STS SRB oscillations

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aphh

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<p>Recently there have been some info about potential vibration problem with the SRB powered new manned spacecraft.</p><p>Is it just me, or is this problem already visible and demonstrated in the STS launch videos?</p><p>At some point during the ascent the whole STS looks like it starts to shake very clearly. Then when the booster rockets are discarded and the exhaust starts to fade, you can clearly see that the exhaust is pulsed and the frequency appears quite low.</p><p>Is this correct and how could this escape the designers of the new manned rocket? </p>
 
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CalliArcale

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Recently there have been some info about potential vibration problem with the SRB powered new manned spacecraft.Is it just me, or is this problem already visible and demonstrated in the STS launch videos?At some point during the ascent the whole STS looks like it starts to shake very clearly. Then when the booster rockets are discarded and the exhaust starts to fade, you can clearly see that the exhaust is pulsed and the frequency appears quite low.Is this correct and how could this escape the designers of the new manned rocket? <br /> Posted by aphh</DIV></p><p>As I understand it, vibrations are an issue with all solid boosters.&nbsp;&nbsp; And yes, the SRBs do vibrate a lot; astronauts have commented in interviews about how rough the ride is for the first couple of minutes, and how smooth it is after SRB separation.&nbsp; This is also clearly visible in ascent cockpit videos.</p><p>The exhaust isn't pulsed, though.&nbsp; Solid rockets are fairly crude and I think it would be very challenging to get it to pulse on purpose.&nbsp; (They can vary the thrust during the flight, but only by casting the solid propellant with a carefully chosen grain -- basically, by varying the shape of the hole down the middle.)&nbsp; I'm not sure what exactly we're seeing on those videos, but I don't think it's variations in the thrust.&nbsp;</p> <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>
 
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MeteorWayne

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>As I understand it, vibrations are an issue with all solid boosters.&nbsp;&nbsp; And yes, the SRBs do vibrate a lot; astronauts have commented in interviews about how rough the ride is for the first couple of minutes, and how smooth it is after SRB separation.&nbsp; This is also clearly visible in ascent cockpit videos.The exhaust isn't pulsed, though.&nbsp; Solid rockets are fairly crude and I think it would be very challenging to get it to pulse on purpose.&nbsp; (They can vary the thrust during the flight, but only by casting the solid propellant with a carefully chosen grain -- basically, by varying the shape of the hole down the middle.)&nbsp; I'm not sure what exactly we're seeing on those videos, but I don't think it's variations in the thrust.&nbsp; <br />Posted by CalliArcale</DIV></p><p>It might be variations. Don't forget, after the point of SRB sep, the boosters are almost out of propellant, and what's left probably burns irregularly.<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>
 
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qso1

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In addition to what MeteorWayne and Calli said, there may be a problem with SRB oscillations, especially in a 5 segment design. But I'm sure it will be a manageable one and if not, NASA will go back to the drawing board. <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>
 
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aphh

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>In addition to what MeteorWayne and Calli said, there may be a problem with SRB oscillations, especially in a 5 segment design. But I'm sure it will be a manageable one and if not, NASA will go back to the drawing board. <br /> Posted by qso1</DIV></p><p>I think if you look at the STS launch videos you can see that the flame of the exhaust is not nice and continuous unlike the main engine flame, that seems really really stable.</p><p>Because the fuel inside the SRB is a long cylinder, a pipe even, the ignition occurs at both ends of the pipe and also in the middle of it. All these flames having been ignited over the length of the pipe are fighting to get out of the same hole. It's probably a bit like having a combustion engine detonate, which means there are multiple ignitions occuring in the chamber causing the flame fronts to reach the walls of the cylinder at different times.</p><p>Would a hybrid rocket be any better when it comes to oscillations? <br /><br />It has the fuel distributed in the same manner, so at first it would feel that it could not be better, as the ignition might also happen distributed along the length of the pipe. But if the flow of oxidizer only allows the ignition to occur at one point inside the pipe and proceed as the fuel is burnt, then it might be a different situation compared to SRB. </p>
 
