How a Qassam-type artillery rocket is made.
How do they manage to create a rocket? They use some common household ingredients such as sugar and potassium nitrate fertilizer. They start by melting the sugar mix in the fertilizer and pour the resulting mixture into a mold to form a properly sized propellant slug. This propellant is subsequently placed into a round tube which serves as the rocket's body; typically made from locally available water pipes. Sugar contains a substantial amount of energy but it releases this energy slowly when it burns. The fertilizer serves as an oxidizing agent. This causes a quicker release of energy. When the right proportions are used it burns fast enough to propel a rocket yet not so quickly as to cause an explosion.
Apparently the more expensive and larger Iranian provided artillery rockets are bigger but just as crude.
We don't know exactly which rocket this was, but this gives us an idea of what may have gone wrong. The solid rocket fuel slug is inserted into a water pipe. The boost phase is very short - 1 to 5 seconds according to this video.
So here are some things to think about:
-I'm wondering if the propellant slug was mixed improperly and part of it burned too quickly. Could this have fractured the pipe (the rocket body) near the rear of the rocket body?
What happens next?
-Does the rear part of the rocket body continue upward with a partial slug of propellant still burning? Is this what we see in the sky throwing out light and sparks and eventually becoming unstable in flight? If the propellant were badly mixed, a residue could have continued to burn slowly, or it was burning slowly because it was not properly contained. There could have been just enough pressure to keep the lightened section under some kind of boost. Or maybe its just fizzling away.
-Does the top section of the rocket body, without stabilizing fins, continue on its way in an aerodynamically unstable trajectory? Unseen, because, while it still has part, or most, of the propellant slug intact inside, the propellant is not burning. Unstable because there are no fins.
-There might be two such fragments, which would explain the two visible ground explosions.
-These parts started falling as the tail fragment continued under boost. This would explain the vexatious issue of the estimated altitude of what we see in the sky versus the physics of falling objects. The timing issue discussed earlier in this thread.
The rocket may have broken up due to any number of other reasons. Maybe one or more fins fell off. It was unstable and started to tumble. Aerodynamic forces would fracture it. Maybe it fractured because it got caught up in the launching frame. Maybe it was already cracked.
In any scenario, a fragmentation of the rocket body during the short boost phase could serve to explain the two ground explosions and the timing issue.