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Vertical Skyhooks and Static Orbital Rings

Bronx Skyhooks
Image from Steve Bowers
Three linked skyhooks orbiting the gas giant Bronx; a scoopship is approaching with a cargo of gas for export. The ship will land on one of the lower platforms and transfer its cargo to the far end of the tether. Over time many more of these structures were constructed and linked into a continuous ring around the planet, forming the first static orbital ring (completed 2753 AT)

Introduction

Tethers find many applications in space travel and satellites, ranging from providing a simple expedient for stabilizing and orienting satellites via gravity gradient stabilization to 'beanstalks' and hydrogen scoops on gas giants. This article addresses close siblings of orbital elevators known as 'vertical skyhooks' (herein referring specifically to non-rotating orbital tethers that do not reach the ground) and a less well-known, but still serviceable, application of tether technology: the static orbital ring.

A complete static orbital ring usually consists of a pair of rings that encircle a planet, the rings mutually supporting each other through tethers. Contrary to the name, static orbital rings are anything but unmoving because they do in fact orbit a planet at normal velocities. But in comparison to dynamic orbital rings, which are actively supported, these largely-passive structures are, indeed, static.

To understand static orbital rings and their unique qualities, it is usually helpful to first consider the physics of non-rotating orbital tethers.

Tether Physics

Skyhooks and static orbital rings can be considered non-rotating orbital tethers or, more correctly, they can be considered tethers that rotate once per orbit and thus appear to remain with one end pointed toward the ground, a condition naturally introduced in long orbital structures by gravity gradient stabilization.

The classic implementation of a skyhook is, basically, a very elongated artificial satellite. As with any satellite, the skyhook's center of mass is in freefall and determines the orbital period of the skyhook. The center of mass of a skyhook is placed in a high altitude orbit. As orbital velocity decreases with altitude, this means the skyhook is orbiting the planet more slowly than a satellite or shuttle in a lower orbit - a point that is key to the utility of the skyhook. Tethers are then constructed from this zero-G core of the station to both dangle above and below the core.

An oddity of orbital mechanics not often recognized in compact spacecraft is that only the center of mass of the spacecraft is truly in freefall. The rest of the structure is either closer or further from the planet (or other object) being orbited. Because the rest of the structure is restricted to the velocity of the center of mass, this means that portion of the satellite closer to the planet - i.e., at a lower orbital altitude - is moving slower than it should for that orbit and thus is inclined to fall into a lower orbit. The portion of the satellite further from the planet is moving faster than it should for its altitude and thus would prefer to climb to a higher orbit. Ideally, the structural strength of the spacecraft will prevent parts from flying off but, again, the effect is weak in small structures. This effect, incidentally, is responsible the Roche Limit. It also makes gravity gradient stabilization and tidal locking possible, as the lower end of the satellite is more forcefully held by gravity than the upper end.

Skyhooks exploit these tidal forces to extend their upper and lower tethers and keep the tethers taut. This rigid orientation and an orbital period based on the altitude of the skyhook's core open creates one of the key features of skyhooks: a low altitude docking platform that is moving considerably slower than a normal circular orbit at that altitude. Depending on the altitude of the core, the low end of the skyhook may be moving some kilometers per second slower than a free orbit at the same altitude. This in turn creates a useful opportunity for Middle Tech launchers to effectively reach orbit for significantly less fuel than if they had to climb to orbit only with their internal fuel supplies. For launchers with a low specific impulse (e.g., typical Middle Tech chemical rockets), several kilometers per second of saved delta-V can dramatically increase payload and make single-stage-to-tether vehicles possible.

A typical skyhook around a planet about the size of Terra will place its low altitude end, the docking platform, within 100 to 300km of the planet's surface (and thus the skyhook will be several thousand kilometers long overall). This altitude is selected to minimize atmospheric drag on the skyhook while minimizing the gravity losses of skyhook shuttles. Docking occurs at very low relative velocities between the shuttle and skyhook, typically at the peak of a parabolic sub-orbital path flown by the launcher. At this point, the shuttle is nearly motionless with respect to the skyhook and may be captured by a free-flying, guided grapple; latch onto a dangling hook; or (for larger docking platforms) settle onto a landing pad atop the docking platform.

Because the skyhook's landing platform is moving in a circular path compared to the parabolic arc of the shuttle, the shuttle will experience a period of acceleration following docking as its course is corrected to follow the skyhook's path. The skyhook in turn experiences some loss of momentum when it picks up the shuttle, but the skyhook is so much more massive than the shuttle (at least hundreds of times) that the loss of orbital velocity is minor. (It should also be noted that raising material from the docking platform to the zero-G core costs the skyhook some momentum.) This lost velocity can be recovered over a period of hours or days by high efficiency, low thrust engines available to Middle Tech civilizations (like ion rockets, electrodynamic tethers or early fusion rockets). Because a skyhook is not subject to the mass or volume restrictions of a shuttle and may use very large solar or radiator arrays to power or cool its heavy orbital maintenance engines.

