It doesn't look phony. It looks exactly like a real explosion.
To YOU, my friend, TO YOU.
To ME it looks like hokey 1950s-style badly faked film footage.
Besides the points I've already mentioned, where are those vehicles and trees shown being blown away in the cutaway shots? Why don't we see them in the first shot? And, why are they destroyed when the smoke trails right beside the blast are barely affected? And, why do the plants in the foregtround of the blast remain prefectly still? Where is the shock wave?
Here is another fake blast. It also has the frozen ring of smoke around the bottom: