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Space Forum / Shuttle / September 2003The SSME throttle-up?
When the shuttle lifts off just before the SRB's kick-in the ME's are at full power, which is 100% with the throttles wide open, and yet a little over 2 minutes after lift off the SSME's go to throttle up and the ME's increase thrust to 106%. If the ME's are already wide open at 100%, where does the extra 6% come from? Christopher +++++++++++++++++++++++++ "Kites rise highest against the wind - not with it." Winston Churchill > When the shuttle lifts off just before the SRB's kick-in the ME's are > at full power, which is 100% with the throttles wide open, and yet a [quoted text clipped - 7 lines] > the wind - not with it." > Winston Churchill Actually, no. At launch, SSMEs are typically at 104% of rated power. At about 1 minute into launch (may be a bit earlier or later, depending on launch weight, trajectory, etc.) the SSMEs throttle down to reduce acceleration while passing through the regime of maximum dynamic pressure ("MaxQ") (which is defined as q = 1/2*rho*V^2 where rho is atmospheric density and V is velocity). After MaxQ has passed, the engines throttle back up to 104%. See <http://www.aerospaceweb.org/question/aerodynamics/q0025.shtml> for a very good explanation.  SignatureHerb Schaltegger, B.S., J.D. Reformed Aerospace Engineer "Heisenberg might have been here." ~ Anonymous > Actually, no. At launch, SSMEs are typically at 104% of rated power. > At about 1 minute into launch (may be a bit earlier or later, depending [quoted text clipped - 7 lines] > See <http://www.aerospaceweb.org/question/aerodynamics/q0025.shtml> > for a very good explanation. Is the throttle up/down done manually or automatically? > Is the throttle up/down done manually or automatically? Automatically, by the computers. The crew monitors and confirms it occurs as expected. And to add, full power is 104%; they are capable of going to max power which is 109% but never done except in certain abort modes. There's been only one in-flight launch abort so far (STS 51-F, Challenger, 1985) but I don't think they went to 109% on that one... I recall they just simply inhibited (prevented the computers from allowing) automatic engine shutdown for the other two engines to prevent faulty sensors from causing real problems -- they didn't want the second engine's sensors to fail too early which would have caused the ET to land in Europe or Saudi Arabia -- and accepted a lowered initial orbit due to being 119 feet per second (fps) short of the target orbital velocity. They made up this shortfall by doing an OMS-1 burn (gained an extra 194 fps) after main engines shutdown and ET separation, to achieve desired orbit. Pretty much the entire mission is done by computer control; only very few phases are done manually by the crew such as the last two minutes for the landing and performing a rendezvous with another object (eg space station, satellite, etc). Even then, the computer is constantly calculating trajectory and making advisory cues to help out the crew achieve their goals while the crew manually gives input to fine-tune things. At least one flight crew has done the entire (re-)entry/TAEM/approach/landing phase manually as a DTO (designated test objective) -- perhaps Engle and Truly on STS-2? -Dan So all that is part of an auto sequence then? Two supplementary questions. If they are at 104 percent of rated power, then why is the rated power not called 100 percent? What I mean is, what is the point in pretending the engines can gove more than their rated power? Seems daft to me, almost like a marketing claim, Our watts in our amplifier sound louder than normal ones etc... and, I understand that some engines can be designed to work better in the upper atmosphere, and some to work better at the ground and up to a point. Given that is so, how are the SMEs designed? Is it just a compromise, or do mixtures or pressures change as height is gained to keep things efficient? Brian -- Brian Gaff.... graphics are great, but the blind can't hear them Email: briang1@blueyonder.co.uk ____________________________________________________________________________ __________________________________ > So all that is part of an auto sequence then? > [quoted text clipped - 5 lines] > a marketing claim, Our watts in our amplifier sound louder than normal ones > etc... That's a good question and I think part of what Christopher was asking. It has more to do with what the original baseline was. The original engines were designed to put out X amount of thrust. Upgrades permitted a higher amount of thrust. In order to make documentation easier, etc, they kept the 100% = to X amount of thrust and simply went over that. So when they throttle