gaetanomarano,<br /><br />You are focusing on one factor, the ability to start and stop a liquid engine, and losing sight of other factors that are just as important.<br /><br />Remember, the capability that your are trumpeting, the ability to shut down a liquid engine if it does not come properly to thrust level, is not really a factor for a solid.<br /><br />My dissertation advisor was fond of saying that he liked digital electronics over analog because they either worked or the didn't. Solids are much like that, if they light, they go. There is little or no chance that they will not come to thrust.<br /><br />There are no moving parts in the combustion process, no complicated plumbing or rapidly spinning pump blades to come apart in a fit in a solid. There are no such wonderful features for solids that liquids, with great names like "hard start" have in their history.<br /><br />There are a lot of factors that go into the reliability analysis of solid versus liquids. Literally hundreds of safety and performace issues that need to be understood in detail. When people who have been working in the industry tell you that there is more to it than "but you can turn it off on the pad" - you don't have to take it as gospel, but at least try to show that you are given a moment of pause before preceeding to what seems like a pre-ordained conclusion.<br /><br />Its funny, this reminds me of an argument my father and I used to have. He used to tell me that cars, if they only have one air bag (and they did back then a lot of the time), should put that air bad on the passenger side, because the statistics showed that was the most dangerous place to ride. For him, that was the end of the discussion. I used to try and tell him - "Dad, I would say the probablity that a car in a wreck has someone in the driver's seat is pretty close to 100%, and it is probably somewhat lower on the other side. Shouldn't a factor in your thinking be having the airbag actually protect someone?" - he w <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything." Admiral Jackie Fisher</p> </div>