Well...

It's your comment about "An airburst nuke delivers all the energy from a hemisphere (the bottom half of the explosion) to the target..." that doesn't fit the facts.

The target, ground zero, receives very little of the blast. If it's a circle with a radius equal to the length of a football field (300'), and the air burst is at 3,000 feet, and 80% of the bottom half of a 1 MT explosion is delivered within a 10,000 ft radius of ground zero, then...

1. 1 MT * .5 * 0.80 = 400 kt

2. Total area = pi * r² = 113,097,335 ft²

3. Target area = pi * r² = 282,743

4. Ratio of total area to the target area: 400.

Thus, the target area would receive 1/400th of the 400 kt blast, or just 1 kt. It would be as if a 1 kt (2,000 tons of TNT) were surface-detonated at the target.

Therefore, an airburst does NOT deliver "all" the energy from a hemisphere (the bottom half of the explosion) to the target. It only delivers a tiny portion.

Rather, an airburst distributes it's power over a wide area, and the altitude is calculated such that the triple point occurs around the distance where the initial and or reflected waves cease to cause the desired damage. In that way, an airburst extends the radius of destruction.

But if you want decimation of a smaller target, a ground burst works better.

__________________
If I set the budget, we'd have Ares and more. Unfortunately, I don't set the budget, and Ares is just too expensive and too far out for us to accomplish our goals within the budget we were given.

If we halt the ISS, all versions of Ares, and transport Orion and Altair aboard DIRECTv3's Jupiter family of Shuttle-Derived Launch Vehicles, we just might make it back to the Moon by 2020.