K
kyle_baron
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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>The flexseal surrounds the nozzle, near the throat (forward). The actuators are about midway down the nozzle, The flexseal serves to determine the pivot poiint of the nozzle, and in the case of the shuttle is a bit aft of the flexseal itself -- the flexseal basically consists of a set of sections of concentric spherical shells and the center of those spheres is the pivot point. The actuators serve primarily to apply a force normal to the axis of the nozzle, creating a torque which causes the nozzle to rotate about the pivot point. The construction of the flexseal serves to maintain a gas seal on the combustion chamber.During the ignition transient the chamber is pressurized and a portion of that pressure is reacted by the nozzle structure and flexseal internal to the motor. That pressurization compresses the flexlseal a bit, and thereby causes the actuators, which are fixed in length at that time, to exert a force with a resultant torque on the nozzle. That torque causes a small rotation, which is the difference between cold null and hot null. Once the motor has reached operating pressure there is no longer a change in the torque from the fixed actuators and things progress from there in a normal fashion. So the question of the duration of the rotation caused by the difference between cold null and hot null is equivalent to the question of the duration of the ignition transient. I don't have the SRB ignition transient curve in front of me, but I think it ought to be over in less than a second (I am not certain of this however). My guess, lacking hard data, is that your 1/2 second estimate is in the ball park. The actuator is not at all a shock absorber. It is in fact very very stiff and needs to be. The best everyday analog would be a hydraulic jack. It really is basically a hydraulic jack with a high pressure fluid source and an electrically conntrolled valve to control the forces.Holding down the vehicle while the motor is firing has other implications. It comes up to pressure pretty fast and is exhausting a lot of very hot gas. It the vehicle is held down that hot gas will impinge on the aft end of the motor and heat things up pretty quick -- rather a hurricane of gas and glowing alumina that can cut metal pretty quickly. You might have a problem that is more severe than the fishtailing that you are trying to cure. If one accounts for the difference between cold null and hot null it is possible to create a straight trajectory during ignition. On the Peacekeeper missile we did that and managed a hot fly-out of the third stage with a very large nozzle from the interstage. That required a very straight fly-out, almost any significant tip-off would have damaged the nozzle, which was a relatively fragile carbon-carbon design.Dealing with the actual dynamics is probably preferable than trying to eliminate them and living with unintended consequences. <br />Posted by DrRocket</DIV></p><p><strong>Thank you. That's an excellent analysis to further clarify the flexseal, actuator, nozzle (TVC) system. <img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-smile.gif" border="0" alt="Smile" title="Smile" /></strong><br /></p> <div class="Discussion_UserSignature"> <p><font size="4"><strong></strong></font></p> </div>