As noted previously, the low altitude platform is moving at velocity below that of a circular orbit of the same altitude. Its orbital period is determined by the altitude of the skyhook's center of mass. With a cable stretching several thousand kilometers to a docking platform, a substantial counterweight is required to keep the center of mass at a high altitude. This counterweight can be simply a very long cable, but more often is a large platform with some ballast - anything from large tanks of fuel to an asteroid. With even a small skyhook massing approximately ten thousand tons the shift in the center of mass caused by a shuttle arriving at the docking platform may be ignored, but some skyhook designs will include shifting ballast for precise altitude control.

By not being in freefall and moving slower than its orbital velocity, the docking platform will be subject to natural gravity at level that increases rapidly as the platform's velocity decreases. Using the example of a skyhook orbiting Old Earth (or a world of equivalent parameters) a docking platform 200 km above the planet moving at 5.5 km/s (vs. the local orbital velocity of 7.8 km/s) would experience approximately 0.5 G, while a 6.8 km/s docking platform would experience 0.25 G, and a 3.9 km/s platform would experience 0.75 G. A similar effect applies to high altitude platforms, which are subject to centripetal forces higher than local gravity. However, while the low altitude platform feels the force of gravity pulling toward the planet (and thus the planet appears "below" the station), the high altitude platform would find the centripetal force pulling away from the planet (which would appear overhead).

Ever-lengthening skyhooks provide larger and larger launch benefits as their center of mass is placed in higher, slower orbits. Taken to an extreme, when the skyhook's center of mass is moved to a synchronous equatorial orbit, then its low end can touch the ground in a stationary manner, becoming a beanstalk.

Static Orbital Rings

Bronx Static rings
Image from Steve Bowers
The inner and outer static orbital rings around Bronx are linked to a central geostationary ring by by carbon nanotube cables. The inner ring is only 265km above the clouds, while the outer ring is 75,600km from the planet's core.
As occupancy on a skyhook increases, it may become necessary to extend the structure of the skyhook in both directions along its orbital path. Taken to its logical conclusion, this can result in a complete ring encircling the entire planet: a static orbital ring.

Nieuw Amsterdam
Image from Steve Bowers
The completed ring at Bronx, known as Nieuw Amsterdam, includes two large habitable decks (this deck, known as Downtown, is suspended 265km above the planet's clouds; the second one, the Heights, hangs outwards thousands of kilometres above geostationary level

Vertical Skyhooks and Static Orbital Rings - Engineering

Vertical Skyhooks and static orbital rings offer many of the same functions as conventional dynamic orbital rings but with lower technology requirements and greater passive safety. Both skyhooks and static orbital rings tend to have five major elements: a low altitude docking platform, a zero-G core, and a high altitude counterweight platform; the cables that bind the platforms together; and elevator cars that travel between the platforms.

The low altitude platform usually originates as a docking platform. In the simplest skyhooks, this may be little more than a bare truss platform to hold shuttles while cargo is transferred to elevators. However, the low altitude platform's local gravity - which is not produced by a dizzying spin system but rather natural gravity - offers an attractive location for orbital resorts and even high-priced real estate in much the same fashion as dynamic orbital rings. In star systems lacking habitable planets, or even lacking planets with surfaces habitable to Middle Tech civilizations, a skyhook can provide a habitat with gravity close to Terran-normal. It is usually in these environments that skyhooks grow into complete static orbital rings.

The core platform may take many shapes and sizes. Their primary function is to mount the orbital maintenance motors and associated systems. Another important function is that of a spaceport. Because the core is in zero-G, visiting spacecraft do not have to visit under thrust (as interceptions of the docking and counterweight platforms would require). Finally, secondary activities include zero-G industrial operations, research, and freefall hotels. In static orbital rings, the core region is often mostly unoccupied, with orbital maintenance and factory modules studding the connection cables every few hundred kilometers. The mass of the core is not significant because of its position at or near the natural orbital radius of skyhooks and static orbital rings.

The counterweight platform is often comparatively underutilized in skyhooks. It primarily stores mass to maintain the core's orbital altitude. Because the counterweight is moving at a supra-orbital velocity, it can offer a free launch boost to spacecraft released from its hangars. (More correctly, the boost is free to the spacecraft; the skyhook or static orbital ring will lose some momentum.) When counterweights stations are made of asteroids, mining activities may be conducted. Maintaining orbital altitude would then require reeling out the counterweight station to higher altitudes as mass is removed.

