BACKGROUND:

The GEO-elevator "rope-trick" design put a counterweight mass beyond Geostationary Earth Orbit for lifting objects on a sky-cable from Earth surface to 26.2K miles, 6.9K mph, where they would be released in GEO (orbit); -or returned similarly to Earth;- a simple tall elevator under perpetual high-speed collision threat, stretching technology by decades and deferring interest in cheap launches.

(It had a cute aside-proposal for launching huge fleets of space-technology personnel, equipments, supplies, for building spaceships to the moon and beyond, in one decade, by letting each lifted mass continue beyond GEO to act as a countering weight for the next up its sky-cable, running a continuous pulley 50/hr, 4 million in ten years ... using Earth's rotation-momentum to upload a virtual population.)

(It also had a potential advantage of starting the whole process with a thread, pulling up ever-thicker...till it'd upped the main cable.)

The LEO "pinwheel" required a cubic mile of asteroid core orbiting just outside the atmosphere over a great circle of the Earth, with 100-mile spokes dipping into the atmosphere to reach objects waiting aground,- capturing up, throwing them outside, onward at double orbit-velocity, 41% faster than escape velocity ... the asteroid also raising Earth ocean tides, tending to cause or trigger earthquakes, and ultimately eventually entering the atmosphere itself,-- end of exodus, asteroid, and much of Earth surface-living, under tsunamis.

PROPOSAL:

By separating the space-base from any space-station, and putting the base mass in MEO (orbit), and raising its elevator bottom floor to high atmosphere altitude at supersonic jet speed, it becomes contemporarily launchable with a self-growing capability of uploading megatons and millions of supplies, equipments, astrovoyagers, by nuclear-power ... for inexpensive arbitrary-duration mission-stays in regular high-turnover commercial hotel and deep-space launch facility and research and educational university scenarios ... [*]

DESIGN THEORY:

• The space-base consists of a high-output nuclear powerplant (standalone megawatt), comprising most of its mass, redundant cable tanks of nonstick non-snagging cable (coiled-reeled or scrambled; 1000+ miles each) and an aeroglider bottom-car (attachable by supersonic jet on the fly), and linear cable-drive motors and brakes, and digital-radio communication-and-control packages.
• The space-base sends its elevator car down by sending it back, where by orbital mechanics it descends to forward, drawing the full cable length, thereupon stopping, and swings down to the high atmosphere, where a supersonic jet attaches the car, either to itself or to its sky-payload, and lets that swing back up at a few gees as the cable top (base) continues forward at orbit speed.
• The long ascent to MEO (15 min.) slows the vertical velocity to zero, consumed by gravity (for base mass 8× transport mass), and the sky-payload then proceeds to the space-station, higher or lower, in about an orbit.
• The space-base repumps its own orbit by "boom-tendency" swing-dropping and relifting an elevator car as an energy-plumb. (*)

VANTAGES: (MEO, cf GEO, LEO)

1. MEO elevator cable total length is feasibly short (1/15th-to-20th of GEO)
2. Kevlar™ or ruby thread (AlO_x) has sufficient tension strength for feasible thickness at the elevator's space-base
3. Space-base mass can be launched in 3-4 contemporary Space Shuttle missions; and can then upload more mass to itself if needed
4. Air-speed-to-space-orbit elevator velocity difference is similar among MEO options
5. The supersonic transport rotates upward to reduce bottom-floor gravity, comparable to takeoff gees
6. It programmably puts the upload destination anywhere, eg. up to about 1500 miles high for a space station hotel [*]
7. The transport's own momentum is recovered regeneratively in departure at the end of the cable in lower "faster" orbit pulling the space-base forward while slowing itself, going suborbital, toward vertical
8. Additional momentum is gained by pulling the transport back, down faster, reaching the atmosphere flying supersonic westward instead of eastward; approximating the freight tons remaining at station orbit speed (for transport mass 1500 mph 6× payload)
9. The transport reenters the atmosphere at airflight speed, pushing supersonic
10. In case of cable break the transport glides back at "safe" suborbital speed, and the second twin-elevator sends down its cable
11. The cable is easily repairable, needing only tension strength; no tractors, guides,...
12. MEO is adaptable to scheduling for orbiting debris,- unlike LEO which keeps its rotation, and GEO which keeps its position
13. (There may be programmable scenarios for operating both elevators simultaneously: one up, one down)

The main disadvantage of the skyhook itself is scheduling more than one per planet, asynchronously ....

