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How to Make End-Burner Rocket Motors




There are three common types of simple, black powder, charcoal-tailed, rocket motors: cored, nozzleless, and end-burner.

A cored rocket motor is the traditional motor, typically with a clay nozzle, and a hollow core going up through the nozzle and into the fuel grain for some distance. This is how the typical skyrocket is constructed. If you take the nosing off the bottom of a commercial skyrocket and look up into the end of it, you'll see about a half inch tall ring of clay, which forms the nozzle, and then a longer core going up into the black fuel grain.

A nozzleless motor is a fairly recent development as far as I know, and it has no clay nozzle, but does still have the core going up into the fuel grain. It uses a hotter fuel than the cored rocket motor.

An end-burning motor has a clay nozzle, but no core going up into the fuel grain. The fuel burns only from its end, and this type of motor typically uses a hot fuel and has a smaller hole, or aperture, in the clay nozzle than does a cored motor. Often, full strength 75/15/10 (potassium nitrate, charcoal, sulfur) black powder is used as the fuel in these motors, and typically the diameter of the nozzle aperture is one quarter of the inside diameter of the motor tube (3/16" nozzle aperture for a 3/4" ID rocket tube).

It is this latter type of motor, the end-burner, which I intend to use to test the power of fuels made with 5 different charcoals in an upcoming article, Some Notes on Experiments with Various Charcoals.

The nice thing about these motors is that they burn for a fairly long time, up to about 10 seconds, with a constant amount of thrust. That thrust varies considerably from one charcoal to another, in my past experiences.

Below is a shot of a motor that I carefully sliced in half with a hacksaw, outdoors. You can see how solidly the fuel packs when dampened just a bit, with no separation between fuel increments. You can also see how the top of the nozzle is shaped to direct the flow of the burning gasses smoothly out of the hole.

End Burner Rocket Motor
End Burner Rocket Motor

End burning motors are useful for more than just rockets, though. They can be used as short duration gerbs (fountains, pronounced like the first part of "gerbil"), or as drivers on wheels or girandolas (horizontal flying wheels). When I want metal sparks in the motor's exhaust, I add 6% fine ferro-titanium or spherical titanium powder to the fuel.

End Burning Gerbs with Ferro-Titanium in Them End Burning Gerbs with Ferro-Titanium in Them
End Burning Gerbs with Ferro-Titanium in Them
(Photos by Nancy Stewart)

Small Girandola Using End Burner Drivers Large Girandola Using End Burner Drivers
Girandola and Wheel, Using End Burner Drivers
(Photo on right by Mike Hrnciar)

The methods for making end-burner rocket motors are very similar to the ones for making regular gerbs and less powerful wheel drivers. Those devices typically are made on a larger diameter spindle, and their fuels are less powerful than the rocket motors.


Making End-Burning Motors


For the fuel, I've made 75/15/10 ball-mill-dust using each of 5 charcoals (See Ball Milling 101 for details), and now I'm going to make 3 end-burning rocket motors using each type of mill-dust.

After milling, I add 1-3% water by weight to my mill-dust fuel, and screen it in well, to minimize dust during the ramming process, and to produce a very hard, solid fuel grain. Most folks do not dampen their fuel prior to making rockets. Some just use the mill-dust as-is. Some dampen it, then granulate it through a screen and then allow the granules to dry, thereby cutting down on the dust.

During the dampening process, I find a bit of difference when it comes to how much moisture each type of fuel needs to dampen it a bit. The willow and commercial hardwood mill dusts each required 1.5% or so....Whereas the other 3 charcoals required about twice as much water to achieve the same degree of moisturization. I know the fuel has enough water in it when it stops being dusty and free-flowing.

I press my motors in standard, 7.5" long 1 lb. (3/4" ID) rocket tubes. If I am using full-strength, hot black powder I use strong-walled tubes from New England Paper Tubes. These tubes are very strong and resist side burn-through. For weaker powder, I can use Skylighter's standard 1 lb. rocket tubes. During the pressing I find that each motor requires 3 oz. of fuel for all the charcoal mill dusts, except the whitewood charcoal, which only needed 2.75 oz. of fuel to fill the tube.

Here's a quickie tutorial on pressing end-burning motors. We covered Making Nozzle Mix in Skylighter Fireworks Tips #89. Here are some end-burner spindles and rammers, from Steve LaDuke and Rich Wolter (wolterpyrotools.com).

Tools for Ramming End-Burning Motors
Tools for Ramming End-Burning Motors

The tapered-end rammer with the hole in it is for pressing the nozzle, and the flat end rammer is used to press the fuel.