down, they know, "ok, throttle to 60% power" and don't have to keep track of "is that 60% of the new engines, or the old, or what?" So, the current engines provide 4% more thrust than the original design. If they developed yet another design that could put out say 200% of the original rated thrust, you'd see engines at 200% power. > and, I understand that some engines can be designed to work better in the > upper atmosphere, and some to work better at the ground and up to a point. Correct, mostly the nozzle shape I believe. > Given that is so, how are the SMEs designed? Is it just a compromise, or do > mixtures or pressures change as height is gained to keep things efficient? It's a compromise mostly, though I think they compromise more towards upper atmosphere than sea-level since the SRB's dominate the thrust at sea level. > Brian > > -- > Brian Gaff.... > graphics are great, but the blind can't hear them > Email: briang1@blueyonder.co.uk ____________________________________________________________________________ > __________________________________ > > --- > Outgoing mail is certified Virus Free. > Checked by AVG anti-virus system (http://www.grisoft.com). > Version: 6.0.520 / Virus Database: 318 - Release Date: 18/09/03 >> So all that is part of an auto sequence then? >> [quoted text clipped - 22 lines] > they developed yet another design that could put out say 200% of the > original rated thrust, you'd see engines at 200% power. I've wondered about that for years! Somehow it sounded like they were deliberately pushing the engines harder than the amount of thrust for which they were designed. While I didn't think that was the case, I never knew what it *did* mean. Thanks. Steve <snip> On Sat, 20 Sep 2003 20:11:02 GMT, "Greg D. Moore \(Strider\)" <mooregr@greenms.com> wrote: >> So all that is part of an auto sequence then? >> [quoted text clipped - 22 lines] >they developed yet another design that could put out say 200% of the >original rated thrust, you'd see engines at 200% power. Yes that was the line Christopher wanted to know. Thanks for the clarification. >> and, I understand that some engines can be designed to work better in the >> upper atmosphere, and some to work better at the ground and up to a point. [quoted text clipped - 22 lines] >> Checked by AVG anti-virus system (http://www.grisoft.com). >> Version: 6.0.520 / Virus Database: 318 - Release Date: 18/09/03 Christopher +++++++++++++++++++++++++ "Kites rise highest against the wind - not with it." Winston Churchill Brian Gaff spewed out: > So all that is part of an auto sequence then? > [quoted text clipped - 5 lines] > a marketing claim, Our watts in our amplifier sound louder than normal ones > etc... That's a great question and one I've wondered about many times. If they can push out 109% (as mentioned by Dan Foster) then _that_ should be the nominal 100% mark. What's now known as 94% would become 86% and 104% would become 95%. At full power what's now known as 109% would simply become 100%. My guess is that the percentages are rated against the original specifications. If continuous improvements are made that can squeeze out more performance then the 100% mark would keep changing. That would make it difficult to compare performance numbers over the life of the program. There are also probably too many rules-of-thumb and years of experience behind them to change it now. By the way, mine goes up to 11. Signaturebp Proud Member of the Human O-Ring Society Since 2003 > By the way, mine goes up to 11. Your wife must be very happy. ;-) David Higgins spewed out: >> By the way, mine goes up to 11. > > Your wife must be very happy. ;-) LOL Signaturebp Proud Member of the Human O-Ring Society Since 2003 | > By the way, mine goes up to 11. | | Your wife must be very happy. ;-) Is that an O ring size? Brian -- Brian Gaff.... graphics are great, but the blind can't hear them Email: briang1@blueyonder.co.uk ____________________________________________________________________________ __________________________________ Brian Gaff spewed out: > | > By the way, mine goes up to 11. > | > | Your wife must be very happy. ;-) > | > Is that an O ring size? She says "O" quite a lot, but no, it was an obscure reference to the movie "This Is Spinal Tap". It was a satiric look at a fictional rock group from the 80's. There was a funny scene where 2 rockers are comparing their amplifiers. Both have gain control knobs that turn through 270 degrees or so, but one is marked from 1 to 10 while the other goes from 1 to 11. The guy with the latter infers that his amplifier is better because his "goes to 11", and tries to convince the other guy that that's a valid comparison. The wording of the original question made me think about that scene. It