Launch Question

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juliemac

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In a video showing the shuttle a few seconds after liftoff, the craft seems to move at an angle instead of straight up. Seems to drift toward the tank.<br />Optical illusion or the real movement?
 
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vulture2

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There's no drift. The Shuttle pitches over about 12 degrees from vertical starting a few seconds after launch.
 
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juliemac

Guest
Ok, then the movement is what I saw in the first of Sally Rides launches. Its a close up of the crew cabin with the launch structure behind it. The tipping of the shuttle would then make it look like drift.<br />Thanks!
 
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rfoshaug

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There is a real movement toward the direction of the tank after liftoff.<br /><br />Because of the asymmetrical design of the shuttle, the three Space Shuttle Main Engines don't point straight down. Their thrust is vectored towards the tank.<br /><br />This causes a very real translation sideways ("down" toward the tank relative to the orbiter) as the shuttle moves off the pad. <br /><br />Your observation is totally correct and real. <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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rfoshaug

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BTW, this (just like the "twang" that happens between SSME ignition and liftoff) is something that is quite easily visible if you know about it and remember to look for it.<br /><br />Look at this video - the first few seconds after liftoff (before tower clear) - 28-31 seconds into the video:<br />http://youtube.com/watch?v=2Ok3-_K3XUU<br /><br /> <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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juliemac

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Man what a blast (no pun in tended). Thats the video.<br />Does it use the surface of the tank as part of the flight surface or is it just due to the shifted CG?<br />If so then as fuel leaves the tank, it must shift back toward the center line right?
 
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edkyle98

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'Says at this link that the SSME null positions are not straight up.<br /><br />http://spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/prop/thvector.html<br /><br />"The installed null position for the left and right main engines is 10 degrees up from the X axis in a negative Z direction and 3 degrees 30 minutes outboard from an engine centerline parallel to the X axis. The center engine's installed null position is 16 degrees above the X axis for pitch and on the X axis for yaw. When any engine is installed in the null position, the other engines cannot collide with it."<br /><br /> - Ed Kyle
 
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bobw

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I am pretty sure that the null position is not a flight position. When the engines start they move in their mounts and the null position prevents them from banging together. After the engines start they are swiveled to point straight back.<br /> <div class="Discussion_UserSignature"> </div>
 
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edkyle98

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But it says here that the engines can only be gimbaled plus/minus 10.5 degrees in pitch/yaw. The previous link said that the upper/center engine null position was 16 degrees from the x-axis. Doesn't that mean that the center engine can't point straight up the x-axis?<br /><br />http://spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/prop/overview.html<br /><br />"Each engine can be gimbaled plus or minus 10.5 degrees in the yaw axis and plus or minus 10.5 degrees in the pitch axis for thrust vector control by hydraulically powered gimbal actuators."<br /><br />It also seems apparent from numerous launch images that the SSMEs are vectored slightly off-vertical, at least at liftoff.<br /><br />http://mediaarchive.ksc.nasa.gov/detail.cfm?mediaid=6881<br /><br /> - Ed Kyle
 
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edkyle98

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O.K.. So there while this initial gimballing from start to ascent is underway, there must be a momentary horizontal force component applied that causes the vehicle to "slip" sideways slightly, away from the orbiter-side of the stack, for just a moment. <br /><br /> - Ed Kyle
 
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rfoshaug

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<blockquote><font class="small">In reply to:</font><hr /><p>"Because of the asymmetrical design of the shuttle, the three Space Shuttle Main Engines don't point straight down. Their thrust is vectored towards the tank. "<br /><br />Actually they do point straight up. The thrust line is vertical. The twang is from the fact that the Shuttle hold down bolts are on the SRBs. The entire stack rotates with the fixed point being the SRB skirts. Once the SRBs ignite their 6,000,000 plus pounds of thrust and the SSMEs total of about 1,000,000 pounds of thrust have their thrust vectors gimbaled to keep the vehicle C.G. going straight up until the pitch over starts. <p><hr /></p></p></blockquote><br /><br />First of all, the SSMEs do not point straight up, as you can clearly see from the SSME plume in the attached image.<br /><br />Secondly, what then causes the horizontal movement of the shuttle stack after liftoff as seen in the YouTube video from my previous post?<br /><br />If the combined thrust from SSMEs + SRBs was perfectly vertical and pointed through the center of mass, the vehicle would go straight up off the pad. But as you can see in the YouTube video as well as every other launch it does not go straight up. It translates quite a bit toward the tank (north) every time.<br /><br />The most important thing is that the engines' total thrust vector must point through the center of mass (center of gravity), or the vehicle would start rotating. Due to the offset position of the SSMEs, the center of thrust is not in the center of the tank (in the same plane as the boosters) but some distance toward the orbiter. At the same time, the weight of the orbiter causes the center of mass to be shifted toward the orbiter as well.<br /><br />Since the SSME's have more thrust than the weight of the orbiter, the center of thrust is further toward the orbiter than the center of mass (the mass of the orbiter is a smaller fraction of the total vehicle mass than the thrust of the SSMEs is as a fraction of tota <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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rfoshaug

