Sounds like a joke.  Maybe it is.  But it works.  Better than I would have thought.

The Tape

First, let me introduce you to a new tape.  Well, new to me anyway.   It is Nashua 322 and 324A by .  It is definately NOT the common plastic "duck tape" used for everything except what it is good for.  This tape is a heavy aluminum foil with a very sticky glue, and some kind of stretchy polymer film in between. It is backed with a peel-off paper to keep it from sticking to itself.  If you do not have this tape, get some.  It is good.

Used as Grain Inhibitor
 
In fact, it was recommended by another rockteer on the SugPro list (I forgot who, my apologies!) who suggested it as a possible grain inhibitor.  Found some at Home Deep Hole, tried it, and sure enough, 5 layers served pretty well on 38mm candy grains in the Dr. Rocket casings.  But it seemed way too easy to possibly work reliably, so I went back to using posterboard/epoxy inhibitors for most of my tests.  But in the 5 motors 38mm I have tested with it, all burned as if properly inhibited, so I may do more definitive tests one day, perhaps even with the 54mm motor.

Used as Grain Packages

Back in November I was making a bunch of grains for a launch (which got rained out) and realized that this tape would make a good packaging for inhibited grains.  Four 38mm grains were lined up on a long strip of it, another piece wrapped the rest of the way around, and the ends folded shut.  This made a tightly-sealed "ready-load:"  just cut off the ends and slip it in the casing.  Perhaps the case liner could go inside the package too, but I haven't tried that.
 

Here is the 38mm load kit	Grain package is inserted into the 38-480 motor casing
Note that all the parts fit into the PVC tube,  keeping everything ready for quick loading in the field	Since this is a simulated loading,  the ends are not yet cut off the tabe tube

You can't see it, so I will tell you that the grains in this package are inhibited with posterboard/epoxy.  The tape is not intended to provide inhibition, although it probably helps some.  It is intended simply to keep the grains together and keep out moisture.  Oh, it might keep stray sparks off too.
 

Used as Model Rocket Grain Inhibitor/packaging

While developing the model rocket motor website, I realized that this tape would make a good inhibitor for these quick-burning, low pressure motors.  One layer performs admirably, allowing me to make sealed propellant grain packages that are quickly turned into Bates grains just by cutting off the tape at the ends, then cutting the grain in half.

Here are a grain and a header, packaged and ready to be installed in a motor casing.
 

The Revelation:

While rolling one of these, it occurred to me that enough layers of this tape might serve as the motor casing.  Quick and dirty is my style, so I tried it.  Worked remarkably well!  I am still looking for the first one I fired across the field.  I think it is in the woods beyond, still hiding from me despite the dayglo red stick.  It was the kind of behavior I would expect from the better-made model rocket motors.
 

Used as Motor Casing:  Duct Tape Model-Rocket Motor

So here it is:

I make a propellant grain and a header grain as per the model rocket motor.  The propellant is catalyzed with 1% red iron oxide to facilitate quick ignition. It may contain a few percent Ti flakes, or not.  I have found that Ti often burns holes in paper tubes, but so far has not done so in these tape tubes.  The grains are sitting in their cute little molding tubes just over the foil strip.

The tape is 2.5 inches wide.  This strip is cut 14 inches long.  The paper backing is peeled off the tape, and the mold liner removed from the propellant grain.  This grain is 1.5 inches long, 1/2 inch in diameter, and has a 1/8 inch hollow core.

The propellant grain is stuck on the end of the tape, and the header grain placed against it.  Two point five inches is just barely wide enough for this project, so the header is placed very close to the edge with just a little tape overlap.  Most of the extra tape is left at the nozzle end, where it will be needed.  The grains are rolled tightly in the tape.

It is kinda hard to get the grains started exactly straight.  So when the tape starts rolling off to one side, I just cut it at that point, stick the remainder on straight, and continue rolling until it is all a tidy silver tube.

So now I crimp the header.  There is very little overlap here.  I would like more, to ensure that it will not be spit out, but will use it anyway.  The edges are folded in with the fingers, and the end rolled on a hard surface to make a nice crimp.

Nozzle end.  One of my favorite oragami moves is to mash a round tube square, then make it a four-pointed star.  That is what I do here.  Then press the "wings" flat, but not before installing a fuse.  Yeah, I am using fuse here.  Yes, I know electric ignitors are more acceptable, but fuse is quick, easy, cheap, effective, and for this application reasonably safe.  Electric ignitors work just fine too, and for model-rocket type launches I will definately use them.
 

Pliers are used to crimp the flanges nice and flat.  Then I fold each flange down in a clockwise direction as far as the pliers will go, then press them down a bit further with fingers.

The pliers are then used to squeeze everything down around the fuse very tightly.

This little motor is just too cute to static-test.  So I wrapped a little tape around the head-end, dumped in a bit of black powder to simulate ejection, and taped it to a 4-foot red dowel.

Launch Time!

Click Here to view a video of this launch  (1 meg download, 9 seconds of video, the first few seconds of which were of my lens-cap coming off.)