relates to how different ranges of values are scaled and that certain measurements are relative to what you happen to define "100%" as. Sort of. Just because one model goes from 0 to 100% does that mean it does more than one that goes to 110%? Depends on how you define 100%. Signaturebp Proud Member of the Human O-Ring Society Since 2003 > Brian Gaff spewed out: > > | [quoted text clipped - 15 lines] > define "100%" as. Sort of. Just because one model goes from 0 to 100% does > that mean it does more than one that goes to 110%? Depends on how you define 100%. I remember that. The musician points out that standard amplifiers go to 10, but his was special because he had it custom made to go to 11. The person he is showing it to asked why he just didn't make 10 louder. The musician not understanding the question, after a moment draws attention to the knob again and says "Yes, but this one goes to 11" as if it made a difference. I have a musician friend, and hear this every time I move equipment for the band. Its actually quite funny when you see long haired hippie musicians say it. JNICHOLS spewed out: > ... I remember that. The musician points out that standard amplifiers go to > 10, but his was special because he had it custom made to go to 11. The [quoted text clipped - 4 lines] > equipment for the band. Its actually quite funny when you see long haired > hippie musicians say it. Thanks for some good details I'd forgotten surrounding the scene. It was subtle humor delivered deadpan. I can absolutely imagine real musicians saying it with totally straight faces. Now I have to go find the movie to rent; I haven't seen it in years. Signaturebp Proud Member of the Human O-Ring Society Since 2003 > 10, but his was special because he had it custom made to go to 11. The > person he is showing it to asked why he just didn't make 10 louder. The It's Nigel Tufnell showing his guitar collection to Marty, the documentary maker: NIGEL: This is a top to a, you know, what we use on stage, but it's very...very special because if you can see... MARTY: Yeah... NIGEL: ...the numbers all go to eleven. Look...right across the board. MARTY: Ahh...oh, I see.... NIGEL: Eleven...eleven...eleven.... MARTY: ...and most of these amps go up to ten.... NIGEL: Exactly. MARTY: Does that mean it's...louder? Is it any louder? NIGEL: Well, it's one louder, isn't it? It's not ten. You see, most...most blokes, you know, will be playing at ten. You're on ten here...all the way up...all the way up.... MARTY: Yeah.... NIGEL: ...all the way up. You're on ten on your guitar...where can you go from there? Where? MARTY: I don't know.... NIGEL: Nowhere. Exactly. What we do is if we need that extra...push over the cliff...you know what we do? MARTY: Put it up to eleven. NIGEL: Eleven. Exactly. One louder. MARTY: Why don't you just make ten louder and make ten be the top... number...and make that a little louder? NIGEL: ...these go to eleven. From Mary Pegg: > > 10, but his was special because he had it custom made to go to 11. The > > person he is showing it to asked why he just didn't make 10 louder. The [quoted text clipped - 41 lines] > > NIGEL: ...these go to eleven. Excellent! Thanks to these blokes, you can actually buy guitar amps with this much "power": "...like all Soldanos, the controls on this little guy go to 11." (From http://www.soldano.com/reviews/astroguitar98.htm) I'm sure that when Spinal Tap toured, they used amps that went to 11. Here's an idea... It would be way cool if the lead guitarist for Max Q would mod the amp to go to 104% (with item entry blistering leads at 109%, of course). ~ CT > Brian Gaff spewed out: > > So all that is part of an auto sequence then? [quoted text clipped - 11 lines] > 100% mark. What's now known as 94% would become 86% and 104% would become 95%. > At full power what's now known as 109% would simply become 100%. But it's not that linear. If you simply went by a 9% "shift" then what's known now as 9% (which technically the SSME can't achieve for other reasons) would then become 0%. 0% would become -9%. Which obviously makes no sense. No, you have to figure out what a 9% increase at each previous % level is. (so 1% becomes 1.9%, not 10%). And now you're redoing a LOT of things. > My guess is that the percentages are rated against the original specifications. > If continuous improvements are made that can squeeze out more performance then > the 100% mark would keep changing. That would make it difficult to compare > performance numbers over the life of the program. There are also probably too > many rules-of-thumb and years of experience behind them to change it now. That's it. > By the way, mine goes up to 11. > If you simply went by a 9% "shift" then what's known now as 9% (which > technically