heat content = (Pi/6)*(d^3)*P/0.4 (SWTC, explosions section)
Firstly we need to know the height of the cockpit viewport. From the film we can see it is at least as tall as the modified stormtrooper helmet worn by the crew. The original stormtrooper helmet is 35cm x 35cm x 48cm. i.e~35cm.
We can also estimate the viewport height directly from the battle scene. If a walker is 23m tall, the ‘head’ is ~4.5m; therefore from the macrobinocular image the viewport is < 40cm. From Luke’s strafing run the head appears to be ~4.2m (again using 23m overall height). And from ICS the head appears to be 4.2m putting the viewport at 35cm. In short it is 35cm in height.
Secondly using the ICS/TJ diagrams we can obtain the maximum distance from the viewport. It cannot be greater than ~1m in the following shot, given the position of the Driver/gunner relative to the camera (and that it is in front of General Veers station).
We can now estimate that the generator subtends >0.1 radians (generator 55 pixels, viewport 165 pixels high). If the distance is 17.28 Km, the generator is >1.7Km across. However this shot takes place at 28:54, General Veers walker reaches 17.28 Km at 31:34. Assuming the walkers made steady 5m/s progress (guestimate based upon walker taking ~2 seconds to half enter a shot) with no time cuts (208 s) the generator is at 18.32 Km thus >1.8 Km across. This now gives us a rough idea of the diameter of the explosion in the images that follow.
In image02 the generator is still intact and therefore the fireball is solely from the energy content of two bolts. As this is ~1Km in diameter, this requires ~16 KT per bolt. This is however only the inital stage (note the blinding flash that immediatly follows it), the fireball expands further:
In images 5-7, the fireball expands to its maximum of ~2Km (Image 7) before shrinking to reveal the generator is still there (image 8 ). This is followed by the second larger explosion (Image 10), possibly the generator 'cooking' off?. In any case, as the generator is still there and a secondary explosion occurs, it suggests the 2Km fireball was probably caused by the bolts themselves, thus maximum firepower is ~130 KT per bolt
The second larger explosion is captured in image 10 above, the fireball must subtend a minimum angle of 0.35 radians (constrained by the cockpit layout, fireball 191 pixels, viewport 104 pixels) and thus at 17.28 Km is ~6Km i.e an explosion 6.8 MT. It is not clear weather or not this is due to the generator cooking off or the result of the AT-AT shots.If it is the generator cooking off why the relatively small explosion? after all this generator was capable of holding off any bombardment from the entire Death Squadron.
To summarise:
A "Maximum a firepower" AT-AT shot appears to be 16KT per bolt taking into account only the first stage of the explosion, 130 KT per bolt considering the maximum fireball diameter, and possibly as high as 3.4 MT per bolt if the final fireball was not the result of the generator 'cooking off'.
If any of the images crap-out (they all worked fine in the preview) try viewing them from the index here:
http://man1ac0.tripod.com/index.html
Note that the whole explosion takes place over 52 frames before the scene change to Veers Walker. A seven second DivX clip (5.02 codec required, 874kb) I made and captured the images from is here (right click, save as):
http://man1ac0.tripod.com/GenExplosionI.avi
Image Links all fixed-Bean
