Model Rocket Motors
Nozzles - Rammed Bentonite Clay
(Click on a small picture to view a larger version)


Tools: A definition:  

"Ramming" refers to the technique of compressing something by the use of a hammer, mallet, rock, heavy stick, forehead, or some other pounding device, where the force is transferred to its target by a rod called a "ram." or "rammer."  In this page the ram is a section of wooden dowel, although metals, certain plastics, and perhaps other substances could be used.  

You may see the term "pressing" or "pressed" which seems to refer to the same thing.  Not quite...  Pressing means to transfer steady or increasing force by means of a mechanical device such as a hydraulic press, a screw press, stacking heavy weights, running over it with a truck, etc.  Sometimes I get sloppy in my terminology and use the term "pressed" when I mean "rammed."  Please do not let this confuse you!

The big difference is that ramming is done by pounding the ram, pressing is done by the application of steady force.  Both work.  Pressing has the advantage of being quicker and more precise...if you have the equipment and have it set up properly.  Thus presses are used in the manufacture of commercial rocket motors for speed and consistency.  It is much better for mass production.  But ramming has one great advantage:  you do not need a press to do it.  A simple hammer and straight stick will suffice, and work quite well once you have acquired a "feel" for it.

The Procedure

Here is a motor tube made yesterday.  It is 2.75 inches long (70mm) and 0.71 inches in diameter (18mm)  The inside diameter is a little over 1/2 inch (13mm).  These tubes vary some, but inside diameter averages about 0.515 inches.
1/4 teaspoon (1.5ml) dry Bentonite clay is funneled into the tube which is politely standing on end.  Wooden dowel (rammer) is inserted slowly and pressed down to contact the dry clay, while holding the tube down firmly so that the clay does not exit the open end.  If you push the dowel in too fast, the clay will squirt out like a thin liquid and laugh at you.

I usually put a  piece of masking tape over the bottom of the tube to keep the clay from running out, but in this case I simply held the tube firmly against the pounding surface until the first taps made it solid.
A few taps of the mallet, say 8 or 10, and the first part of the clay plug is set.  The another 1/4 teaspoon of clay is added, the rammer re-inserted, and the second bit of clay compressed with 8 or 10 more whacks.
A third increment of clay is added, the dowel inserted and tapped 8 or 10 times.  This makes a solid plug of clay that is a little over 1/2 inch long, and very tightly wedged into the paper tube.  It would seem like these nozzles would blow out easily, but they don't.  When a case like this fails it is usually the header that blows out, or a flaw in the casing which allows a burn-through.

To be fair I must tell you that I did not do much hammering on this counter.  That was set up for the photo.  After a couple of light taps I took this gizmo outside, put it on a concrete block, and hammered it well.  This is not a delicate operation.  You can use most any hammer or mallet, but use heavy ones with restraint.  It is quite possible to destroy the casing - hammer too much or too hard, and the clay will split the casing.  This rubber mallet works OK, as does a steel hammer.  But I think a wooden mallet might be ideal.  I will try that one day.
Dsc00111.html The magic of Bentonite is that it becomes solid when rammed.  It does not need to be wetted and dried.  Drying clay shrinks, and may become loose.  It also takes time for clay to dry:  rammed Bentonite nozzles can be used right away.  There is no need to defer gratification, or to put off your experiment for several days while the clay dries.  Find out if it works today, so you can think about it until next weekend.

My Favorite Clay

Bentonite is remarkable stuff.  A few years ago I had a new well put in.  After the driller had left I was rummaging around the trash pile (a persistent habit I am unable to break) and found some bags that said Bentonite on them.  I looked inside and in a corner found a little bit of the same stuff I've been purchasing from pyrotechnic suppliers for years.  Doing a little research, I discovered that Bentonite is also known as "well-driller's clay" and used for sealing well casings in loose sand, among other things.  It is a prime ingredient in kitty litter, and in "Oil-Dri" absorbent clay.

Bentonite is volcanic ash that was deposited eons ago and weathered into an ultra-fine silica.  Here is a very nice photo of Bentonite hills in Utah.  It is named after the Fort Benton formation in eastern Wyoming.  And here is a description of how it is formed.  The Colin Stewart Minchem site gives a description of its chemical structure, as well as a list of uses.

I was running low, so I called a local well driller's supply and said I wanted some Bentonite.  The clerk asked:  "What kind?"  I was unprepared for that question, and hesitated. He said "It comes in several grades.  What do you want it for?"  I said "rocket nozzles."  He said "For WHAT?" but recovered his composure rather quickly.  I admitted not knowing much about the stuff, and told him that I wanted the finest, most compressible grade.  He said: "You want 'quick gell."

So I went and got some Quik Gel.  $13.00 for a 50 pound bag.  And the clerk was right. It is the best Bentonite I have ever used.  But I was surprised to see on the label that Bentonite is considered a carcinogen, and breathing the dust is cautioned against.  Guess its sort of like asbestos - chemically inert but a physical irritant.  So I will now take care to avoid breathing the dust and recommend that you do so too!

On the other hand, a Google search for Bentonite Clay generates a list of dozens of natural-health sites promoting Bentonite for a variety of health purposes:  to dry oily skin, in the bath, for making soap, for cleansing the bowel....  I didn't see inhaling it as a recommended practice.  It is used as a fabric softener, a pond-sealing liner, and to make rocket nozzles!

So we must now make a nozzle in this clay plug.

Since your nozzle drill is completed (right?) that will be quick and easy.  The reamer is inserted into the plugged tube and twisted until it drills its way through the clay to open air.  This usually pushes out a flake or two from the bottom of the plug creating an unsightly irregularity.  We will take care of that shortly.

The first reamer I made  used an abused old drill bit.  It took a lot of twisting and turning to make a hole.  I sharpened it by hand and improved it a little, but sharpening drill bits is not my forté.  I am much better with knives and scissors.  Then I tried a masonry bit.  It worked OK, but was slow.  I tried a spur-point wood bit.  It was very slow.

The best?  A plain old high-speed twist bit is excellent.   But please get a new, sharp one.  It's well worth the $2.00 or so. 
An exit cone is made by reaming the nozzle pilot with a suitably-shaped knife blade, or in this case a pair of surgical scissors.  Their taper is a bit too steep.  I need to find or make a better reamer for this purpose.  Any suggestions?

Since the end of the nozzle was chipped, I decided to make it pretty and scrape with a knife blade to make a shallow taper.  This is primarily for aesthetics - I don't think a few greeblies at this end matter very much.  I am willing to be corrected on that matter if anyone would like to enlighten me.  A good alternative is to rub the nozzle-end back and forth across a sheet of sandpaper a few times.  This makes a nicely smoothed, pretty end.
Case is ready for the propellant!
Next...Making the Grain

Jimmy Yawn

rev. 10/10/04
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