Okay, by now you may have figured out that I like things that go bang. Fortunately for me, DF!'s publisher tells me that you do, too. So let's go...
Those of you with military backgrounds are very likely familiar with ground burst simulators. For the rest of you, these are comparatively small explosive charges meant to provide lots of flash and bang with a minimum of hazardous shrapnel. As the name implies, they are used to imitate the blast of true munitions, generally for training purposes. However, depending upon the size of the charge, a simulator can be used for a variety of applications. A small simulator might be used as a 'flash/bang' grenade for disorientation. JBT's are fond of using them during 'dynamic entry' raids. Imagine their surprise if the raidee were to return the favor. Or you might find such a device useful as a distraction: Divert the JBT's attention to where you aren't. You might also consider using one of these mini-bombs as the propellant charge for an improvised mortar. They also make dandy Second of July noisemakers.
Or you could even use one as an actual ground burst simulator for your own training sessions.
First, you need to gather some materials; none of which will be too difficult to acquire, which is the point after all:
You'll also need a knife and something to apply the epoxy with.
If you aren't already familiar with rocket motor igniters, you should become so immediately. These little beauties are too useful to the munitions improviser not to keep on hand. An igniter is basically a piece of bent wire coated with an incendiary material. When launching rockets, you insert one of these into the rocket nozzle, touching the solid propellant within. Then you connect the igniter to a firing circuit consisting of a safety switch, firing switch, and a battery. With all switches engaged the wire heats up until the incendiary material ignites, in turn touching off the rocket fuel. In our GBS, the igniter will serve exactly the same purpose.
Igniters and firing circuits are readily available at toy and hobby stores. The igniters are packaged with rocket motors; the motors can be useful too, although not in this particular project; save them for later. Rocketry starter kits containing a rocket, motors and igniters, and firing circuit can often be found in discount toy shops for five to ten FRNs.
Back to the GBS. This is going to be a 'fuel/air explosive' (FAE); instead of gun powder, or some more exotic explosive, our blast will come from burning a mix of propane and air.
Start with the soda bottle cap and the tire valve. The valve can be removed from any old bicycle inner tube; the tube may be shot, but if the valve holds air, great. When you cut the valve free, keep about an 1/8th of an inch of the surrounding tube rubber; this will help seal the valve into the bottle cap. Drill a hole (I find that 11/32 works fine for most bike valves) in the bottle cap, and push the valve into the hole from the inside of the cap. Just before the valve seats against the cap, squeeze in a bead of RTV adhesive (or other similar glue) around the hole. Now press the valve into the glue. Once set, this is supposed to make an airtight seal against several pounds of pressure; do it right.
On to the bottle and igniter: Approximately mid-way down the side of the bottle, make a small cut about a quarter of an inch long. Wiggle your knife slightly to widen the cut just a little bit- you're going to slide the incendiary end of your igniter into this slot, and you want it wide enough that it doesn't scrape the flammable mixture off the igniter. With the igniter in place, seal the hole with epoxy. I've found that it's easiest to mix the epoxy well in advance, so that it is thickened enough not to run.
Now it's just a matter of waiting for the glues to set. Once that's done, it's time to charge the simulator. You want to begin by completely filling the bottle with propane; air (oxygen) comes later.
The business end of most propane torches usually consist of a threaded burner with relatively large air holes in the side which allow air to mix with propane, creating a combustible mixture. For our purposes, we don't want to mix the air and propane yet. Unscrew the air mixing fitting and set it aside. Attach the plastic tubing to the exposed propane jet. If the air mixing portion of your torch isn't removable, simply cover the air holes with tape, and firm the tubing to the end of the torch. Don't light it. Stick the free end of the tubing down into the upright soda bottle. Open the valve of the propane cylinder and let the gas hiss into the bottle. Propane is somewhat denser than air, so it will displace the air in the bottle, leaving only propane. When the bottle is full of propane put the tire valve-bedecked cap on; twist it on tightly.
How do you know when the bottle is full? Propane is colorless, after all. With a middlin' full propane cylinder, you can approximate this by just letting the propane run for about 3 seconds per ounce capacity of the bottle. One of those dinky 8 ounce water bottles needs around 24 seconds; 12 ounces will be 36 seconds or thereabouts.