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DrRocket

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I think if you look at the STS launch videos you can see that the flame of the exhaust is not nice and continuous unlike the main engine flame, that seems really really stable.Because the fuel inside the SRB is a long cylinder, a pipe even, the ignition occurs at both ends of the pipe and also in the middle of it. All these flames having been ignited over the length of the pipe are fighting to get out of the same hole. It's probably a bit like having a combustion engine detonate, which means there are multiple ignitions occuring in the chamber causing the flame fronts to reach the walls of the cylinder at different times.Would a hybrid rocket be any better when it comes to oscillations? It has the fuel distributed in the same manner, so at first it would feel that it could not be better, as the ignition might also happen distributed along the length of the pipe. But if the flow of oxidizer only allows the ignition to occur at one point inside the pipe and proceed as the fuel is burnt, then it might be a different situation compared to SRB. <br />Posted by aphh</DIV></p><p>The thrust of the solids is quite consistent.&nbsp;&nbsp;The exhaust is not really a flame but is a more-or-less isentropic expansion of very hot gasses produced by the combustion process inside the rocket motor and upstream of the throat of the nozzle.&nbsp; There is&nbsp;a little&nbsp;bit of chemical reaction going on in the plume, but most of the combustion has taken place within the rocket and upstream of the nozzle throat.&nbsp; &nbsp;I absolutely guarantee that in television pictures you&nbsp;cannot see any pulsing or acoustics that might be occurring, at least until after the pressure and thrust have dropped to negligible levels.&nbsp; You may see some shock waves in the plume as the supersonic gas drops back below sonic velocity, but that effect is not seen in the thrust.&nbsp; You see a lot more action in the plume from the STS solics than from the liquids because the solid plume contains very hot solid particles (primarily aluminum oxide) and visible gasses, while the plume from the liquids is basically hot water vapor.&nbsp; </p><p>Most solid rocket motor acoustic pressure oscillations are on the order of a few psi.&nbsp; As I understand the ARES oscillation problem it is related to a hypothetical low frequency oscillation that is hypothesized to couple with the payload and affect the human cargo.&nbsp; The problem has recently been re-evaluated and much of the concern has gone away.&nbsp; One "fix" that had been considered was to provide greated shock and vibration isolation in the crew seats.&nbsp; </p><p>In the SRM the grain is not quite a pipe.&nbsp; It is more complicated than that, because of the need to tailor the thrust-time curve to mission requirements, and to optimize the curve for performance.&nbsp; Solid rocket propellant produces gas at a rate that is primarily dependent on exposed surface area, with some effects of pressure figured into the design.&nbsp; If the grain were a simple pipe, then the surface area would increase dramatically as the diameter of the inerior "pipe" increased.&nbsp; That would cause what is called a progressive pressure-time curve.&nbsp; One generally tries to avoid a progressive curve in favor of a more neutral (flat) curve.&nbsp; This is because the inert weight of the motor is largely a function of the maximum pressure, while performance is more closely related to the average pressure.&nbsp; In addition, the SRM grain is modified to reduce thrust to limit what is called max q -- maximum dynamic pressure -- for aerodyamic reasons.&nbsp; This tailoring is done by the inclusion of features such as fins or grooves in the grain.</p><p>Ignition in&nbsp;a solid motor is performed by a device called an igniter.&nbsp; There are two primary types of igniters.&nbsp; Pyrotechnic igniters are basically baskets of pellets of what is called BKNO3 a mixture of boron and potassium nitrate.&nbsp; The pellets themselves are ignited through a device called a through-bulkhead initiator (TBI) which receives a detonating impulse and transmits it through a solit steel bulkhead to a receptor on the otheer side, igniting the pellets.&nbsp; The pellets burn rather quickly and generate a short pulse of hot gas that ignites the grain, usually from the front end.&nbsp; Pyrogen igniters are like little rocket motors.