Ares 1: Lift Off Drift

[vulture4]

<p>If we throw enough money at the Ares it can be made to fly.&nbsp; But what's the point? Obviously lift-off drift isn't a problem with the Delta IV, as of course the all-liquid engines are at full thrust before the hold-down bolts are released.</p><p>&nbsp;</p><p>&nbsp;</p><p>&nbsp;</p>

 

[trailrider]

<p>Kyle Baron wrote, in part, "On the two sides where the rams are attached, the rams hold the nozzle firmly in place, but on the opposite side of the nozzle, where there are no such devices providing support, the nozzle deflects/rotates by a significant amount."</p><p>Hunh?&nbsp; Do you mean the nozzle deforms like an egg around its longitudinal axis?&nbsp; If that is true, then it would also be true during flight.&nbsp; In point of fact, the nozzle's joint is NOT a ball-joint, but (IIRC) </p><p>a Flex-seal joint composed of alternating conoidal rings of steel and flexible material in between.&nbsp; It is flexible, but not to the extent that the nozzle pivots like the ball-joints on your car's suspension system!&nbsp; The actuator rods provide plenty of support to the exit cone.&nbsp; If, indeed, there is any temporary "egging" of the nozzle at the moment of ignition, it can be nulled out ahead of time by biasing the actuator's position in the direction required.&nbsp; If that turns out to be insufficient, opposing actuators could be added in opposition to the others, though it would, of course, add weight.&nbsp; The TVC's response time to angular accelerometer inputs to the guidance system can probably be tweaked.</p><p>As to holding the vehicle down until the thrust builds up to full, I was under the impression that that happens on the Shuttle launches anyway.&nbsp; (Shuttle Guy, please jump in here if I am all wet on these items!)</p><p>Please understand, I am NOT a fan of Ares I, or other solid rocket boosters of this size, on "high value" payload missions.&nbsp; But, until the problems&nbsp;defy solution, technically and/or budgetarily, we are probably stuck with this beast!</p><p>Only time will tell!</p>

 

[shuttle_guy]

<p>

Kyle Baron wrote, in part, "On the two sides where the rams are attached, the rams hold the nozzle firmly in place, but on the opposite side of the nozzle, where there are no such devices providing support, the nozzle deflects/rotates by a significant amount."Hunh?&nbsp; Do you mean the nozzle deforms like an egg around its longitudinal axis?&nbsp; If that is true, then it would also be true during flight.&nbsp;"</p><p>Yes it does deform as a process of start when the thrust is building up. This is true with all rocket engines. it will not happen in flight.</p><p>&nbsp;</p><p>&nbsp;</p><p>&nbsp;</p> <div class="Discussion_UserSignature"> </div>

 

[MeteorWayne]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Kyle Baron wrote, in part, "On the two sides where the rams are attached, the rams hold the nozzle firmly in place, but on the opposite side of the nozzle, where there are no such devices providing support, the nozzle deflects/rotates by a significant amount."Hunh?&nbsp; Do you mean the nozzle deforms like an egg around its longitudinal axis?&nbsp; If that is true, then it would also be true during flight.&nbsp; In point of fact, the nozzle's joint is NOT a ball-joint, but (IIRC) a Flex-seal joint composed of alternating conoidal rings of steel and flexible material in between.&nbsp; It is flexible, but not to the extent that the nozzle pivots like the ball-joints on your car's suspension system!&nbsp; The actuator rods provide plenty of support to the exit cone.&nbsp; If, indeed, there is any temporary "egging" of the nozzle at the moment of ignition, it can be nulled out ahead of time by biasing the actuator's position in the direction required.&nbsp; If that turns out to be insufficient, opposing actuators could be added in opposition to the others, though it would, of course, add weight.&nbsp; The TVC's response time to angular accelerometer inputs to the guidance system can probably be tweaked.As to holding the vehicle down until the thrust builds up to full, I was under the impression that that happens on the Shuttle launches anyway.&nbsp; (Shuttle Guy, please jump in here if I am all wet on these items!)Please understand, I am NOT a fan of Ares I, or other solid rocket boosters of this size, on "high value" payload missions.&nbsp; But, until the problems&nbsp;defy solution, technically and/or budgetarily, we are probably stuck with this beast!Only time will tell! <br />Posted by trailrider</DIV></p><p>The Shuttle Main Engines start about 7 seconds before liftoff, to build up to full power and ensure they are operation correctly. Then when the solids are lit, it is released within a few tenths of a second. That's what I see anyway.<br /></p> <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

