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.
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.