13mm Snake Eye Bomb Rocket Build Thread

Aslansmonkey

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A while back I got it into my head to design a model rocket based on the MK82 Snake Eye bombs of the Vietnam era. In my research on the topic I came upon the "Mk82 High Drag Bomb Rocket" thread on this forum (https://www.rocketryforum.com/threads/mk82-high-drag-bomb-rocket.64002/) and the work Chris Attebery did on the idea. While he worked with wood, I had a 3D printer available so I used that instead, basing my designs on what I could see on photos of the actual bomb.

The first one was an 18mm motored version...which failed. The second one was also an 18mm version, which almost worked, but still failed. But that one was close enough that my third design for 18mm motors worked. That design is here: https://www.thingiverse.com/thing:4847720

For my next design, I wanted to go bigger, so I designed a version that looked very much like the original bomb using a lot of 3D printed parts including the petals (the entire petal was 3D printed with a built in taper) and designed it around 18mm motors. It failed...twice. There was never a good chance of success with this one because it came out really heavy, but I had to try. The first flight went up ok, but came down too fast and damaged the inner tube of the rocket. I rebuilt it and tried again and the second flight was...well...almost, but not quite, completely unstable. That rocket survives, but is good only for a display.

The third design, also 18mm, was submitted to Apogee rockets. I tried to minimize the 3D parts this time and used mostly purchasable parts with an eye to reducing weight and increasing the float performance. You can read all about that one here: https://www.apogeerockets.com/Peak-of-Flight/Newsletter551

I make a lot of rockets based on Pringles cans, so it was only natural I make a version for those as well. The first design was meant to simplify the construction, particularly the fins and though it worked, it came down to fast. I added webbing between the petals which helped, but I was unhappy with the design. So I made another version which can be found here: https://www.thingiverse.com/thing:4982215

But then I thought...18mm...24mm...I need to try a 13mm to "complete the set" of the smaller black powder motors, and this thread will show that build.

I will point out, that at the time of posting this thread I have completed two versions of the rocket and flown them each twice successfully, so what you will be reading here is not a thread of an experiment but rather the build instructions for a working design.

The design for the 13mm rocket can be found here: https://www.thingiverse.com/thing:5028681
 
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Aslansmonkey

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For this build you will need the following parts. Get the body tubes where you will, I just post the Apogee links because that's where I tend to order things.

BT-5 (13mm) body tube for center tube like https://www.apogeerockets.com/Build...wer_Tubes/13mm_x_18_Body_Tube_Estes_BT-5_size
BT-50 (24mm) body tube for main tube like https://www.apogeerockets.com/Build...er_Tubes/24mm_x_18_Body_Tube_Estes_BT-50_size
1/8" Launch Lug like https://www.apogeerockets.com/Building_Supplies/Launch_Lugs_Rail_Buttons/Launch_Lugs/1_8_Launch_Lug
1/2" rubber bands like https://www.amazon.com/gp/product/B003H54TY8/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1
Thin cardboard, like from a cereal box. Mine was around .5mm thick and came from the divider cardboard in a canned cat food box. If you have cats and feed them canned food, this is where I got my cardboard: https://www.amazon.com/Purina-Fancy...001G0NKV4,B06XZYHSS9,B002CJG1YQ&srpt=PET_FOOD

You should be able to make 4 or 5 of these rockets if you ordered those parts from the links above.

Print 4 Bottom arms with support (only part that needs support)
Print 4 top arms
Print 1 of everything else

The fin template and cutting guide are helpers for construction and are NOT used on the finished rocket.

Note that the Bottom and Top arm holders appear identical. The Top arm holder has a slightly larger inner diameter, however as it needs to slide freely over the 13mm tube. There is a small circular dimple on one of the upper surfaces that can be used to tell it apart. I marked that dimple with a paint pen to make it easier to see.
 

Aslansmonkey

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To start, we're going to glue the "CageSlider" to the top of the 13mm tube, but not flush with the top. You want it down a little bit. What I did is use the CateTop piece to push the slider where it needs to be, then make a mark at the bottom of the slider. Then I moved the slider out of the way, applied the glue, and pushed it back down to the mark.

02_CageSlider2.jpg 03_CageSlider3.jpg 01_CageSlider1.jpg
 

Aslansmonkey

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While this is drying, you can cut the main 24mm body tube into three sections. The CuttingGuide part is an excellent guide for this as you can run an xacto knife along the flat side to make the cut. You will probably need to temporarily take this guide in place while you make the cut.

The three tube lengths you will need are 105mm, 85mm and 15mm.

