Sunday, 20 April 2014

Tunnelling in loose soil methods

I recently read about the difficulties encountered by the prisoners escaping from Stalag Luft III (the Great Escape) due to the looseness of the soil. When digging downwards the walls of the shaft would cave in before sufficient depth had been dug to put in place the next pieces of wall, and likewise with the horizontal shafts, both the ceiling all walls would cave in before the next pieces of ceiling and walls could be put in place.

It occurs to me that there might be a design solution that could address these problems. To my mind, the key is to have walls that move down (or extend sideways) continuously, and which are able to interlock to withstand the pressure of a cave-in.

The diagram below illustrates walls that interlock (and could actually be bolted in place) sitting inside the existing walls of the tunnel. These would be continuously advanced as the digging continues (for vertical shafts the force of gravity would be sufficient, but for horizontal shafts some other force, e.g. a spring mechanism, would be required). Once the new section of wall has cleared the old section, it is expanded and corner pieces are added to form a rigid structure (probably bolted together).

Obviously a key requirement is that the soil be loose enough that it can accommodate the force of compression to move the sections of wall outwards.

This approach should also work with other shapes. For example, to achieve a cylindrical-like tunnel, four quadrants of a circle would be expanded with straight sections.

Additionally, for horizontal shafts a rhomboid type structure may be preferable as the risk of ceiling cave-in is more significant that side wall cave in.

Friday, 18 April 2014

Using hydrogen in airships

Having read recently about the scarcity of helium has led me to give some thought to whether hydrogen should be reconsidered as the lifting gas for airships.

The advantages of hydrogen over helium are that it is less, cheaper to produce and less scarce (particularly on Earth, but also in the Universe at large). The downside is that it is flammable.

However, I think that good engineering could overcome the issues of flammability. Here are some ideas:

  • Make the envelope out of flame retardant material
  • Make the envelope into a cellular structure, such that one cells can be burst and catch fire without causing all of the other cells to do likewise
  • Fill the outer cells of the aforementioned cellular structure with cheap inert gas (e.g. Nitrogen). This means that outer damage doesn't result in the release of flammable gas, and also means that if both an outer cell and an inner cells are burst, the burn rate is lowered as there's less oxygen in the immediate vicinity of the hydrogen
  • Build in fire suppression systems
  • Structure the gondola to a glider, such that it detaches from the balloon in the event of a fire and safely returns to Earth
  • Add parachutes to both the gondola (as an alternative to the glider functionality) and the balloon. The latter is intended to prevent damage to people or property on the ground when, after a fire, the ballon falls to Earth
  • Deploy such airships as AUVs / drone (at least until such time as a strong safety record could be established)
  • Only use such airships in isolated areas / over the sea (at least until such time as a strong safety record could be established)
Would these safety features be effective? Would they be cost-effective compared with alternative lifting gases and alternative modes of transport?