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To further illustrate what I mean, take a look at this image (it took me only 5 minutes to make, so it's not perfect, but it shows the principle).<br /><br />If there was no orbiter, the center of mass and center of thrust would both be along the centerline of the tank. The thrust would be vectored straight up and the vehicle would lift off straight up just like any symmetrical rocket.<br /><br />But when we add the orbiter, the center of mass is shifted toward the orbiter. The center of thrust is at the same time shifted toward the SSMEs.<br /><br />According to Wikipedia:<br />The 3 SSMEs generate a total thrust of 5.4 MN<br />Total vehicle liftoff thrust is about 30,3 MN<br /><br />The thrust from the SSMEs amount to about 17% of the total vehicle thrust.<br /><br />The orbiter has a mass of about 109 metric tonnes (Endeavour according to Wikipedia).<br /><br />Total shuttle stack mass at liftoff is about 2,000 metric tonnes.<br /><br />The orbiter mass is about 5% of the total vehicle mass.<br /><br />This means that the addition of the orbiter with SSMEs shifts the total center of thrust more than the total center of mass, causing the thrust vector (line between center of thrust and center of mass) to shift a bit away from the orbiter, causing the vehicle to translate a bit off the pad.<br /><br />In my illustration, I have not done any calculations to find out exactly where the center of mass and center of thrust are. It is just for illustration purposes so that you can see what I mean.<br /><br />If I'm not mistaken, this is also the reason why the shuttle flies heads-down, as it reduces the angle of attack that would otherwise have been. <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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rfoshaug

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<blockquote><font class="small">In reply to:</font><hr /><p>Why are the launch wind constraints lower from the southern sectors of direction? <p><hr /></p></p></blockquote><br /><br />Because north is the direction the shuttle will translate anyways, so margins are greater? Just a guess. <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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propforce

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<blockquote><font class="small">In reply to:</font><hr /><p>First of all, the SSMEs do not point straight up, as you can clearly see from the SSME plume in the attached image. ........<br /><br />Disclaimer: I am not a rocket scientist, and I cannot prove these claims. But nobody is going to make me believe that the shuttle translates due to wind or that it doesn't translate at all. <p><hr /></p></p></blockquote><br /><br />Allow me to clarify. <img src="/images/icons/smile.gif" /><br /><br />Imagine your automobile. Even though the front wheels are not aligned perfectly straight, you can still compensate it's direction (of where the car is going) with the use of steering.<br /><br />It's the same with the shuttle. <br /><br />It goes where it needs to go via steering, e.g., thrust vector control, as well as the orbiter wing flaps along the way. <div class="Discussion_UserSignature"> </div>
 
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MeteorWayne

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Are the aerodynamic surfaces of the shuttle used during ascent.<br /><br />That would not seem to be a good idea to me, it's stressful enough.<br /><br />With the ability to vector the enormous amount of thrust available, it would seem to me to be unnecessay and dangerous in several ways to move the aero sufaces from a fixed position.. <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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rfoshaug

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<blockquote><font class="small">In reply to:</font><hr /><p>In that case I will not try to convince you.<p><hr /></p></p></blockquote><br /><br />Then let me try to convince <i>you</i>. <img src="/images/icons/wink.gif" /> I asked the same question on page 13 of this thread at NasaSpaceflight.com (in the lower part of page 13. It also continues a bit on page 14) and the responses there were that the movement is indeed both real and not caused by wind or anything else than the positioning and vectoring of the SSMEs. <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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trailrider

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S.G.: "Since the vehicle C.G. is nearly on the ET centerline the the Shuttle stack actually flys with the nose of the Orbiter pitched up."<br /><br />Me: Does the resultant positive alpha (angle of attack of the wings) contribute significantly to pitch-up or translation in the direction of the orbiter's dorsal side? I would imagine if it does, this, too is taken out by the booster TVC's.<br /><br />As an aside, I think we can do without further "Navajo stack" configurations. (If anyone wants to know to what I refer, see the old cruise missile designed by North American Aviation.)<br /><br />Ad Luna! Ad Ares! Ad Astra! (And the sooner the better!) <br />
 
