The Reload

Tidy packages for sugar propellant grains.

This page pulls together several techniques developed over the last few years to produce sugar-propellant reloads for Aerotech - type casings.  

An executive summary might read:
  1. Make tubes:  inhibitor and case liner
  2. Make the propellant
  3. Mold propellant into the inhibitor tubes
  4. Insert grains in inhibitor tube
  5. Seal with foil tape

The only "special tools" one will need are two hardwood dowels.  Lots of other fiddly bits will be used as we go along, guess I should make a list of them eventually.

Dowels:  one 1-1/4 inch diameter for making the case liner tubes, and another 1-1/8 inch diameter, for making the inhibitor tubes.

Two hardwood dowels, 12 inches long, one 1.25 inch diameter, the other 1.125 inch dia.  

Find nice hardwood dowels, straight and reasonably round.  Cut sections about a foot long, and sand them nicely.  
The larger dowel is for rolling case liner tubes, and is left unfinished.
The smaller one is for rolling inhibitor tubes.  It is covered with plastic adhesive tape, because it will be exposed to wood glue and we don't want it to stick.  

I am sure that metal or plastic rods could work just as well, but this is what I have, and it works.

Duct Tape !

Another essential is Aluminum Foil Duct Tape.  Note that this is NOT the plastic kind!  I am using Nashua 322 and Nashua 324A from Tyco Adhesives.

Two types of aluminum foil tape

The only differences between these two that I can ascertain is that the 324A is 2-1/2 inches wide, 322 is 2 inches wide.  
Also, that 324A has its name written all over it, but 322 is plain foil-colored, much prettier!  I wish the 324A did not have lettering.  
This is incredibly useful stuff.  

Inhibitor tube

Inhibitor tubes should be made first, as they need to dry a day or two before using.

Obtain a piece of common posterboard.  I am using Royal Brites # 24301 (white) which measures 0.011 inch in thickness.
This is available at my local supermarket - the white board is $0.59 for each 22x28 inch sheet.  
Office Depot has them in packs of 10 for about $4.00
Colors cost a little more - I use them sometimes because I like them, and because it makes the different tubes more apparent in these photos.

I begin by cutting a piece of posterboard 8-1/2 inches long by 5-1/2 inch wide.  Be sure to determine the bias of the board first, and cut so that the bias is with the long side.  Bias is the direction that the board "wants" to roll.  It will roll much more smoothly and easily in one direction than the other.  Usually, you can determine the bias by simply rolling the board into a tube.  It will be much easier one way than the other, which will tell you the bias.

Some posterboard does not have a pronounced bias and doesn't like to roll in either direction.  Guess that is good if you are going to use it as a poster and want it to lay flat, but it is not good for these tubes.  The board pictured below is of this type.  It did not roll very well.  If you find your board is like this, do the best you can - it is possible to make it work.  But buy another kind next time.


Now we mix a little glue with water, roughly half and half.  I usually use Titebond wood glue.  But Elmer's works fine,  it's just a little thinner so don't dilute it as much.  Now that I say that...I forgot it was Elmer's, poured in too much water and had to add more glue.  Oh well.  It's cheap.


The glue/water should be mixed to a creamy texture, perhaps a bit frothy.  Slather it onto one side of the posterboard nice and even, roll the board once around the 1-1/8 inch dowel, and be sure to "tuck" the leading edge under the dowel.  It will make a "squinch" sound and feel as it tucks under the edge.  Press and roll the dowel to roll up the rest of the posterboard.

What do I mean by "tuck" and "squinch"?  Here are a couple of photos:

First one is untucked.  Second one is tucked. Work your fingers down to where the action is to make sure the leading edge is well tucked before continuing the roll.  Otherwise, it will be oversized, and may have delaminary gaps.   Click here for a movie of "the squinch."  It is the clicking noise you hear as I am tucking the ear under.  


Let the slathering begin.  Remember I said to "obtain" posterboard?  I suggest buying it, and usually do.  Money tends to facilitate these kinds of transactions.  In this case it is only a little money, so be generous.  But this particular posterboard was obtained.  I saw it lying around in a classroom for weeks after the the presentations on that topic were done.  When Winter Break came and we did a little classroom cleanup, it was fair game and I absconded with it.  Hee hee hee!  I love Education!


Secure the newly-rolled tube with several rubber bands, slide it off the dowel, and stand it on end to dry.  If you lay it flat, it is likely to dry in an elliptical shape.

