Digging tunnels through rocks is a slow and expensive process. Either you have to inch you way forward with a boring machine or remove the rock in batches using explosives. That's why long tunnels often take decades to build.
A new method:
Set up a very large array of solar mirrors. Concentrate the sunlight into a single powerful parallel beam with a 6 m diameter. Direct the beam into the mountain where you want to have the tunnel. Sit back and wait for a month. Voila. Tunnel is ready.
OK, it's probably not quite as simple as that. The design is similar to solar furnaces, which can easily reach 5000 degrees C. This should be more than enough to vaporise the rock. But you probably still need some kind of ventilation system to get the vaporised rock out (suggestions welcome). Also, the method will only be suitable for load-bearing rocks, otherwise the tunnel would keep collapsing.
Still, it should be faster and cheaper than the traditional methods, because it's less complex, it requires less human labour, and energy costs are zero. The initial investment is higher, but the solar reflectors can easily be dismantled and moved to new sites, and can be expected to last for decades.-- kinemojo, Sep 01 2007 Won't it be tough to keep the beam coherent beyond a few dozen meters? This would be good only for short tunnels. Obviously, a more powerful beam could compensate for less beam coherence. One would therefore need more and more powerful beams as one cut further. The tunnel would flare out larger and larger as it went.-- bungston, Sep 01 2007 It is not just temperature you need. How much energy does it take to vaporize a ton of rock? A few months will not be enough to drill a tunnel.-- kbecker, Sep 01 2007 >>How much energy does it take to vaporize a ton of rock?<<
Heat of vaporization of SiO2 (silica) ~ 20 MJ/kg
For a 50 km tunnel volume of rock extracted is 3^2 * 3.14 * 50000 = 1.5 M m^3
density of silica = 2200 kg/m^3
1.5e6 m^3 * 2200 = 3.3e9 kg of silica
3.3e9 kg * 2e7 J/kg = 6.6e16 J (total energy needed)
Let's say we want to burn the tunnel in a month.
there are 60*60*24*30 = 2600000 seconds in a month
6.6e16 J / 2.6e6 s = 2.5e10 W (minimum power output of solar furnace)
So the solar furnace would need a power of at least 25 GW-- kinemojo, Sep 01 2007 ...Assuming it's sunny all the time.
Psst, [kinemojo], the sun doesn't shine at night.-- Texticle, Sep 01 2007 Could you get some usable energy from the stream of lava pouring out of the tunnel?
[Edit] and why vaporis(z)ed? that seems like overcooking it to me.-- marklar, Sep 02 2007 ... imagines condensation problems in the ventilation system for removing vaporized rock...-- lurch, Sep 02 2007 [lurch]:
perhaps you can get it to condense into a fine dust.-- kinemojo, Sep 03 2007 The last mirror bringing all the 60GW (extrapolated from the night-ignoring calculation) into its final parallel form could use some good cooling methinks. And most modern tunnels have a slight bend, an endeavour for which most optic solutions are most unsuited.-- loonquawl, Sep 03 2007 One place this might makes sense is on the Moon's Mt. Malapert.
Mt. Malapert is just near the South Pole and its 5,000m high summit is lit for over 90% of the time. All that unfiltered sunlight is handy for power generation.
Once you had dropped in your giant solar panel your rock-vaporizer could get going blasting out a base inside the mountain. This moon-cave might be ready in a few years for humans to move in to, safe from the cold, radiation and meteorites. I am guessing that the 1/6 gravity and vacuum would help out with the blasting too.
(Malapert is close to a possible ice cap and his direct line of sight to the earth all year, so its THE spot for a base.)-- aide, Nov 30 2007 Okay, [aide], that last was just confusing. Turns out that you are talking about the Moon and a mountain near its southern pole. You coulda said so right away.-- baconbrain, Nov 30 2007 Ok [baconbrain] fair point - fixed now I hope.-- aide, Dec 19 2007 The sun is half a degree wide. Anything done with mirrors directly has to account for that. With 6 meter diameter hole at about 500 meters into the mountain the mirror optics will start to spread wider than the hole. (about 89.5 x 6 as a first guess)
So...
Instead of mirrors you power rock cutting lasers with sunlight maybe through solar cells.
Carve a rock doughnut 6 meters in diameter and whatever depth seems to work.. Remove a doughnut hole the size of a explosive charge. Blast so as to shear off the doughnut. Remove doughnut.
repeat that is
With lasers you can go as deep as you wish and turn corners if need be.
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(But we can't go to the sun, it too hot. Sure we can you dummy, We just go at night. )-- popbottle, Dec 29 2014 An ancient Roman method of tunnel boring involved building a fire against the rock face, letting it get the rocks hot, and then using water to cool/shatter the rocks. Of course that also put the fire out, making it take longer to get the fire going again for the repeat performance, but the shattered rock needed to be removed, anyway....
Here the solar heater could take the place of the fire -- no need for such heat as to cause melting. Water is still generally available for rapid cooling/shattering of the rock. And modern machinery would make quick work of removing shattered rock. If the machinery was automated, it might even be able to do it while the rock face was getting heated up again.-- Vernon, Dec 29 2014 [Vernon], I like that. (I recall that Hannibal's crossing of the Alps involved heating rocks and dashing vinegar on them.)-- baconbrain, Dec 29 2014 random, halfbakery