@Yup, solid fuel
Actually the SRB on Challenger never exploded, the o-ring failed (due to extreme cold on the launch pad) on one of the section joints allowing exhaust gas to escape which cut through the supports which held the SRB to the external tank. They had had this o-ring fail on previous flights but the hole was blocked by debris, which did happen on Challenger but was dislodged due to large amounts of steering caused by wind shear. In all likelihood if the oring had failed on the other side of the SRB then there wouldn't have been an accident. I suspect that if something similar happened with Ares I then it would not cause a major failure.