04_BodyTube1.jpg 05_BodyTube2.jpg
 

Aslansmonkey

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When the Cage slider you glued to the 13mm tube is dry, you can complete the cage assembly by sliding the Cage part over the bottom of the 13mm tube and gluing it to the CageTop.

NOTE: DO NOT GLUE THESE PARTS TO THE 13mm tube. They need to slide freely up and down on the top of that 13mm tube. These parts basically retain the inner 13mm tube in the rocket and restrict it's movement.

In these photos you will also note that I drilled through the 13mm tube where the holes are. These drill holes are to release the exhaust gasses from the ejection charge, so don't forget to do that as well.

06_CageAssembly1.jpg
07_CageAssembly2.jpg
 

Aslansmonkey

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Next we're going to start working on the heart of this rocket. The petal assembly. The parts of this assembly should move freely and easily, but not be soo lose that they're sloppy.

We'll start with the BottomArm pieces, which you should have printed 4 off. Once you remove the support needed to print these parts you'll have to sand the outside rounded. This surface will be glued to the inside of the petals you will cut from the 24mm tube (the 85mm long section) later. I did this by putting the spine of the part in a pair of channel lock pliers and rocking them against a piece of sand paper as shown in these photos.

08_LowerArmSanding1.jpg
09_LowerArmSanding2.jpg 10_LowerArmSanding3.jpg
 

Aslansmonkey

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Note that the BottomArms have a "fat" end and a "skinny" end. You can tell them apart by the position of the hole. If it's closer to the rounded surface, it's the "TOP" (Skinny) end of the part. The hole that is further away from the rounded surface (fat) is the "BOTTOM" end of the part. This is important in the later assembly.

11_LowerArmProfile.jpg
 

Aslansmonkey

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The next step is to attach the BottomArms to the BottomArmHolder. As I mentioned earlier, the BottomArmHolder is the one that fits snuggly to the 13mm tube (does NOT slide freely). That piece will eventually be glued to the 13mm tube forming the bottom of the rocket.

For now, though, we're going to attached the BottomArms to the BottomArmHolder. This is done by running a dowel through the BottomArmHolders and through the BOTTOM (or FAT) end of the Bottom Arms. Cut the dowels off so there's none protruding through the plastic on the sides in the BottomArmHolder as rubber bands will need to go there.

NOTE: You may need to drill out the holes in these parts. Remember, these need to move freely, so DO NOT GLUE these.

12_LowerPetalAssembly1.jpg
13_LowerPetalAssembly2.jpg
 

Aslansmonkey

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Now attach the UpperArms to the BottomArms, again with a dowel. Clip off the extra down. The Upper arms are symmetrical so it doesn't matter what side goes out.

14_UpperArms1.jpg
15_UpperArms2.jpg
 

Aslansmonkey

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To complete the petal assembly, attach the TopArmHolder with dowels. When done, the entire assembly should be able to expand and contract freely. If it's a little tight, you may need to work it a bit to loosen it up.

16_UpperArmHolder.jpg
17_PetalAssyExtended.jpg
18_PetalAssyCompressed.jpg
 

Aslansmonkey

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Now we can move onto the main body. We'll start by taking the cage assembly we made earlier with the 13mm tube. This needs to fit inside the 105mm piece of 24mm tube we cut earlier. Before gluing it in, however, make 4 marks corresponding to the open sides of the cage assembly. We'll eventually need to drill some relief holes in the 24mm body tube for exhaust gasses from the ejection charge, and these marks will help you line up where to drill the holes.

Once you've done that, glue the cage assembly into the 24mm tube down far enough so that the nose cone can still go in. You can (and should) leave a little extra room for the nose cone shoulder, but you want the cage weight as far up in the rocket as you can.

Once that is done, you can glue the BottomSlide part to the 24mm tube ONLY (NOT to the 13mm Tube) flush with the bottom. When done, the 13mm tube should slide back and forth about 1/4" inside the 24mm tube.

19_MainBody1.jpg
20_MainBody2.jpg
21_MainBody3.jpg
22_MainBody4.jpg
23_MainBody5.jpg
 

Aslansmonkey

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Now take that 15mm piece of 24mm tube you cut earlier and make a slice length wise. This is the petal retainer that will keep the petals in while the rocket is going up. Slide it around the 24mm body tube and glue it so that sticks out over the end by about 5mm.

It'll leave a gap that just happens to be the perfect size to glue our launch lug. Cut the bottom of the launch lug at 45 degrees (which I did AFTER the photo) so that the petal on that side doesn't get hung up on the bottom of the launch lug.