In full static orbital rings, the counterweight ring is often fully developed into real estate matching the low altitude ring. The rings are rarely equal in width. For complete rings, the lower ring, which has a smaller circumference, can be wider and still of equal mass to the upper counterbalance ring.

Radiation shielding is often an important engineering aspect of Middle Tech skyhooks and static orbital rings. Their length often puts their core and counterweight deep in planetary radiation belts, which necessitates radiation shielding to shroud those platforms and the elevators that service them.

Finally, if a ring does not regularly aid planetary launches; keeps its low altitude ring well above the atmosphere (1000km+); and is not interested in maintaining a particular orbital inclination, it is generally a stable structure over a time scale of centuries to millennia. In practice, static orbital rings are busy structures and active maintenance is required. Onboard engines work to maintain the rings precise inclination and altitude while active ballast systems help to keep these tension-supported structures from wobbling due to internal and external stresses.

As noted above, skyhooks are structures that can grow easily as each new section is largely independent of neighboring sections - minimal loads are imposed on the older sections as new one is added, assuming simultaneous expansion of high and low altitude platforms. Because of the disadvantages of complete static orbital rings (see below), there are far more skyhooks than complete static orbital rings.

Advantages

Skyhooks are less technologically rigorous than beanstalks and are more forgiving to dock with than rotovators. Skyhooks offer a Middle Tech means of accessing higher gravity planets, like gas giants, when a beanstalk cannot be constructed due to materials limitations. A complete static orbital ring, though, represents a massive logistical exercise beyond most other tether systems of the same technology level and is thus quite rare.

As noted previously, static orbital rings are relatively passive structures and thus include an element of safety not found in some other, more dynamic structures. Beyond this, they share many - but not all - of the advantages of dynamic orbital rings, like natural gravity (on the inner ring) and launch assist to planetary spacecraft.

A second advantage is that an incomplete static orbital ring will not occupy an entire orbital band, as a dynamic orbital ring must. A skyhook may have very small platforms, though its long inter-platform cables need right of way. Dynamic orbital rings must fling continuous mass streams around a planet (or star), even if the dynamic orbital ring is not a complete platform.

Third, skyhooks offering 2 to 3 km/s of launch assist require an order of magnitude (or less) strength from its materials than a beanstalk and several orders of magnitude less material for the same payload. Depending on how rapidly the Middle Tech culture develops spaceflight, skyhooks may be viable for decades or more before a beanstalk is possible. (This, of course, means that skyhooks and static orbital rings are quite rare in the modern Terragen Sphere. Few cultures are so regressed that they cannot build beanstalks.)

Even in cultures that have possess carbon nanotube technology to an extent that they could build a beanstalk there are circumstances where nanotube technology may be insufficient, such as tethers providing access to gas giants rich in helium-3. If active structures like space fountains and dynamic orbital rings do not appeal to the engineering capabilities of the culture, then skyhooks provide an effective alternative. Generally, though, there is only a narrow window to utilize skyhooks before a culture is able to deploy more advanced alternatives.

Finally, while skyhooks that rotate more often than once per orbit (aka, "Rotovators") offer greater launch assistance in shorter, lighter forms than non-rotating skyhooks, they also present narrower docking windows and remove one of skyhooks' secondary values: high value real estate without the headache of spin-derived "gravity."

Drawbacks

As previously noted, there is often only a relatively limited period when skyhooks and static orbital rings will seem viable to a culture. Generally by the time a Middle Tech civilization can build a skyhook offering a 4-5 km/s launch assist around a Terran-sized planet, the civilization is also capable of building beanstalks, which simplify space access compared to skyhooks. For civilizations interested the least logistics burden in building a space-based launch-assist tether, rotovators require less mass in orbit than skyhooks. Active mass stream technology may also be more convenient, depending on the civilization's ability to flawlessly maintain such systems.

Static orbital rings are also enormous logistical challenges compared to even dynamic orbital rings. While much of the technology required for a complete static orbital ring is within the grasp of Middle Tech civilizations, the actual logistics of completing a space-based structure that may be kilometers wide and tens of thousands of kilometers long is usually beyond them. Such civilizations thus tend to produce skyhooks with habitat platforms of no more than a few kilometers square before replacing them with dynamic orbital rings.

A critical drawback of static orbital rings is that their tethers and twin rings consume a lot of orbital real estate, much like Saturn's rings. While dynamic orbit rings can be nested and placed in orbits of many different inclinations, static orbital rings occlude a stretch of thousands or tens of thousands of kilometers of orbital space through which intersecting orbits are impossible. Typical Middle Tech implementations may demand supporting tethers every kilometer or less, forming a very dense web of long (and massive) structures through which satellites, stations, and ships cannot pass. It is also difficult to put several static orbital rings around one planet because the outer rings will be in a much lower gravity environment unsuited for replicating the gravity of the inner ring. The tethers can find some useful work as anchors for very large solar arrays and other lightweight structures, but they are generally wasted orbital space.

