I've tried to do some calculations to determine the liftoff mass of the new shuttle-derived heavy-lift launch vehicle that will get NASA out of low-earth-orbit, by comparing it to Saturn V and the Shuttle.<br /><br />According to wikipedia.org, the mass and thrust of the Saturn V and the STS are as follows (with their thrust-to-weight ratio calculated for those values):<br /><br />Saturn V: Thrust: 7,500,000 lbf<br /> Weight: 6,500,000 lbf<br /> T/W: 1.15<br /><br />Shuttle: Thrust: 7,820,000 lbf<br /> Weight 4,500,000 lbf<br /> T/W: 1.74<br /><br /><br />Of the shuttle's thrust each SSME gives 400,000 lbf, and each SRB gives 3,300,000 lbf thrust.<br /><br />The new heavy-lift launch vehicle will have two 5-segment SRB's and 5 SSME's. That extra segment (and thus extra fuel) on the SRB's could be used for increased thrust over the same 2-minute-6-seconds time, or they could have the same thrust for a longer burn - or something in between.<br /><br />If we assume that they have the same thrust as the current STS SRB's, total thrust of the new launch vehicle would be:<br /><br />(2 x 3,300,000 lbf) + (5 x 400,000 lbf) = 8,600,000 lbf<br /><br />If we instead assume that the 5-segment boosters have 5/4 as much thrust as the 4-segment boosters, each booster gets about 4,125,000 so we get:<br /><br />(2 x 4,125,000 lbf) + (5 x 400,000 lbf) = 10,250,000 lbf<br /><br />So the total thrust of the heavy-lift launch vehicle should be in that range somewhere.<br /><br />It's probable that the thrust-to-weight ratio of the new vehicle will be higher than the Saturn V, but it probably will not jump off the pad quite as rapidly as the shuttle. If we assume the highest of the T/W-ratios with the "version" with the least power in the calculations above and vice versa (ie "worst-case" scenario with the least amount of power combined with a requirement for high T/W compared to "best case" scenario with lots of extra power and a very low T/W), I guess that total launch mass <div class="Discussion_UserSignature"> <p><font color="#ff9900">----------------------------------</font></p><p><font color="#ff9900">My minds have many opinions</font></p> </div>
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Best to consider 104% the maximum. IIRC, 109% is only for emergency use, like RTLS or TAL.<br /><br /><br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>
The new unmanned launcher will be 9.1 million lbs of thrust. Not quite as much as the N-1, but we'll just pretend that never existed ;-)<br /><br />The 5-segment SRBs give 3.6 million a piece at liftoff.<br /><br />By the way, your numbers are, for once, right on the money, thank you! I'm tired of misquotes giving it as 6.6 (which is just the SRBs) 
<br /><br />I'm also curious about the thrust to weight ratio, expecially on the CEV stick launcher. It looks like it would give the crew a lot of Gs. But without knowing its weight I can't figure it out. <div class="Discussion_UserSignature"> <p>-Ben</p> </div>
<br /><br />I'm also curious about the thrust to weight ratio, expecially on the CEV stick launcher. It looks like it would give the crew a lot of Gs. But without knowing its weight I can't figure it out. <div class="Discussion_UserSignature"> <p>-Ben</p> </div>Michael Griffin specifically stated during his presentation that the heavy lifter's SRBs would burn for two minutes, seven seconds -- just like always. <img src="/images/icons/smile.gif" /><br /><br />Of course -- although it seems unlikely -- he may simply have been recalling the four-segment values.<br /> <div class="Discussion_UserSignature"> <font color="#0000ff"><strong>Just tell the truth and let the chips fall...</strong></font> </div>
I've got a question about max q.<br /><br />How much drag do you really get?<br /><br />How does the drag force compare gravity?<br /><br />In other words, there are two major losses of converting thrust into altitude and horizontal velocity during a launch. Gravity and drag.<br /><br />If the thrust to weight ratio at launch time is higher, then the acceleration is higher. This means that the time to orbit is lower and the gravity losses are lower.<br /><br />But if the acceleration is higher, than the vehicle will travel faster during it's initial ascent through the thickest part of the atmosphere. My assumption is that that makes for higher drag forces.<br /><br />Does it make sense reducing the liftoff thrust to weight ratio a little to reduce drag even if it increases gravity losses?
How much would the total mass to orbit increase by going to a 6 segment SRB? Or would that actually decrease mass to orbit?
