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=scale= Nike Smoke Fin Dimensions?

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rocketguy101

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I am getting ready to build my Centuri Nike Smoke kit (its been open since the 70s, I just never got around to it). I plan to use built-up fins, rather than carve out solid balsa fins. I like to model these parts in SolidWorks to create the flat patterns.

I was looking through my data and cannot find the dimensions of the bevels on the fins. I have G.H. Stine's drawing from the Oct 69 Model Rocketry, and ROTW. I scanned the MRm drawing and was measuring the fins when I found there appears to be a difference between the forward bevel (towards the nose) and the aft bevel (about 0.25 inch). I always thought they were symmetrical. The ROTW drawing looks like they are symmetrical. The Centuri instructions give the same dimension FWD and AFT.

I started searching the web (and the forums) and can't find any data about this. I have seen several reports that the Stine drawing has some errors (nobody points out what they are!). On the scaleroc group there is a thread about Nike Smoke dimensions, and people offering some drawings, but the date on the thread is 1999. I saw on scaleroc mention of some data on NARTS, but I can't find mention of it on the NARTS site.

Does anybody here have good dimensional drawings of the Nike Smoke fins? Could you post scans?

As I model this w/ the CAD, I find another issue. If I use the dimensions scaled from the Stine drawing, the fin bevels have a warp (or twist) in them (I produced it by drawing the root profile and the tip profile, then lofting the profiles along the leading edge of the fin). The head-on profile looks like the 2D drawing, but I am not sure the real fin has a twist. If I cut the fin with a flat plane, then the head on view does not match the 2D drawing.

I have posted some images showing the comparisons. Those dark images are a SolidWorks plot of the surface curvature--if it is solid black, the surface is flat. The colored area show the twist.

So, does anybody know if the fin has that twist? I am guessing the fin is hollow, with sheet metal skins, which could be forced into a twist, I guess. If the fins are solid, I would guess the surface is milled flat and the 2D views are off.

The pics Micromeister posted http://www.rocketryforum.com/showthread.php?t=2076 aren't quite clear enough to see the bevel details.

I know, I am probably over thinking all this, but now curiosity is killing me!

NikeFin_Stine.jpg


NikeFin_Sweep_Top.jpg


NikeFin_Sweep.jpg


NikeFin_Sweep_Curvature.jpg


NikeFin_Sweep_Tip.jpg
 
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rocketguy101

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Here are pics of the plane cuts (why the limit of 5 attachments??)

NikeFin_PlaneCut2_Top.jpg


NikeFin_PlaneCut2.jpg


NikeFin_PlaneCut2_Curvature.jpg


NikeFin_PlaneCut2_Tip.jpg
 

rocketguy101

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I tried a second method to sweep the profiles, by projecting them on a plane perpendicular to the leading edge. The warp goes away but now the head-on profile looks different to the 2D drawings...

NikeFin_Sweep2_Setup.jpg


NikeFin_Sweep2_Top.jpg


NikeFin_Sweep2.jpg


NikeFin_Sweep2_Curvature.jpg


NikeFin_Sweep2_Tip.jpg
 
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MarkII

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Dave,

I don't know what you mean by a "twist" in the Stine drawing. In your first attachment, you emphasize (with red arrows) the line around the airframe which almost (but not quite) lines up with the inner edge of the top fin bevel. If you look at the N-S fin area straight on, as it is shown in the drawing, and draw a line across from the inner bevel edge of one fin to the inner bevel edge of the fin opposite it, that "bevel line" is not even with the line shown on the airframe. Is this what you mean? That line on the airframe indicates some sort of seam and it has nothing to do with the "bevel line" as far as I can tell. I believe that it is simple coincidence that it is nearly, but not quite, level with the inner edge of the top fin bevel.

