Sunday, May 06, 2012

SEVERED TETHER





























Several Severed Scenarios are described below.
BACKGROUND:  SE Takes Tremendous Tether
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. 
3 Ways to Ensure Tether Tautness
1) No counterweight means much more tether.
2) Replace some tether with captured asteroid as inert counterweight.
3) Replace asteroid with well designed Habitat as  multi-featured Apex Anchor.
Tether Can Sweep the Sky
Anchored at equator, tether will eventually encounter any non-GEO orbital debris to impact it. Hopefully, all such debris will embed in tether for hosted nanobots to usefully dispose.
SCENARIO-1: ABSOLUTELY BEST CASE
CNT Tether exceeds our expectations and never, ever breaks.
As well as excellent climber service, an eternal tether could provide several other useful functions.

• 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.
To enhance eternity of these tethers, several SELF HEALING SOLUTIONS are proposed.  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.

BAD NEWSThe 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
Can on board tether reels mitigate risk of severed tether???.
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.
① TE assumes Habitats to be constructed in GEO, to later deploy to AA and beyond.② Due to inevitable atomic defects, tether will eventually snap like a rubber band to sting anyone holding either end.③ At 100,000 km away, Tether bound AA's linear velocity = 7.7 kps; but escape velocity is only 2.7 kps.④ Standby Habitat moves "on top" of GEO Node to deploy up to 80,000 km of tether.
SCENARIO-3: SEVERED OUT-LINK
If Out-Link tether breaks, perhaps it can recover.
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.
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  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
If Up-Link tether breaks, it might burn!!!
 If Up-Link tether snaps, TE assumes a deadly recoil (“de-tensioning” wave) would reach both MA and GEO Node in a few hours.
 &  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.
 Up-Link segments are at less than escape velocity; thus, total tether mass (~2,000 mTs) would likely fall to Earth. Some scientists say 'minimal impact' because tether remnants would burn up in atmosphere.
Perhaps burning tether segments would encircle Earth equator for short lived “Ring of Fire”; a spectacular sight; but what about other consequences?

POSSIBLE HAZARDS: If tether burns, 2,000 tons of CNT soot fall on us.
Climate impact? Likely none; NOTE: 1883 Krakatoa eruption.
Soot inhalation? Soot level is far less than experienced during house fire.
Short term carcinogen? Ingested CNT particles might cause cancer.
Food chain contamination? Farm fish and animals might ingest CNT.
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: Unfortunately, Up-Link tether is gone, and we must deploy a new one.
RING OF FIRE
Up-Link tether extends unbroken from Marine Anchor (MA) to GEO Node. ① Tether snaps like rubber band to start  dangerous recoil to impact both ends. 2 Free  Segments.
Both  MA and  GEO Node must self sever to avoid recoil.
④ Entire tether falls to Earth to burn up and extinguish in sky around equator.  
Tether intact and spare tether reel standing by and available. Up-Link tether breaks and disappears in
a ring of fire.
New Up-Link tether deploys from GEO Node.Old AA uses thrusters to orbit elsewhere as habitat.
SCENARIO-5: DEPLOY NEW UP-LINK
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.
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.
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.
Old AA Uses Thrusters to leave AA position, and then start life as a habitat in an orbit somewhere. 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 o 80,000 km of new tether for new Out-Link. 
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 are destined to "snap" anytime, anyplace along their considerable length.





Thus, it makes sense to plan for such contingencies.  To routinely recover from severed tether requires considerable infrastructure to be in place as described in this chapter.
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.

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