3 inch 29mm Scratch Build

Dane Ronnow

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I got the idea for this build about a month ago when I was looking at the 3" School Rocket from BMS, and saw that Bill had an 11" polystyrene nose cone. I'm partial to PS because it takes adhesives and paint much better than polypropylene, and PS cones always have a true shape on the long axis. PP cones frequently don't.

Anyway, I wanted to take that nose cone and build a 3" rocket along the lines of the Lance Delta, one of my favorite designs from North Coast Rocketry, with those long, ultra swept fins, and cool canards.

The first thing I did is rough in a basic design in OpenRocket, adding my recovery gear, an ejection baffle, an altimeter and a motor retainer. Once I had a preliminary design in place, I ordered parts from BMS—nose cone, body and motor tubes, couplers, bulkheads, and centering rings. I picked up two sheets of .125" basswood from Hobby Lobby, a 29mm Aero Pack retainer from Apogee Components, and eye bolts from Lowes.

IMG_1601 (2).jpg

With parts in hand, I went back into OR and entered exact weights and measures of all components. I added a mass object to account for adhesives and paint, then set that at 2 oz. and placed it about a third of the way up from the aft end of the rocket.

Once I had the total weight of the rocket, I started tweaking fin shape and canard placement until I got the stability factor where I wanted it with a G80 loaded—a shade over 1 caliber.

3 inch OR design view.JPG

Then I started running sims with the G80, and five other single-use motors, adjusting the weight of the rocket until I came up with three motors—G74-6, G78-7 and G80-7—that gave me deployment velocities well below 10 mph.

3 inch OR sim view.JPG

With the design pretty much nailed down, I ran a fin flutter analysis using a trick I learned from John Cipollla for calculating flutter with swept fins—measuring semispan from the center of the root chord, to the center of the tip chord. I set the fin thickness to .158, which is the thickness of .125" wood, papered and painted.

Flutter analysis a.jpg Flutter analysis b.JPG

Flutter threshold is 386 fps—263 mph. That's 28 mph over VMAX on a G80, which is not the 25 percent margin Cipolla recommends, but close enough for me.

Finally, a calculation of ejection pressure to make sure the parachute ejects. The body tube ID is 2.93", length of area to pressurize is 13.5", and the BP charge is .7 grams, which gives me 15 psi, with a force of 101 lbs on the bulkhead.

Ejection charge pressure.JPG
 

Dane Ronnow

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With all that out of the way, and before any assembly takes place, I wanted to get some idea of how much work might be involved in just getting parts to fit right. So, a quick test fit of everything, beginning with the CRs and motor tube.

IMG_1602.jpg IMG_1603.jpg IMG_1604.jpg IMG_1605.jpg

As you can see from the scuffing on the lower end of the tube, where burn residue from the laser cut rubbed off, the rings fit tight, but not so tight they're hard to slide. One pass with sandpaper and they'll be perfect. They're also very uniform, and concentric.

Here's the CR to BT fit:

IMG_1610.jpg

And the bulkhead to coupler fit:

IMG_1611.jpg

The only problem with any of the components was the tight fit of the coupler in the BT, too tight for reliable deployment. I fixed that by peeling away the top layer of paper (got a clean separation, thankfully). Then I measured and marked 3" from the end and coated that part of the tube with thin CA. I let that dry for a half hour, then sanded it with #320 paper. Now the coupler slides in and out of the BT beautifully.

IMG_1615.jpg IMG_1617.jpg IMG_1618.jpg

I'll have to do that to the other coupler as well, since it will house the ejection baffle. But other than that, this build is looking like it will be fairly smooth. I've got to say, these are some of the nicest fitting components I've worked with. Very pleased with BMS parts.

The one thing I haven't done before is cut fins out of basswood. Big fins. I'm thinking a straightedge and a heavy hobby knife, or even a box cutting blade, might do the trick. I'll find out soon enough.

I'm going to take one last, close look at the design before I cut the fins, just to make sure they're good all the way around. Then I'll start the build process. Meanwhile, thanks for checking this out.

