DOWN-LINK: GEO to Surface

Solar Power Rules!!!
	Downlink climber has a solar powered, electric driven, "linear motor drive" to give it an average velocity of 215 kph and total descend time of about seven days (same as Uplink climber). At the GEO platform, climber is 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 to 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, gravity assist. Below GEO, downward bound vehicles can use Earth's gravity to help 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.
Dedicated Downlink 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 makeEN
Enhance Mission Effect
Downlink Tether (Dnlink) enhances mission effectiveness for the entire Space Elevator Enterprise (SEE). 1) Fully Functional SEE.  If both Uplink and Dnlink are fully functional; then, the SEE can sustain a continuous cyclic rate of one climber/day. Without the Dnlink, rate would be one climber per 15 days, substantially slower. 2) Degraded SEE.  There is substantial risk of either Dnlink or Uplink going down for scheduled maintenance or for unscheduled disaster. Either way, degraded SEE can provide much slower service with remaining link providing less frequent round trips in following manner. ---a) Exclusive Uplink Service. A climber ascends tether for 7.5 day journey to Geosynchronous Equatorial Orbit (GEO). A limited number of climbers can follow in one day intervals. ---b) Park at GEO Node. When each climber reaches GEO, it parks in/near GEO node until all ascending climbers reach GEO. ---c) Exclusive Dnlink Service.  A climber leaves GEO daily and returns to Marine Anchor. SUMMARY: Downlink can reduce risk of isolation at GEO node by alternating service as described above.
Essential Risk Reduction
	Torn Tether is a Real Risk!!!! 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, a substantial risk to the 7 fully loaded climbers which could be on the tether. Mitigations for fixing flaws include: 1) Flaws are detected/repaired during each climber transit. 2) Tethers can host billions of nanobots to rapidly repair defects. LIFEBOAT MODE: However, a disastrous event could rupture, even break, the tether in spite of above mitigations.  Thus, any of the 7 climbers possibly on the tether must always be prepared to become a lifeboat and autonomously fly back to Marine Anchor. Thus, any climber must be able to disengage from tether and fly through space and atmosphere back to Earth's surface.

SUMMARY Dedicated Downlink Tether presents benefits and risks.
Many benefits include: Gravity assist Increasing climber frequency Backup to up link tether.
However, the eye popping risk is the significant likelihood of a ribbon rip with multiple co-located climbers. Thus, the requirement for climbers to autonomously assume "lifeboat" mode. NOTE: Lifeboat mode is later discussed in depth.

SLIDESHOW

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

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