Also note that the fins, in addition to having the double bevel, do taper in thickness from the root edge out to the tip edge. Because of this, the slope of each bevel effectively gets lower (more shallow) from root to tip. The angle of each bevel only appears to change, while in reality they all remain constant from root to tip. Compounding the issue is the fact that the fin also tapers in width from root to tip. (IOW, the tip edge is shorter in length than the root edge.)

I have never seen a Nike Smoke in real life, but I can imagine that if you were to look at a fin tip edge on, the profile would look pretty strange. I can also imagine what a headache it would be to try to represent this clearly in an engineering drawing of the fin profile, and then trying to reproduce that view in a drawing meant for publication in a periodical that had the resolution quality of ModRoc in the early '70s. (And reproducing that fin shape in a piece that you are sanding by hand is daunting to say the least!)

I view the drawing in the Stine article as a good general reference, but not as a true engineering drawing of the rocket. In order to determine the actual shape, I use the dimensions that are shown. When I reproduce the fin shape using the actual dimensions that are provided, I don't see any twist or warp.

I have seen several drawings and photos of the Nike Smoke, and they don't all match in every dimension and shape. And representations of the Nike Smoke's booster, which was repurposed from the Nike Ajax, doesn't always completely match drawings of the booster portion of the Nike Ajax itself, or in drawings of other Nike booster applications, such as the Nike Cajun. In particular, the fin shapes don't always match. Instead of the double bevel airfoil shown for the Nike Smoke fins in the Stine drawing, some other representations of the booster show the fins with a true diamond-shaped airfoil. The diameter of the booster isn't always exactly the same from one drawing to the next, either. Having never seem a real Nike booster, let alone several of them in different applications, I can't say for sure why this is so. Given the exacting detail-orientation (or -fixation) of scale modelers, I rather doubt that different modelers, looking a different rounds of the same booster, would arrive at such different shapes for the fin airfoils, for instance, by error. My guess is that some small details of the booster's design varied slightly during the entire production run of the several thousand boosters that were made. So any scale technical drawing made by a modeler after examining a Nike booster in a museum could only be trusted to be completely accurate for that one round. While they would or should all agree on all of the major dimensions and details, some variation in a few small details is not surprising. This doesn't necessarily mean that one drawing is less accurate than another.

I do have a question of my own. That seam in the motor case down near the fins that I mentioned earlier troubles me a bit. I have seen at least one photo that shows it with a row of flush-mounted screw heads running along beneath it. The Stine drawing is one of only two that I have seen that indicated that there was even a seam at all, but it doesn't show the screws, not does he mention the screws or the seam in the accompanying article. The other, a drawing in ROTW, is more detailed and shows two seams that are perpendicular to each other, with a row of screw heads running alongside one side of each one, but no mention of them is made. The drawing gives the impression that there is a shroud wrapped around the aft end of the motor case, which is held in place by a row of screws along its upper edge and by another row of screws along the seam where the two ends of the shroud come together after being wrapped around the motor case. Other drawings that I have seen do not indicate this seam at all, and it is never mentioned in any text. Since I don't happen to have a Nike booster nearby to examine, I have to ask: what's up with this feature, anyway? Does the booster really have a pair of seams there? If it does, are there a row of screw heads along each one? Details, please, details.

MarkII
 

rocketguy101

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Mark
The twist I am referring to is not in Stine's drawing, but in the geometry I get when I constrain the solid model to the dimensions I measured off the 2D drawing. I have seen this twist before in other diamond airfoils on a compound taper (yes I am aware of the multiple tapers). It basically come from the fact that a line and point in space define a plane. If I force the geometry to 4 points that aren't coplanar, then the surface has to warp. I am just saying the dimensions I got off the Stine drawing aren't coplanar.

And yes, there are multitudes of fin planforms for Nike boosters. Perusing ROTW you can see the Nike-Asp and Nike-Cajun are similar to the N-S but the dimensions are different. The Nike-Tomahawk and Nike-Apache have the full diamond x-section.