It's not a bad idea to wipe the glue off the dowel before starting another one.  I have ruined more than a few strips of posterboard by gluing them to my stick.

I use the disposable foam brush not because it is cheap, not because it is convenient, but because it is the best brush I have found for this purpose.  And you don't have to throw it out - just wash it well before the glue hardens and it will be as good for the next round.  You can just throw it in the washing machine with a load of towels, but don't let the S.O. see you do it.  And you had better be the one to get it out too.


That tube needs to dry for a day or two.  But if you are in a hurry, let it dry for an hour and then put it in a low, low oven, perhaps 150 degrees, and leave it for another hour.  That should dry it fairly well.  Bruce Clemens says he puts them in the microwave for a few seconds to set the glue.  I haven't tried that yet, but will!

Case liner

This is also made from posterboard, but it does not need to be glued. Instead, it is wrapped with the aluminum foil tape mentioned above.  I am using a different color in these photos so that it is easy to tell which tube I am working on.  In practice, the color matters only in that some of the colored boards are a bit thinner than the white ones.  

Cut a strip of posterboard 5-5/8ths inches long by 12-1/2 inches wide.  Again, cut across the bias so that it will roll easily the long way.  Cut two strips of Nashua 324A, each about an inch longer than the strip is wide, say 7 inches.  


Twelve and a half inches is enough to circle the inside of a 38mm motor tubing three times, with just a little overlap.  
I find that two thickness is usually enough, three gives us a bit of insurance.


The tube is rolled, and the end trimmed to leave about 1/4 inch tape all around.  The dowel is used to press the tape nice and square, getting ready for the next operation.


Note that one end of the larger dowel is tapered.  The last 1/8 inch or so has been sanded to a 45 degree angle, making a truncated cone.  That is to assist in this pressing procedure, to ensure an even squeeze.  

Insert the tapered end into the tube, pressing the edge of the tape back against the tube wall.  Do both ends, slide the tube back on the dowel and roll it on the table a time or two.  Remove the dowel and notice what a pretty crimp has been made.  Nice, smooth, even, O-ring-supporting edges.


Making the Grain

Because of the different sizes of dowels used in making the two different tubes, they are matable.  The inhibitor tube slides into the case liner tube with little effort, but also with little wasted space.  


Here we are getting ready to make the propellant grain.  It will need to happen quickly once started, partly because I am impatiently using 5-minute epoxy.  Also because it is a chilly today in our "Winter Wonderland" and I am out in the breeze.  The propellant is heating in the toaster oven at 200 degrees, but once out it will cool very quickly.  This batch has 1 percent Ti added, so it will look a bit dowdy, gray and grainy.  But wait until you see the exhaust plume!  It is also a slow-burning batch of toaster-oven propellant, but that reeks of another experiment.


OK, here is a heads-up on how things will go together.  The case liner will serve as a support for the inhibitor tube while we press soft propellant into it. The inhibitor tube will be coated on the inside with epoxy before the propellant goes in.

The liner will hold a 1-1/4 inch "fender washer" which has a .281 inch diameter hole in the middle of it.  The dowel you see is our coring rod.  The end has been sanded to a point, then most of the point sanded off to make a short truncated cone.  The cone is then pressed real hard into that washer to make a little shelf.  This will serve to keep that end of the coring rod centered while we press in the propellant.


Note once again that the inhibitor tube fits nicely into the liner.  But before it goes in, we must coat the inside of the inhibitor tube with epoxy.  

Epoxy?  Why?  That seems like an awful lot of trouble...   Why?  To make the propellant stick!  Candy propellant will often stick to paper products, but not reliably.  Yeah, I know - some people cast directly into the paper with no glue and get away with it.  But they are usually using a melt/cast propellant which may well be stickier than mine.  So if it's important, I hedge my bets and glue the tube.

Topside of the washer is greased before inserting into the liner tube.  We don't want epoxy sticking to it, or propellant!


Make a tool.  Take a section of doweling at least 8 inches long, cut a strip of paper toweling, wrap tightly around the end and secure with a rubber band.  This is the epoxy swab.  Here I am dancing it in a glob of 5-minute epoxy, about 1/4 teaspoon.  It doesn't take much, just enough to get a complete, even coating inside the tube.  I move the swab around and around, then back and forth, then around again and look at it.

That looks good!  If you see any dry spots, big globs or struggling insects, feel free to take corrective action.  Small insects can stay in the tube.  They will probably die from the epoxy coating anyway, since insects breathe through their carapace.  Might as well go in a blaze of glory.   Large insects should be removed.  They might reduce ISP.


