<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'> Understood, back-pressure is involved. Aren't SRB's simple motors, involving vanes near the bottom? I would think those could be beefed up. Understood, something similar would be needed for an SRB. Understood, but the nozzle on the SRB is wide, and the water would provide some back pressure to the nozzle, compared to air. In this instance, back pressure is good. Yeah, yeah, yeah, an explosion, I get it. <br />Posted by kyle_baron</DIV></p><p>SBRs are probably more complicated that you think, but the complication is in the design and not in a lot of moving parts. There are no vanes in the SRBs. You might be thinking of fin-like features in the propellant grain, but they are there to provide initial surface area and are not relevant to the questions at hand. In the case of the shuttle, the fins slots in the grain are in the head end.</p><p>The problem with igniting underwater is not just a little back pressure. You have to move the water out of the way to get the gasses flowing, and until that is done the back pressure communicates with the interior pressure of the motor. That water has a lot of mass and takes too long to get out of the way. In the meantime the motor pressure is going up very rapidly and the burn rate is going up with it. The motor will probably burst. It will, technically, probably not be an explosion, but the distinction will be lost on a nearby observer.</p><p>The Titan IV SRB was tested vertically in a test stand with a water spray. Steps were taken to keep any significant amount of water away from the nozzle exit plane at ignition. That was a long way from a submerged ignition. </p><p>The flow in a rocket nozzle is choked at the throat and supersonic in the nozzle cone. The transition from nothing to choked flow takes place very quickly, as soon as chamber pressure hit the critical value which is only about double ambient pressure. You want to get the flow established quickly and to get any nozzle obstructions out the way as soon as possible. You simply cannot tolerate having to move a lot of water out of the way. Creating a bunch of shock waves in the nozzle cone is a bad idea, unless it is very carefully controlled as in liquid injection thrust vector control systems (but that uses a very small amount of injectant).<br /></p> <div class="Discussion_UserSignature"> </div>