Grable detonation was an unique test in many aspects. For one is was the first test of a nuclear
artillery shell. Also, the test results provided very useful data for nuclear weapons use and tactics. The shell used in
Grable test was Mk-9 11.02"(280mm) AFAP(Artillery Fired Atomic Projectile). 54.4" inches long, it weighted 803lb.
Mk-9 employed Gun-Type design, and matter of fact, it was the first weapon after
Little Boy to use Gun-Type scheme. Fissile material -
Oralloy. The cannon that fired Mk-9 was an enormous artillery piece,
weighting 85 tons. It's muzzle velocity of 2060ft/sec allowed up to 20 miles shooting range.
The Mk-9 shell traveled 11,000 yards and was detonated 500 ft above the ground. The time fuze was employed to ensure precise bombing time. The actual yield of 15kt was very close to the predicted one - 14kt.
What was interesting with Grable was the result of the detonation. Preceding Grable, there was Encore shot, that yielded 27kt. Encore was detonated in the same test area at Nevada etst site, #5. Now, comparing the results of those two explosions, Grable inflicted a lot more damage than the Encore shot, even though the Encore shot was almost twice as powerful as that of produced by Grable. The difference was in the detonation altitude. Encore was detonated at 2423ft, i.e. relatively high above the ground. Basically it was an airdrop, delivered by B-50 bomber. Grable has been detonated at much lower altitude, 500ft. That produced a very abnormal waveform, referred as precursor. Basically precusor, or the precusor loads, are very strong dynamic winds, and when dragging through the target they produce very extensive damage. For instance the Jeep at a given level from the detonation point that successfully survived Ecnore blast, practically undamaged, was completely torn to pieces and thrown to the distance as much as 500ft during the Grable test.
All that can be seen on the declassified footage restored in the Atomic Bomb Movie. It sure looks very impressive, and somewhat scary as well. First the painting on the surface cars and poles around them ignites all of the sudden, then the shock wave arrives hitting with immense power. The Jeep starts its last flight loosing pieces while in the air. The bus and an RV both are in the air. Of curse they won't be thrown as far as the lighter Jeep, nevertheless to watch them flying and rolling like tin-cans is an interesting experience. Houses and other test structures get shredded into pieces in an instant, but first just like everything else around, their surfaces are burning, all at once. Well, unless you see it there's no way to describe all that.