Now the coring rod has been dipped in water (yes, water!) and wiped off.  This is the mold release agent that will allow the rod to be withdrawn.  It is environmentally friendly, cost-effective, and it works.  Please do use it.  

Flash forward, November 2005:   I have since realized that Vaseline works much better than water.  Vaseline is more persistent, and does not moisten the grain.  Now I am recommending this finger-friendly grease as the coring rod release agent.

The propellant patty is out of the oven, the epoxy is starting to cure... it's showtime!

A chunk of propellant is cut off with a table knife, and formed into a "snake."  Snake is slithered around the tropical hardwood dowel and eased into the tube.  


The warm, soft propellant is packed to the bottom of the mold with a slender stick.  If the propellant sticks to the stick, stick the stick in some water and wipe most of it off.  That should end the sticking for a few pokes.  


When you get to the top of the tube, you might notice that the coring rod isn't perfectly centered at that end.  Not to worry.  Just press the propellant DOWN on the near side to move the stick toward the middle.  That will make a little crater, so press in another little blob of propellant to bring that end of the grain up level.  

Side note:  I didn't do it here, but using a poking stick one web-width in diameter will help to keep the coring rod centered.  In this case the web thickness is 3/8ths inch, same as the diameter of the coring rod.  Is that a coincidence or what!

To make the top-end grain pretty, I press another 1-1/4 inch washer down on that end.  This washer has a larger hole, about 1/2 inch.  I hope one day to obtain some washers with a 3/8ths inch hole so they can be used for centering as well.

Now before you walk off, be sure to give the coring rod a twist to break it free from the propellant.  If you don't do it now, you may have a heckuva time doing it later.


At least with rcandy, if the rod gets totally stuck you can just put the whole thing back in a 200 degree toaster oven, soften the propellant, and remove the stick.  You will probably have to remake the grain, but can recover the propellant.  On the other hand, this stuff is cheap, and a grain with Ti in it would make quite a sparkler, don't you think?  And the coring rod would make a good handle....

Cooling of the grain is patiently supervised by Mr. Buckley.

As dawn breaks, the new grain emerges from its casing.  This grain could be fired as it is for a progressive burn, and I am tempted...  

But I have an experiment in mind, and so will cut it into three equal sections.  


You were wondering about the chef's knife, weren't you?  It's one of my favorite tools.  The inhibitor tube is marked into exact thirds, the casing is scored all the way around, and the knife blade rolled around and around until the inhibitor is cut completely through.  The grains can then be broken off by hand.  Yes, the ends are a bit uneven, but I don't think it matters a whole lot.  


Wanna see some action?  Here is a video of me breaking up that grain:  <><><>

Grains get dropped into the case liner tube.  Here is the experiment.  I am placing squares of black match paper between the grains and at the ends.  I suspect this will improve ignition by spreading flame at the grain junctions more readily than would be spread by the propellant flame itself.  


All three grains are in, time to seal up this package.  Note that the case liner tube is now slam full.  Doesn't that warm your heart?  I'm cutting another piece of foil tape to cover one end.


Might as well put a piece of match paper at the ends too, else the tape might be a problem.  This foil tape serves as a mediocre inhibitor.  It works very well by itself in model rocket motors, moderately well in 29mm motors, and not very well in 38mm motors.  I haven't tried it in my 54mm motor, and probably won't.  But sticking this tape directly to the end of the grain might delay or even prevent its ignition.  The match paper keeps the tape off the grain, and should spread flame.  Maybe I should have cut it round to fit better.  Darn!  Another experiment!

The tape is hand-pressed onto the liner tube.


Hand pressing leaves some folds and gaps, so a motor casing is used to swage the tape all the way flush.  It works pretty well - what a handy tube!  Then I roll the package a time or two on a flat surface to even things out, and it is done!  

The finished load a thing of beauty.  It should be airtight, watertight, spark resistant, loyal, trustworthy, and brave.  I imagine that the propellant will keep inside one of these for a long time.  Just how long?  Well, I need to do some tests on that issue.  

Loading the motor casing

Here are some photos of a load I made and fired a few days earlier.  Click here to see details of this test.


The load is placed in the motor casing with its end-seals intact, thus the propellant is still sealed from air until the moment the ignitor needs to be inserted.


The forward bulkhead and nozzle assemblies still need to be installed.  It is hoped that some of these parts can be included in the package at a future date.