From my perusing yesterday on scaleroc, there was some discussion about the seam you see in the drawings--I don't have the reference bookmarked, but it was a can that slid over the M-5 Nike motor nozzle that the fins attached to. You have to join the scaleroc Yahoo group, but there is a ton of information there, and some real gems in the photo and files section.
 

MarkII

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Thanks, Dave. Actually, I am a member of scaleroc, but I haven't been keeping up with it (or most of the other Yahoo groups that I subscribe to) in the past year or so. In the past, I have often found it difficult to find specific answers to scale questions by searching through the archives (kind of like another forum that I belong to...), but the membership is extremely knowledgeable and they readily respond to questions. So I'll look there some more.

Sorry if I went over too much stuff that you already knew. My post (like many others that I have made to questions) was just a brain dump, basically listing everything I know about the subject. You probably already knew much more than I do about these rockets.

I don't how the twist occurs. (I sort of understand your explanation, but the last time I studied geometry was also back in the Paleozoic Era.) When I study the drawings and try to figure out how to recreate a scale version of the fin and what the end result will look like in three dimensions, I never see it with an inherent warp, based on my understanding of the shape. I have never used a CAD program to model parts, but I have done some very simple drawings in GIMP. Back in jr. high, I took some drafting classes (which I enjoyed very much), but that was over 40 years ago. I might be able to recall about 15-20% of the stuff that I learned, if I'm lucky.

I have been doing a bit of research into the Nike Smoke myself, because I am preparing to build a 1:8 scale version of it. (The biggest hold-up is reproducing its most significant feature, the nose cone.) The GHS Astroscale article in Model Rocketry, the article in ROTW and the Centuri and FSI kit plans have been my only sources for scale info so far. There is no chance that I will get to see an actual round anytime soon (too far away), so I have to rely on published scale information.

MarkII
 
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stantonjtroy

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I reciently restored my L1 cert rocket, a 3" Nike smoke, and this time incorperated a correct cross section for the fins. Originally I just had a flat "slab" fin. I encountered the warp of whitch you speak right off. You have addressed the reasons already so I won't restate. Sufice to say, I shaped the fins as drawn, with the warp. Oddly, it's not visually noticeable on the finished fin. It's there, just not redily apperent. Not sure what point I'm trying to make. Sorry. Think I need to go to bed.:blush:
 

MarkII

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I'm still confused about this warp issue. When I visualize shaping the fin, I don't see it. When I look at the images that Dave posted, I don't see it in them, either. What am I missing here?

MarkII
 

rocketguy101

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Mark in post #1, look at the "curvature" image (the mostly black one). Towards the tip, see the bluish areas? That is where the flat bevel has to twist from the root to meet up with the tip. If you look at the top image in that post, you can see a shadow on the bottom bevel near the tip area.

edit: look at the tip view and you can see the angle of the bevel at the root is different than the angle at the tip, so the "flat" bevel surface has to twist as it runs from the root to tip. Compare that view to the other methods.

I saw this phenomena when I built a fin for a semi-scale Terrier-Sandhawk using balsa ribs and cardstock skin. I thought the warp was due to glue shrinkage, but when I went back to look at the CAD file, the fin surface had a similar warp when I moved my light source to the right spot. My oldest son is a CAD/3dsMax guru--I pointed this out him and he was the one who said "Duh, Dad, a line and point create a flat plane, you are trying to fit a plane to four points that aren't coplanar".
 
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MarkII

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Could the warp that you see in the solid fin be an optical illusion?

How about this: instead of thinking of the fin as a slab that has but cut down to a shape, sanded to a profile and then beveled, imagine it as a hollow box. Now open up the box and unfold it so that the entire pattern lies flat. Can you figure out how to make a hollow box (or a built-up fin) with that shape? Imagine the tip edge as the center shape between the two halves, with the top skin and the bottom skin attached to it. (The skins being the two broad flat sides of the fin.) Does this work? Can you design a shape on a flat piece of material which, when folded, replicates this shape?

