Constantly I hear people saying that solid rockets should never be used for manned flight and how unsafe they are. But looking at it objectively, to me it seems a little unfounded; In terms of statistics, every shuttle launch consisted of two solid rocket motors. Ok so one of the shuttle explosions was the result of a solid rocket failure, granted. But if you look at it there were numerous faults with the RS-25 engines thought out the shuttle history. statistically the solids did twice as many launches, and only 1 failure (albeit spectacularly). You have to consider however the other shuttle explosion was a result of falling ice damaging the orbiter, a result of using cryogenic liquids required for both kerosene and hydrogen fuels. Add to that Apollo 13 was a result of having cryogenic fuels, requiring stirring, electrical systems to keep them frozen etc.. And it's not just the fuel you have to worry about, with liquid oxygen, almost anything will ignite, don't forget Apollo 1. Both hydrogen and oxygen slowly turn to gas even with the best insulation, this means you also have to worry about venting to avoid dangerous pressure build up. I mean to use any liquid fuels you need lots of high pressure, high vibration tolerant machinery, and plumbing, high speed moving parts, electrics, just a whole series of points of failure. To me it seems like mixing a high explosive liquids (hydrogen or kerosene) and a cryogenic highly reactive oxidiser under extremely high pressure is a lot more dangerous, and causes a lot more failures than a relatively inert fuel rubber mixture.