There are various types of tube supports which can be used when ramming or pressing motors, but I don't find them to be necessary when using a mallet to hand ram these motors. I do use a support when pressing motors on a hydraulic press. (See notes on presses in Tiger Williw Shells in 2 1/2 Days - Part 2). Here are two types of commonly used tube supports. The reason they are used is to prevent the paper tube from splitting under the extreme pressures of hydraulic pressing these motors.

Two Types of Motor Tube Supports, Wolter Aluminum Clamshell and PVC Pipe and Hose Clamps
Two Types of Motor Tube Supports, Wolter Aluminum Clamshell and PVC Pipe and Hose Clamps

Before I ram the nozzle, I plug the hole in the spindle with the end of a bamboo skewer, and then install the tube onto the spindle.

Spindle with Hole Plugged for Nozzle Ramming
Spindle with Hole Plugged for Nozzle Ramming

Then I ram the nozzle using 1/2 tablespoon of the nozzle mix, with 8-12 good blows with my rawhide mallet. Each time I add an increment of fuel, I ram with the same number of blows from my mallet, using approximately the same force each time. It is important to keep the hole in the tapered rammer cleaned out.

Cleaning Nozzle Mix Out of Rammer with Drill Bit
Cleaning Nozzle Mix Out of Rammer with Drill Bit

I then pull the spindle out of the tube, remove the bamboo, insert a doubled piece of thin blackmatch which projects out of the spindle 3/8", and re-insert the spindle into the motor-tube to ram the fuel.

You might notice that in the photo of the different spindles above, there are two types of spindles which are identical except one type has holes in them, and one does not. If you have several tools like this, you can press the nozzle with the solid spindle, remove it, insert the other spindle with the blackmatch inserted in it, and then press the fuel. This eliminates the need for the bamboo plug.

Spindle with Blackmatch Prior to Loading Fuel
Spindle with Blackmatch Prior to Loading Fuel
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The blackmatch gets embedded into the fuel grain for a very reliable, ignition priming system. This is a piece of one of the five strands of thin blackmatch that can be found in flat quickmatch.

The fuel is rammed, about a tablespoon at a time, to within 1/2" of the end of the tube, and then a clay bulkhead is rammed on top of the fuel.

Ramming Fuel in Tube Fuel in Tube Rammed
Ramming Fuel in Tube

Rod Pushing Fuel through Funnel into Tube for Final Increments
Rod Pushing Fuel through Funnel into Tube for Final Increments

Inserting Clay into Tube Clay Bulkhead Mix Rammed in Tube
Inserting Clay into Tube, and Clay Bulkhead Mix Rammed in Tube

If a header is going to be installed on the rocket, a drill bit is hand-twisted into the clay bulkhead mix to create a "passfire" hole. The passfire simply directs fire from the burning rocket motor to the contents of your header. A header can be a star shell, or any other pyrotechnic effect that you can get your rocket to lift.

Drilled Passfire Hole in Rocket Motor Bulkhead Drilling Passfire Hole in Rocket Motor Bulkhead
Drilling Passfire Hole in Rocket Motor Bulkhead

A simple rocket heading can be made with some aluminum-foil duct-tape, 1/2 teaspoon of FFg BP, and some stars.

Simple Tube Heading Measure of Black Powder
Stars for Simple Heading Simple Star Heading on Rocket
A Simple Starburst Rocket Heading

I like to use two sticks on a rocket for balance. Home Depot sells some nice 1/4" x 3/4" pine lattice which works well for these little motors. Sticks are necessary, because without them, the rocket will not fly in any particular direction. Sticks act like a kite-tail. They help your rocket fly straight.

Cutting Rocket Stick Stick Stabilized Rocket
Rocket Stabilizing Sticks
Rocket Stabilizing Sticks

Blackmatch Embedded into Nozzle after Spindle is Removed
Blackmatch Embedded into Nozzle after Spindle is Removed

The last step is to add some visco fuse to your finished rocket. Just cut a 4-5-inch length and insert it in past the blackmatch all the way into the nozzle. Your fuse should fit into that hole snugly and not fall out.


"Dialing In" the Motors and Fuel


For years, these motors performed very consistently for me. Then, while making and testing the motors for this article, some of them started to blow up on me, obviously exerting too much pressure in the motor tubes.

Well, "Dang It." I had changed something: either the potassium nitrate I was using, or the charcoal, or the darn alignment of the stars in the heavens. But something had changed and now my new batch of fuel was too hot, too powerful.