Here's the .ork file if anyone is interested:
 

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Dane Ronnow

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I made a change in the design—moving the fin's root trailing edge back so it's flush with the end of the body tube.

I've been trying to decide between two versions of this build—one, the version pictured in post #1, with fins 1.5" above the bottom of the body tube, and two, a version with the tube end and root trailing edge flush.

I liked the extended tube, but from a visual standpoint only. It did nothing other than trim 85 feet off the simmed apogee because of the weight I had to add up front to get the stability above a 1-caliber minimum. But it looked cool. And it kind of enhanced the canards, because it gave the fins a bit of canard look.

Anyway, I hate adding weight to a rocket just to get stability, so I finally decided to move the fins back and be happy with it. It's lighter, it flies better, and it gets rid of a CR that was used to close off the recessed body end.

Here's a before-and-after:

Fin Placement G80.jpg

Now, back to printing a template, and cutting the fins. I'll be back in a day or two.
 
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SolarYellow

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I made a change in the design—moving the fin's root trailing edge back so it's flush with the end of the body tube.

I've been trying to decide between two versions of this build—one, the version pictured in post #1, with fins 1.5" above the bottom of the body tube, and two, a version with the tube end and root trailing edge flush.

I liked the extended tube, but from a visual standpoint only. It did nothing other than trim 85 feet off the simmed apogee because of the weight I had to add up front to get the stability above a 1-caliber minimum. But it looked cool. And it kind of enhanced the canards, because it gave the fins a bit of canard look.

Anyway, I hate adding weight to a rocket just to get stability, so I finally decided to move the fins back and be happy with it. It's lighter, it flies better, and it gets rid of a CR that was used to close off the recessed body end.

Here's a before-and-after:

View attachment 540236

Now, back to printing a template, and cutting the fins. I'll be back in a day or two.
Moving the canards back or reducing their size should also improve stability.
 

waltr

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Make the booster section longer to move the Ebay to nose out farther, moves CG forward.
Maybe it will move CG enough to move the fins forward like you had them originally.
 

Dane Ronnow

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So, @David_Stack just took a load off my mind. Regarding the fin cutting, Dave let me know that James at Rocketry Works does custom fin cutting. I contacted James for a quote, and in the process of comparing the cost of cutting my basswood sheets (which I would have to send to him), and cutting the same design from birch ply, I found out that the actual weight of the fins in plywood is just a tad more than what OpenRocket was giving me for basswood. So, in addition to getting the fins cut for a great price, I get plywood instead of bass. Perfect for this build, since I'm launching from a dry lake bed—very unforgiving on fins upon landing.

Thank you, Dave. You saved me a lot of work in cutting fins, and also in possible repairs down the line.
 

SolarYellow

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Lengthening the body tube is one of the big factors I find reduces apogee, playing with the little rockets I usually play with.

I downloaded your .ork and played with it a little. With the extreme sweep of your fins, you're running into the Hi-Flier problem, where there ends up being too much fin area too far forward. I changed the fins and canards so the leading edge is swept 70 degrees from perpendicular to the airframe and stability jumped to 1.61. (There's a research paper out there somewhere that suggests that angle is optimum for drag both subsonic and supersonic.) Still looks nice and zoomy. I pulled the fins back so there's 19mm of airframe stickout, stability went up to again. Pulling the front of the MMT and forward MMT CR back to the new front of the fins reduced stability a little. I zeroed out the forward weight. It left stability factor at 1.19 and added 95 ft to the apogee. So I slid the fins back forward to 38mm from the end of the tube and lengthened the MMT/fwd CR a little. Leaves you at 1.04 and 1583 ft.

If I was building it, I'd probably slightly reduce the canards. It's not in the model I've attached, but I tried scaling them to 80 percent, and stability shot up to 1.21. Did a little more playing around with fins and got good results.

I do way too much of this little tweek OCD in OR. Hope it helps you.
 

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Dane Ronnow

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@SolarYellow --

Oh, sure, NOW you tell me. Just kidding. I had fins laser cut yesterday, so I'm locked into the placement of CRs.

