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
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
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 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
End Burning Gerbs with Ferro-Titanium in Them
(Photos by Nancy Stewart)
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
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
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
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
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
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<
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
Rod Pushing Fuel through Funnel into Tube for Final Increments
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.
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.
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.
Rocket Stabilizing Sticks
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
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
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
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
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
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
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
Till then, have fun and Stay Green,
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