 

[shuttle_guy]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>The Shuttle Main Engines start about 7 seconds before liftoff, to build up to full power and ensure they are operation correctly. Then when the solids are lit, it is released within a few tenths of a second. That's what I see anyway. <br />Posted by MeteorWayne</DIV></p><p>&nbsp;</p><p>Essentially correct, except:</p><p>The main engines are still throttling up at lift off. When they start and reach "main stage" (about 90% thrust) the SRB ignition and hold down post release are commanded at the same time.</p> <div class="Discussion_UserSignature"> </div>

 

[shuttle_guy]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Hey SG, care to comment on this article?&nbsp; Apparently, it's a little more complicated, than just wind speed:http://www.nasaspaceflight.com/2008/11/ares-i-lift-off-drift-curve-tvc/At the base of the rocket, the hot gas flow from the first stage Solid Rocket Booster (SRB) is controlled by a directional nozzle which can gimble to steer the vehicle on a designated path through flight.This nozzle is attached in three locations, via a large bearing deep inside the SRB, and two hydraulic actuator rams mounted 90 degrees apart.One of these rams controls the pitch angle of the nozzle and the other controls the yaw angle.Like balancing a pencil on the end of your finger, the TVC system must balance the thrust as it launches, but at the moment of ignition the 3.6 million pounds of thrust all push against the nozzle.On the two sides where the rams are attached, the rams hold the nozzle firmly in place, but on the opposite side of the nozzle, where there are no such devices providing support, the nozzle deflects/rotates by a significant amount.This ignition deflection means that the rocket starts its launch with its nozzle pushing the rocket off-center, and the TVC must start to fight this effect from the first moments of the flight. As the rocket begins to climb, the rocket pivots around its center of gravity (near the top of the first stage) and the aft of the rocket is pushed off-line. This is known as &ldquo;fish tailing&rdquo;.The TVC computers then counteract that move by gimbaling the nozzle to produce a force to bring the bottom of the rocket back into line, but in the few seconds that takes to occur, the rocket may already have drifted closer to its launch tower.I'll say it again, that holding down the skirt for a 1/2 sec. can't be all that bad......when compared to increasing the distance of the vehicle to the tower, or beefing up the tower. <br />Posted by kyle_baron</DIV></p><p>A launch commit criteria for the wind speed and direction will solve the problem. Moving the tower will allow a higher max wind seed from the south and help reduce tower launch damage.<br /></p> <div class="Discussion_UserSignature"> </div>

 

[MeteorWayne]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>&nbsp;Essentially correct, except:The main engines are still throttling up at lift off. When they start and reach "main stage" (about 90% thrust) the SRB ignition and hold down post release are commanded at the same time. <br />Posted by shuttle_guy</DIV><br /><br />Thanx for the clarification. So the SRB ignition and release are simultaneous!</p><p>As usual, thanks for your expertise. That's why we pay you the big bucks </p> <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

 

[silylene old]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>&nbsp;I don't think 200' berms are realistic in that environment.&nbsp; Wouldn't it be easier to build a fence or structure?&nbsp;&nbsp; Won't be cheap in any case.&nbsp; <br />Posted by tanstaafl76</DIV></p><p>Berms are cheap and quick / easy.&nbsp; But probably there are some environmental constraints.</p> <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center"><em><font color="#0000ff">- - - - - - - - - - - - - - - - - - - - - -</font></em> </div><div class="Discussion_UserSignature" align="center"><font color="#0000ff"><em>I really, really, really miss the "first unread post" function.</em></font> </div> </div>

 

[MeteorWayne]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Berms are cheap and quick / easy.&nbsp; But probably there are some environmental constraints. <br />Posted by silylene</DIV><br /><br />I'm not sure that's an answer though. Obstructions like that can created cylindrical roll currents downstream from them. Such currents can be very hard to predict, and cause different effcts depending on wind speed and direction, probably air density as well. So it just would make things more complicated.</p><p>&nbsp;</p> <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

 