24_PetalRetain1.jpg
25_PetalRetain2.jpg
26_PetalRetain3.jpg
 

Aslansmonkey

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Now take the 85mm piece of 24mm tubing and using the CuttingGuide, mark it into four sections to form the petals. I find it helpful to mark the tube in a way so I know the order of the petals so when I glue them to the petal assembly later, they match up.

DO NOT CUT THE PETALS APPART AT THIS POINT. Only mark them. We need this tube in one piece for the next step.

27_PetalMarking1.jpg
28_PetalMarking2.jpg
29_PetalMarking3.jpg
 

Aslansmonkey

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There is a part named the "ReturnRing" that needs to be glued to the 13mm tube. A rubber band will go from this piece to the BottomSlide you glued in earlier to force the inner 13mm tube to want to be "up" in the rocket. This prevents the petals from deploying during the coast phase.

To position this ring, slide it over the 13mm tube. Then put the petal assembly over the tube and slide the 85mm piece of 24mm tubing over that. Make sure the BottomArmHolder is flush with the bottom of the 85mm tube and push it up the 13mm tube.

Make sure the 13mm tube is "Up" as far as it can go in the rocket while you do this. The end result is that you should be able to push the ReturnRing to about where it needs to be on the 13mm tube.

The goal is that the ReturnRing needs to be far enough away from the BottomSlide for the rubber band to work, but high enough to not interfere with the petal assembly when the petals are closed. That worked out to about 18mm in my case.

Once you have it's position determined, mark it on the 13mm tube, move it aside and apply glue. Then move it back in place. Remove the 24mm petal tube and assembly before gluing.

30_ReturnRing1.jpg
31_ReturnRing2.jpg
32_ReturnRing3.jpg
 

Aslansmonkey

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NOW you can cut the 85mm tube into the 4 petals. You'll probably have to sand the edges to clean them up too.

Using one of the petals, position the petal assembly on the 13mm tube so that the petal goes under the edge of the petal retainer but is ABOVE the BottomArmHolder. Examine the photo to be sure of your positioning.

Once your sure, apply glue to the rounded surface of the BottomArms, being careful to not get any on the moving parts, and glue the petals in place ONLY TO THE ARMS. This is a little tricky so you may want to practice it dry first. It may help to tape the petals back into their original tube form first.

What you're looking for is for each petal to be centered on it's arm. Take your time on this step.

33_Petals1.jpg
34_Petals2.jpg
35_Petals3.jpg
 

Aslansmonkey

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The fins are made from thin cardboard. What I used was close to half a mm thick. A cereal box ought to do. Using the FinTemplate cut 8 fins. It's easiest to cut 8 strips the thickness of the fins first, then cut the angled side and the little notches out.

Then using a ruler, bend the bottom flap of the fins 90 degrees. Bend 4 of the fins to one side and the other 4 to the other side. In the final model, the fins will form a set where2 fins become a single fin when the petal assembly is closed.

36_Fins1.jpg 37_Fins2.jpg
38_Fins3.jpg
 

Aslansmonkey

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Using some small clips (binder clips, paper clips, hair clips, whatever), attach two opposing fins together so the flaps go out. Apply glue to the very outside edge of these flaps and center them over the edge of two petals. Repeat this for the remaining fins. When this is dry you can open the petal assembly and apply more glue to reinforce the fins.

39_Fins4.jpg
 

Aslansmonkey

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You're now ready to glue this petal assembly to the rocket. Slide it over the 13mm tube until the petals are under the petal retainer. Make sure the 13mm tube is pushed up in the rocket as far as it can go too. Then make a mark on the 13mm tube to mark the bottom edge of the BottomArmHolder.

Slide the 13mm tube down to release the petals and open them up. Then slide the assembly up on the 13mm tube so you can apply glue just above the mark you made. Then slide the BottomArmHolder down flush with that mark again and let it dry.

Once dry, you can cut off the excess 13mm tube flush with the bottom of the BottomArmHolder.

NOTE: The rocket in the photo is the experimental "Heli" model where I only used one fin on each petal. Normally there'd be a fin on both sides.

40_PetalAssyGlue1.jpg
41_PetalAssyGlue2.jpg
 
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Aslansmonkey

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I didn't take a photo of this, but glue the nosecone on. If you can't figure out how to do that, you shouldn't be building this (or any) rocket.

At this point you can paint the rocket. If you paint the areas inside the petals, like I do, just be aware that the paint in this area may bind the petal assembly's movement and you will need to work it lose again.
 

