Fuel Mines are mortar tube based effects that use liquid fuel (gasoline, ethanol, etc) and a black powder charge to produce a fireball effect.
Our mortar tubes are custom made (by us) from schedule 40 steel pipe and plate steel. We looked at tubes made by CraigCo and also the info contained in The Liquid Fuel Mine document.
For the charge we use Clear PET Plastic Boston Round Bottle with a 24-410 Neck Finish. A 4oz size (125mL) handles the charge for the 6″ and 8″, while an 8oz size (250mL) is used for the 10″. We designed the 4″ tube but, after testing the 6″, decided not to make any that small.
Both bottles use the same 24/410 cap. 24/410 refers to the bottle’s neck finish, where “24” means the bottle neck is 24mm in diameter, and “410” signifies the specific thread style, meaning it has 1.5 turns of thread per complete rotation around the neck.
We drill a hole in the cap for the shroud of a MJG Firewire Initiator. We epoxy the shroud into the hole and epoxy the wire once it is pushed out far enough to contact the black powder in the bottle. We then put a generous amount of epoxy on the threads of the bottle before screwing the cap on. Finally, we put epoxy around the lid where it meets the bottle. All this care and extra effort is worth it to guarantee no water or fuel leaks into the charge.
ALWAYS SHUNT THE IGNITER BEFORE PLACING IT IN THE CHARGE!
To get consistent results on detonation, the charge needs to be held in place at the center of the bottom of the mortar tube. We have seen people use nuts or a link of chain welded onto the bottom to ziptie the charge too. We wanted a more elegant solution, so we designed a way to capture the lid of the bottle easily and hold the bottle centered tightly against the bottom. We used an EDM (Electrical Discharge Machining) cutter to cut out the holder and then welded it to the bottom plate in the correct location.
The wire coming out of the top of the cap slips through the wedge shaped opening and the bottle cap slide into the hole. Since the cap is tapered, as it is pushed into the hole it tightens up, holding the bottle firmly in place. This produces a consistent effect as the detonation and resulting force are centered in the tube and repeatable.
BLACK POWDER CHARGES
The rule of thumb for lift charge is 1.25oz [40 grams] of 2FA Black Powder per gallon of fuel. we then add a 1/4 tsp of 30-50 mesh spherical titanium. The titanium is highly flammable and ensures that the cloud of vaporized fuel ignites as the burning titanium moves up through it. “So where do I get 2FA Black Powder?” you ask. Well, that’s where things get interesting…
The manufacturers classify Black Powder types and grain sizes by numbers and letters. Most black powder sold in the US is either “sporting grade” (“G” black powders), or “blasting grade” (“A” black powders).
Fireworks manufacturers prefer the “A” series black powder to be used in fireworks. But you are required to have an ATF license to buy that type black powder legally. However you don’t need an ATF license to purchase up to 50 lbs. of sporting grade (G) black powders, which are chemically identical.
Since the composition of the two types of black powders is identical, you can, in fact, substitute G black powder for A black powder in your fireworks. But you need to understand that the black powder manufacturers created completely different grading/numbering schemes for each type of black powder (i.e. sporting vs. blasting powder). Hence, 2FG black powder is not the same size as 2FA.
Hence, 2FG black powder is not the same size as 2FA, but “Cannon” is! So we are using 1.25oz, or 40g, of Schuetzen Cannon Black Powder per gallon of fuel in our fuel mine charges with consistent results.