[kyle_baron]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>&nbsp;Essentially correct, except:The main engines are still throttling up at lift off. When they start and reach "main stage" (about 90% thrust) the SRB ignition and hold down post release are commanded at the same time. <br />Posted by shuttle_guy</DIV></p><p><strong>Ok SG, can we then assume, that Ares 1 will be the 1st and only rocket, that doesn't utilize the "hold down post release"?</strong><br /></p> <div class="Discussion_UserSignature"> <p><font size="4"><strong></strong></font></p> </div>

 

[kyle_baron]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I'm not sure that's an answer though. Obstructions like that can created cylindrical roll currents downstream from them. Such currents can be very hard to predict, and cause different effcts depending on wind speed and direction, probably air density as well. So it just would make things more complicated.&nbsp; <br />Posted by MeteorWayne</DIV></p><p><strong>I would think that the "cylindrical roll currents downstream", would be of a lower linear velocity&nbsp;on the rocket.&nbsp; It might be higher in velocity while spinning, however.</strong><br /></p> <div class="Discussion_UserSignature"> <p><font size="4"><strong></strong></font></p> </div>

 

[DrRocket]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Kyle Baron wrote, in part, "On the two sides where the rams are attached, the rams hold the nozzle firmly in place, but on the opposite side of the nozzle, where there are no such devices providing support, the nozzle deflects/rotates by a significant amount."Hunh?&nbsp; Do you mean the nozzle deforms like an egg around its longitudinal axis?&nbsp; If that is true, then it would also be true during flight.&nbsp; In point of fact, the nozzle's joint is NOT a ball-joint, but (IIRC) a Flex-seal joint composed of alternating conoidal rings of steel and flexible material in between.&nbsp; It is flexible, but not to the extent that the nozzle pivots like the ball-joints on your car's suspension system!&nbsp; The actuator rods provide plenty of support to the exit cone.&nbsp; If, indeed, there is any temporary "egging" of the nozzle at the moment of ignition, it can be nulled out ahead of time by biasing the actuator's position in the direction required.&nbsp; If that turns out to be insufficient, opposing actuators could be added in opposition to the others, though it would, of course, add weight.&nbsp; The TVC's response time to angular accelerometer inputs to the guidance system can probably be tweaked.As to holding the vehicle down until the thrust builds up to full, I was under the impression that that happens on the Shuttle launches anyway.&nbsp; (Shuttle Guy, please jump in here if I am all wet on these items!)Please understand, I am NOT a fan of Ares I, or other solid rocket boosters of this size, on "high value" payload missions.&nbsp; But, until the problems&nbsp;defy solution, technically and/or budgetarily, we are probably stuck with this beast!Only time will tell! <br />Posted by trailrider</DIV></p><p>&nbsp;Your description of the flexseal is correct.&nbsp; As usual Kyle is all wet.</p><p>There is an assymetry in the loads on the nozzle, as you noted.&nbsp; It arised because the actuators are located on only one side and because the nozzle and flexseal compress a bit as the motor pressurizes.&nbsp; That results in a difference between what is called "cold null" (the zero vector position of an unpressurized motor) and "hot null" (the zero position of a pressurized motor).&nbsp; As you noted the usual procedure is for the actuators to apply a bit of force and move the nozzle just prior to ignition so that the nozzle moves to the hot null position as the motor comes up to pressure.&nbsp; This is all standard stuff.</p><p>BTW the flexible material used is just rubber.</p> <div class="Discussion_UserSignature"> </div>

 

[Testing]

So we have been using an unproven, unreliable system of Thrust Vector Control for over 40 years? The frequency response lives up to the specification.&nbsp; <div class="Discussion_UserSignature"> </div>

 

[shuttle_guy]

<p>[responding too?OTE]So we have been using an unproven, unreliable system of Thrust Vector Control for over 40 years? The frequency response lives up to the specification.&nbsp; <br />Posted by Testing[/QUOTE]</p><p>&nbsp;</p><p>What post are you responding too?</p> <div class="Discussion_UserSignature"> </div>

 