The NARTS scale pack for the Sandia Sandhawk, written by Matt Steele and Craig Beyers, uses this approach for that rocket's fins. The Sandhawk's fins have some of the same issues (tapering in thickness from root to tip, symmetrical knife-edge bevel along the leading edge). In their diagram, they unfold the "box" at the leading edge, but it's the same principle. I really couldn't get a handle on the Sandhawk's fin shape until I saw that diagram. Perhaps the same approach might work here, too.

MarkII
 

Meatball 1

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My guess is the "twist" in the bevel is quite possibly there on the real thing, although you would probably never notice it, nor think about it, were it not for the precision of 3D CAD programs. There would be a skin over a framework; the fin would not have been milled as a solid piece--at least not any photos of Nike fins I've seen.

As for the shroud on the fin area, there were several types; most of the ones I've seen data for (or in person) have a single seam. Some have screws and some are welded. I've never seen the Nike-Smoke fin/shroud assembly up close. It appears similar to the Nike Fins on the 2nd & 3rd stages of the Argo D-4 Javelin, but without the rows of obvious socket-head cap screws holding the fin on (I have seen photos of one Javelin round that uses fins without those screws--might be the same type as the Nike-Smoke, but no good close-ups that properly reveal the construction of that particular assembly).
 

georgegassaway

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I think I get what RocketGuy means.

And it is pretty simple.

Look at the angle of the leading edge of his 3-D image at the ROOT. It is a sharp angle.

Look at the angle at the leading edge at the TIP. It is a much blunter angle.

Therefore, the surfaces which join those two have to be twisted in order to match up.

Let me make it sort of (?) simple to understand. Let’s pretend only for visualization that the leading edge angle at the root is 10 degrees, and the leading edge angle at the tip is 30 degrees.

I am talking about both sides. Now, break it down to just one side, where at the root instead of 10 degrees it would be 5 degrees per side, and at the tip rather than 30 degrees it would be 15 degrees per side. So, there is a 10 degree “twist” in the surface of the leading edge on that side, from root ( 5 degrees) to tip (15 degrees). Remember again, I am only using those degrees as examples to understand the twist, not the REAL angles.

So, two possibilities here. If the data is correct then the fin was built like that and the surface is indeed twisted.

Two, the data might be incorrect. So that either the tip thickness should be THINNER, or that the flat area in the middle is supposed to have a much smaller chord, to allow for the leading and trailing edges to have the same FLAT angle for root to tip, without being twisted.

I’m not into Nike history enough to know which is correct, but it has to be one of the two.

And I do see what you mean about the twist, based on the information given.

During a visit to the Udvar-Hazy Center (During NARAM-50), I got some pics of a Nike-Cajun on display there. Attached are some pics. It seems to use the same fin type.

Ever hear of the phrase “Be careful what you wish for, you might get it”? :)

You will see some details you probably did not want to know about (nothing to do with airfoiling).

I will say that based on what I see in the pics, the tip thickness of the real thing seems to be thinner than the tip thickness of your drawing. Indeed, visually, it LOOKS like the angle at the tip is about the same as the angle at the root. So it may be that it is not twisted at all, that the angles are the same, but the tip thickness data is way wrong. I even did a quick copy and paste of part of the photo into a drawing program and overlaid some lines, and the lines seem to match, or VERY close to a match. Like, in the area of 4.5 to 5.0 degrees per side for both root and tip. I swear I wrote the example of the degrees before even looking up the photos....

BTW - I am also on Scaleroc, but I won’t be repeating this over there since there is little point of posting the same thing twice. Lot easier to do it with photos over here anyway.

- George Gassaway

IMG_0747.JPG


IMG_0752.jpg


IMG_0753.jpg


IMG_0754.jpg
 
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rocketguy101

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Could the warp that you see in the solid fin be an optical illusion?

