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Space Fountain
Tech Level: 14
Loftstrom Loop
Tech Level: 15


Both the Loftstrom Loop and the Space Fountain are closely-related concepts using similar technologies. Space Fountain-based structures have also been called Launch Towers or Orbital Towers.

Tech Level: 14

The Space Fountain concept was pioneered as a colloborative effort between Robert L. Forward, Marvin Minsky, John McCarthy, Hans Moravec, Roderick Hyde, and Lowell Wood. Almost all the information on Space Fountains here comes from Robert L. Forward’s Book, Indistinguishable From Magic.

A Space Fountain uses a continuous stream of electromagnetically accelerated metal pellets to hold things up at extreme altitudes using the same basic physical principles that a water fountain uses to suspend a plastic ball at the top of its vertical jet of water.

Metallic pellets by the hundreds of thousands would be shot up to a "deflector" station far overhead, which would use magnetic field scoops to catch the pellets, curve them back down with an electromagnetic accelerator, then shoot them back down to the ground. The ground station would in turn magnetically catch the pellets, curve them back up with a powerful electromagnetic accelerator, and shoot them back at the station in one continuous loop. The pressure exerted on the magnetic fields of the scoop and curved EM accelerator by the continuous stream of pellets would keep the station aloft.

The key to understanding the Space Fountain is that it uses a continuous, as in non-stop and never ending, stream of pellets to constantly exert pressure on the station it is holding up. Think back to the water fountain analogy; the way it can continuously keep the ball suspended on its jet of water is by continuously recycling the water; the water that falls back into the fountain is sucked up by the fountain’s intakes and fed back into the water jet in a never-ending loop. The same with the metallic pellet "jet" of the Space Fountain.

Also, the pellets and the suspended station never actually make physical contact; the magnetic fields of the scoop and curved accelerator act as a kind of buffer, preventing any physical damage from the pellets screaming at the station at over 4 km/second. Yet the pellets exert pressure on the magnetic fields as they pass through them, and this force is in turn passed on to the physical structure of the station, holding it aloft.

Using this technique, it is thought the Fountain could hold up a full-sized, fully-equipped space station of 40 tons or more at almost any altitude, even space elevator heights of 30,000 km plus. However, the higher the station, the higher the required start-up and maintenance energy (see below.) A Space Fountain of about 2000 km height was mentioned to require the constant input energy of a modern city to maintain; it therefore may not be too economically viable to build Fountains much higher than this. For the purposes of building a tower capable of launching vehicles into orbit or creating super-large buildings for arcologies and such, however, 40 to 200 km or so may be more than sufficient height.

One advantage of the Fountain is that once the system is set up, the energy needed to maintain it would be minimal compared to its start-up energy. The loss of momentum from gravity as the stream is shot up is exactly balanced by the gain in momentum due to gravity as the stream is shot down to the ground station again, but at no time does the total momentum of the system change. Entropy does dictate that some energy will eventually be lost over time, but this can easily be continually compensated for by auxiliary power stations at a small fraction of the energy needed for the system’s initial set-up. Thus, even if all power were cut to the pellet stream, it would still function normally for a while, and may take up to several hours for the suspended station 1000 km or more up to feel even a wobble.

Space Fountains structures, if ever built, will in all likelihood be built with multiply-redundant support Fountains; Forward in his book mentioned a minimum of three, with up to six or eight per individual structure, each with its own independent power supply. In Forward’s SF novel Starquake, the alien Cheela built a number of hexagonal Space Fountain structures, with six seperate fountains each holding up a "side" of a hexagonal deflector station.

Another advantage is that the system can literally be built from the ground up. The ground station with its accelerator and the station with its accelerator would both be fully constructed on the ground, the station placed on top of the ground station. Then the Fountain is powered up slowly, the force of the pellet stream eventually lifts the station first a few centimeters, than a few hundred meters, and then is pushed up kilometer after kilometer. The process can be suspended at any altitude, from a few centimeters to a few thousand meters, indefinitely, allowing for calibration, maintenance, etc.

If the support stream(s) are sufficiently over-powered, ie carrying more than the needed energy to hold the deflector station aloft, the Fountain can also be used to support side structures such as elevators or walls along its length. Like at the deflector station, electromagnetic accelerators/decelerators are can be built vertically along the stream as the Fountain is slowly built up, drawing upon the force of the pellets passing through the magnetic fields for support. Since the wall sections (and any internal structure they themselves may support) are held aloft by the internal stream passing through them, they are under no extraordinary structural stress themselves, as one might suspect from a structure that may be hundreds or thousands of kilometers high.

Thus Space Fountains can be used to create truly gigantic structures and towers as well as being used "merely" to hold a space station aloft. And unlike a Space Elevator, it would not require any revolutionarily strong materials to hold together; modern-day alloys and composite materials would do just nicely.

The most obvious use for such a super-high structure would be as an orbital launch tower; line the outer walls with electromagnetic accelerators and shoot a payload up the side. A Fountain tower about 40 km high would be sufficient to launch passenger-carrying vessels with less than 3 g’s of acceleration. A Fountain tower 100 km or higher could simply fling the cargo directly into orbit without exceeding one g.

A Fountain tower could also be used as a super-sized arcology, research facility, industrial center, etc, as a fountain 100 KM high and 100 meters wide at its base the would have about 7.85 entire cubic kilometers of volume the designers could use for just about anything. Higher and wider towers, with many thousands of cubic kilometers of interior space, are also possible.

Forward suggests that Space Fountains would first have to be demonstrated on a "small" scale, first used in building kilometer-high transceiver towers and such for radio and televison broadcasts. The technology can then be scaled up to support super-highrises, and then finally a true Space Fountain.

Tech Level: 15

Named after Keith Lofstrom, who did the first detailed calculations on the concept.

A Loftstrom Loop as a concept actually predates the Space Fountain by a number of years. In fact, the Space Fountain was originally conceived as an extremely simplified version of the Loftstrom Loop.

The Loop is based on a stream of magnetically accelerated metal bars, each about 1 meter in length, loosely linked together with bendable joints. The stream is shot into space, via electromagnetic launchers, where a station "rides" it using electromagnetic scoops and accelerators much like a Space Fountain deflector station "rides" a pellet stream. The station then redirects the metal bar "stream" horizontally to another station, which sends it downwards to a receiving station on the ground. From this station the stream is then sent back to the launch station.

Like a Space Fountain, a Loftstrom Loop could be built on the ground and incrementally "pushed" up to the desired altitude. Also like the Fountain, it would probably incorporate redundant multiple bar "streams" for maintenance and safety, the metal bar "streams" could be over-powered to help hold up side structures such as elevators or walls, and it would require a constant input of energy to help make up for entropy losses once the system was set up.

Unlike a Space Fountain, however, the Loftstrom Loop has a built-in horizontal component (this is also possible with a Space Fountain, but the designers of Fountain concept concentrate almost exclusively on the vertical,) creating a trianglular projectile loop. The two upper "horizontal" stations would be pushed away from each other even as they are pushed up in altitude. Once the system was set, the positions of both stations would be stable. In fact, a Loftstrom Loop can have any number of stations supported by it, as the projectile stream is shot from deflector to deflector to deflector, possibly looping all the way around Earth. Unfortunately, the more stations added, the more energy is likely to be lost from the system, requiring greater constant energy input.



Indistinguishable From Magic, by Robert Forward

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