[trailrider]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>&nbsp;Your description of the flexseal is correct.&nbsp; As usual Kyle is all wet.There is an assymetry in the loads on the nozzle, as you noted.&nbsp; It arised because the actuators are located on only one side and because the nozzle and flexseal compress a bit as the motor pressurizes.&nbsp; That results in a difference between what is called "cold null" (the zero vector position of an unpressurized motor) and "hot null" (the zero position of a pressurized motor).&nbsp; As you noted the usual procedure is for the actuators to apply a bit of force and move the nozzle just prior to ignition so that the nozzle moves to the hot null position as the motor comes up to pressure.&nbsp; This is all standard stuff.BTW the flexible material used is just rubber. <br />Posted by DrRocket</DIV></p><p>Wasn't sure if they were still using rubber or some other material. It's been 36 years or so since I was involved with FlexSeal nozzles, on the C-3 Poseidon missile.&nbsp; In order to get the "compliance" (flexibility) in the nozzles to permit actuation with the available actuator forces, due to the size of the hydraulics packages (required by the missile size envelope), we had to use natural rubber.&nbsp; The problem with that material was that it age hardens, and initial tests showed the forces going up at an alarming rate!&nbsp; Fortunately, the force/age curve leveled off below the maximum permissable.&nbsp; I don't know what rubber (natural or synthetic) they use on the Shuttle's SRB nozzles.&nbsp; Was only involved in the "north" end of the boosters (the parachute recovery system).&nbsp; I would almost expect they would use a different type of rubber on the Shuttle SRBs since they reuse them, and probably have more available space for larger hydraulics packages/actuators.&nbsp; But I don't KNOW that.&nbsp; The Titan 34D and IV (Stage 0) SRMs used fixed nozzles with fluid injection TVC.&nbsp; Again, IIRC, there were four sets of injection ports every 90 degrees around the nozzles, so they could continuously inject differentially for "null" settings.&nbsp; Might be a better system, but, again, would add weight that Ares I probably couldn't afford.&nbsp; And besides which, would require additional development (read money)!</p><p>Be interesting to see what the Obama administration does with the whole thing!</p><p>Ad LEO! Ad Luna! Ad Ares! Ad Astra!</p>

 

[kyle_baron]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>&nbsp;Your description of the flexseal is correct.&nbsp; As usual Kyle is all wet.<br />Posted by DrRocket</DIV></p><p><strong>Hey Dr. Rocket, when you're going to insult someone, make sure it's the right person!&nbsp; What you are referring to is a quote from Chris Bergin (Managing Editor of the National Science Foundation NSF).&nbsp; It's the 10th paragraph from the top of this article:</strong></p><p>http://www.nasaspaceflight.com/2008/11/ares-i-lift-off-drift-curve-tvc/</p><p><strong><font size="4">ROFLMAO!!!</font></strong><br /></p> <div class="Discussion_UserSignature"> <p><font size="4"><strong></strong></font></p> </div>

 

[lampblack]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Hey Dr. Rocket, when you're going to insult someone, make sure it's the right person!&nbsp; What you are referring to is a quote from Chris Bergin (Managing Editor of the National Science Foundation NSF).&nbsp; It's the 10th paragraph from the top of this article:http://www.nasaspaceflight.com/2008/11/ares-i-lift-off-drift-curve-tvc/ROFLMAO!!! <br /> Posted by kyle_baron</DIV></p><p>Ummm... just a note to help clarify. The individual to whom you refer is a journalist and not an engineer -- as he happily admits. Indeed, he has been an occasional contributor to these forums.</p><p>And NSF stands for NasaSpaceFlight -- as noted in the domain name. It is a privately owned (and reasonably well-done) web site, and is in no way tied to NASA or any aspect of the government.</p><p>&nbsp;</p> <div class="Discussion_UserSignature"> <font color="#0000ff"><strong>Just tell the truth and let the chips fall...</strong></font> </div>

 

[DrRocket]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Hey Dr. Rocket, when you're going to insult someone, make sure it's the right person!&nbsp; What you are referring to is a quote from Chris Bergin (Managing Editor of the National Science Foundation NSF).&nbsp; It's the 10th paragraph from the top of this article:http://www.nasaspaceflight.com/2008/11/ares-i-lift-off-drift-curve-tvc/ROFLMAO!!! <br />Posted by kyle_baron</DIV></p><p>You made the post.&nbsp; You misunderstood the article.&nbsp; You quoted bad information. &nbsp;And as noted above you also misunderstood your source.&nbsp; Very wet indeed. Typical.<br /></p> <div class="Discussion_UserSignature"> </div>

 