How about this: instead of thinking of the fin as a slab that has but cut down to a shape, sanded to a profile and then beveled, imagine it as a hollow box. Now open up the box and unfold it so that the entire pattern lies flat. Can you figure out how to make a hollow box (or a built-up fin) with that shape? Imagine the tip edge as the center shape between the two halves, with the top skin and the bottom skin attached to it. (The skins being the two broad flat sides of the fin.) Does this work? Can you design a shape on a flat piece of material which, when folded, replicates this shape?

The NARTS scale pack for the Sandia Sandhawk, written by Matt Steele and Craig Beyers, uses this approach for that rocket's fins. The Sandhawk's fins have some of the same issues (tapering in thickness from root to tip, symmetrical knife-edge bevel along the leading edge). In their diagram, they unfold the "box" at the leading edge, but it's the same principle. I really couldn't get a handle on the Sandhawk's fin shape until I saw that diagram. Perhaps the same approach might work here, too.

MarkII
Its not an optical illusion, because the profile at the root and tip have different bevel angles in my first sweep method (post #1). I started with this method because the 2D drawing gives me the starting points for the profile at the root and tip.

In the NARTs data you referenced, Figure 3 shows the fin has a constant angle for the bevel shown in section A-A. That is how I created the second sweep in post #3, by placing it on a sweep plane that is perpendicular to the leading edge of the fin.

Modeling the Sandhawk was where I discovered my first sweeping method introduced the warp. After seeing the NARTs diagram, I realized I couldn't just use the end profiles, I had to use the true cross section rotated like their A-A section. Here is a figure of the Sandhawk modeled from the NARTs data.

I bet a factory drawing of the Nike Smoke fin will show a similar section as the Sandhawk drawing, rather than a cross profile parallel to the root as shown in the Stine drawings.

SandhawkFin_SldMdl_Iso.jpg
 

rocketguy101

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During a visit to the Udvar-Hazy Center (During NARAM-50, I got some pics of a Nike-Cajun on display there. Attached are some pics. It seems to use the same fin type.

Ever hear of the phrase “Be careful what you wish for, you might get it”? :)

You will see some details you probably did not want to know about (nothing to do with airfoiling).

I will say that based on what I see in the pics, the tip thickness of the real thing seems to be thinner than the tip thickness of your drawing. Indeed, visually, it LOOKS like the angle at the tip is about the same as the angle at the root. So it may be that it is not twisted at all, that the angles are the same, but the tip thickness data is way wrong. I even did a quick copy and paste of part of the photo into a drawing program and overlaid some lines, and the lines seem to match, or VERY close to a match. Like, in the area of 4.5 to 5.0 degrees per side for both root and tip. I swear I wrote the example of the degrees before even looking up the photos....

BTW - I am also on Scaleroc, but I won’t be repeating this over there since there is little point of posting the same thing twice. Lot easier to do it with photos over here anyway.

- George Gassaway
Thanks George! Those photos pretty much confirm my thought above (guess I was writing while you were posting)...the angle is constant along the edge of the fin.
 

MarkII

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Mark in post #1, look at the "curvature" image (the mostly black one). Towards the tip, see the bluish areas? That is where the flat bevel has to twist from the root to meet up with the tip. If you look at the top image in that post, you can see a shadow on the bottom bevel near the tip area.

edit: look at the tip view and you can see the angle of the bevel at the root is different than the angle at the tip, so the "flat" bevel surface has to twist as it runs from the root to tip. Compare that view to the other methods.

I saw this phenomena when I built a fin for a semi-scale Terrier-Sandhawk using balsa ribs and cardstock skin. I thought the warp was due to glue shrinkage, but when I went back to look at the CAD file, the fin surface had a similar warp when I moved my light source to the right spot. My oldest son is a CAD/3dsMax guru--I pointed this out him and he was the one who said "Duh, Dad, a line and point create a flat plane, you are trying to fit a plane to four points that aren't coplanar".
I'm still not quite sure that I get it. I don't know what that last sentence means and I don't know what the term "coplanar" means. (I'm not kidding. I'm a simple bumpkin from the sticks; that explanation is all Greek to me.) Are you talking about the slope of the bevel changing angle as it goes from root to tip?