&nbsp; The use TBIs&nbsp;in the same manner as a pyrotechnic igniter and and a small receiving charge of BKNO3 pellets to ignite a smaller solid propellant grain which then discharges hot gasses into the the free volume of the main motor, igniting the grain.&nbsp; That ignition usually starts at the head end of the grain and very rapidly (milliseconds) progresses to include the entire grain itself.&nbsp; Uniformity of ignition is very very good and is needed with a system like the shuttle so that the thrust from the two solid boosters is balanced.</p><p>Once ignited, the propellant grain burns pretty uniformly over the surface.&nbsp; There are no "competing" flame fronts or interenal shock waves of note.&nbsp; One can get some second order variations in burn rate due to rheological effects of the casting process, probably alignment of ammonium perchlorate particles with flow lines of the uncured propellant.&nbsp; These effects are somewhat understood and are accounted for in ballistic models by what is called a BARF curve -- a&nbsp;completely ad hoc empirical correction for slight differences in burn rate throughout the propellant bulk, but an effect that is usually quite reproducible.&nbsp; It is nothing like having a combustion engine detonate.</p><p>By the way, the word "detonation" has a rather specific meaning, particularly in the solid rocket motor industry.&nbsp; It is a really scary word.&nbsp; Most combustion with which you may have had everyday experience is deflagration.&nbsp; Deflagration is ordinary burning.&nbsp; It may be quite rapid, as with normal burning of gunpowder.&nbsp; Deflagration basically progresses with the heat required being supplied by radiation from from the flame.&nbsp; Detonation is far more rapid.&nbsp; Detonation is a combustion process in which the heat required is supplied by adiabatic compression of the solid or gas fuel.&nbsp; High explosives detonate.&nbsp; It is really fast.&nbsp; As a comparison, typical solid rocket fuels burn (via deflagration) at a rate on the order of 0.3-0.5 inches per second.&nbsp; High explosives detonate at rates on the order of 5,000 to 8,000 meters per second.&nbsp; Some very high performance solid rocket propellants are capable of detonation and when they do the result is really exciting -- and leaves a large hole in the ground.&nbsp;Those propellants are described as class 1.1.&nbsp; Propellants used for space launchers such as the STS are class 1.3 and are generaly not&nbsp;susceptible to true detonation.&nbsp; </p><p>Hybrid rockets are generally, most assuredly not, better when it comes to oscillations.&nbsp; While solids use an oxidizer that is mixed uniformly into the propellant grain composition (ammonium perchlorate is the most common oxidizer) hybrids inject a separate oxidizer, usually pure oxygen, at a discrete point or points.&nbsp; Where typical pressure oscillations in a solid are only a few psi, I have seen oscillations of over 100 psi in hybrids.&nbsp; That is a lot.</p><p>There is an old phenomena in solids, unstable acoustic burning, that has received some attention.&nbsp; This is a phenomena in which acoustic oscillations interact with combustion chemistry to produce a positive and unstable feedback that can result incatastrophic failure.&nbsp; There were problems with this phenomena in the early Minuteman days and a lot of work was done to understand it.&nbsp; The early Minuteman used a propellant formed from what was called casting powder combined with a "solvent" that caused the powder to stick together-- basically the grain was formed by pouring nitroglycerine over a nictrocellulose-based powder.&nbsp; That propellant was apparently quite susceptible to acoustic burning problems.&nbsp; More modern propellants use a polymer binder into which is mixed an oxidizer (commonly ammonium perchlorate) and a fuel (aluminum is the main ingredient for space launch applications).&nbsp; This "slurry" propellant seems to be far less susceptible to acoustic instability.</p><p>The purported issue with AREAS is not related to acoustic instability.&nbsp; My information is that is was not based on data from static tests of the motor either, but rather was based on some experience from the Mercury, Gemini and Apollo days and some preliminary analysis.&nbsp; The concern was over a physiological effect on the astronauts (hence the potential fix by modification of their seats).&nbsp; It apparently was also focused on relatively low frequency oscillations.&nbsp; My understanding is that the concern is now minimal and will be addressed with some instrumentation in upcoming testing with no human cargo involved.<br /></p> <div class="Discussion_UserSignature"> </div>
 