But your post got me thinking. Instead of moving the fins forward, why not extend the aft end of the BT? @waltr suggested something along these lines (post #12), but when I did that, all internal components stayed where they were relative to the bottom of the lower BT, thus having a net effect of pushing the CG farther aft, and a loss of stability. So it didn't click in my mind that by changing the relative position of all those internals from the bottom of the parent component to the top, then adding 1.5" to the lower BT, I'd get the extended BT at the bottom, but nothing else moves. The CP location is exactly the same, while the CG shifted forward 1/8". The stability with the G80 improved slightly, from 1.02 to 1.06.

I also added 1.5" to the MT to reach the back end, and a fourth CR to support it, but I only had to add 1 oz. of weight up front to compensate, instead of the 3 oz. the other design needed. The other three CRs stay put, so there won't be a problem with the fins I had cut.

I did lose 70 feet in the G80 simulation. But frankly, I'm not as concerned with apogee as I am with deployment velocity, which dropped from 8.5 mph to 6. Ground impact velocity also dropped from 9.6 mph to 8.7, which helps when you're landing on a dry lake bed.

Anyway, thanks for your post. Without it, I never would have gone back to @waltr's suggestion, and looked at it from a different perspective.

Here's the new rear end with the G80 loaded:

Fin Placement G80 b.jpg

And the revised .ork file:
 

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SolarYellow

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Oh, sure, NOW you tell me. Just kidding. I had fins laser cut yesterday, so I'm locked into the placement of CRs.

But your post got me thinking. Instead of moving the fins forward, why not extend the aft end of the BT? @waltr suggested something along these lines (post #12), but when I did that, all internal components stayed where they were relative to the bottom of the lower BT, thus having a net effect of pushing the CG farther aft, and a loss of stability. So it didn't click in my mind that by changing the relative position of all those internals from the bottom of the parent component to the top, then adding 1.5" to the lower BT, I'd get the extended BT at the bottom, but nothing else moves. The CP location is exactly the same, and the CG shifted rearward less than 1/8". The stability with the G80 improved slightly, from 1.02 to 1.06.
Changing the position reference for body tube components from bottom to top is something I had to do when I shortened the body tube a little after shortening the fins with the ~5 degree steeper angle on the LE. There was a big gap between the forward CR/MMT and the bottom of the baffle, so I closed things up to keep the gap about how it had been.

If you wanted to change the LE angle of the fins and canards to a 70 degree sweep, that should be easy enough. It's just one straight line. You could even stack them and do all four at the same time, then sand them so they are perfectly matched, just like we all used to do in the bad old days of die-crushed kit parts.
 

Dane Ronnow

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The fins and canards arrived in yesterday's mail. A great product from James at Rocketry Works. At his prices, I won't be cutting plywood parts by hand from now on. Super pleased with these!

IMG_1619.jpg

I guess I've run out of things to do before I start the build. First up, the motor tube and CR assembly. I'll be back in a day or so.
 

Dane Ronnow

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I cut the motor tube yesterday and marked it for CRs, then cleaned up both ends with light sandpaper and swabbed them inside with CA. Then I epoxied the forward CR in place with J-B Weld. I let that sit for a day, then this afternoon, I epoxied the mid CR in place.

IMG_1621.jpg IMG_1622.jpg IMG_1628.jpg IMG_1631.jpg

The CRs fit the tube beautifully. They're a tight fit, but with the tube's glassiene coating, they slide nicely with practically no wobble. And they stay where they stop. On the mid CR, I was able to fillet the bead of epoxy right after I got the ring placed, instead of having to wait until it cured. That was nice. I'll set the assembly aside for a day, then do the fillets on the bottom sides of the CRs.

After that, I cut the corners off the fin tabs.

IMG_1632.jpg IMG_1634.jpg

After cutting the lower BT to length, I marked it for fin slots and rail buttons, then cut the slots.

IMG_1635.jpg

That's it for now. I'll be back after I install the motor mount.
 

Dane Ronnow

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I finished the CR fillets yesterday (aft side of the forward and mid rings), then let them cure overnight so the assembly would be ready to install in the body tube today. Before installing, I painted the face of the forward ring with several coats of high temp Rustoleum for a little protection against ejection charge heat.