Imagine this construction:

Start with two identically-sized rectangles and join them together along one short edge, then fold them together at that joint so that one rectangle lies directly on top of the other. Now spread them open so that they are separated at the end opposite the joint by some small amount.

Now take two long, narrow isosceles triangles that are also identically-sized and joint them together along one of their long sides. Then fold one over onto the other, but keep them separated by a small amount at the other long side.

Place the tip of that pair of triangles at one of the corners of the joint between the two rectangles. Spread the open side of the two triangles so that they straddle the long edges of the two rectangles along one side. Keep the joined tips of the two triangles in contact with the two corners of the rectangles on that side. Bond the pair of triangles to the rectangles where they touch them.

Now trim the base edge of the two joined triangles so that they are even with the spread-open end of the two joined rectangles.

Then trim away the portions of the triangles that extend above and below the long sides of the two rectangles that they are straddling.

Create two more isosceles triangles that are the same size as the previous pair, and join them together in the same manner as the first pair. Place their joined tip at the other corner of the joint between the two rectangles, have them straddle the other long edges of the pair of rectangles, one above and one below just as was done with the first pair. Just as with the first pair, join them to the rectangles where they touch. Trim the bases so that they are even with the open end of the pair of rectangles, and trim off any portion of the triangles that extend beyond the two long edges of the rectangles. Stand that assembly up on its open end.

You end up with a polyhedron with two sloping rectangular sides that meet at the top and spread out at the base. On each side are two triangular sides that meet at the upper corner of the two rectangles. A long side of each triangle joins a rectangle along one of its vertical sides, and then the two triangles that face each other are joined together along their other long sides. This is symmetrical on both vertical sides of the two rectangles.

Now take that polyhedron and clip off the peak and a short section just below it. Make the cut parallel to the base of the polyhedron. Fill in the top of the clipped polyhedron so that it is solid and flat across the cut edge. That's your fin. Note that every side is flat; none are twisted or bent. All joined edges are straight along their entire length; none exhibit any curve.

MarkII
 

Meatball 1

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I've never seen the Nike-Smoke fin/shroud assembly up close.
Okay, then... I take back what I said. I have seen that vehicle in George's photos up close, but I forgot. :blush: And I agree that it looks to be the same fin configuration as the Nike-Smoke.

And just to benefit those who may not be on scaleroc, I responded with a link to a brochure with Nike Fin dimensions:

A few years back I scanned Bob Biedron's copy of the Nike fins brochure. I was planning on putting a link to it whenever I get around to making a Nike scale data page, but here's a link to the 12-page pdf file. (about 1.7 MB).

http://www.meatballrocketry.com/nike_data/nikefins.pdf

Happy [very early] fourth day of Christmas. :hohoho:
 

MarkII

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I just constructed a very crude mock-up of what I described in my last post. I only created one bevel though. Everything went together just as I described. No bending or twisting anywhere. You have to start with a box in which the sides all meet along one edge. To create it, extend each of the fin's "faces" out well beyond the tip edge. Eventually they all intersect. You end up with a sort of 7-sided pyramid. (6 sides plus the base.) The two flat rectangular sections on the sides eventually come together. Each face of each bevel eventually tapers out to a point while remaining flat all the way, and the face assumes the shape of a long triangle. The peaks of each pair of these triangles meet at the outer corner of the extended rectangle where the rectangles meet. Set that polygon upright on its base (the root edge of the fin). Do you see how it forms a pyramid with a straight line across the peak? Now chop off the peak along with part of the pyramid below it. Make the cut parallel to the base (root edge). That clipped pyramid is your Nike fin. I partially constructed such a pyramid within the past hour as I described above in order to prove that at no point do the bevel sides ever warp or bend in order to remain in contact with the flat rectangular section and with the corresponding bevel on the other side. If you think of the 3-dimensional fin as a pyramid with the top clipped off, then this all makes sense.

