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What is a Space Fountain?

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  • Written By: Michael Anissimov
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  • Last Modified Date: 19 October 2014
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A space fountain is a hypothetical active structure that would use the momentum of a stream of projectiles to hold a structure aloft. Because a space fountain would need to be hundreds or thousands of kilometers in height, it is referred to as a megastructure, and the cost of its construction would be prohibitive for the next couple of decades at least.

In contrast to a space elevator, which uses tensile strength to stay aloft, a space fountain's means of suspension would be the same means that a hypothetical Frisbee being barraged by a stream of water from a hose would use. For a space fountain, the projectile stream would need to be encased in a vacuum to prevent massive power loss. The vacuum would be contained by a tube that uses magnetic levitation to remain disengaged from the projectile stream. For this reason, one of the only serious studies of a space fountain uses a continuous, segmented iron ribbon just 2 inches (5 cm) across as the projectile stream.

Despite the initial energy investment required to get the projectile stream going, the space fountain ultimately holds more promise than the space elevator because its height would not be limited by the tensile strength of the material. Exotic materials like carbon nanotubes would not be necessary to hold the tower aloft -- conventional materials would be sufficient.

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A space fountain has been alternatively envisioned as a huge loop or as a single tower with two paths for incoming and outgoing pellets. In a tower, pellets would be slowed as they reached the top by electromagnetic drag devices which would harvest energy from the accelerated objects. Then they would return to the Earth's surface through gravity, where a portion of their momentum would again be harvested. The energy gained through this process would then be used to power a mass driver which again launches the pellets up into space, where they transfer momentum to a floating platform and resume the process all over again.

The method of construction would also vary from the space elevator. Instead of being built from space down, the tower would be built incrementally from the ground up. For a loop, an extremely long hollow tube would begin at rest on the Earth's surface. Then, the projectile stream would start slowly, with pellets being fed into the tube at a slight upward incline, causing the tube to slowly but surely rise off the Earth's surface. Eventually, the apex of the loop would reach into space. For a tower, a closed loop would also be used, but the outgoing and incoming streams would be located alongside one another.

The power supply and projectile streams for any space fountain would need to be redundant. If the loop broke and the projectiles were freed, massive disaster could ensue. A space fountain loop falling towards the ground at reentry speeds would be similar to lashing the Earth with a huge flail.

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anon325011
Post 5

I don't think you have to penetrate the earth's mantle. We build structures which put way more pressure onto the earth's surface than a big platform high above the earth. It is not much more weight than a lot of things we have already build, I think. And, you can divert the stream of projectiles into more little ones.

I can understand why we don't use this for building something into space, but why not put it into a tall tower to stabilize it? Or to test it? Are there any practical tests?

anon78479
Post 4

First Post: The force isn't really that massive. It's just the total weight of the structure. Yes, the foundation would need to be substantial, but there are plenty of conventional designs that can withstand more than enough pressure.

Are you picturing this as a solid building? It's really just a giant tube loop, not particularly thick, nor heavy. While it is tall, it would not weigh that much in comparison to say, a convention building built right up to orbit.

Second post: Same response.

Third post: The Coriolis effect does indeed come into play. Keep in mind that whatever projectiles are in the tower, they are magnetically suspended so that there is less resistance.

A computer could feasibly control the output of the electromagnets to counteract the Coriolis effect's affect on the projectiles.

anon25387
Post 3

What about the Coriolis effect? You can't expect the projectiles to just fall straight down.

anon11932
Post 2

The main stumbling block that hasn't been addressed is that whatever reduction in weight is experienced by the structure will be experienced as a downwards force on whatever turns the pellets around. Unless the pellets will be turned around over a much greater distance than the building is tall, the same limitations on material properties will be encountered. However if the building is to be tall enough to make low orbit cheap and easy, a vertical underground shaft deep enough to effect a manageable rate of change in momentum would probably penetrate into the earth's mantle.

anon11931
Post 1

The problem with this is that however much of the building's weight is relieved by the momentum of the pellets, this exact same amount of weight would be experienced by whatever is turning the pellets around at the bottom of the loop.

If the pellets are shifted to a horizontal plane at the bottom of the building then a massive amount of force will be acting downwards on whatever is causing that change in direction, same again but a horizontal force when the pellets are redirected to vertical ascent.

The only way to alleviate this is to have the pellets slowed down on their original vertical path as they travel down an underground vertical shaft which is multiple times deeper than the building is high, to spread the pressure. Problem is if this building is supposed to be high enough to reach a low orbit then surely that shaft would need to penetrate the earth's mantle.

Joe

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