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usn_skwerl

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friendly bump...<br />rfoshuag, <br />think of it like this, if nothings going to convince you wind doesnt have a factor in drift of the shuttle.<br /><br />the shuttles essentially suspended in the air by the thrust in the lateral direction. now imagine a helicopter hovering in the air. you mean to tell us all that a helicopter wont drift (without compensation, same as the shuttle) with a breeze? if so, i applaud you for thinking inside the cardboard of the box.<br /><br />now, my questions about shuttle launches are these....just so i understand what ive been wondering for years. please clarify these. <img src="/images/icons/laugh.gif" /> thanks much in advance.<br /><br />1.) watching this video, i believe the announcer has his info off, as he states at 7:71 into the video that the shuttle's travelling at 19,000 mph. at about T+7:30 launch time..isnt that a little fast?.<br /><br />2.)and then at about 8 minutes into the video, you see traces of what look like plasma around the outer rim of the camera lens. <br /><br />3.)maybe about 8:10 into it, you see what looks like maybe a pulsing flame (possibly) in the back, by the tail (it may be from the nose, im not quite sure)...is that plasma from the atmosphere as well, or exhaust from the APU(s)?<br /><br />4.)at about 8:38 in the video, white plume appears from the tail..please clarify what this is.<br /><br />5.) at 8:41 video time, a sudden orage glow appears around the ET/orbiter...MECO, i assume?<br /><br />6.) 8:50 several things happen. the shuttle seems to shift, vapors and small bits of foam debris appear, the shuttle pitches down a couple degrees (for ET sep, yes?), and theres some vapor apparently around the leading edge of the left wing (over on the right side of view)<br /><br />9.) 9:00 vid time, more orange glow on left, please clarify. 9:03-9:15 white vapor vectoring around on the left wing again, what is it? i heard him say they were the RCS, but only to one side like that? <br /><br />thanks again. <br /><br />Jeph <div class="Discussion_UserSignature"> </div>
 
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jimfromnsf

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"Are the aerodynamic surfaces of the shuttle used during ascent. "<br /><br />No, they only move for load relief
 
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jimfromnsf

Guest
Don't know which video but I can still answer the questions<br /><br />1. Shuttle never goes that fast unless he said 19,000 fps vs MPH<br />2. the "plasma" is the SSME exhaust expanding with altitude<br /><br />5 MECO and RCS thrusters<br />9 yes<br />
 
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usn_skwerl

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thanks for the replies, guys. its nice to be able to add a few more answers to the grey matter filing cabinet. i wasnt sure of what i was seeing in the videos of the puffs of vapor approaching meco. this helps a lot. <img src="/images/icons/smile.gif" /><br /><br />SG..i had a feeling 19,000 mph was off, i didnt think 19,000 *fps*. i know ~17,500mph for orbit speed. <br /><br />on #9, i was referring to the left wing in reference to pilot POV, the port side. the camera is under the shuttle's right wing, unless the video is mirrored, as it sometimes can be. with the O2 feedpipe being on the right side of ET...unless im bass ackwards? <div class="Discussion_UserSignature"> </div>
 
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rfoshaug

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<blockquote><font class="small">In reply to:</font><hr /><p>think of it like this, if nothings going to convince you wind doesnt have a factor in drift of the shuttle.<br /><br />the shuttles essentially suspended in the air by the thrust in the lateral direction. now imagine a helicopter hovering in the air. you mean to tell us all that a helicopter wont drift (without compensation, same as the shuttle) with a breeze? if so, i applaud you for thinking inside the cardboard of the box. <p><hr /></p></p></blockquote><br /><br />I'm not saying that the shuttle can't be affected by wind, BUT the fact that the shuttle visibly translates north (toward its belly) at liftoff on every single launch, is not due to wind but due to the asymmetrical configuration of the engines.<br /><br />Check out this video:<br /><br />http://www.youtube.com/watch?v=wUG2jbRxYoQ<br /><br />Or any one of these:<br />http://www.youtube.com/watch?v=n7djROivZjo<br />http://www.youtube.com/watch?v=on7_7WZmdgI<br />http://www.youtube.com/watch?v=ZLOVlz-XUa8<br /><br />Or the video I posted earlier in this thread:<br />http://youtube.com/watch?v=2Ok3-_K3XUU<br /><br /><br />Watch just as the shuttle lifts off. You can see some part of the fixed service structure on the launch pad just to the right of the tank, up at the top of the tank.<br /><br />As the shuttle rises off the pad, this structure is obscured by the tank as the shuttle not only moves straight up, but translates a little bit north.<br /><br />Also notice the launch pad lightning rod (white structure on top of the FSS) becoming visible between the orbiter and the tank as it translates sideways.<br /><br />This effect is NOT due t <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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lampblack

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Isn't that the famous "twang" phenomenon? The force exerted by the main engines pushes the entire stack noticeably forward prior to liftoff. <div class="Discussion_UserSignature"> <font color="#0000ff"><strong>Just tell the truth and let the chips fall...</strong></font> </div>
 
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rfoshaug

Guest
Yes, the twang is prior to liftoff. From SSME ignition until T-0 the stack is pushed forward by the force of the main engines. It then returns more or less the vertical position again (the SRB's acting as springs). The time the twang takes to get back to near vertical was part of what determined the exact SSME ignition time prior to liftoff.<br /><br />In fact, on the Flight Readiness Firings you can see the stack "twanging" fore and aft several times.<br /><br />Check this out:<br />http://www.youtube.com/watch?v=AUHLdJsoUOE<br /><br /><br />What I'm talking about is after liftoff as the vehicle rises off the pad and translates a bit north as it does so. <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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usn_skwerl

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oh, yes, i totally agree. i think i may have misunderstood. my mistake.<br /><br />its been mentioned on this board before that the shuttle translates northward after liftoff. it can be seen from the side angle showing the gap between the ET and orbiter. it can be noticed by using the lightning rod as a reference point. its sort of like sideslip on regular aircraft. vectored thrust, in a way. <div class="Discussion_UserSignature"> </div>
 
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