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kyle_baron

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<p>&nbsp;</p><p>Wow, you make "Shuttle Guy" (from the old forum) look like a kindergardener.&nbsp; I guess you're the new "Shuttle Guy".<img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-wink.gif" border="0" alt="Wink" title="Wink" /><img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-smile.gif" border="0" alt="Smile" title="Smile" /></p> <div class="Discussion_UserSignature"> <p><font size="4"><strong></strong></font></p> </div>
 
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MeteorWayne

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>&nbsp;Wow, you make "Shuttle Guy" (from the old forum) look like a kindergardener.&nbsp; I guess you're the new "Shuttle Guy". <br />Posted by kyle_baron</DIV><br /><br />Just a suggestion, there's no particular reason to quote the whole post for a comment like this, either save just a line or two or the poster's name, or address the reply to the person. It makes the page count lower and repeating it serves no useful purpose. It's hard enough to maneuver around here without extra stuff.</p><p>If you are going to respond to specific points, and are worried about being taken out of context, that's different.</p><p>It's a habit I'm trying to teach myself to follow <img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-smile.gif" border="0" alt="Smile" title="Smile" />&nbsp;. I often go back and edit excess stuff out from the quote section when I forget.</p><p>And yes indeed, DrRocket appears to be someone who knows that subject very well; I too appreciate his education!</p><p>Wayne</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>
 
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aphh

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>The thrust of the solids is quite consistent.&nbsp;&nbsp;The exhaust is not really a flame but is a more-or-less isentropic expansion of very hot gasses produced by the combustion process inside the rocket motor and upstream of the throat of the nozzle.&nbsp; <br /> Posted by DrRocket</DIV></p><p>Thanks for all the info. It's going to take a while to digest all that.&nbsp;</p>
 
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shuttle_guy

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>&nbsp;..................... My information is that is was not based on data from static tests of the motor either, but rather was based on some experience from the Mercury, Gemini and Apollo days and some preliminary analysis.&nbsp; The concern was over a physiological effect on the astronauts (hence the potential fix by modification of their seats).&nbsp; It apparently was also focused on relatively low frequency oscillations.&nbsp; My understanding is that the concern is now minimal and will be addressed with some instrumentation in upcoming testing with no human cargo involved. <br />Posted by DrRocket</DIV></p><p>The concern is based on taking the SRB induced vibrations in the Shuttle stack and trying to determine what the effect will be on a single stick SRB with a 5th segment. No 5 segment SRB has&nbsp;even been tested. A static&nbsp;test (firing the motor on the ground) of the 5 segment booster is planned for next year. This test should give the Ares team a&nbsp;much&nbsp;determination of how big the problem is.&nbsp;</p> <div class="Discussion_UserSignature"> </div>
 
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DrRocket

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>The concern is based on taking the SRB induced vibrations in the Shuttle stack and trying to determine what the effect will be on a single stick SRB with a 5th segment. No 5 segment SRB has&nbsp;even been tested. A static&nbsp;test (firing the motor on the ground) of the 5 segment booster is planned for next year. This test should give the Ares team a&nbsp;much&nbsp;determination of how big the problem is.&nbsp; <br />Posted by shuttle_guy</DIV></p><p>That is not quite correct.&nbsp; A 5-segment motor was tested about&nbsp;4 1/2&nbsp;years ago at the Promontory test facility.&nbsp; It was successful.<br />http://www.spaceref.com/news/viewpr.html?pid=12865</p> <div class="Discussion_UserSignature"> </div>
 
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