Yes, Griffin is correct on the 5-segment burn.<br /><br />Increasing the length of an SRB increases the thrust, while increasing its diameter increases burn time.<br /><br />The 5-segment booster has the same burn time, but gives more thrust. If the Shuttle had 5-seg boostrers, then it would be higher and farther away (and faster) when they burned out compared to the 4. <div class="Discussion_UserSignature"> <p>-Ben</p> </div>
"If the thrust to weight ratio at launch time is higher, then the acceleration is higher. This means that the time to orbit is lower and the gravity losses are lower.<br /><br />But if the acceleration is higher, than the vehicle will travel faster during it's initial ascent through the thickest part of the atmosphere. My assumption is that that makes for higher drag forces. "<br /><br /><br />It seems logical that the most efficient way of launching a vehicle would be as much power as possible for the first seconds of flight (overcoming gravity - you definetely don't want to hover over the pad longer than absolutely necessary), then throttling down around max q, because there's no sense in forcing the vehicle too hard through that region of flight (and besides that would mean that you'd have to build it stronger and heavier), and then throttle back up to full power when gravity again is the dominant force.<br /><br />Makes sense why they do exactly that on the Shuttle. <br /><br />In the days of Saturn V, it didn't have throttleable engines, so maybe a lower thrust-to-weight ratio was more efficient on that vehicle.<br /><br />In any case, I would think that if you have a vehicle with a T/W-ratio of, say, 1.75, you would be able to lift a heavier payload into orbit by adding payload and extra fuel. It wouldn't be as efficient (kilograms to orbit per kilogram of fuel), but fuel efficiency isn't a launch vehicle's strongest point anyway, and it might be more economically efficient to have a slightly underpowered rocket (as opposed to adding more SSME's or using 4 boosters instead of 2).<br /><br />So it might well happen that the new heavy-lifter has a T/W ratio closer to that of Saturn V than that of the Shuttle, I guess. Which, given the thrust as a more or less known factor, means a large liftoff mass.<br /><br /><br />I'm also wondering how the payload capability compares to Saturn V. Griffin said the new booster will have a LEO capacity of 125 metric tonnes (and it seems fro <div class="Discussion_UserSignature"> <p><font color="#ff9900">----------------------------------</font></p><p><font color="#ff9900">My minds have many opinions</font></p> </div>
<br /><br />In the days of Saturn V, it didn't have throttleable engines, so maybe a lower thrust-to-weight ratio was more efficient on that vehicle.<br /><br />In any case, I would think that if you have a vehicle with a T/W-ratio of, say, 1.75, you would be able to lift a heavier payload into orbit by adding payload and extra fuel. It wouldn't be as efficient (kilograms to orbit per kilogram of fuel), but fuel efficiency isn't a launch vehicle's strongest point anyway, and it might be more economically efficient to have a slightly underpowered rocket (as opposed to adding more SSME's or using 4 boosters instead of 2).<br /><br />So it might well happen that the new heavy-lifter has a T/W ratio closer to that of Saturn V than that of the Shuttle, I guess. Which, given the thrust as a more or less known factor, means a large liftoff mass.<br /><br /><br />I'm also wondering how the payload capability compares to Saturn V. Griffin said the new booster will have a LEO capacity of 125 metric tonnes (and it seems fro <div class="Discussion_UserSignature"> <p><font color="#ff9900">----------------------------------</font></p><p><font color="#ff9900">My minds have many opinions</font></p> </div><blockquote><font class="small">In reply to:</font><hr /><p>That is true if the grain port design is unchanged, which it will not be.<p><hr /></p></p></blockquote><br /><br />If the grain design is changed, how much of the trackrecord can be claimed then? Because I seem to remember that sollid feulled engines are pretty sensitive to changes in the grain design....<br /><br />Could be wrong though
<font color="yellow"><br />The max G's will be 3.0, the same as the Shuttle. <br /></font><br /><br />Will this apply to the heavy lift vehicle as well?
There's nothing fundamental limiting the number of segments; it would be determined by other design factors, most likely. <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>
I have a table here from ATK which says the "average" thrust of the SRB is 2.6 million lbs. This is the four-segment version. Is the lift-off thrust higher? Also, what is the liftoff thrust of the SSME's (at surface pressure)? I'm asking because I thought the shuttle stack had quite a bit less take-off thrust than the Saturn V.
Liftoff thrust should be the minimum thrust available.<br /><br />Rocket Engines are the most efficient when they have zero backpressure... IE. They work best in a vacuum.<br /><br />They will have the lowest thrust and efficiency at sea level.