If you want to be sure how long, you can fill a spare bottle of equal capacity with water and hold it upside down in a basin of water; so long as you keep the mouth of the bottle in the water, the bottle stays full. Then you stick the plastic tubing (attached to your propane cylinder) into the bottle, and turn on the gas. The propane will displace the water. If you time the process, you'll know how long to run the gas when filling a GBS. You could gas up your GBS directly this way, but you'll need to do it before you install the igniter, as igniters and water don't work and play well together.
Okay, you have a propane-filled soda bottle with funny looking appurtenances. Cool. But propane all by its lonesome isn't going to make much of a bang. You need oxygen. Fortunately for life on Earth, some 20% of the atmosphere happens to be oxygen. This is where that tire valve comes into play. Hook it up to a regular tire pump and start pumping. Bring the pressure up to approximately 70 pounds per square inch (PSI).
Pressurizing the bottle not only provides air for the propane to burn with, but also pre-stresses the bottle, giving an improved pop when you set off your GBS.
Propane needs five molecules of oxygen (O2) for every molecule of gas (C3H8) to burn completely. Air is, more or less, only one part oxygen out of five. Pressuring the bottle crams more oxygen in for a more complete burn; but not a totally complete burn unfortunately. For that, peak efficiency of the GBS, you'd need more air pressure than most old soda bottles are going to handle, not to mention your improvised valve and igniter seals. But feel free to up the pressure as much as you can manage. The more, the merrier.
This is too easy. All you have to do is connect a battery to the igniter leads sticking out of the bottle; it'll go boom. Of course, you'll probably go boom with it if that's all you do. I suggest that you use a model rocket firing circuit to detonate the thing; these gadgets normally come equipped with a set of wires at least ten feet long, for safety. I won't be too surprised if you opt to extend the length of the wires for more safety. As general rule, model rockets aren't supposed to go boom.
If you're using the GBS as a GBS or as a distraction, just set the sucker out on the ground and blow it. If you want to use it as mortar propellant, slide it down into the mortar tube with the wires trailing out, place wadding over the charge, and insert the projectile. Fire at will. Of course, Will may fire back.
If you mean to use one of these as a flash/bang grenade, consider mass. Have you ever tried throwing an empty soda bottle for distance? They generally don't make it too far. Neither will the basic GBS. So you need to add a little weight to the sucker. The easiest technique is probably to tape something comparatively heavy to the bottle. The downside to that is that anything you tape onto it can all too easily become unwanted shrapnel. Try wrapping the bottle with a few layers of duct tape, making it just heavy enough to pitch well. Any resulting shrapnel from that will be little more hazardous than the plastic of the bottle itself.
Particularly in grenade-type applications, an electrical ignition system can be somewhat less than completely convenient. So... You did go out and buy some cannon fuse after reading my thermite article, right? If not, do so now; same reasoning as that behind acquiring igniters. And once you've got it, punch a hole in your bottle, insert a length of fuse, and seal the hole with epoxy. Light it, toss it, duck. You can determine the required fuse length by timing the burn of a known length, and extrapolating from there. Obviously, a GBS with a fuse doesn't need the rocket igniter.
I haven't specified a particular sized soda bottle, because it doesn't matter in assembly. Tactically, it's another matter. For a hand-thrown flash/bang, I don't much care to use a two liter bottle; that's too much blast too damned close to suit me. Try a twelve ounce bottle, even one of the dinky eight ouncers that yuppie water comes in. For big booms out in the field, scale up to three liter bottles. For a mortar, you will likely want a selection of 'calibrated' sizes for different ranges.
On the off chance that you don't have access to propane (yeah, right), you can charge a bottle with oxy-acetylene from a welding torch, and you'll get a bigger bang. If it doesn't suit you to keep pressurized tanks of flammables laying around, consider calcium carbide. Just add water, and get acetylene. Check out chemical suppliers, or 'Bangsite' carbide cannon fuel.
You can get more bang for your bottle by pumping more propane into it. Try replacing the previously mentioned plastic tube (on the torch fitting) with the pressure line scavenged off a tire pump, and filling the bottle through the valve. And so long as you're doing that... You can finish pressurizing the bottle with pure oxygen instead of air. You needn't have access to medical supplies or oxy/acetylene torches (but if you do, wonderful); Bernzomatic makes small oxygen cylinders similar to their propane tanks. They're readily available at good hardware stores.
Rocket Igniters, kits: Discount Rocketry, http://www.discountrocketry.com
Cannon Fuse: Sportsman's Guide, http://www.sportsmansguide.com
Calcium Carbide: Big Bang Cannons, http://www.bigbangcannons.com/
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