FYI: If you fail at any point to obtain a Huge Materia before gathering the Huge Materia from the Rocket, the materia you get will be from somewhere else (i.e. Huge Materia from Corel, Huge Materia from Fort Condor, Huge Materia from Underwater Reactor), and even if you fail to get all three previous materias, you only get the one materia out of the rocket (which meaninglessly is always represented by the Support Materia colored crystal). So, I for one would assume that no matter which scenario plays out, the Shinra only manages to load the useless Support Huge Materia onto the Rocket and that's why if you fail to get it out, the materia has no effect on the explosion caused by the impact.