SEVERED TETHER
Several Severed Scenarios are described below.
| BACKGROUND: SE Takes Tremendous Tether | |||||||||||||
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A successful space elevator (SE) will consistently and reliably move payloads into Earth orbit space at much lower cost with much less risk than current rocket based methods.
SE's tether requires material with very high strength and low density; fabric made from recently discovered carbon nanotubes (CNTs) seems to be the best candidate. At 100,000 kms in length, tether will contain billions of CNTs.
A tapered tether reduces total weight and maximizes strength; thus, we seek a proper “taper ratio”, i.e., max cross section area (at GEO) over its minimum value (at Earth’s surface). This ratio varies greatly for different materials:
---For steel, this value is an enormous 1033; definitely not considered. ---For kevlar, it is 2.6×108; better than steel, but still impractical. ---For CNTs, it is a feasible 1.9; thus, tether fabric could be perhaps one meter wide at GEO, but only about .55 m at Earth surface. SUMMARY: CNT is certainly a much better tether candidate than others (such as steel or kevlar); perhaps, it might work for theoretical best case scenarios. |
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| SCENARIO-1: ABSOLUTELY BEST CASE | |||||||||||||
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As well as excellent climber service, an eternal tether could provide several other useful functions.
To enhance tether persistence, consider a few of many SELF HEALING SOLUTIONS. 1) ARTIFICIAL INTELLIGENCE: Tether can host swarms of vigilant nano-bots to detect any anomalies and quickly repair ribbon rips to keep the fabric functioning. 2) GENETIC ENGINEERING: Joan Slonczewski's novel, The Highest Frontier, depicts SE with self-healing cables of anthrax bacilli. She proposes genetically engineered bacteria to quickly regrow more fabric to treat tears in the tether.• Sweeper. Remove orbital debris (see figure at left.) • Catcher. Gracefully receive incoming payloads from space. • Launcher.Host ports at different altitudes to release payloads to go to the Moon, Mars and beyond. BAD NEWS: The cable, about 100,000 km long, has numerous carbon nanotubes (CNTs), each about 100 nano-meters long. Relevant literature discusses different deterministic and statistical models which predict atom level defects in any given CNT. Even one atomic defect could compromise entire CNT which could, in turn, impact entire tether. CNT simulations now indicate tether strength to be at most 30% of the theoretical nano-tube baseline strength, now erroneously assumed in the cable design. CONCLUSION: Even with self healing methods, tether will inevitably break which leads to following scenarios. | ||||||||||||
| SCENARIO-2: REEL IN POTENTIALS | |||||||||||||
With reel-in/out capability, stations can quickly adjust total tether's Center of Gravity (CG) to ensure GEO Node remains at Geosynchronous Equatorial Orbit (GEO).
• ON BOARD REEL DIMENSIONS: Assume 6 m long with max of 2 m diameter provides mechanical leverage for 80,000 km of tether. • SPOOLING SPEED: For a large mechanical device (described above), an optimistic rotating speed might be 1,000 rpm. EXAMPLE: Typical retraction rate might be as fast as 3 km/min or 180 km/hour for 200 hours (~ 8 days) to reel in 36,000 km. NOT FAST ENOUGH. A tether tear likely causes a pair of waves (i.e., "recoils") to move apart at enormous speed. If we assume 1,000 m/s (exceeds muzzle velocity of M-16 round), recoil would travel 36,000 km in 10 hours with devastating consequences. CONCLUSION: Tether reels can make minor adjustments, but rapidly retracting total tether in response to breaks is not likely. |  | ||||||||||||
| SCENARIO-3: SEVERED OUT-LINK | |||||||||||||
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① TE assumes Habitats constructed in GEO, near GEO node, plan to elevate to Apex Anchor (AA), eventually launch from AA to another orbit around Earth or another celestial body. Thus, makes sense for standby habitat to patiently wait its turn "on deck" at GEO.
② When Tether eventually breaks, all SE components must react quickly; recall de-tensioning waves travel 1 kps with a heavy impact. ③ Current AA Habitat must quickly react with tether in tow. It can use inherent linear velocity to escape Earth orbit. On board thrusters can assist as required. It will try to reel in as much tether as possible, but will likely separate considerable tether before wave reaches it. ④ GEO Node must also cleave off considerable tether to escape impact from incoming wave; then, attach new tether. Standby habitat uses thrusters to assume position above GEO node and rapidly deploy new tether from internally stored reel until it reaches AA position. CHALLENGES: Where have all the tethers gone??? Gone space-ward, everyone!!! Separated tethers from steps ③ and ④ will most likely have enough collective velocity to escape Earth's orbit; hopefully, to never return. TOUGHEST CHALLENGE: Ensure all components are in place and properly do their deeds in the short time (less than 10 hours) available after the tether breaks. | ||||||||||||
| SCENARIO-4: SEVERED UP-LINK | |||||||||||||
① If Up-Link tether snaps, TE assumes a deadly recoil (“de-tensioning” wave) would soon reach both MA and GEO Node. ② & ③ MA and GEO Node try to reel in as much tether as possible, but both stations must separate significant portions of tether before recoil arrives. ④ Perhaps burning tether segments would encircle Earth equator for short lived “Ring of Fire”; a spectacular sight; but what about possible hazards? Pollution: Perhaps 2,000 tons of burnt CNT might fall. Short term carcinogen? Ingested CNT soot particles might cause cancer. Food chain contamination? Farm fish and animals might ingest CNT which might cause cancer for human consumers. If unburnt tether portion impacts Earth, tether remnants might hit Earth at 5.7 km/sec (12,759 mph) to impact both sea and land. TRANSITION: If Up-Link tether is gone, we must deploy a new one; perhaps as shown in next frame. |
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| SCENARIO-5: DEPLOY NEW UP-LINK | |||||||||||||
| ① Normal Ops: To best recover from a tether break, spare tether must be readily available. Spare reel with 80,000 km of tether must stand by GEO Node to quickly mount and use. ② Tether Breaks:Per Scenario-4, Up-Link tether snaps, and both ends recoil like a rubber band. To avoid this deadly recoil, Marine Anchor and GEO Node must soon sever the tether. Resultant two tether segments float down to encircle the Earth to burn up in the atmosphere. ③ New Up-Link Tether deploys from GEO Node perhaps with lifeboat from previous chapter. Descending to Earth for 12.5 hours at low gravity until about midway, 20,000 km from Earth, velocity increases to about 15 kps. Then, it must decelerate for net force of -1G for about 1/2 hour to arrive at Marine Anchor at zero velocity. ④ Old AA Uses Thrusters to become a habitat in another orbit elsewhere. GEO Node uses thrusters to maintain GEO position. As in Scenario-3. Standby Habitat goes on top of GEO Node and uses centrifugal force to deploy up to 80,000 km of new tether for new Out-Link. ④ Old Out-Link tether tends to drift backwards due to orbital forces. GEO Node might try to reel it in. At 64,000 km, optimistic reel-in time might be about 320 hrs = 13.33 days. | ||||||||||||
SUMMARY
Eminent scientists insist
that CNT tethers are temporary at best;
they will inevitably "snap"
at some time, some place
along their considerable length.
Thus, it makes sense to plan for such contingencies.
To routinely recover from severed tether
requires considerable in place infrastructure
as described in this chapter.
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CONCLUSION:
CNT tethers are certainly possible and perhaps even feasible; but considering their considerable risk, are they practical???? Perhaps there is a less risky and much more practical solution to routinely lift payloads from Earth to GEO and beyond. | ||||||||||||
posted by JimODell at 5:23 AM
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