<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I am just curious... if separation is such an issue and such a focal point of launch failures, why not have the upper stage fire while still attached to the 1st stage instead of waitinf for separation?To separate the stages before hand, you need to time the separation event. You need to rely on sometimes unreliable pneumatic pushers or multiple small solid motors. You also need to ensure these separation devices provide sufficient ullage settling effect so you upper stage fuel is covering your fuel pick up points.If you fire the upper stage while it is still attached to the 1st stage, triggering it not by pre-determined timing but simply but an accelerometer detecting the fall off in acceleration at the end of the 1st stage burn, you get rid of the aforementioned problems. The stages are not secured vetically, they are simply stacked with a rod in tube type interface and you can pull them apart longitudally at any time. The weight of the upper stage and the compressive loads during ascent is what keeps them together. Instead of an enclosed staging adapter, you simply have an open truss or a cylinder with blow out panels towards the lower end. A cone in the middle, below theupper stage nozzle, deflects thrust to the sides. When the 1st stage engine burns out and its thrust falls off, the accelerometer based simply fires the upper stage.Why can't it work that way? <br />Posted by dwightlooi</DIV></p><p>If the Space-X design can be pulled apart vertically at any time then that is a most unusual design, and I don't understand the role of the explosive bolts in such a design. Most launchers are not constructed in that manner. I would think stiffness an control would be an issue with a design like that.</p><p>But in any case it is not necessary to have the stages widely separated before the upper stage is ignited. In fact the system that you are describing is not really unusual. You need to do a couple of things. First, you need to have the stages separated in the sense of not being firmly attached to one another before the upper stage is ignited. Second, you need to have the lower stage be essentially non-propulsive so that you do not have recontact after separation, or you need to at least have very low thrust and some means of absorbing a light recontact without damage to the upper stage. It is quite common, and in fact normal, for the second stage ignition to be triggered by a sensor that tells you that the lower stage is not longer providing thrust. You can do that with an accelerometer or with a pressure gauge, if you know something about the expected thrust or pressure profile (and you should know this). Triggering second stage ignition off of nothing more than timing is a serious mistake.</p><p>I would not rely on pneumatic pushers or on multiple small rocket motors (or even one auxiliary motor) for staging. All that is needed is good engineering of the overal event. An open truss for the interstage has been considered, but I don't know of any design like that that has flown. There would be some serious aerodynamics to consider before jumping into that. You don't really need blow-out panels either. </p><p>If there the stage connection has been severed, you can ignite the upper stage and fly away from the lower stage. You need to be a bit careful about the ignition pressure transient and pressurization of the interstage (the interstage is the coupling between the two stages. You also need to be careful about the aerodynamics during staging so that there is no relative rotation between the two stages that could cause damage to the upper stage. This can be handled by the design of the staging event. You can use rails to guide the separation, you can use springs to push the stages apart, or if you are high enough that aerodynamic forces are not too bad, you can simply fly away. Also there is some leeway in how much of the interstage you carry with you. All this takes some careful thought and some sophisticated fluid dynamics modeling. But it can be done and has been done many times.</p><p>Basically there are several ways to skin this cat. It takes a bit of engineering and some discipline during the design phase. The problem is that Space-X apparently lacks the expertise and the discipline. The mistakes that they have made are not the sort of mistakes that ought to be forgiven. They simply reek of amateur incompetence.</p><p>The problem is not so much the individual engineers, but the overall lack of discipline and expertise in the organization. A good organization would have tight discipline and then a mix of experienced and newer engineers. But the tight discipline is absolutely crucial and Space-X has demonstrated that they simply don't have it.<br /></p> <div class="Discussion_UserSignature"> </div>
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