If realized, the space elevator will revolutionize current methodology to take payloads into space at much lower cost. UNFORTUNATELY, empirical evidence now indicates tether has unrealistic requirements.
About 100,000 kms in length, tether will most likely contain numerous carbon nanotubes (CNTs). Different deterministic and statistical models predict atom level defects in any given CNT, about 100 nano-meters long. Even one atomic defect could compromise entire CNT which could, in turn, impact entire tether.  CNT simulations now indicates tether strength to be at most 30% of the theoretical nano-tube baseline strength, now erroneously assumed in the cable design. 
SUMMARY:  Tether will have inevitable defects. 
CONCLUSION: Tether tears are very likely with complete tear-throughs soon to follow. 
POSSIBLE SOLUTION:  Tether can host swarms of vigilant  nano-bites to detect any and all ribbon rips and quickly repair them before the cable completely ruptures.

UPLINK RIBBON RUPTURE
is much more likely than outlink ribbon rupture, and it could have disastrous consequences.
Tether portion above break can endanger spacecraft in equatorial plane due to ribbon’s high speed.
Tether portion below break  can reach Earth surface with high speed to endanger ground based objects.
Possible Solutions:
Climber can become a lifeboat to return safely back to Earth.
GEO Node can quickly reel in upper portion of tether above the break.
Marine Anchor can quickly reel in lower portion of tether below the break.
SUMMARY: Even the strongest tether might break at anytime with disastrous consequences.POSSIBLE SOLUTIONS: Nano-bots; Super quick rewind motors; Methods still unknown.




VOLUME 0: ELEVATIONAL
VOLUME I: ASTEROIDAL
VOLUME II: INTERPLANETARY
VOLUME III: INTERSTELLAR




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