Truly large static orbital rings may be beyond the limit of nanotubes and other conventional materials to support, at least not without using inhibitive numbers of tethers. This puts an upper limit on their size that, when combined with other restrictions, often leads a high tech civilization to select another option rather than use a static orbital ring system.

Vertical Skyhooks and Static Orbital Rings as Habitats

The habitat platforms of skyhooks and static orbital rings may take many forms. They may be modeled as fully enclosed Stanford torii, or flat platforms with Bluesky Worldhouses "tenting" the buildings (often with taller structures poking through the worldhouse), or walled and tented platforms. It is not unusual for structures to be mounted on both sides of a ring, both built "up" and "dangling" from the underside. This is particularly popular on skyhook platforms where "real estate" is sparse and skyscraper-like structures offer a great expansion of available window area to view the planet below.

History

Skyhooks were used for a brief period in the Sol system before the Technocalypse. The first were built c90AT around Old Earth. While these small, limited payload skyhooks were useful proofs of concept, they were vulnerable to the large amount of orbital debris orbiting Terra at that time. The technology found more use elsewhere, such as the helium-3 mining efforts around Uranus and Neptune in the second and third centuries AT. They were later superseded by more advanced technologies, including the Tanna Bintang beanstalk and high-thrust fusion rockets.

Skyhooks have subsequently found regular use in several applications, specifically gas giant and brown dwarf orbital atmosphere mining scoops, but these may use Ultratech materials. Their original role as a means of assisting launches from planets recurs periodically when technological, economic and logistical conditions make tether systems preferable, but they are usually abandoned as soon as more sophisticated launch systems are available or economical.

As large-scale habitats, however, skyhooks had to wait until the Middle Federation and 1452 AT, when Takicorb used a skyhook to ease access to a very large (5 Terran masses) pelagic-type terrestrial world (now known as Sargasso) during terraforming operations. Almost a super-terrestrial, Sargasso's gravity was too high for Takicorb to build a complete beanstalk (with then-available technology) and the surface was too turbulent during terraforming for support a dependable space fountain or similar active structure, and so a skyhook was used. The skyhook's docking and counterweight platforms came to host industrial facilities and tens of thousands of workers until Sargasso was opened for colonization in 1903 AT, when the skyhook population boomed with transiting colonists. The skyhook was scrapped ten years later when Lofstrom Loops (possible thanks to calmed surface conditions) began launching payloads from Sargasso.

The first complete static orbital ring, Nieuw Amsterdam, was completed more than 1000 years after the Sargasso skyhook. Built by slow-traveling Expulsion-era exiles from North America, Nieuw Amsterdam grew from industrial/habitat skyhooks in a system some 50 light-years from Terra. The system, New Brooklyn, had no planets or moons that could offer the nearbaseline refugees a near-Terran gravity or environment - the terrestrial planets of the system were too small and the other planets were gas giants. With restricted resources, the refugees could not build both large habitats and the industrial facilities they needed and if they had built dedicated habitats first, they would be starved of resources needed to start industry. Accordingly, they settled in gas mining skyhooks orbiting 'Bronx,' a gas giant in New Brooklyn's life zone that was anomalously rich in helium-3. Over the centuries, these skyhooks developed into sizable orbital cities and were eventually joined into a continuous ring in 2753AT. It has been suggested that Nieuw Amsterdam was an inspiration for Metropolis Ring City, but Urban Cloudscapes has never given a clear answer on the matter.

Occurrence

High tech civilizations that may have recently suffered a catastrophic failure of a more active orbital megastructure tend to select skyhooks and static orbital rings. Civilizations (particularly those with large numbers of baselines and nebs) with a preference for Dumbtech also tend to adopt these structures to improve orbital real estate. In practice, static orbital rings (or skyhooks) tend to have a short life of a few decades or centuries before they are replaced by more advanced structures.

The most common application of static orbital rings is in systems lacking planets suitable for Middle Tech colonization by nearbaselines and clades requiring Terran-like environments. Skyhooks and, eventually, static orbital rings tend to grow around gas giants to host industrial facilities (like helium-3 and hydrogen mining scoops) and colonists. A fully developed static orbital ring can offer nearly as much land area as a Gaian world, but without depending on active (and hence fallible) systems.

 
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Development Notes
Text by Mike Miller
Initially published on 25 July 2013.

 
 
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