Sunday, August 16, 2009

DOWN-LINK: GEO to Surface

Space Elevator Downlink
Solar Power Rules!!!
As in Uplink, Downlink climber's drive train is solar powered, electric driven, "linear motor drive" with an average velocity of 215 kph and total descend time of about seven days.
Climber initiates descent from GEO in a zero g state; thus, it will remain at GEO until an applied force moves it.  Thus, the same solar powered, electrical motor and gripper system which tugs the climber upward on the Uplink tether can propel the climber downward toward the Earth. Spaceborne solar mirrors is a practical measure which could greatly enhance solar rays received by collection panels at each climber. 
Less Practical Power Sources might include:
1. Use climber motion through radiation belts to generate electricity. this is a significant health hazard; it's best to avoid them or even to shield against them.
2. Laser banks (both ground based and space borne) requires complex infrastructure; solar mirror uses much simpler system for same benefits.
3. Air Friction (i.e., "wind") can turn small onboard turbines; however, this only produces power during the slight duration in Earth's lower atmosphere.

 Downlink  Kinetic Benefits
Unlike Uplink, Downlink climber gets a bonus.  Below GEO, downward bound vehicles can use Earth's gravity to power their descent. As the climber descends closer to Earth, gravity force ever increases to accelerate climber. Since solar power will be less needed as gravity increases, climber's motor system can change function.  As well as grabbing ribbon to propel climber, it can also:
1) Generate Electricity.  As Earth's gravity pulls climber closer, friction turned wheels can rotate magnets and electrical conductors (i.e. generator) to further power climber.
2) Control Speed.  To avoid damage to itself as well as tether, motor-gripper system will regulate a given speed limit, perhaps 200 kph.
3) Disregard Earth's Shadow.  Due to this non-solar power source, climber will not have to "hibernate" during brief "night times" which it must do during ascent.
Constant Ribbon Maintenance
During every tether transit (both Uplink and Downlink), climber inspects ribbon; then, repairs as required.
Downlink Marine Anchor
Downlink climber's destination will likely be a second, equatorial Marine Anchor dedicated to Downlink operations; it will perhaps be located 500 km from the first Marine Anchor, dedicated to Uplink ops.

Space Elevator Enterprise will use Ocean Going Vessels (OGV) cruising at 30 knots to transit between these two Marine Anchors in perhaps nine hours.

Recall that large retired vessels, such as aircraft carriers or oceanic oil rigs, would make suitable Marine Anchors.
To Spare or Not To Spare
Downlink Tether can contribute whether or not the entire Space Elevator Enterprise (SEE) is fully functional.
1) Fully Functional.  If both Uplink and Dnlink are fully functional; then, the SEE can sustain a continuous cyclic rate of one climber/day.
2) Partially Functional.  Assume scenario of Uplink tether going down for maintenance or perhaps down due to disaster and awaiting replacement. Either way, degraded SEE can provide at least degraded service with Downlink acting as both Uplink and Downlink; perhaps as follows:
---a) Uplink service for 7 days. A climber daily ascends tether for 7.5 day journey to Geosynchrous Equatorial Orbit (GEO).
---b) Park at GEO Node. When each climber reaches GEO, it parks in/near GEO node until all 7 ascending climbers reach GEO.
---c) Dnlink service for 7 days.  A climber leaves GEO daily and returns to Marine Anchor.
SUMMARY: Downlink can back up Uplink by alternating service as described above.
Downlink Disasters
Previous chapter indicates that any ribbon (either Uplink or Downlink) can rupture at anytime. Consider a tapered ribbon from GEO Node to ocean surface, a length of 35,760 kim with gross weight about 2,400 Metric Tonnes (MTs), see ISEC report. Any flaw (even at atomic level) seriously degrades tensile strength; unfortunately, flaws are very likely.

Mitigations for fixing flaws include:
1) Flaws are detected/repaired during each climber transit.
2) Tethers can host billions of nanobots to rapidly repair ribbon rips.

WORSE CASE:  However, a disastrous event could rupture, even break, the tether in spite of above mitigations.  Thus, any or all 7 fully loaded climbers must always be prepared to autonomously return to Marine Anchor. (See more in "Lifeboats" chapter.)



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