This is an all-too-common experience in fireworking. It can be minimized by consistent manufacturing techniques, and the use of consistent quality and type of chemicals. Unfortunately, we hobbyists do not usually buy large quantities of chemicals, so, as we re-supply, variations can be introduced, resulting in the type of problem above. (You can reduce this problem by shopping with reputable vendors who carry the same grades of materials for years.)

This ends up requiring and cultivating diligence, persistence, flexibility, patience, and carefulness in the budding fireworker, as we recognize and work to solve these problems. Meticulous note-taking will help you reproduce successful results. Unless you record everything you do, it will be next to impossible for you to achieve success repeatedly.

The fuel that is currently too hot for my motor tubes was made from ball milled black powder, using a combination of willow and pine charcoals. I have milled up quite a few batches of this fuel and now have a 5 gallon bucket of the fuel, which has been thoroughly mixed so it is consistent throughout.

I know that fuel made with commercial airfloat charcoal is less powerful than the above fuel, so I mill 2 batches of it. My plan is to make up some fuels which are mixtures of the above two fuels, until I hit on a proportion of the two which results in a high-performance motor which does not blow the tube to smithereens.

I have an electronic scale I use to measure rocket thrust, to be detailed in a forthcoming article, Some Notes on Experiments with Various Charcoals. I also time the motor-burn with a stopwatch.

I have found in the past that a reliable, powerful motor burns for 9-10 seconds with 1.3-1.5 pounds of thrust. Beyond those parameters, the motors start to blow up.

I ram a motor, made with fuel containing only commercial airfloat charcoal. It burned for 12.3 seconds and produced .9 pounds of thrust on average. (This is better performance than I've had in the past with this fuel, so apparently this new batch of potassium nitrate that I’m using results in a more powerful BP.)

Next I ram a motor with 3 parts of the weaker, commercial charcoal fuel, above, and 1 part of the hot fuel containing the pine/willow charcoal. This motor burns for 11.5 seconds with 1.15 pounds of thrust.

The next motor has 50/50 of the two fuels, and it burns for 10.8 seconds, at 1.3 pounds of thrust. We’re gettin’ somewhere. This motor, with this amount of thrust is usable for a driver or girandola motor.

I want to end up with nice, consistent, high-thrust motors, but not have them at the edge of blowing up with some of them failing. I have a 40-motor, 36" girandola planned for this summer. I'm going to enter it in the competition at the Pyrotechnics Guild International's annual convention in Gillette, Wyoming; I don't want it to fail because some of the motors blow up.

I decide to try one hotter motor, stepping up to 1 part of the slow fuel, and 2 parts of the fast fuel. This motor burned for 10.5 seconds and had a bit over 1.4 pounds of thrust. It also had that "sound" of a motor being on the edge of blowing up.

I'm going to back off to the 50/50 ratio of the fuels, press up 9 motors with ferro-titanium in them as I would with girandola motors, let them dry out for 2-3 days, and then re-test them to make sure they are working consistently with that fuel mixture.

When these motors were dry, I took them out to my testing grounds and burned all of them. Three of them, mounted on the digital-scale test stand, burned for 10.5 seconds, with an average of 1.2 pounds of thrust, and they were all consistent.

Then I launched the rest of the rockets with and without headers, and with sticks of various lengths on them, so that they each had different weight payloads. Typically these motors fly well when their thrust is 2.5 to 3 times the total weight of the rocket.

For a 1.2 pound thrust (19.2 ounces), then, a rocket ought to weigh in the 6.4 - 7.7 ounce range. This is also the way to calculate how much a girandola ought to weigh, depending on how many drivers are on it.

For the above girandola example, with 40 of these drivers, the total thrust will need to be 40 x 19.2 = 768 ounces. The final 'dola ought to weigh between 256 and 307 ounces, or 16-19 pounds. The lighter it is, the faster it will climb and the higher it'll fly.

Of the sample rockets in this current batch that I launched, all flew well when their total weight was 6 -7.75 ounces, and they failed to fly well when their weight was 8 ounces or more. The above calculations would have predicted that.

I think that these motors, rammed with 50/50 cool/hot fuel have enough thrust, but are not near the "edge" where there is a risk of a percentage of them blowing up.

In the next article, I'll be using these end-burning rocket motors to test black powders made with the 5 different charcoals.

Till then, have fun and Stay Green,

Ned

Materials Needed
Read and review these Fireworks Safety Articles before starting any fireworks project.


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