ADDITIONAL: (notes, calculations, drag, obstacles)

1. At straight vertical hang, the elevator bottom would travel at proportionally 4/5ths MEO speed, (about 13Kmph). Exact atmosphere height is daily changeable, and five miles overall cable length is a hundred miles lead, six degrees off vertical.
2. Ascent vehicles, suborbital in the atmosphere, must advance to catch the elevator bottom stage car at this forward position, and pull forward, ahead until rotation,- minimizing atmospheric drag on the fast-moving cable by increasing the leading angle.
3. Then subsequently in space, draw potential energy from the base; and once up at orbit altitude, additional from elevator pull.
4. (At the end of the slower base-orbit-pumping-only regime the aeroglider avoids entry, and elevator power hoists the cable.)
5. The space-base loses altitude in lifting the elevator car with the jet sky-payload, but total mass and extremities remain in orbit.
6. The elevator cable remains taut while the vehicle rises high above the base to reach the station orbit (matching ephemerides).
7. On return, braking to "boom-tendency" exact position, elasticity, powered pullbacks and repeated brakings, affect entry speed.
8. The station is a plate-wheel (eg. 100 m dia. rotating twice per minute simulating .22 gee at its perimeter) orbiting edge-on in a great circle around the Earth,- thus minizing frontal exposure: Only the tire needs whipple armour.
9. Alternatively, To expedite station arrivals and departures, the station and base may be tethered as a tall tower stabilized by orbital mechanics to vertical orientation: The velocity difference, both proportional and direct orbital, facilitates transfers (eg. 100 mile at 1000 differs about 460 mph total), and departing transfer vehicles drop at 6% gee.
10. The -contemporary- disadvantage of MEO is the high density radiation belts, much less dense at GEO,-- but there are ample technological remedies to come, in terms of mu-glass shielding, electrostatic repulsion, per NASA design, etc. [*]

NB. The GEO-elevator has a major disadvantage usually undisclosed: Velocity in GEO is 1.9 mi/sec, much faster than the Earth surface 0.3 mi/sec, that must be gained or lost by each elevator car,-- But without direct horizontal acceleration the proposed fast-scent cars will swing far, plucking the cable with great mechanical advantage at 0.26 mph/mile-climb, (0.002 gee at 600 mph, 37-hour trip);- the Earth base doesn't pull the cable but to sub-gee tension-reel it, to compensate said swing and low frequency oscillations due to diurnal GEO-positioning under solar and lunar tidal-draw (and of near planets) ... the car will sway for enough angle (44 miles at low altitude,- redoubling till high up). ... #2. If a car air-pressure tank ever blows, it may swing wild thousands of miles freed from its tension-reel, in satellite space. ... #3. If cars travel oppositely, simultaneously, they will contend horizontally: coming up west, going down east; and in passing, flip over, -unless they crawl passed- ... wilder and weirder than any skyhook, on 20× longer -exponentially tapered- cable.

* [High-voltage proton "radiation" at MEO is ameliorated by dousing, trapping, deflecting,- by designs similar to interior surfaces of hot fusion plasma containment vessels, and designs special for this environment, eg. superconductive nano-whiskers, trillions per sq'are ("beach fuzz"), maintaining a free-electron surface cloud at subzero work function where each incoming proton accelerates thousands of individual electrons, gradually slowing the proton; or eg. thick heavy high-tesla mu-surfaces deflecting both protons and electrons]
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This skyhook-spacehook would be usable on Earth. On the Moon, maneuvering across its airless surface to catch an elevator car might be less effectual than electro-rail-launch, but the spacehook might be sufficiently controllable for precision touchdown. On Mars the thin vacuous atmosphere hampers flight, but precision touchdown may work as for the moon; On Venus the thick lower atmosphere may support floating-platform cities in the clouds (54Km up) but its global stormy multiple jetstreams hamper precision touch-down.