3 ms·

Well, something to consider is that Little Boy (the hiroshima bomb) had a yield of roughly 15kt. Obviously a nuke has different characteristics from a fuel-air

by ericd 9y ago

Well, something to consider is that Little Boy (the hiroshima bomb) had a yield of roughly 15kt. Obviously a nuke has different characteristics from a fuel-air explosion, most of which I haven't considered and would not qualified to comment on anyway, but the comparison doesn't make me very comfortable with 15 miles. It probably wouldn't be within the blast radius (depending on how the burning fuel propagated), at least, but the the amount of radiant heat generated would be nothing to sneeze at. I could certainly be overestimating the issue - I'm guessing the rapidity of a nuclear explosion vs. a fuel air explosion affects its pound for pound destructiveness significantly. Sibling comment has a good point that hyperloop could make spaceports at safe distances more accessible.

btilly 9y ago
Hiroshima was total destruction within 1 mile, the farthest away that glass broke was 12 miles, no damage past that. So 15 miles is definitely out of the blast radius. What about the fireball? Google told me the following. 15 miles is roughly 24 km. The mass of air is 1.2 kg per cubic meter. The volume of a sphere is 4/3 pi r^3. The energy of a kt is about 4.184e9 joules. It takes about 1 joule to increase the temperature of air by 1 C. When I put all of that together I calculate that a 15kt explosion has enough energy to raise the temperature of a 15 mile ball of air by about 0.9 C. Please double check my figures and calculation, but I think that your average city can survive that. :-)
- ericd 9y ago
  Ha! Thanks for running through the numbers, very interesting. The air in that 15 mile ball is almost transparent to infrared, though, so I'd probably assume that the surface area of the sphere would receive the lion's share, rather than the volume within it. So the energy delivered should fall off roughly with the square of the radius rather than the cube. Might be enough to give a sunburn, if nothing else. Edit: On my phone I'm getting ~5.8 joules per square meter. Google seems to be mangling the units, though, so I can't be sure it's not dropping or adding in a factor of 1,000, though. Edit 2: assuming that figure is correct, unless it's released in <1/200 of a second, it's not competitive with the sun at the equator (>1,000 Watts/Joules per m^2 per second)
  - btilly 9y ago
    I think that there are some decimal places missing. But that said, I'd expect a chemical explosion to deliver itself mostly as a shockwave and a ball of hot gas, with relatively little delivered by light. Because the part exploding is surrounded by smoke and chemicals that absorb light.
    - ericd 9y ago
      Yeah, I was going for worst case assumptions, where all of the energy turned into infrared, which is definitely not going to be the case. Here's the math, if you want to double check: (10 x 4.184 x (10^9 joules))/(4 x pi x (24000 meters)^2)
  - fpoling 9y ago
    Atmosphere is only transparent to infrared in 7-13 mkm window. Yet I expect the fireball to producing heat at above 500 C that gives peak of radiation below 4 mkm.
    - ericd 9y ago
      Thanks, good to know. I was going for worst case calculations, assuming complete transparency. Even that doesn't seem bad.

by ericd 9y ago

btilly 9y ago

Hiroshima was total destruction within 1 mile, the farthest away that glass broke was 12 miles, no damage past that. So 15 miles is definitely out of the blast radius. What about the fireball? Google told me the following. 15 miles is roughly 24 km. The mass of air is 1.2 kg per cubic meter. The volume of a sphere is 4/3 pi r^3. The energy of a kt is about 4.184e9 joules. It takes about 1 joule to increase the temperature of air by 1 C. When I put all of that together I calculate that a 15kt explosion has enough energy to raise the temperature of a 15 mile ball of air by about 0.9 C. Please double check my figures and calculation, but I think that your average city can survive that. :-)

ericd 9y ago
Ha! Thanks for running through the numbers, very interesting. The air in that 15 mile ball is almost transparent to infrared, though, so I'd probably assume that the surface area of the sphere would receive the lion's share, rather than the volume within it. So the energy delivered should fall off roughly with the square of the radius rather than the cube. Might be enough to give a sunburn, if nothing else. Edit: On my phone I'm getting ~5.8 joules per square meter. Google seems to be mangling the units, though, so I can't be sure it's not dropping or adding in a factor of 1,000, though. Edit 2: assuming that figure is correct, unless it's released in <1/200 of a second, it's not competitive with the sun at the equator (>1,000 Watts/Joules per m^2 per second)
- btilly 9y ago
  I think that there are some decimal places missing. But that said, I'd expect a chemical explosion to deliver itself mostly as a shockwave and a ball of hot gas, with relatively little delivered by light. Because the part exploding is surrounded by smoke and chemicals that absorb light.
  - ericd 9y ago
    Yeah, I was going for worst case assumptions, where all of the energy turned into infrared, which is definitely not going to be the case. Here's the math, if you want to double check: (10 x 4.184 x (10^9 joules))/(4 x pi x (24000 meters)^2)
- fpoling 9y ago
  Atmosphere is only transparent to infrared in 7-13 mkm window. Yet I expect the fireball to producing heat at above 500 C that gives peak of radiation below 4 mkm.
  - ericd 9y ago
    Thanks, good to know. I was going for worst case calculations, assuming complete transparency. Even that doesn't seem bad.