Domestic Structures

why you don't want to make buildings and bridges with loop technology

Some people have suggested using loop technology to build dynamic structures like towers or bridges or inter-city transportation launchers near urban areas.

Not safe enough. This would be like building a bridge out of dynamite - too much stored energy. A kilogram of rotor moving at 14km per second stores as much energy as 23 kilograms of TNT. The deflection force is proportional to the kinetic energy. The slower you make the rotor, the more mass you need to support the load and the rotor itself. Thus, a slower, heavier rotor stores even more kinetic energy per unit of external load force. Something like an office tower or a road bridge would have a much higher static load than a launch loop, so the rotor weight and stored energy per meter would be enormous.

A 3kg/meter, 5600km round trip launch loop stores about 400 kT of energy. I've spent a LOT of time thinking about how to keep that energy contained, or how to dissipate it safely when something prangs. The energy will not be released in one titanic blast, like a nuclear weapon, but over a period of 400 seconds. Still a firestorm can be as deadly as an explosion.

For comparison, the Saturn 5 produced 150GW for 150 seconds. That is about 5 kT of energy. A worst case explosion would have released all that energy in a fraction of a second. Pad 39 is 3 miles from the press site, and "safe blast distance" goes up as the cube root of the energy, more or less. So I would want a large population at least 15 miles from an above-ground loop, not driving on top of it every day.

In 1980 Mount Saint Helens erupted 50 miles northeast of me, releasing 24 MT of thermal energy over a few hours, but most of it in the first minute. I was working indoors that Sunday and didn't find out until I went home. So a mere 400 kT can be dissipated underground or underwater without causing a major catastrophe outside the blast zone. But be sure to be outside the safe blast zone!

"Safe blast distance" assumes the rotor is designed to disintegrate when it fails, instead of throwing red hot globs of iron. Otherwise, a few segments could come down just about anywhere on the planet. Rotor pieces released horizontally at 80 km altitude and 14 km/s velocity will go into solar orbit, and eventually (perhaps in billions of years) collide with the Earth. Hopefully, they will also disintegrate in the upper atmosphere. But it is better to dispose of them downwards, quickly, with some semblance of control.

The answer for safe energy dissipation is diversion, depth, and distance. This is hard to do with a structure near lots of people. Much easier on the ocean, far from land, or tunneled underground. If the mass stream can be dissipated downwards, it will boil sea water and tear up some ocean floor, not level cities. Or make a small pool of lava if sited underground. It is also important to site launch loops away from large concentrations of sea life, at least if you are a fuzzy-minded eco-nut like me. Fortunately, large parts of the ocean are virtual deserts, including the parts of the eastern Pacific that are optimal for a large array of Launch Loops.

One remaining problem is that the resulting iron-rich water may cause an explosive bloom of plankton, attracting fish and sea mammals (and fishing boats). Hopefully the currents will carry the iron away from the launch loop before it is rebuilt and capable of another high-energy failure. The danger zones will be a few kilometers wide, and with currents of a few hundred meters per hour, the iron will be hundreds of kilometers away by the time the launch loop is rebuilt. Additional measures to humanely drive away sea life from the underwater danger zones may be desirable.

These are not viable options for domestic towers and bridges near populated areas, or areas dense with life.

Other failure hazards

The launch loop danger zones will be near the east and west end upwards deflectors. These are hundreds of kilometers from the bottoms of the elevator cables, where major cargo operations will be taking place. The main deflection cables for east and west stations will pull the falling stations towards the center of the loop (hopefully slowed by large parachutes) so they will miss the surface platforms.

The elevator cables, weighing a few hundred tons, will probably fall vertically, partly onto the surface platforms, with Coriolis forces pushing them a few meters to the east and winds pushing them mostly west. Many armored shelters for workers, within a few seconds walking distance, will be needed near all work areas on the surface platforms.

DomesticStructures (last edited 2009-09-14 21:14:15 by KeithLofstrom)