Look up how Bransons virgin galactic doesn’t need heat shielding to understand the difference between an orbital velocity and a parabolic trajectory.
He man I wasn’t involved In the Manhattan project so I’m just basing it on what I know, but critical masses can be calculated by the neutron flux of the radioactive material and the fraction of fissile material in the material.. Nuclear reactors do this all the time and are kept by refinement and buffers below the possibility of criticality. Japan’s boys were plutonium bombs I’m pretty sure both of them, but nevertheless less uranium has a higher critical mass but otherwise all the physics is the same. When the neutron flux density is high enough in fissile material criticality will take place…explosively. At least that’s what they tell me. Modern bombs are fusion bombs don’t even produce radiation fallout.