The tube remains sealed inside the motor casing.  If needed, it can be removed from the casing without breaking the seal.  

But if one were using a delay-grain in the forward bulkhead, it might be a good idea to poke a hole in the forward end of the package before loading it in the casing.  Then again, the propellant does not seem to have any problem burning through that one layer of tape, and the tape would ensure that the delay grain was ignited by the motor, not by the ignitor.  This may require some experimenting!

Test Firing

Here the motor is placed in a test stand, and I am about to make a hole in the foil so the ignitor can be inserted.  The pointy thing is a chopstick.  Very useful things, chopsticks.  Besides providing us Americans with humorous dining experiences, they can be used to poke and prod where fingers don't easily go.  This one has a little epoxy on the end because I used it to mix the stuff once.  And it reinforced the tip pretty well.


In this experiment, I ran the pointy thing all the way down the motor, to make a hole through each of the little pads of black match paper.  

The Test

Click Here for a video of this test
(900K .mpg, 3.5 seconds of video)

Click Here for an Excel spreadsheet describing this motor's performance.

Hey!  I'm not used to getting pretty thrust-curves.  I need to try that again

Another Test

3/5/05 - Similar test using even-slower burning propellant
(no photo yet - it's getting developed.)

Click Here for the Excel spreadsheet on this motor test

That's not too bad either.  I suspect that the black match paper is helping to get the grains going by providing a strong ignition source between the grains.  Otherwise they may lag behind, generating a progressive thrust curve when the burn should have been "neutral."  Check out my tests page for these.



Quick Quiz: What item was missing in my assembly photos?  

Answer:  Aerotech users got this one 100% - It's the fiber washers that protects the O-rings from exhaust gasses.  

And guess what?  You don't really need them in these loads. The foil tape apparently provides enough protection for at least a 38-360 firing.  Note that the nozzle-side O-ring is not badly burned.  Under a microscope, the O-ring shows a few shallow zorched spots, but nothing mission-threatening.  It could be used again.  I have seen much worse burns with commercial Aerotech reloads and washers.  

The inhibitor tubes are burned away at the ends, but that is to be expected.  They are intact in the middles, which is to be hoped.  The thrust curve suggests good inhibition.  

Case liner is burned somewhat.  The first layer of posterboard is about 50 percent burned away.  The second layer is about 1/3 gone, the third layer is almost entirely still there.  It has a scorch-mark here and there, but has not allowed burn-through at any point.  The aluminum-foil-tape covering is completely intact.  

The motor tube needs little cleaning.  Soaking in water and a wipe inside and out is generally adequate.  But I have found some buildup of sticky stuff inside the 360 motor casing, probably glue melted out of the foil tape.  It is soft, so probably not epoxy.  WD-40 cleans it off with a little scrubbing.  

One more test

Oh, here is today's experiment.  Essentially the same thing but using the 38-480 casing with four Bates grains.  


Click Here  for the Tests page, containing an analysis and video of this test.

The motor performed well, but the thrust curve was truncated by my leaving the INA125 amp at too high a gain level.  That needed to happen at some point, I had to prove that one needs to reserve some headroom in these tests.  That will teach me.  Well, probably not much.

Again, liner held OK, case cleaned up with a rinse and a wipe.  Mid-H performance despite the flat top curve.

This page is submitted for amusement and enlightenment purposes only.  Any other use is strictly forbidden by some law, somewhere, no doubt.

Comments, criticisms, additions, and bad jokes are welcomed!

Flash Forward!  
September 5th, 2005

After four months of Florida swelter, I pulled out a "tidy package" moonburner load I'd made for launch back in June but did not get a chance to fire.  It has been sitting in my magazine since it was made on 6/5/05.  

I decided that it was time to fire it.  I expected it to be all gummy because the 54mm loads I had assembled back then had a thick coat of goo, despite being "sealed" inside their commercial paper/phenolic case liner tubes.  Somehow they had leaked a little moist air.  I scraped off the soft stuff down to hard propellant, and they worked OK.  

But this one, when opened, was crisp.  No trace of moisture, no gummy stuff.  It was as dry as the day it was made!  I fired it as test 9-5-05B, and its performance is nominal.  

Now this is just one test.  I have not yet done quality-control sampling of this technique, but the fact that it worked once suggests that it might work again, perhaps reliably.  

Recrystallized Rocketry home page

Jimmy Yawn
rev. 12/10/05