How To Make Black Powder Rockets
By John Werner
There are basically two types of black powder rocket engines. One type, the "End Burner," rocket uses a relatively fast-burning black powder propellant and often incorporates a high percentage of commercially made black powder in its formulation, or requires the black powder rocket propellant to be ball milled before it is loaded into the rocket tube. The term "End Burner' is used to describe the way in which the rocket propellant progressively burns from one end of the rocket tube to the other in a straight linear fashion. Typically, this type of rocket engine produces a fairly short tail as the rocket rises into the sky.
This article will describe how to make and fly a second type of rocket engine, the "Core Burner." Core burning rocket engines are made by forming the rocket propellant around a thin metal core or spindle, which extends up through the center axis of the engine tube. Removing the spindle after the rocket engine is loaded results in the black powder rocket propellant having a long hollow cavity, which greatly increases the available surface area for burning. More surface area means that a slower burning composition can be employed for the propellant and still provide adequate thrust. Core burning rocket engines can now be made using only simple hand-mixed black powder compositions of potassium nitrate, charcoal and sulfur. As a bonus, core-burning rocket engines are characterized by having very long, beautiful golden tails and an impressive distinctive sound as they take off.
Before You StartThe best use of black powder rockets is firing them in groups or "flights." So this article is geared toward production techniques that allow lots of rocket engines to be made quickly without machinery. I particularly like the methods used by the Mexicans to make their rockets. It is straightforward and produces rockets that are reliable and consistent. They produce thousands of hand-rammed black powder rockets for their displays and in my mind are the best rocket makers in the world today. My video "Fireworks From Mexico" shows many of their rockets being fired.
This article is intended for beginners and is based on using 4-ounce rocket tooling (TL1204). No commercial black powder or ball milling of a composition is required, and the black powder formulation is straight out of "Dictionary and Manual of Fireworks" by George Weingart (BK0025). The rocket engines are rammed instead of pressed and reviewing the article on hand-ramming "Basic Pyrotechnic Hand Ramming Technique" is suggested.
The Tools Needed to Make a Black Powder Rocket
Preparing Rocket Tooling. The 4-ounce rocket tooling available from Skylighter consists of a stainless steel spindle mounted on a large aluminum base and four varying length aluminum rammers (or "drifts" as they are sometimes called). Prior to using the tooling the rammers need to be marked in conjunction with the rocket tube that will be used, in this case a 5" long tube (TU1028). Slip the tube onto the spindle and base. Start by sliding the longest rammer into the rocket tube until it lightly bottoms out on the spindle shaft. Withdraw the rammer about 1/8" and place a mark with a felt tip pen on the shaft of the rammer where the top of the rocket tube falls. The mark should be drawn as a ring around the shaft, so it is visible from all directions. Mark the other three rammers in a similar fashion. It is extremely important that the rammers are accurately marked in order to prevent them from striking the spindle when filling a rocket tube with black powder.
Use a Ramming Sleeve. The paper tubes for this project do not have a wall thickness heavy enough to withstand the ramming process and require a metal sleeve be placed over the tube while it is being rammed. The sleeve prevents the tube from expanding and splitting and is removed after the rocket engine is completed. I used a 5" section of brass tubing obtained from the McMaster Carr Company, No. 8950K721, with an I.D. of .811" and an O.D. of 7/8". Since the rocket tube O.D. is only 3/4" a spacer made from a 3-3/4" x 4-7/8" piece of file folder (tag board) is rolled around the tube to increase its diameter and make it a sliding fit into the brass sleeve. Make sure the brass tube ends are cut square and are beveled smoothly on the inside. Naturally, for those who have access to a metal lathe, a sleeve can be machined to an exact I.D. to accommodate the rocket tube without using the tag board spacer.
FunnelsA small plastic funnel is needed to neatly and easily load the clay and black powder into the rocket tube. Paper cone drinking cups work great as funnels too, just snip off the bottom to the correct size hole to fit the tube.