[kyle_baron]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>You made the post.&nbsp; You misunderstood the article.&nbsp; You quoted bad information. &nbsp;And as noted above you also misunderstood your source.&nbsp; Very wet indeed. Typical. <br />Posted by DrRocket</DIV></p><p><strong>Now you be-little a journalist, with access to most of Nasa's technical data?&nbsp; TYPICAL.&nbsp; By the way, here's what you're arguing about:</strong></p><p><table border="0" cellspacing="0" cellpadding="3" width="100%"><tbody><tr><td class="windowbg"><table border="0" cellspacing="0" cellpadding="5" width="100%" style="table-layout:fixed"><tbody><tr><td rowspan="2" width="16%" valign="top" style=""><strong>James Lowe1</strong> <div id="msg_329411_extra_info" class="smalltext">Lead Moderator<br />Administrator<br /><img src="http://forum.nasaspaceflight.com/Themes/nsf/images/staradmin.gif" border="0" alt="*" /><img src="http://forum.nasaspaceflight.com/Themes/nsf/images/staradmin.gif" border="0" alt="*" /><img src="http://forum.nasaspaceflight.com/Themes/nsf/images/staradmin.gif" border="0" alt="*" /><img src="http://forum.nasaspaceflight.com/Themes/nsf/images/staradmin.gif" border="0" alt="*" /><img src="http://forum.nasaspaceflight.com/Themes/nsf/images/staradmin.gif" border="0" alt="*" /><br />Offline<br /><br />Posts: 850<br />Location: New York City<br /><br /><br /></div>

 

[DrRocket]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Now you be-little a journalist, with access to most of Nasa's technical data?&nbsp; TYPICAL.&nbsp; By the way, here's what you're arguing about:James Lowe1 Lead ModeratorAdministratorOfflinePosts: 850Location: New York CityRe: Ares I Lift Off Drift Curve focuses on the TVC - not just high winds &laquo; Reply #32 on: 11/06/2008 07:05 AM &raquo;Couple of hundred megabytes worth of SRB/RSRM presentations on L2. There's a lot to go through, but found this immediately. I know there's some good full system overviews, just a case of going through the Bibles. AttachmentsI assumed NSF stood for National Science Foundation, when it stands for NasaSpaceflight.com&nbsp; So, sue me.&nbsp; <br />Posted by kyle_baron</DIV></p><p>I know what we're&nbsp;arguing about.&nbsp; We built the damn things.&nbsp; I've been throught the manufacturing processes, for several different flavors of flexseals.&nbsp; I've seen the test data.&nbsp; I've evaluated TVC systems.&nbsp; I've worked on conventional and advanced designs.&nbsp; I know the failure modes.&nbsp; Been there,&nbsp; Done that.&nbsp; </p><p>NASA does not build or design this hardware; the solid propulsion contractor does.</p><p>You really do need to get a little education and stop believing everything that you read. </p> <div class="Discussion_UserSignature"> </div>

 

[Testing]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>[responding too?OTE]So we have been using an unproven, unreliable system of Thrust Vector Control for over 40 years? The frequency response lives up to the specification.&nbsp; Posted by Testing</DIV>&nbsp;What post are you responding too? <br />Posted by shuttle_guy[/QUOTE</p><p>Sorry SG, guess the BS got to me. I get to go in and fire up the mass spec today to chase down a leak.&nbsp; Are you still coming out for turkey day or has that slipped?</p> <div class="Discussion_UserSignature"> </div>

 

[kyle_baron]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I know what we're&nbsp;arguing about.&nbsp; We built the damn things.&nbsp; I've been throught the manufacturing processes, for several different flavors of flexseals.&nbsp; I've seen the test data.&nbsp; I've evaluated TVC systems.&nbsp; I've worked on conventional and advanced designs.&nbsp; I know the failure modes.&nbsp; Been there,&nbsp; Done that.&nbsp; NASA does not build or design this hardware; the solid propulsion contractor does.You really do need to get a little education and stop believing everything that you read. <br />Posted by DrRocket</DIV></p><p><em>Chill out Dr. Rocket, this simplified diagram, was meant for everyone else in this thread.</em><br /></p> <div class="Discussion_UserSignature"> <p><font size="4"><strong></strong></font></p> </div>

 

[shuttle_guy]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>&nbsp;What post are you responding too? Posted by shuttle_guy[/QUOTESorry SG, guess the BS got to me. I get to go in and fire up the mass spec today to chase down a leak.&nbsp; Are you still coming out for turkey day or has that slipped? <br />Posted by Testing</DIV></p><p>&nbsp;</p><p>If all goes as planned I will be flying to Edwards on turkey day.<br /></p> <div class="Discussion_UserSignature"> </div>