IMG_1638.jpg IMG_1639.jpg

My standard method for installing the motor tube assembly is to apply a bead of epoxy with a 1/4" dowel about an inch behind where the forward CR will come to rest, slide the motor tube halfway in, apply a second bead an inch behind where the mid CR will end up, then slide the tube the rest of the way in.

But I've never had a forward slot to content with. So I decided the best way to keep the epoxy from dripping out of those slots while I'm applying the rear bead, was to cover the forward slots, and two inches of the aft slots, with masking tape. That worked nicely.

IMG_1641.jpg IMG_1645.jpg IMG_1646.jpg IMG_1649.jpg IMG_1651.jpg

Then I test fitted the fins.

IMG_1652.jpg IMG_1654.jpg

Before attaching the fins, I'm going to paper them with full-sheet Avery label paper. (I need more than .125" thickness to get a good fin-flutter threshhold.) And I need to cut a fin guide out of foam core. So, it'll be a couple of days.
 

T-Rex

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This is reminiscent of the old NCR Lance Beta. I like it a lot and may have to use your design if I ever get to scratch build again. (Too many kits in the pile already)
 

Dane Ronnow

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May I ask how you cut your fin slots. They are terrific. Thanks

1. I make my own fin marking template.
2. I use a mechanical pencil with .5mm lead for thin, precise lines.
3. I use an aluminum angle to draw lines on the tube.
4. After lines for the slots are drawn, I frame them with blue masking tape for a cutting guide.
5. I cut the slots with a #2 X-Acto blade, making several shallow cuts, rather than a single, deep cut. Then finish with a #11.

If that answers your question, you're good to go. If not, read on. I'll explain the steps above in detail. (This info spans two posts because of the number of images.)

1 & 2. Making a fin marking template:

Note: A template made from letter-sized copy paper is long enough to mark a 3" dia. tube. A larger tube will require a longer sheet of paper. Just make sure that one edge is absolutely straight. And keep track of which edge is the straight one.

Start with a sheet of 8.5" x 11" paper. Heavy is better; I use 28 lb. Cut a 2" wide strip lengthwise. Mark the uncut edge. This edge is the reference for all the marks you'll be making.

IMG_1689.jpg

Now we need to measure the exact circumference of the tube. Wrap the strip around the tube with the straight edge up, pulling it tight as you align the top edge of the overlap with the top edge underneath. Make sure the edges are even. Then put a mark on the overlap where it crosses the edge underneath. Be precise. This is your circumference mark. It is also the mark for the first fin.

IMG_1691.jpg IMG_1692.jpg

Next, lay the strip flat and measure it from the circumference mark to the opposite end. Divide that measurement by the number of fins. This is your fin spacing. Now mark the straight edge of the paper with a short, straight line where each fin will be. (Don't make a second mark for the other side of the slot. We'll take care of that in a minute.)

IMG_1693.jpg IMG_1694.jpg

That is your fin marking template.

From the bottom of the tube, measure and mark where the trailing edge of the fin tab will be.

IMG_1690.jpg

Now wrap the strip around the tube again, pulling it tight as you align the top edges. Tape the loose end of the strip, being careful not to change the alignment of the strip. Then slide the strip up to the tab trailing edge mark you just made, and place a piece of tape at the bottom edge of the strip to secure it to the tube. You don't want the paper moving while you're marking the tube.

(Tip: I use Scotch Magic tape because it doesn't distort like masking tape can. And before I place a piece on the tube, I press the tape to my pants or shirt to pick up a little lint. This goes a long way to preventing the tape from pulling up the top layer of glasseine when you remove it.)

IMG_1695.jpg IMG_1696.jpg

Using the fin spacing marks on the strip of paper for reference, make a vertical mark on the tube where each fin will be, and a horizontal mark to indicate the bottom of the cut you'll be making.

IMG_1697.jpg IMG_1698.jpg

Repeat those two marks for each fin.
 