MarkII
 

rocketguy101

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@Mark--here are some links about planes and coplanar --- basically you are constructing exactly what I had to do in post #3 to make sure everything is planar...sorry I have confused the issue with my first construct--I am mainly looking for some detail dimensions of the N-S fin which leads to...

@ Meatball 1: woo hoo! you da man! Is that you (drumn4j) that posted the link on scaleroc?

note to all ez2cdave on scaleroc contacted me and is sending some drawing info, his computer is down so he faxed it to my work, I am off all week, it will be after New Year's before I get to see it--just wanted to give him public props

Thanks everybody!!
 
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rocketguy101

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After measuring several photos, it appears both the 2 sq ft fin planform listed in the ARC catalog, and the fins dimensioned in G. Harry Stine's scale data were used on flight vehicles.

I created templates to make built-up fins for the "Modified Diamond" fins listed in the ARC catalog (pg 11), plus a 2.5 sq ft fin based on the ARC planform. I also made a set using Stine's dimensions.

I modified my Centuri NS by adding a piece of cardstock to simulate the fin shroud (pg 3 and 5 of the ARC catalog). It ends up creating that little gap between the fin and the body, as seen in the various NS photos.

The last photo shows the 2 sq ft fins compared to the 2.5 sq ft I used on the model.

Thanks everyone for your help and input!

View attachment NikeFinFlatPatterns_ARC.PDF

View attachment Fin_NikeSmokeStine_Sweep.PDF

DSC_2650MR.jpg


DSC_2653MR.jpg


DSC_2654MR.jpg
 

MarkII

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I'm glad that it all worked out, but I am still in the dark about what the original issue was. The edge on view of the tip looks exactly the same to me in both sets of drawings you provided early on. I don't understand where you saw a twist? This is very relevant to me because I, too, have a scratch-build of an N-S coming up in my queue. I seem to be missing what everyone else is seeing, and that has me rather irked.

I don't see how all of the planes of the fin don't stay flat from the root all the way out. I have read and reread the thread all the way through several times, and I still don't get what the issue was.

Mark K.
 

Meatball 1

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...I don't see how all of the planes of the fin don't stay flat from the root all the way out. I have read and reread the thread all the way through several times, and I still don't get what the issue was.

Mark K.
It's actually fairly simple (though difficult to visualize). A plane is defined by 3 points. The fin facets are trapezoids, which are made up of four points. All four points do not lie in the same plane, which leads to the twist when trying to loft between the fin root and the tip.

The only way to prevent the twist--assuming the fin root and fin tip are on parallel planes (which most rocket fins are)-- is for the line segments that making up the facets to be parallel. I'll see about adding a graphic for you in a secondary post. It's easier to show visually than to describe it. :)
 
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Meatball 1

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Here's a quickie graphic that should help visualize the effect of the twist. Obviously it's much more extreme than what would occur in a real Nike fin, where the effect would be subtler.

FinPlanes.jpg
 

MarkII

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It's actually fairly simple (though difficult to visualize). A plane is defined by 3 points. The fin facets are trapezoids, which are made up of four points. All four points do not lie in the same plane, which leads to the twist when trying to loft between the fin root and the tip.
OK, I understand a bent or curved/warped plane. The way I understand and visualize the fins, each of the six facets remains flat from the root to the tip edge. That's the shape that I have always taken them to be. I never thought that they could be seen any other way. Please, I'm not being argumentative here; I'm just trying to see in the diagrams what Dave, George and you - all of who I highly respect, were seeing. If you all could see it, then it must have been real; you all have much greater knowledge about solid geometry than I do. What piece of the picture am I missing? Which planes are warped, and how would I detect it?

Mark K.
 

