The killers did not need to plant explosives as the planes themselves were the bombs.
Two entirely different types of buildings and two entirely different types of speed/weight/size of aircraft.
The Empire State Building was a steel post and girder frame and 1 foot thick granite/brick (8" brick, 4" granite) building with a lot of mass in bricks, plus it's of boxes made of massive steel girders that were larger than they really needed to be for a more modern building. The building weighs in at 364,000 tons
The World Trade Center Towers were constructed entirely differently with the floors being truss and girder design hung on the outside curtain wall with the wall being a main load bearing structure, hence the narrow windows as each vertical between the windows acted as the post to transfer weight to the ground. This eliminated all interior columns except for the massive concrete elevator column which was another load-bearing structure to which each floor was supported. Although the buildings were much larger than the Empire State Building, the weight of materials was only 490,000 tons, less than a third more, for more than three times the capacity due to the more efficient construction techniques.
The B-25 Mitchell bomber, with a full load of fuel, crew, and bombs, at maximum overload could weigh only 41,800 pounds and its max speed lightly loaded was 275 MPH. It had a wing span of 67 feet.
The Boeing 767 that crashed into the World Trade Towers weighed 300,000 lbs, hit at 590 MPH, and had a wing span of 156 feet.
Calculating for the energy released by the impact, we find:
- The B-25 results in ~140 million joules (Watts/second) of energy just from the impact, not counting any fuel fire.
- The B767 results in ~4.6 BILLION joules (Watts/second) of energy just from the impact, not counting any fuel fire.
That's 32 times more energy put on one of the World Trade Tower than was put on the Empire State Building back in 1945.
Here's a look at the damage that was done back then:
