Spring-loaded RC bomb

Description

Here is a spring-loaded bomb intended for dropping from RC airplanes, but could be used for any purpose. Normal (passive) RC bombs are unimpressive. Most are a piece of foam or plastic that hits the ground with a dull bounce. I have also used various break-apart plastic bombs, which can be filled with something like chalk or rice flour, and scatter a bit of dust on impact. This is an improvement, but still the dust generally splatters horizontally due to downward momentum (dust doesn’t really bounce). It leaves a nice mark on the runway, but still, it is not very bomb-like. So I thought it would be good to embed a spring inside a bomb, to throw a bit of debris upward. Here is a design for that. See youtube video with instructions and demo at: https://www.youtube.com/watch?v=-SKtauXVptM A second video for updated files is at: https://youtu.be/n8TuWNpvReU CAUTION : DO NOT POINT A COMPRESSED SPRING AT YOUR EYE WITHOUT EYE PROTECTION. It isn’t very much energy, but a flying spring or other piece could definitely injure your eye. The spring goes in between the nosecone and the plate. A ridge inside the cylinder halves holds the plate in place as long as the cylinder halves are held in place, which is accomplished by the tail piece. A 2.8mm (.11 inch) diameter carbon fiber rod runs along the length of the bomb, and sticks out the front. When the bomb lands on the nose, the rod pushes the tail off. With nothing to hold the cylinder halves together, they open up and release the spring and plate. The plate pushes the debris up. One bomb consists of 2 cylinder halves, 1 nosecone, 1 plate, and 1 tail. You then also need: • a length of carbon fiber rod (I used 2.8mm diameter) • a spring • some dust or powder • a piece of pipe or rod to use as a tool for loading (see text below) I included a short, normal, and long cylinder half, so that you can have some choice of aspect ratio of the finished bomb. I only used the normal size so far. Of course, you can also scale all the parts to any size. The spring I used is this one (1/2inch diameter, 1.5inch length), but any similar sized compression spring should work. You might try different springs. I have not yet. https://www.amazon.com/gp/product/B008RG3P7E By the way, don’t order this type of “light load” compression spring. https://www.amazon.com/gp/product/B07LBWDRQ3 What “light load” means is that it barely compresses (it does NOT mean “low compression force.”). It cost me $6.62 to learn this, so I thought I would pass it on for free  To load the bomb, I found it useful to use a few-inch piece of PVC pipe, to press down on the plate. Any hard tubular object will work. You could just use a solid dowel, but then you would have to insert the carbon fiber “trigger rod” after setting the bomb. I found it easier to put the trigger rod in place from the beginning, and then press down on the plate with a hollow tube rather than a solid dowel. Then hold the cylinder halves in place with your hand, fill it with powder, and then place the tail on. NOTE1: I intended the 45 degree wall of the plate to engage with the 45 degree wall of the cylinder halves. So the little protrusion on the plate faces the nosecone. I guess it would probably work with the plate upside-down without a lot of difference. It would leave 1.5mm less room for the spring, though. NOTE2: You have to make sure the plate actually goes under the ridges on the cylinder halves. The plate can “catch” just on the sides of the cylinders, and it seems loaded, and it will kind of work. But the spring isn’t fully compressed and will have a lot less energy. The holes are big enough to pass a 2.8mm rod easily. Depending on your print settings, you might have a tighter or looser fit. Drill out if it is too tight. If it is too loose, you might have problem with the rod falling out when the bomb is held upside down. For me the rod seemed to stay in place just fine after the bomb was set (even with the holes drilled out to 3mm), probably because it is touching several places (nosecone, plate, tail). But it could be held in place with a tiny ribbon of tape on the nose if required. It would be possible to glue the spring to the plate or nosecone or both. I didn't do this, because it isn't necessary, and I thought it might reduce the released energy. See video for loading and a demo “explosion”. The key to a good visual effect is dispersion of the powder. Throwing a few clumps up is less impressive than throwing up a cloud. For me, the thing that worked best was Holi Powder (for Indian Holi Festival). Rice flour is also very fine, but it did not disperse well for me – but this might not be a fair comparison because the rice flour was old and the Holi powder was new (had only been exposed to air/humidity for minutes before I used it). Baking and/or storing with a desiccant might help. Post in the comments what dust you found that works well. I am pretty much settled on the Holi powder - it disperses well and looks good. Confetti or glitter are options also (see video). UPDATE SEPT 8, 2019: I uploaded a new files and also a video showing some alternatives: 1. I tried a couple stronger springs 2. I uploaded a “cup” that can be used instead of the “plate”. I thought a cup might prevent material from escaping to the side before the spring pushes it up. It does kind of seem to work (see video), but in retrospect I would put some holes in the side so the material comes out more when the cup is flying through the air. The new files I uploaded are: - Cylinder_normal_7mm_ridge.stl - Cylinder_normal_10mm_ridge.stl - Cup.stl The original file “Cylinder_normal.stl” has the plate-holding ridge at 5mm from the bottom. The two new Cylinder files have this ridge at 7mm and 10mm. When the bomb is loaded, the room available for the fully compressed spring is 11.5mm plus the ridge height (i.e., 5mm, 7mm, or 10mm, depending on which version of the Cylinder you use). So between these three ridge options, and scaling, you should be able to use a variety of springs. Numbers for my examples are below. Note that if you calculate the compressed spring height as number of turns x wire diameter (which is absolute minimum), you need to add some margin between that compressed spring height and the “room available for spring” to be able to actually load the thing. The sizes shown in the video are: A. Small bomb: Cylinder file = Cylinder_normal.stl (5mm ridge height). Print scale = 100% Room for spring = 16.5mm Spring type = ½ x 1.5”, wire = 0.041in, 10turns Compressed spring height = 10 x 0.041in = 0.41in = 10.4mm Spring force = 6.82lbs Link to Spring: https://www.amazon.com/gp/product/B008RG3P7E B. Medium Bomb: Cylinder file = Cylinder_normal_7mm_ridge.stl (7mm ridge height). Print scale = 130% Room for spring = 18.5mm x 130% = 24.05mm Spring type = ¾” x 2”, wire = 0.08in, 10turns Compressed spring height = 10 x 0.08in = 0.8in = 20.32mm Spring force = 23lbs Link to Spring: https://www.amazon.com/gp/product/B00G34QJDO C. Large Bomb: Cylinder file = Cylinder_normal_10mm_ridge.stl (10mm ridge height). Print scale = 170% Room for spring = 21.5mm x 170% = 36.55mm Spring type = 7/8” x 4”, wire = 0.08in, 15turns Compressed spring height = 10 x 0.08in = 1.2in = 30.48mm Spring force = 24.9lbs (but also much longer stroke) Link to Spring: https://www.amazon.com/gp/product/B008RG80Y2 Note that the “indents” in the Cup.stl are intended to line up with the crack between the cylinders, since there is some structure at the cylinder edge that needs extra room. The second video named above shows all these in action

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