the SSME can't achieve for other reasons) Can you clarify that? Are you saying the SSMEs can't run at 109%, or am I just misreading it and need to take Reading Comprehension 101 again? ;) I'm just curious for more details behind that assertion if that's what you meant. Rocketdyne originally designed the SSMEs to run at 109% for full power (ie nominal power used for launch) but then later decided to back off a bit to 104% since there were concerns about immense stresses sharply reducing the lifetime (and hence, more checks, maintenances, replacement expenses, associated downtime, failures at a bad time, etc) if run at 109% more than once or twice. Subsequently, NASA designated 109% to be used only in a matter of life-and-death emergency (certain abort modes) - which would have been rare and yet not outside of the revised engine use rules. However, they have run the engines at 109% plenty of times during ground tests so they're confident that it can indeed run at that level if needed; just that it wasn't a real good idea to *ordinarily* do so. In Dr. Feynman's Appendix F to the Rogers Commission report (for the Challenger accident), he spoke briefly about blade cracking in the high pressure fuel turbopump... and there was apparently also some concern in certain quarters that running the engine at 109% rather than 104% for a nominal launch would reduce the blades' safe lifetime by a factor of 2. (I seem to recall this was also only very briefly discussed in the sequel to the 'Surely, You're Joking, Mr. Feynman' book.) There was also some talk of a newer version that would yield 109% full power and 115% abort power. Not sure what came of that. I also vaguely recall some talk of recertifying the current SSMEs to 106% for full power a while ago to better assist with the ISS building missions -- was that from the Block II upgrades? -Dan > > If you simply went by a 9% "shift" then what's known now as 9% (which > > technically the SSME can't achieve for other reasons) > > Can you clarify that? Are you saying the SSMEs can't run at 109%, or am I > just misreading it and need to take Reading Comprehension 101 again? ;) You need to take Reading Comprehension 101 again. ;-) You snipped most of what was pertinent. > I'm just curious for more details behind that assertion if that's what you > meant. The engines can not be run at 9% of rated power. Most engines can't throttle below a certain point. Below that point they basically can't sustain the ignition due to lack of pressure I believe. > Rocketdyne originally designed the SSMEs to run at 109% for full power (ie > nominal power used for launch) but then later decided to back off a bit to > 104% since there were concerns about immense stresses sharply reducing the > lifetime (and hence, more checks, maintenances, replacement expenses, > associated downtime, failures at a bad time, etc) if run at 109% more than > once or twice. No, Rocketdyne originally designed the SSMEs to run at 100% for full power, not 109%. > There was also some talk of a newer version that would yield 109% full > power and 115% abort power. Not sure what came of that. I also vaguely > recall some talk of recertifying the current SSMEs to 106% for full power a > while ago to better assist with the ISS building missions -- was that from > the Block II upgrades? Not sure. > -Dan >> In article <2Y4bb.83654$yG2.45798@twister.nyroc.rr.com>, Greg D. Moore > (Strider) <mooregr@greenms.com> wrote: [quoted text clipped - 14 lines] > throttle below a certain point. Below that point they basically can't > sustain the ignition due to lack of pressure I believe. Ahh! So you *did* mean 9% of rated power. I saw that, but thought I must've been misreading that since it's so low that it's clearly not feasible ;) Someone once said that they set 65% as the lower limit cut-off to avoid chuffing. I don't know what exactly chuffing translates to in a SSME context but I know that from an APU context, chuffing is when the APU's hydraulic pump switches on/off in a fast-paced cycle and is audible post-landing. Presumably, below 65% SSME power, not (very) usable. I haven't been able to independently confirm the above from a reputable source, so I wouldn't necessarily take that as gospel but it certainly sounds within reason (with respect to SSMEs and 65% power). (Unrelated side note: hydraulic pumps like the Airbus jets' Power Transfer Units (PTUs) are also often audible to passengers in the cabin when they do sudden pressurization of the hydraulic lines such as for brakes or opening cargo doors, and drives a motor with a very noticeable and unique sound.) As for Rocketdyne originally designing for 100% flight power, that's correct. I *meant* to say that the first revision (to the original design plans) was