gaetano,<br /><br />Stop a minute, and take a deep breath.<br /><br />We have a saying around work, never get emotionally attached to a engineering concept, it clouds your judgement. (Borrowed from that classic movie, "Wall Street")<br /><br />I understand your - preoccupation with the ability to actively throttle a liquid (or for that matter a hybrid engine) - for years I too thought that was the most important thing in manned rocketry. In part, this stems from the teaching of von Braun and my father.<br /><br />As years went on, and I was involved more closely with the guts of rocketry, I began to see that there are other factors in the decisions. I was taught the concepts of fault tree analysis, and understood the concept of the factor that is the probability of occurence convolved with the consequence.<br /><br />And you start to realize that making cut and dried assessments for what is safer is more involved that just "you can shut it down"<br /><br />As has been pointed out, solids and liquids each have positives and negatives, and those can and do change depending on the application.<br /><br />We love a good, rational technical discussion on the merits of different designs.<br /><br />What we do not like a discussion that starts from a strong statement about the engineering, and attempts to argue that position into correctness. Ignoring when the statements you make, like the abort saved the shuttle, when you are told that it did not such thing just makes you look like someone who wants to prove that their thesis is true, not to discover whether it is true.<br /><br />That is the nature of the scientific process, the examination of a thesis for its truth. An objective analysis of the facts. <br /><br />I am pretty good at examining my postulates for faults, as I am married and by definition that makes me wrong most of the time. <br /><br />Wayne <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything." Admiral Jackie Fisher</p> </div>