Dane Ronnow

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[Cutting fin slots cont'd]

3. Using aluminum angle to draw lines on the tube:

Aluminum angle ensures that your lines are parallel to the body tube, with zero cant. But you need to avoid squeezing the tube while you're holding the angle in place, otherwise the line will be slightly canted.

Place the angle on the tube with the edge right at the fin spacing mark. This means allowing for the width of the line you're going to draw. Start your line at the mark, then check the placement. If the line is a little off from the mark, adjust the placement of the angle. When the line and the mark are aligned, draw the rest of the line upward, past the point where the leading edge of the fin tab will be by about an inch.

IMG_1699.jpg IMG_1700.jpg

Rather than simply measuring .125" (for 1/8" wood) to mark the other side of the slot, I use the actual fin tab to measure width. (1/8" wood is not always 1/8".)

I place the tab just inside the line, then draw a short line on the opposite side of the tab, right next to the wood. Lightly sanding the tube where the tab is going to sit makes it easier to hold the fin still while you mark the other side.

IMG_1702.jpg IMG_1703.jpg

Then place the aluminum angle on the new mark, and draw that line the same length as the first line. Then measure from the bottom slot mark, and make a second mark at the top of the line where the leading edge of the tab will be.

IMG_1705.jpg IMG_1706.jpg

4. I use blue masking tape as a cutting guide, laying a strip along the line so most of the line is covered, with just the barest amount visible along the tape. When we measured the width of the cut with a fin tab, the lines were on the outside of the tab. So we want to cut along the inside of the lines.

IMG_1707.jpg

5. I start my cut with a #2 X-Acto blade, with the tip of the blade against the blue tape, using very little pressure. It's easier to keep the blade straight if the cut is shallow. With subsequent cuts, also shallow, with light pressure, the blade will follow the first cut. Hold the knife handle at about a 45 degree angle to the tube.

I make three or four passes with the #2 blade, then finish the cut with the lighter (and narrower) #11 blade, at about a 60 degree angle.

When that cut is complete, I flip the tube around (I can see the posistion of the blade against the tape better if the tape is on the left side of the cut. Go figure.), then cut the other side of the slot. When both sides are cut, I finish by cutting the ends of the slot.

Repeat for the rest of the fins.

When peeling up masking tape from a body tube—even when the tape has lint on the adhesive—I pull the tape back on itself, slowly, rather the pulling it up. This helps keep the glasseine finish from tearing. And I start the peel by lifting an outside corner, so the tape is peeling toward the cut edge of the slot, rather than away from it. This helps avoid tearing the paper at the edge of the slot. Also, pay close attention when crossing a spiral that meets a slot edge, so as not to start peeling a layer.

The tube I used for this instructable is slated for another project, so I didn't actually cut the slots. But here's a pic from earlier in the build, showing the slots I cut using the exact same method. The slots are straight, even, with very little cleanup needed.

IMG_1637.jpg

Anyway, that's it. Sorry for the length of the post, but when I get in teaching mode, I get into the details too. I hope this helps you, or anybody else who may read it.
 

Dane Ronnow

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This is reminiscent of the old NCR Lance Beta.

This build was inspired by NCR's 2.6" Lance Delta. I love those big, highly swept fins and canards. I took a few liberties with the original design, tweaking the size and shape of the fins and canards, and the placement of the canards. I've also got the root chord trailing edge 1.5" up from the bottom of the body tube instead of flush with the bottom.
 

waltr

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Yes, very nice description of how to do fin slots.

I do the same with one exception:
I lay the angle on the bench with the upright side towards me. Then lay the BT in the angle to mark and then use the edge of the ange as a guide for the X-acto knife. Always get straight cuts parallel to the BT that way.
 

Dane Ronnow

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I lay the angle on the bench with the upright side towards me. Then lay the BT in the angle to mark and then use the edge of the ange as a guide for the X-acto knife.

I tried that same method on my 2.6" scratch build, but I couldn't keep the tube from moving in the aluminum as I cut, without squeezing the tube, which changed the shape slightly. Drawing a line with a pencil was no problem, so I went with that.
 
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