Space fountain

A space fountain is a type of active structure. This means that it requires power to retain structural integrity (ie: Not fall down!)

It basically works by having a hollow core through which pellets are shot upwards. A device at the top of the building (most likely a magnetic diverter) will reverse the pellet's direction. Doing this will impart some of the kinetic energy in the pellet to the structure, providing lift.

Therefore some of the weight of the structure is supported by the pellet stream rather than the building, allowing for less bulky building techniques.

Space fountains can be used as a launch pad for spacecraft by reducing the amount of fuel needed to be carried.

A related technique is the space elevator.