Aslansmonkey

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You'll need rubber bands to make it all work. I used 5 1/2" rubber bands. Depending on your 3D printer, you may also need to clear the "hooks" so they stay on. I did this by heating up a piece of servo wire and pushing it in the hook ends (carefully) but there are probably other things you can do too.

Attach the rubber bands as shown in these photos.

42_ReturnBand.jpg
43_PetalBands.jpg
 

Aslansmonkey

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One last, but very important step. Glue the ThrustRing up inside the 13mm tube far enough so that a 13mm motor sticks out by 1/4".

44_Final3.jpg
 

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Now you're all done and ready to fly. Note that this rocket is designed to eject the motor at apogee. The ejection should move the 13mm tube down enough for the petals to clear the petal retainer allowing the bands to pop the petal assembly open. You may want to cut the tops of the petals at angles as I did to assist them going under the petal retainer.
 

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And as a final treat, here is my entire "Snake Eye" family.

Left to right they are:
24mm version as featured at Apogee Rockets
24mm Pringles Version (Final published version)
13mm Regular Version
24mm Experimental (too much plastic!) failed version
13mm Helicopter Version (I omitted half the fins to make it helicopter down, didn't quite work...yet)
18mm version
24mm Pringles Single Tube version (which needs plastic webbing to fall slow enough)
45_SnakeEyeFam1.jpg
46_SnakeEyeFam2.jpg
 

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Wow, so many pages! I don't know if I can read them all.
From your brother. 😁
I've actually been part of the Rocketry Forum for about 8 years now. So I get the daily notification for the previous day's posts and saw your post for this.
 

Aeronerd

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And as a final treat, here is my entire "Snake Eye" family.

Left to right they are:
24mm version as featured at Apogee Rockets
24mm Pringles Version (Final published version)
13mm Regular Version
24mm Experimental (too much plastic!) failed version
13mm Helicopter Version (I omitted half the fins to make it helicopter down, didn't quite work...yet)
18mm version
24mm Pringles Single Tube version (which needs plastic webbing to fall slow enough)
View attachment 487013
View attachment 487014
Such an awesome fleet! Really creative. Great engineering getting it all to work!
 

Aslansmonkey

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I missed this, but you can see it in the photo on item #15 when you're gluing the petals on, it helps to wrap some rolled up paper towel and put it between the petal assembly and the 13mm tube to create some outward pressure to help hold the BottomArms against the petals. Remove this paper towel when done.
 

KenECoyote

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Woo hoo! Nice collection! (understatement lol)
I've earlier started printing out the parts for the 24mm Pringles can one, have it almost all printed and I hope to build it after the Rear Eject Bomb II one is done and if everything goes according to plan, I'll launch them at my club's last launch in November. :)
I can't wait to build the 13mm one after that!
 

Aslansmonkey

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I'm almost done with my 3rd 13mm one. The "Helicopter" one more tumbled than helicoptered, though it did spin. Unfortunately the first launch kinked the tube. I thought it'd be ok for the second launch and it mostly was as the rocket still went up mostly straight. But it still didn't helicopter right. Because of the kink, however, I tossed that rocket so I'm building it's replacement.

I noticed on that previous one that the fins were past 90 degrees in relation to the fuselage, so I'm going to make sure the fins on this one are a true 90 degrees and see if it does better. I may have to play with the angle from there if necessary.

If anyone builds one of these, I'd appreciate a photo, and even a launch video making it to this thread. I get a lot of joy out of sharing these with the world and it's always great to see other examples in flight.
 

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Launched the 3rd "Helicopter" version again today but I'm not impressed with it. It does, indeed, spin as it comes down, but so much so that it basically goes into a spinning tumble. This ruins the effect of this rocket for me as it's supposed to fall nose first like any self respecting bomb would. So I've given up on the "helicopter" idea, but if anyone else wants to try it, just use half the fins. Make sure you use the SAME half on each petal, of course.

I also launched a new version based around a 33mm (BT-55) body tube. This one has petals that taper in at the bottom and looks a LOT more like the actual original bomb. It needs work, however. First, the petal by the launch lug stuck out a little bit so it slowed the rocket at launch. But second, I think the rocket needs nose weight. It wasn't stable. The one "good" launch of the three attempts it went up, but lost stability at apogee. Still, the motor ejected and it FELL correctly. I think I skipped the spin test on this rocket, so it likely needs nose weight. It's enough under the max limit of the A3-4T's I'm using that I think I can get it to work.

I also launched the regular version again today, it flew great!
 
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