According to the Space Shuttle News Reference Manual, the current Space Shuttle SRBs provide 3,300,000 lbs of thrust apiece at liftoff, and that they provide 71.4% of the total thrust at liftoff. The system is designed so that the thrust drops below 60,000 lbs prior to separation.<br /><br />The SSMEs can be throttled from 65% to 109%. 100% is 375,000 lbs at sea level, or 470,000 lbs in a vacuum. Engine throttling is restricted at sea level because of flow separation in the nozzle (I'm not sure exactly what that means, but perhaps someone else can explain) and while the manual didn't say what the restricted range is, it did indicate that 65% is forbidden at sea level, so this suggests that for the SSMEs at least, it's the low thrust settings that are bad at sea level. The maximum (but not allowed in nominal circumstances) thrust of 109% corresponds to a thrust of 417,300 pounds at sea level and 513,250 pounds in a vacuum.<br /><br />The SSMEs are throttled down to a value not specified in the manual at about 26 seconds, and back up at about 60 seconds. The next major event is when the SRBs abruptly throttle back due to propellant depletion and are jettisoned. Then at about 7 minutes and 40 seconds, the SSMEs are throttled back again to reduce loads on the crew. (The target is 3 gs.) 10 seconds before MECO, the SSMEs throttle down to 65% in 10% increments. That is held for 6.7 seconds and then they shut down entirely.<br /><br />That's basically all I could find at the reference manual. <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>
<font color="yellow"><br />Liftoff thrust should be the minimum thrust available.<br /><br />Rocket Engines are the most efficient when they have zero backpressure... IE. They work best in a vacuum.<br /><br />They will have the lowest thrust and efficiency at sea level.<br /></font><br /><br />This is true IF the rocket is not specially designed otherwise.<br />The reason why you do not want the minimum thrust at liftoff is that as the flight progresses, the mass of the vehicle decreases and therefore for constant thrust, the acceleration would increase (a=F/m). However the STS is limited to a maximum acceleration of 3Gs to avoid overloading the structure and more importantly the crew. Therefore it would be advantageous to have a large initial thrust, decreasing as mass decreases. There is also the issue of Qmax where you want to reduce acceleration.<br /><br />The SRBs achieve this control by having a specially designed core. Imagine a solid cylinder with a bore down the middle, so the cross-section looks like two circles. This is the "basic" shape of a solid rocket booster. The fuel burns on the inside surface, throughout the whole length of the booster. Thrust is roughly proportional to the inside surface area of the bore. As the flight progresses, the bore widens, its surface area increases, and thrust goes up.<br />However the Shuttle SRBs (and several others) are built with a different-shaped bore, which has a cross section in the shape of a multi-pointed star. This has a very high surface area so initial thrust is high which helps to get the shuttle off the pad quickly. Then as it burns, the "points" of the star are burned away, decreasing the surface area to reduce acceleration for Qmax and keep within the 3G limit.<br /><br />By carefully choosing the shape of the cross-section, the thrust-versus-time profile can be controlled to a fairly fine degree. Clever, isn't it? Of course this makes it hard to guess what the initial liftoff thrust is. I'm
Thanks SG re: the grain.<br /><br />3.3 is the max thrust it produces, 2.6 is the average. The shuttle's SRBs are at full 3.3 when it lifts off. It throttles down to about 2.5 or 2.3 for maxQ and then again (and more so) near burnout, if I remember correctly. <div class="Discussion_UserSignature"> <p>-Ben</p> </div>
Either an extendable nozzle or variable nozzle would add a fair amount of additional thrust at lower altitudes, where it is needed. The SSME, itself, would remain unchanged.<br /><br />Whether it is enough to be worth the added complication is another thing. If it was intended for commercial use I would say it would definitely be needed, but for NASA and this vision thing, it's probably not a major issue. <div class="Discussion_UserSignature"> </div>
Thanks, that answers my question. With one eye on history, this launcher should be named: "Nova"! Yee-HAA! I want to be there when they launch this one! <br />9 MILLION POUNDS OF THRUST!!
Halleluah, Brother. I'm going to be there for the first launch of that sucker, that's for sure.<br />According to my calculations, it'll have 9.287 million pounds thrust...<br /><br />Uuggh, Uuggh!! (Tim the Toolman Taylor grunts).<br /><br /> <div class="Discussion_UserSignature"> <p> </p><p>One Percent of Federal Funding For Space: America <strong><em><u>CAN</u></em></strong> Afford it!! LEO is a <strong><em>Prison</em></strong> -- It's time for a <em><strong>JAILBREAK</strong></em>!!</p> </div>
My favorite names for it are: HERCULES, POSEIDON, NOVA and NEPTUNE. <div class="Discussion_UserSignature"> <p> </p><p>One Percent of Federal Funding For Space: America <strong><em><u>CAN</u></em></strong> Afford it!! LEO is a <strong><em>Prison</em></strong> -- It's time for a <em><strong>JAILBREAK</strong></em>!!</p> </div>
Hercules was known for his great strength. That would seem to make sense for a heavy-lifter.<br /><br />Of course, the titan Atlas carried the world on his shoulders. But both "Titan" and "Atlas" are already taken, eh? <img src="/images/icons/wink.gif" /><br /> <div class="Discussion_UserSignature"> <font color="#0000ff"><strong>Just tell the truth and let the chips fall...</strong></font> </div>
<blockquote><font class="small">In reply to:</font><hr /><p>..... Engine throttling is restricted at sea level because of flow separation in the nozzle (I'm not sure exactly what that means, but perhaps someone else can explain)..... <p><hr /></p></p></blockquote><br /><br />Excellent explanation by the Shuttle_Guy.<br /><br />Here's more info and background on nozzle design. It's good for SRB and/or liquid rocket engines. Some would say it's the bible of nozzle design. <img src="/images/icons/smile.gif" /><br /><br />http://www.rocketdynetech.com/articles/nozzledesign.htm <div class="Discussion_UserSignature"> </div>
I was thinking more along the lines of the new heavy lifter. <div class="Discussion_UserSignature"> </div>
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