 

[kyle_baron]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>&nbsp;Your description of the flexseal is correct. There is an assymetry in the loads on the nozzle, as you noted.&nbsp; It arised because the actuators are located on only one side and because the nozzle and flexseal compress a bit as the motor pressurizes.&nbsp; That results in a difference between what is called "cold null" (the zero vector position of an unpressurized motor) and "hot null" (the zero position of a pressurized motor).&nbsp; As you noted the usual procedure is for the actuators to apply a bit of force and move the nozzle just prior to ignition so that the nozzle moves to the hot null position as the motor comes up to pressure.&nbsp; This is all standard stuff.BTW the flexible material used is just rubber. <br />Posted by DrRocket</DIV></p><p><strong>You say the actuators apply a bit of force, and move the nozzle prior to ignition.&nbsp; Then you say that the flexseal compresses a bit as the motor pressurizes.&nbsp; The article just says that the nozzle rotates by a significant amount.&nbsp; My question is, there are 2 movements of the nozzle, one by the actuator at the bottom of the nozzle (before ignition) and one by the flexseal at the top of the nozzle (after ignition).&nbsp; Is this correct?&nbsp; Therefore, it would seem that the nozzle does move at ignition.&nbsp; There is compression at the top (flexseal), and I assume there is compression of the actuator (shock absorber) at the bottom.&nbsp; Is that correct?</strong></p><p><strong>Another question is, at ignition, approximatly how long does it take for the nozzle to not move at all?&nbsp; 1/2 sec?&nbsp; Less than 1/2 sec?&nbsp; Because that is how long the SRB should be held down at the skirt, to avoid the fish tailing (in the article).</strong><br /></p> <div class="Discussion_UserSignature"> <p><font size="4"><strong></strong></font></p> </div>

 

[DrRocket]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>You say the actuators apply a bit of force, and move the nozzle prior to ignition.&nbsp; Then you say that the flexseal compresses a bit as the motor pressurizes.&nbsp; The article just says that the nozzle rotates by a significant amount.&nbsp; My question is, there are 2 movements of the nozzle, one by the actuator at the bottom of the nozzle (before ignition) and one by the flexseal at the top of the nozzle (after ignition).&nbsp; Is this correct?&nbsp; Therefore, it would seem that the nozzle does move at ignition.&nbsp; There is compression at the top (flexseal), and I assume there is compression of the actuator (shock absorber) at the bottom.&nbsp; Is that correct?Another question is, at ignition, approximatly how long does it take for the nozzle to not move at all?&nbsp; 1/2 sec?&nbsp; Less than 1/2 sec?&nbsp; Because that is how long the SRB should be held down at the skirt, to avoid the fish tailing (in the article). <br />Posted by kyle_baron</DIV></p><p>The flexseal surrounds the nozzle, near the throat (forward).&nbsp; 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.&nbsp; 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.&nbsp; The construction of the flexseal serves to maintain a gas seal on the combustion chamber.</p><p>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.&nbsp; 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.&nbsp; That torque causes a small rotation, which is the difference between cold null and hot null.&nbsp; 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.&nbsp; 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.&nbsp; I don't have the SRB ignition transient curve in front of me, but I think it ought to be over in&nbsp;less than a second (I am not certain of this however).&nbsp; My guess, lacking hard data, is that your 1/2 second estimate is in the ball park. </p><p>The actuator is not at all a shock absorber.&nbsp; It is in fact very very stiff and needs to be.&nbsp; The best everyday analog would be a hydraulic jack.&nbsp; It really is basically a hydraulic jack with a high pressure fluid source and an electrically conntrolled valve to control the forces.</p><p>Holding down the vehicle while the motor is firing has other implications.&nbsp; It comes up to pressure pretty fast and is exhausting a lot of very hot gas.&nbsp; 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.&nbsp; You might have a problem that is more severe than the fishtailing that you are trying to cure.&nbsp;&nbsp;</p><p>If one accounts for the difference between cold null and hot null it is possible to create a straight trajectory during ignition.&nbsp; 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.&nbsp; 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.</p><p>Dealing with the actual dynamics is probably preferable than trying to eliminate them and living with unintended consequences.<br /></p> <div class="Discussion_UserSignature"> </div>