looking like 109% for nominal flight power but later backed off to 104%. Unfortunately, hands got ahead of brain and that qualifier 'first revision' went missing. -Dan snipetty snip... Ok, I understand all that then, a further question. The SMEs are aimed away from the stack, presumably so the thrust when used with SRBs is roughly centred in the right direction, hence the sway when they start up. But after SRB shedding, how does the remaining stack cope with the trim adjustments that surely need to be done? Brian -- Brian Gaff.... graphics are great, but the blind can't hear them Email: briang1@blueyonder.co.uk ____________________________________________________________________________ __________________________________ "Brian Gaff" wrote in message: > But after SRB shedding, how does the remaining stack cope with the trim > adjustments that surely need to be done? I don't have the engineering background to be sure about this, but I have pondered this question a couple of times. This best explanation I've come up with suggests that the SSME trim is a gradual process leading up to SRB sep, since the sep time is determined by the internal pressures of the SRB's (50 psi I think). As the SRB's begin to burn out, the resulting thrust would be declining for at least a few seconds prior to separation. For this reason, I suspect that the SSME's will already be reacting to this change before the SRB's are cut loose, resulting in a smooth transition in the thrust angle in relation to the remaining stack. Of course, I could be totally wrong about this; so I'll be watching this thread along with you to see what the experts think. Cheers SignatureSent to you by Ken at: kenwho?@sympatico.ca replace 'who?' with 'b2' for email > For this reason, I suspect that the SSME's will already be reacting to this > change before the SRB's are cut loose, resulting in a smooth transition in > the thrust angle in relation to the remaining stack. > Of course, I could be totally wrong about this; so I'll be watching this > thread along with you to see what the experts think. I read this just recently ... wish I could remember the specifics. Think about it for a second: What happens to the stack CG when the SRBs are shed? The CG will move towards the orbiter, nowhere along the Y axis, and not sure how it moves along the X axis. At the moment the SRBs are shed, the SSMEs would give a slight rotation away from the SRBs, if anything (nose "up" in local coords - due to the new stack CG), and at that moment the SSMEs would be directed to thrust through the new CG. If I run into the article again I'll make corrections if needed. The above is just sort of an off-the-cuff walkthrough. Jon > I don't have the engineering background to be sure about this, but I > have pondered this question a couple of times. This best explanation I've > come up with suggests that the SSME trim is a gradual process leading up to > SRB sep, since the sep time is determined by the internal pressures of the > SRB's (50 psi I think). 50 PSI is the number I've read also, I think from the online shuttle tech manual, Jenkins, or both. Too lazy to check at the moment. :) "Tail-off" appears to start at about six seconds before separation, when the SRB plumes expand and brighten. I haven't gotten a full explanation for this phenomenon yet but I've noticed that both shuttle PAO and the crew call the start of "tail-off" at this point. As the SRB's begin to burn out, the resulting > thrust would be declining for at least a few seconds prior to separation. > For this reason, I suspect that the SSME's will already be reacting to this > change before the SRB's are cut loose, resulting in a smooth transition in > the thrust angle in relation to the remaining stack. There was a really interesting post here late last year that described what (probably) happens to the stack CG throughout ascent. In any case, it sounds like the SSME's are constantly working to keep everything pointed correctly. http://tinyurl.com/o6g1 The charts in the CAIB report show this too... Come to think of it, maybe they can provide an answer to your question. They were used to illustrate early-ascent anomalies but they might show SSME motion all the way up through SRB sep. > Of course, I could be totally wrong about this; so I'll be watching this > thread along with you to see what the experts think. Me too. :) From Brian Gaff: > So all that is part of an auto sequence then? Yes. There is an ability to override the auto sequence and use the manual throttle lever. But that would make for one dicey ascent. > Two supplementary questions. > [quoted text clipped - 3 lines] > a marketing claim, Our watts in our amplifier sound louder than normal ones > etc... PLT Nigel Tufnel: We've got 3 at 