A "ram-through funnel," though not absolutely necessary, is a nice addition to your rocket tooling. Since the funnel remains in place during the ramming process, you can save considerable time by not having to repeatedly insert and remove your funnel. The funnel is machined to sit snugly on top of the rocket tube and its throat is the same diameter as the I.D. of the rocket tube thus allowing the rammers to slide down through the funnel itself. Don't forget to mark the rammers as described earlier, taking into account the added height of the ram-through funnel if you decide to use this accessory.
Mixing the Black PowderThe formula for the black powder propellant is from the Weingart book Dictionary and Manual of Fireworks and is listed under "4 Oz. Rockets." I have converted the formula, which is in parts by weight, to percentages:
Weingart's 4 Ounce Rocket Fuel
|Air Float Charcoal||6||21%|
|80 Mesh Charcoal||3||10%|
Make up a small batch of black powder formula for test purposes. Black powder core burning rockets can be pretty fussy about their exact formulation and much depends on the nature of the chemicals. Each rocket engine will require about 16 grams of black powder, so a small, 58-gram batch will allow you to make up 3 rocket engines. An easy way to get 58 grams is to double each amount in the "parts" formula and then weigh out that amount of chemicals in grams. The total will be 58 grams. With these small amounts it is important that your scale can accurately weigh out the desired quantities.
Make sure all chemicals are free flowing, fine powder. You may need to grind the potassium nitrate if it is lumpy. After weighing out each finely powdered chemical, pass them through the 20-mesh stainless steel screen onto a clean sheet of paper. Mix the chemicals together by hand on the paper and then screen and mix two more times. At this point the mix will be dusty and messy to work with. Later in this article, I'll show you how to granulate the black powder to make it more pleasant to deal with. First, however, it needs to be loaded into a tube and fired before going to the trouble of granulating.
Charging the Rocket TubeAlways isolate the ramming surface from the black powder container to avoid vibration separating the composition (in effect un-mixing your black powder). Keep your container of black powder covered whenever you are not dipping into it. If you have an accidental ignition while ramming, this will reduce the chances of your mixed powder igniting as well.
Slip a rocket tube into the brass-ramming sleeve using the tag board wrapped around the rocket tube to take up the space and provide a nice sliding fit.
Place the tube and sleeve over the ramming spindle. Look down into the tube. You should clearly see the top of the metal spindle in the tube. The spindle must be well centered in order for the different length rammers to slide down over it as they enter the tube to ram the black powder. If it is not centered the most likely cause is that the end of the rocket tube has not been cut square. Replace the tube if necessary.
Using a teaspoon-sized measuring scoop, measure out a slightly heaped amount of dry bentonite clay (4.0-4.2 grams). Use either a removable funnel or a ram-through funnel to dump the clay into the tube.
Slowly slide the longest rammer down into the tube. Since the clay is very fine it will tend to puff out the top if the rammer is inserted too quickly.
Ramming The Black Powder RocketUsing only hand pressure, consolidate the clay as much as possible by slowly pushing down on the rammer, twisting it at the same time, and withdrawing it slightly several times. By withdrawing it, clay that has been forced up inside the rammer hole will be allowed to fall back down into the tube. After a while you will develop a feel for how far to withdraw it. Sometimes pulling it up only an inch or so is sufficient, and other times the rammer needs to be completely withdrawn and the excess clay knocked back into the tube.
Give the rammer 10 fairly good blows with the mallet. Observe the limit mark on the rammer; it should stop about 1/2" above the top of the rocket tube. Once the rocket engines are working well, this point should be marked on the rammer to indicate the height of the rammed clay nozzle in the rocket tube.
Withdraw the rammer, and without taking the rocket tube off the spindle and base, turn the whole unit upside down and rap it on a hard surface over a piece of paper. Some loose clay will fall out of the tube. Return the rocket tube and the spindle to an upright position on your ramming area.
Once again look to see if the tip of the spindle in centered in the tube. If it is not and the rocket tube is cut square, the cause is due to forcing the tube out of perpendicular while ramming the clay. During ramming, always maintain the tube in a vertical position with your hand. Once the clay is compacted in place, the tube and spindle will maintain their relative positions, so it is important to get this right before proceeding further.
Next, scoop in slightly less than a teaspoon of black powder propellant (2.0 - 2.4 grams) and ram as before, using the same rammer.
