Six Pound End Burning Rocket Motors


This project page has been a long time coming, but had to happen sooner or later. It's the fulfillment of many of the childhood dreams that formulated in my imagination as I grew up flying the much beloved Estes model rockets. As many of you who were also bitten by the rocket bug in your youth know, the trend is always toward bigger and more powerful rockets. The black powder motor size topped out at D motors in those days and even today goes no further than E motors. I dreamed of monster motors back then and have only just realized the dream recently with these motors. I have not characterized them extensively, but my rough calculations indicate that these motors have a total impulse of about 200 nt-sec. and could be designated as H40 class motors. Although I have designed these motors to be used primarilly for display pyrotechnic purposes, they could easily be adapted for flying traditional model rockets with little effort. It should be noted that this project is quite advanced and is probably out of reach of most casual pyro hobbyists because of the tools and materials required. For this reason, I will not describe the project with all the detail needed to successfully attempt to do it yourself. Think of this page as an interesting general article about a "born again" rocketeer's experience with a truly impressive black powder rocket motor.


The true secret to this project's success lies with the paper tube that is used for the motor casing. These were custom ordered from New England Paper Tube through a fairly large cooperative effort of a group of fellow hobbyists. They are made from virgin craft paper, parallel rolled very tightly to make a very dense and strong casing. Every other commercially available tube I tried before these failed to stand up to the pressures needed to make the motors fly. The failure mode of these previous tubes was usually pin-hole ruptures in the motor walls that caused size venting of the exhaust gasses during ignition. The failures were never catastrophic enough to cause motor explosions, but, nonetheless, caused poor flight performance. These tubes are 36 inches long, have an inside diameter of 1.5 inches and an outside diameter of just a hair over 2 inches. I usually cut them to 8 7/8 inches long so I can get 4 motor casings from one tube. Longer ones might be used for larger payloads or longer burns, and shorter ones, like the one on the right, would be about right for a stinger missile.
This picture is a close-up of the most important part of the tooling needed for the project. It is the spindle, which is used to form the rocket nozzle throat and cone. It is machined from a piece of 1/2 inch stainless steel and has a pressed aluminum collar. This spindle assembly is then secured to a base plate made of 3/4 inch aluminum. A drawing showing the dimensions of all the tooling can be found here.
The rammers and spindle are pictured here. The rammers are simple cylinders made from 1 1/2 inch aluminum stock. These would also make good murder weapons if you should decide to pursue the profession of hoodlum for hire.
A reinforcement sleeve is made from a piece of split 2 inch PVC pipe. At the pressures used to press the rocket fuel into the motor casing, this sleeve is essential to prevent casing deformation. Eight pipe clamps are used to compress the sleeve tightly against the casing.
Here we have an action shot of my new jumbo press being used to form the clay nozzle. Although this picture is about making a rocket motor, I can't resist the opportunity to brag a little bit about this beast of a machine. It uses a compressed air driven 20 ton hydraulic bottle jack. At the press of a button, I can apply up to 40,000 pounds of force. No more endless manual pumping of the jack handle, as on my original home-made rocket press. On the under side of the top press plate, I have mounted a custom machined hydraulic load cell to measure the exact applied force. The load cell bore has an area of precisely 2 square inches, making the gauge indicate exactly half of the actual applied force. The 10,000 PSI gauge gives me a measurement range of 0 to 20,000 pounds of force. I'll never need to apply that much force to make most rockets and fireworks, but knowing that I can gives me some kind of a warped sense of satisfaction.
After pressing, the clay nozzle looks every bit as good as any Estes rocket motor I've ever seen. For this one, I use a mixture of 60 grams of clumping kitty litter and 30 grams of wax treated kyanite and bentonite clay. It it pressed with 12,000 pounds of force and comes out hard and shiney. After the nozzle is pressed, 250 grams of milled black powder, mixed with 10-100 mesh titanium for a spark trail, is pressed into the motor casing at 8,000 pounds of force. I use the hottest black powder I can make, using black willow charcoal. The finishing touch is the payload. This can be stars, flying fish or flash powder for a nice salute finish.
To close the end of the rocket after the payload has been added, a fair amount of tooling is needed to form a professional looking end cap. I use three layers of moistened cereal box cardboard glued together with wood glue. The layered cardboard disk is pressed into the end cap former using the ram and the press.
After the rammer has been pressed into the former, I use a utility knife to trim off the ragged edges. Because the cardboard has been moistened, it cuts fairly easily by rocking the knife edge against the rammer. The result, after drying for a few hours, is a very neat looking and strong end cap.
Because of the amount of compression in the cardboard, it is very difficult to extract the rammer from the former. I use a short piece of 2 inch PVC and a dowel pin, as shown here, to gently tap the rammer and finished end cap out of the former.
Here is the top end of the finished rocket motor, after the end cap has been glued into place. Now, all that remains is to add a stick and a fuse to finish the rocket. I use a six foot length of whatever I can find for the guide stick. A dowel, a piece of finish molding or even 1/2 inch ID roman candle tubing is simply taped to the side of the rocket casing using wide masking or packing tape. To accomplish motor ignition, a piece of visco fuse or my home-made thermalite fuse is placed into the nozzle throat and held there with a wad of tissue. A good long one is recommended to give the operator time to get at least 100 feet away. The effect of these rockets is hard to appreciate if you are too close.
The finished product doesn't look very impressive, but in action, it will blow your socks off. I wish I could capture the effect on film, but a picture just doesn't do it justice. The best I can do is describe it in words that are woefully inadequate. Upon ignition, the rocket jumps out of its launch tube with a burst of speed, due to a high initial spike of liftoff thrust. The noise is louder than a cluster of D engines and the tail of bright, golden fire is about 25 feet long. The 5 second burn puts these rockets up about 500 or 600 feet before the payload is activated. I enjoy a good salute finish the most because the altitude is so high that visual effects are fairly diminished in size. Well, there you have it. There is a fair amount of detail that has been omitted in this description, but, hopefully, it has been enough to inspire some of you to discover the same fun and fulfillment that I have found in developing these big brothers to the cherished Estes rocket motors.


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