This isn't so much of a finished project as an ongoing exploration into how to turn this fantasy 'April-Fools' product into a functioning dart-throwing toy. For those of you not familiar with it, the project was originally a 2014 fake product at ThinkGeek ( https://www.thinkgeek.com/product/1ba4/ ) I worked on their in-house custom product design team for 7 years and every April first we'd show off fake ideas for fun and to get press coverage during a pretty slow time of year.
The video below shows the final product which was really just a dummy mock-up 3D printed in SLA resin by Quickparts.com and then painted by me. All of the dart throwing effects shown in photos & video are 'movie-magic' by our amazing marketing team.
Step 1: Design Overview
Inspired by the real 1950's US army project to create a 'nuclear bazooka' ( https://en.wikipedia.org/wiki/Davy_Crockett_(nucl... ) I based the toy on a dodechahedron with a bit more sub-division to allow for more than 20 darts at the vertices or 12 at the faces. Each of the pentagon faces is sub-divided into 5 triangles which serve as bases for the darts and then I added another one at each vertex for a maximum of (6x12+20) 92 darts. The top and bottom pentagons are clear of darts to leave room for the nose and tail cone parts meaning we subtract 12 from 92 leaving 80 darts - a nice round number.
Once the darts were laid out, I added the nose and tail which are just simple revolves with planar tail fins. Everything on this model is a multiple of 5, so it's a bit easier to create one section of the pie and then reproduce it 5 times instead of creating every instance all the way around the toy. This only became a problem when I created the radiation symbol on the nose as it's divisible by 6 - I'd have been better off using a 5-sided star, but referencing nuclear energy was important.
Once the main sphere of the body is created and the holes for the darts are created, everything else is just aesthetic detail: counter-sunk holes, pentagonal marks around some holes, fin details, and some extra crenelations to make it look high-tech and realistic.
Step 2: Internal Mechanical Structure
After the overwhelmingly positive feedback for the fake product, I put some effort into seeing how we could actually create a fully functional version that would be safe and reliable enough to produce and sell. I began thinking about how to manufacture this as a real product, working out plastic molding and the mechanics of ejecting 80 darts at once. Since this will probably never be made by traditional injection molding tools a lot of my ideas can go out the window - 3D printing is a whole different way of thinking about production.
My best solution for pushing the darts out of the toy was to use a small CO2 cartridge in the tail section to blast all the darts out with air pressure. This is actually a pretty scary amount of energy we're dealing with here - somewhere around the power of a real handgun's .357 caliber cartridge if I remember correctly. I did a REALLY SKETCHY test by drilling 80 holes into a craft foam sphere and sticking a CO2 cartridge inside with a nail ready to pierce the seal. Please don't try this at home. I was wearing hearing and eye protection, but things could have easily gone very wrong: the foam ball could have straight-up exploded, the wooden rig for holding the cartridge and nail could have shattered, the nail half could have launched into the ceiling - or my neck, the blast could have shot the hammer back at me... too much danger to re-try.
The dart ends at the core of the toy are pretty much what drive the diameter of the sphere and density of their layout. In order to grip the darts and make it so you can't put foreign-objects in the breach of the blaster there is a spherical starburst of center posts. This starburst is broken into 6 layers with 10 posts on the top and bottom layers and 15 on the middle four. These stars incorporate screw holes and stand-off spacers to align the parts but also have air baffles that I imagine would redirect the blast of air from below along each of the dart posts. I honestly have no idea if this would work. I haven't been daring enough to test my 3D printed model - it could easily turn into a literal bomb showering shards of PLA plastic shrapnel all over the place.
My idea for the trigger mechanism, which wouldn't be practical at all, was to have the nose-cone connected to a long spike that would pierce the cartridge seal all at once. This would require throwing the big thing and making sure it lands just right on the nose - not a very realistic scenario. A better idea would be to create some sort of mechanical count-down timer in the base or a remote-detonator using a battery powered servo motor. Honestly, this toy wouldn't be very practical to use in a real Nerf battle - you can't drop it from an airplane and do an airburst, so half of the darts would just go towards the ground.
Step 3: Foam Sphere Slices
For the center sphere, I imagined that it would be made from some sort of castible soft polyurethane foam like in old Nerf footballs. I've never gotten to the point of trying this myself, but Smooth-On does have a product that looks like it would work: https://www.smooth-on.com/product-line/flexfoam-i...
There are 4 different part geometries and you'll need to create two of each except for the middle layer. The shape of each part nests with adjacent parts and is captured around the starburst with 5 long threaded rods and the top and bottom shells. The top shell has space for a spring and the trigger cap to go along with a long steel spike to pierce the CO2 cartridge.
Step 4: Tail Section Assembly
The tail section is relatively simple in comparison to the other half of the toy. There are 5 screws that hold the cartridge holder to the tail shell and capture the 5 accent pieces and tail fins. All of this is pretty simple to 3D print and assemble without much explanation.
The one area that could be improved is the small L-shaped latches that grab onto the sphere section. They're pretty small and wimpy for any sort of digital prototyping method. As you can see from the section view, there's plenty of room for other mechanisms or electronics depending on what dart pushing method would be best. I did a brief search for "high pressure gas solenoid valve" but couldn't come up with anything affordable enough for a toy.
Step 5: Final Thoughts
Check out the files on my GrabCAD page here: https://grabcad.com/library/davy-crocket-aka-nerf...
I have STEP solid files and STL files ready for 3D printing. I would love to see people continue working on this crazy idea or just make one as a desktop sculpture. Please send me feedback if you have any questions or make any improvements on the design and BE SAFE!