Switching to the next rammer, ram two more increments of black powder. Again, watch your limit mark on the first increment. The limit marks on the rammers should ALWAYS be visible when ramming the rocket engine. If the mark disappears inside the rocket tube then there is a good chance that the rammer will be driven into the spindle. The result could be a damaged rocket tool or, even worse, the accidental ignition of the powder being rammed.
Switch to the third rammer and continue with two more increments. Since this rammer is used near the top of the spindle and the rammer is quite small, it is very easy to plug up the rammer hole. Make absolutely sure the hole is clear before ramming each increment (this is important advice on all the rammers being used). An old drill bit can be used to clear out the hole if it should get plugged with powder. Slowly twist the bit into the hole using just your fingers to turn the drill bit. Do NOT use a drill motor to power the bit.
At this point, depending on how much powder was in each increment, you will be close to using the last (solid) rammer. Pay careful attention to the limit marks on this rammer so that the rammer is in no danger of striking the top of the spindle.
Ram approximately 5/8" of solid black powder above the tip of the spindle. This will leave an empty space of approximately 5/8" in the top of the tube. Then ram in a teaspoon of clay to close off the tube.
Removing the Rocket Engine from the SpindleSlowly twist the ramming sleeve while holding the base with the other hand. If the sleeve (and tag board spacer) were not too tight to begin with, they should twist and slide off before the rocket tube will twist and release from the spindle. Remove the sleeve and set aside.
Slowly twist the tube while holding the base. Carefully slide the rocket tube off the spindle once you feel it break free. The tube can be stuck on the spindle quite tightly, but it should release without having to use any type of tools. (Larger rocket engines can be very difficult to remove).
Rap the released rocket engine, nozzle down, on a solid surface. There should be little or no loose powder falling out. It there is, the engine may have been damaged while removing it or it may not have been rammed firmly enough. It may malfunction when lit.
Preparing the Rocket Engine for Static TestingAttach a paper nosing to the nozzle end of the rocket engine by wrapping two turns of lightweight paper around the tube. Use tape, a glue stick, or white glue to adhere the paper to the tube. The paper should extend 1-3/4" past the end of the rocket tube.
Insert an 8" piece of good quality black match into the core of the rocket engine. Slide a 2-1/2" piece of paper match piping onto the match until it hits the nozzle.
Tie a clove hitch around the paper nosing to hold the black match and the piping in place.
Bury the rocket engine 2/3 of its length in the ground or in sand with the nozzle/fuse end up for static testing.
Light the fuse and stand back to observe the results. The rocket engine should quickly fire up and burn smoothly and cleanly. If it does not, read the section on troubleshooting. If the rocket engine burns well, with a loud roar and does not explode, proceed to flight-testing.
Rocket Flight TestingLoad and fuse a rocket engine as described previously.
Cut a 1/4" dowel or stick to 30" and attach to the rocket engine. The dowel does not need to extend the full 5" of the rocket engine length 3-1/2" is plenty. Two bands of masking tape are sufficient to hold the stick in place for preliminary testing. The complete rocket with stick and fusing should weigh approximately 60 grams.
Black Powder Rocket Attached to Stick
Place launch tube in ground straight up. Slide the rocket stick down into the launch tube until the engine section rests on the lip of the launch tube.
Light the fuse and stand back to observe the results. The rocket should quickly come up to full thrust and rapidly leave the launch tube. With no payload, the flight should be straight and very high. Watch for the falling rocket, as it is still aerodynamic as it comes back down and will be traveling very fast. If possible, recover the spent rocket and examine.
TroubleshootingProblem: Rocket lights slowly, starts to take off slowly and then accelerates pretty well:
Solution: The entire surface of the internal core of black powder propellant needs to be lit instantaneously by the black match. The reason for adding a short piece of piping to the match is to get the flame to shoot into the core as fast as possible from the black match. If the core is only lit at the bottom near the nozzle by a piece of green visco fuse, for instance, the takeoff will be poor. See "Modifications and Enhancements" below for additional ways to fuse the rocket engine if good black match is not available.
Problem: Rocket nozzle blows out at or shortly after ignition:
- The clay was not rammed hard enough.