TheAviator

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OK, I understand a bent or curved/warped plane. The way I understand and visualize the fins, each of the six facets remains flat from the root to the tip edge. That's the shape that I have always taken them to be. I never thought that they could be seen any other way. Please, I'm not being argumentative here; I'm just trying to see in the diagrams what Dave, George and you - all of who I highly respect, were seeing. If you all could see it, then it must have been real; you all have much greater knowledge about solid geometry than I do. What piece of the picture am I missing? Which planes are warped, and how would I detect it?

Mark K.
What he is saying is that the way the 2-D scale drawing was drawn, the fin would end up with the twist in the leading edge. As was shown, this is incorrect, and the leading edge should be planar.
 

Meatball 1

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OK, I understand a bent or curved/warped plane. The way I understand and visualize the fins, each of the six facets remains flat from the root to the tip edge. That's the shape that I have always taken them to be. I never thought that they could be seen any other way....What piece of the picture am I missing? Which planes are warped, and how would I detect it?

Mark K.
I understand the need to wrap your brain around it (I didn't catch it at first, myself).

Maybe the modified examples will help you out. I sliced up an even more extreme version in the FinPlanes3.gif file to help you see the shape a little better.

In real life the warp isn't all that important. It wouldn't surprise me if the real fin had a bit of warp. You'd never notice it when forming a fin from plastic or cardstock anyway. But geometrically it is there if the edges that form the fin root aren't parallel to the ones that form the fin tip.

If you still don't see it after this graphic, I have one more visual up my sleeve that I can try. :)

FinPlanes2.jpg


FinPlanes3.gif
 

powderburner

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I just came across this thread, and aside from the mountain of EXCELLENT data (thankyou very much to all those who have shared this info), there are a ton of very nicely, clearly, and generally well-drawn images of 3-D fin shapes that add a LOT to understanding this subject. Everybody involved has done a fantastic job! Thankyou for taking the time to do this
 

MarkII

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I understand the need to wrap your brain around it (I didn't catch it at first, myself).

Maybe the modified examples will help you out. I sliced up an even more extreme version in the FinPlanes3.gif file to help you see the shape a little better.

In real life the warp isn't all that important. It wouldn't surprise me if the real fin had a bit of warp. You'd never notice it when forming a fin from plastic or cardstock anyway. But geometrically it is there if the edges that form the fin root aren't parallel to the ones that form the fin tip.

If you still don't see it after this graphic, I have one more visual up my sleeve that I can try. :)
Well, OK, if you say so. Now where does this occur on the Nike fin? I'm guessing that it must be really, really subtle.

Mark K.
 

rocketguy101

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Well, OK, if you say so. Now where does this occur on the Nike fin? I'm guessing that it must be really, really subtle.

Mark K.
See my first post. That bluish color in the 3rd image is the warp on the front panels. I didn't mean for that issue to become the dominant subject of this thread! I was looking for more accurate dimensions of the fin, which Josh (meatball 1) provided.

There is an interesting drawing of the M5 Nike motor on the yahoo scaleroc site (files section) that provides more details of the forward area of the rocket, and combined with George's lovely photos, gives lots of juicy detail. I see the weld bead now that is mentioned in the M5 drawing.
 

Meatball 1

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Well, OK, if you say so. Now where does this occur on the Nike fin? I'm guessing that it must be really, really subtle.

Mark K.
It is really subtle. I haven't tried, but it's probably so subtle that you wouldn't be able to tell without using a CAD program to draw the lines that make up the root and tip of the fin--and even then, you probably wouldn't notice unless you put the edge lines right on top of each other to see if they're perfectly parallel (or if you use a really expensive 3D program like Solidworks that can highlight subtle features or defects--fancy programs that I do not have access to).

Where would the warp occur on the Nike fin? The extreme examples I gave are all pointing in the same direction as the most normal-looking (blue) example, so use that blue shape as a baseline (since it looks the most like a Nike Fin) and compare with the corresponding twisted areas on the squished shapes.
 
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