I do wonder if you built a fuel air bomb and put as much funding into it as developing nukes had (both R&D) how big that bomb would be. All the money that went into nukes can buy a lot of fuel.
Air fuel bombs have diminishing returns and scale non-linearly because the dispersion of the fuel requires ever larger mechanisms to do so.
The moab and foab require cargo transport planes to drop them. The moab provides an 11 ton yield and weighs roughly 10 tons. Little boy provides for up to 18kT, and only 5 tons in weight.
So you create tech to disperse your dispersal units. The problem is you can only blast fuel out so fast, but if you essentially make it a cluster bomb each element can have a reasonable scale. As far as delivery vehicle you don't have to be honest about that to do "testing". Just fly a plane over a prepared site and drop a dud. Japan helped us with little boy. They were ready to surrender. The emperor couldn't say publicly "this war is stupid, let's just stop." By helping us they negotiated us rebuilding Japan and defending them against post-war retaliations.
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