104. CDR Derek Smalls: Roger, umm, but shouldn't the max be 100? PLT: Well 104 is better, isn't it?! > and, I understand that some engines can be designed to work better in the > upper atmosphere, and some to work better at the ground and up to a point. > > Given that is so, how are the SMEs designed? Is it just a compromise, or do > mixtures or pressures change as height is gained to keep things efficient? The expansion is physically set by the geometry of the engine bell. The infamous aerospike of the X-33 was supposed to achieve gains similar to a "variable geometry" engine bell since the outer expansion was not physically limited. As a wag, I'd guess that the SSME bells are optimized for some intermediate altitude near max-q. ~ CT From Herb Schaltegger: > > When the shuttle lifts off just before the SRB's kick-in the ME's are > > at full power, which is 100% with the throttles wide open, and yet a > > little over 2 minutes after lift off the SSME's go to throttle up and > > the ME's increase thrust to 106%. If the ME's are already wide open > > at 100%, where does the extra 6% come from? > Actually, no. At launch, SSMEs are typically at 104% of rated power. <snip> Here is one graph to show that launch occurs with 100% throttles followed by throttle up to 104% at ~T+05sec: http://history.nasa.gov/rogersrep/v5p1235.htm (Also notice the two stage throttle bucket.) ~ CT From Christopher (mcai9ce2@hotmail.com): > When the shuttle lifts off just before the SRB's kick-in the ME's are > at full power, which is 100% with the throttles wide open, and yet a > little over 2 minutes after lift off the SSME's go to throttle up and > the ME's increase thrust to 106%. If the ME's are already wide open > at 100%, where does the extra 6% come from? I find this topic interesting also... Obviously, 100% isn't the maximum capability for the SSMEs. There are even abort modes where 109% throttles get commanded. So even 106% isn't the "widest" of the "wide open". While the "%" terminology certainly leads to confusion, one way to make sense of it is that as some point in time for the program, 100% was the maximum rated thrust. While this thrust level has remained as the benchmark, over time the performance has increased. One analogy would be to think of a car engine's redline. If when the car is first released, say that a redline of 5000rpm gets tagged as 100%. Then through subsequent improvements over the years, it is determined that the engine can be reliably pushed to a redline of 5300rpm. The new redline gets termed 106%. What puzzles me is the reason why engines only run at 100% at lift off instead of blasting off at 106%. The best guess I could think of is that the deflector shields and other pad constraints have never been rated to anything above this. ~ CT Here are the SSME numbers: The original 1970 spec for the SSME required the engine to produce 550,000 pounds of thrust at sea level and 632,000 pounds at high altitude (the so-called *vacuum thrust* level). These were the nominal or 100% thrust levels. In addition, NASA required that the SSME be capable of throttling from 50% to 109% of nominal thrust. This version of the SSME was to be used with the fully-reusable two-stage version of the shuttle, the one that had the large, winged flyback booster. In 1971 for budgetary reasons, NASA changed from this humongous two-stage shuttle configuration to the smaller partially-reusable 1-1/2 stage shuttle that we've come to know and love. The new SSME 100% thrust levels were now set at 385,000 pounds at sea level and 470,000 pounds at high altitude. The engine throttling spec remained the same. NASA and Rocketdyne worked like hell from 1975 thru 1980 to get the SSME flight certified and in March 1980 the SSME received its *rated power level* (RPL) certification that qualified it to operate at 100% thrust for a few flights before major maintenance and overhaul. It was not until April 1984 that the SSME was certified at *full rated power* (FPL), i.e. at the 109% thrust level. According to NASA's certification guidelines, the SSME was now certified for 20 flights at 104% rated thrust or for 8 flights at 109% rated thrust before major maintenance. The Air Force requirement for 32,000 pound payloads from VAFP to 100 nm circular polar orbits drove NASA to certify the SSMEs for 109% thrust. After the Challenger disaster, this Air Force requirement disappeared and the 109% thrust level became extra margin to be used in emergencies. Later Ray Schmitt > When the shuttle lifts off just before the SRB's kick-in the ME's are > at full power, which is 100% with the throttles wide open, and yet a [quoted text clipped - 7 lines] > the wind - not with it." > Winston Churchill |
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