The problem was that the airframe of the blimp had been coated with a substance that would become known as rocket fuel. That substance is what we see burning while the blimp is on the ground. The hydrogen gas immediately escaped the airship and gained altitude rapidly, but this lightweight gas got the blame for the fire and the project was scrapped. The Fear, Uncertainty, and Doubt about hydrogen has persisted through to today, despite scientific evidence showing that it is impossible to contain a hydrogen gas fire in the open air, for the same reason why a balloon filled with helium gas floats into the sky.