- Clay was too coarse and did not consolidate well. Some people try to use kitty litter for nozzles; it is almost impossible to hand ram kitty litter into a solid enough rocket nozzle. Stick with bentonite clay.
- Propellant is burning too fast; add more charcoal to formula to reduce burn rate.
- Propellant burns too fast; add more charcoal to reduce burn rate.
- Propellant was not rammed hard enough.
- Propellant had internal cracks, from engine being dropped, or from shrinkage due to humidity change.
- Charcoal was damp when it was weighed out but mix was dry when it was rammed. If the charcoal is damp when you weighed it out, you are weighing water AND charcoal. That means that the percentage of charcoal in the mix will be LESS than normal after the water evaporates. Having less charcoal in the mix will cause the mix to burn slightly faster which may be the reason the rocket engine blows up.
- Propellant burns too slow; add more potassium nitrate.
- Make sure the rocket engine is being lit correctly.
- Spindle is not centered on center axis of tube during ramming.
- Stick is too heavy or long. Make it shorter or use lighter weight stick.
- Rocket engine is mounted on stick at an angle.
- Stick is too short.
Modifications and EnhancementsBoth the clay and the black powder propellant can be granulated to cut down on dust when charging the rocket engine. The black powder is dampened with water until it forms a ball when squeezed. Do not over dampen. A piece of 1/4" x 1/4" hardware screen in a frame is used as a kind of "cheese grater" to grate the ball of black powder in a thin layer over a large sheet of paper. Do not add any binder to the black powder when granulating it. The object is to make very soft, small grains of powder that will crush down and ram easily into a solid mass. Granulating often will cause the powder to burn slightly faster, so don't be surprised if the rocket engine performance is a bit more energetic compared with the non-granulated black powder.
Enhance the tail of the rocket: The percentage of the air float charcoal (AFC) can be reduced in relation to the coarser charcoal (80 mesh) to increase the hang time and length of the tail in the air. In larger rockets, 36-mesh charcoal can be tried. The coarser charcoal will also make the mix less messy to mix and ram. Try different ratios of AFC to 80-mesh. The long tail is really the most beautiful feature of core burning rockets, and it is worth tinkering with the formula in order to enhance it.
Use the following tips to streamline the charging process so hundreds of rockets can be efficiently turned out. Remember that flights of rockets are much more impressive than singles.
- Make a hold-down for the spindle and base, so the rammers can be pulled out without having to hold the base with your hand.
- Machine limit-marks on the rammer shafts, as felt tip markers wear off quickly.
- Make a ram-through funnel.
- Extend the lengths and enlarge the striking end of the rammers. The rammer lengths are a bit too short in my opinion, and all should be at least an inch or two longer in order to get a better grip on them. The 1/2" diameter of this set really tears up the face of the mallet and enlarging the striking end of the rammers with a machined cap will extend and enlarge it at the same time.
- This is one of my favorites: Tape your measuring scoop to the end opposite the striking face on the mallet head (see the rocket tools picture above). This really speeds up the ramming process. It avoids constantly setting the mallet down to pick up the scoop and vice versa.
- Arrange the ramming area efficiently so that everything flows in a logical manner when charging the rocket engine. With all equipment set up properly a rocket engine can be turned out in five minutes or less.
Q: Why don't I need to ball mill the black powder?
A: Ball milling is needed when the chemicals are obtained in a very coarse state and need to be reduced to a fine powder. Ball milling is also used when the burn rate needs to be increased above what is obtained by simply mixing the chemicals together. Chemicals bought from Skylighter should be already finely ground, and the burn rate for core burning black powder rockets is low on purpose.
Q: I don't have any good black match to light the rocket engine with; what else will work?
A: You can harvest high quality black match from Skylighter's Superfast Paper Fuse (#GN1205). You can also use either a length of Skylighter's fast yellow visco fuse (#GN1100) or two lengths of Skylighter paper fuse (#GN1200). I still like to use the paper piping and paper nosing as described for inserting the black match when using the fast visco or paper fuse. Make sure the match goes up into the hollow core as far as possible.
Q: Wooden dowels for rocket sticks are expensive; what else is good to use?
A: Cut your own sticks from straight grain pine. For this four-ounce rocket, the stick itself should weigh about 16 - 20 grams maximum.