Tiny Nozzles

These motors work OK, but not very consistently, nor with high efficiency.  I mentioned this technique to Stuart Leslie, who immediately offered an elegant solution.   Get a 1/2 inch phenolic rod, drill a concentric hole through the middle, and cut off little nozzles.  Now why didn't I think of that!  The local hardware store didn't have any phenolic rod, so I bought aluminum.  Not optimal, but OK for a test of concept.  I drilled a 3/16th inch diameter throat and cut off 3/8 inch.  Sanded the ends to get the roughest stuff off.

But I note that the tape is not nearly wide enough for this operation, so I cut 6 pieces, each 4 inches long.  These will be rolled sizeways so that my tape-tube length will be 4 inches.  First the nozzle, then the propellant grain, finally the header grain go into place.  

It is rolled tightly, and subjected to the hardwood ruler to make sure every spot is adhered-to.  Then five more pieces of tape are added and the whole thing rolled again to smooth them down.

With a little dexterity and judicious use of masking tape, I was able to make an ignitor that would just fit inside the 1/8 inch grain core.  The remaining tube end is squeezed shut against the ignitor wire.  I doubt that the exhaust gasses will have any trouble pushing it open.

Here is a static test of this motor.

Click Here for a video of this test
(1.1 meg download, 4.3 seconds of video)

Well that was dramatic, wasn't it!  Please take this as fair warning that the header system on these motors is not completely reliable.  Premature ejeculation is common, and perhaps 50% have failed so far.  But the fact that half of them worked is encouraging.  That means that it can be done, and with further development might be made reliable.

The next example includes a trick that I think will help.

Bates Grain Tape Motor

It's so easy to make little Bates grains... just make one grain that is double-length, cover it with tape, and cut it in half.

But first:  Let's get that header grain fixed.  I cut it a strip of tape about an inch long, roll it tightly, and then rulerize it.  This is to mash out any tiny folds and kinks that prevent complete adhesion.  I suspect that longitudinal folds are the cause of many of the failures...just a hunch.

The tape tube is trimmed down so that the edge height is a little less than half the radius of the grain.  Thus when folded over, they will cover all but a small spot in the middle, so that flame can get to the grain during the burn and ignite it, and it can get out to the ejection charge a few seconds later.  A spoon is used as a light mallet to tap the tape down nice and flush.  Didn't show you this, but the grain was rulerized again to round out the corners created by the spoon-pounding.

Likewise, the propellant grain is covered with a strip of tape, rolled well, and cut in half by rolling it under a sharp knife blade until it crunches through.

Everybody is lined up on the first strip of tape, nozzle to the left, Bates grains in the middle, header grain at the right and it is all rolled up.  Five more strips of tape added to make a stiff tube.

The header is open now, waiting for something to do.  I funnel a little black powder in it just for fun, cram a little piece of paper on top and and crimp it closed as illustrated above.  I am not really expecting the BP to open the head end, instead to give a little kick out the nozzle so we can tell when the header has burned through.

If nothing else, this excercise is giving me practice in making really small ignitors.  This one is just an exploding bridge wire made from network cable, with a pinch of black powder wrapped in a tiny strip of masking tape.  Sometimes there is a struggle getting them in, and once there seemed to be a struggle getting it out - the header blew and the motor flew backwards!  Guess I could make the core a tiny bit larger - it is smaller than the nozzle throat which is usually a no-no, but in these small motors seems to work fine.  Might even help them build up pressure, and I am sure the core enlarges rapidly once the burn is underway.

Flanges are folded, crimped, and mashed flat.

And here it is:

Well, not really, but I got a better picture of this one which is similar.  You will note that the nozzle end of this one is not squashed flat.  I am still deciding whether that is a good thing to do or not.  It certainly is more attractive when the "fins" are left out, don't you think?

Click Here for a video of this static test (1.4 meg file, 5.5 seconds of video)

The show so far...

  These little motors are a breeze to make.  Quick, easy, and cheap.   Juvenile fun.

  Casings are stronger than expected.  None have burst so far, although an occasional header has blown open, usually when folded to the point of cracking.  

  The finished motor is somewhat less than 18mm in diameter, and fits rather loosely in a model rocket motor mount.  Thus they might be upscaled slightly, offering a moderate increase in thrust over the paper-tube motors

  Kn ratios are low, on the order of 50 to 70.  Thus pressure and efficiency are also low.  The paper-tube/clay nozzle motors burn the same grains and seem much more energetic, possibly due to their higher Kn ratios.

  Calculations kindly submitted by Richard Nakka indicate that the tape tube should be able to withstand up to 600 psi, so testing will be done at higher Kn ratios in the future.

  The glue on the tape tends to melt, but only after the motor has burned out.  This technique would probably not be good for long-burning motors, not without substantial effective insulation.

  The aluminum nozzle may improve efficiency, and phenolic might be better.  The present nozzle is a simple plug with a cylindrical hole.  Adding a diverging cone should be easy, and might increase thrust slightly.  No flight tests have been performed so far with the aluminum nozzles, and my static test stand is not accurate at this low level of thrust, so we shall see...

  Burn-through delay header is not reliable.  Needs work.  Or replacement.  Or avoidance altogether.

An Idea:  
Perhaps this method could be upscaled by using a stronger "tape."  Mold a grain, wrap it with an insulating liner, stick on a nozzle and cover it with fiberglass, carbon fiber, or some other strong material.  Could this work?  I am tempted to try!

Jimmy Yawn

4/5/04