Tidy Packages for 54mm loads When I go to fly nowdays, it is usually with a 54mm motor.   Here the techniques used for sealing up 38mm motors are adapted to the Loki-style 54mm propellant load to create a sealed package that is impervious to moisture, and thus can store sugar propellant loads indefinately.

The process starts with a propellant load already made, test-fitted into the motor casing, and taken out again.  

At the bottom of the photo are three (simulated) propellant grains.  The brown thing above above them is the case liner tube.  I purchase casting tube and case liner tube from Loki.  One day I will try making my own for the 54, as I do for the 38mm motors, but for the moment, the purchased tubes are quite good, and reasonably cheap.  

This load is to fit the 3-grain 54mm casing Jon Carter made for me last year.  It was a lot prettier before I started abusing it!

Materials for the casing are plastic wrap and Nashua 324 aluminum-foil duct tape.  

 

Plastic wrap can be any kind.  Saran Wrap, Handi-Wrap, used shopping bags, wax paper.  Here I am using Reynold's version and it is fine.  Even better because it is colored, thus easier to see on the photos.  The primary purpose of this plastic-wrap layer is to prevent the foil tape from adhering to the case liner tube when we seal it up.  Without the plastic wrap, the foil tape would stick directly to the case liner, and getting it off later would be quite a chore.  So I waste a little plastic wrap now in order to save myself a lot of trouble later.  



The foil tape makes the big seal.  Plastic wrap is porous to gaseous moisture.  It seeps even when sealed.  Aluminum provides a near-perfect barrier to atmospheric moisture, but only when there are no creases, folds, or perforations in the package.  So I will take much care to ensure that there are no such defects in our package, or all this work is for naught.  

There is one special tool required.  Good news is that it is easily made from a 1-1/2 inch PVC fitting.  It is a female slip-fit coupling, filed out a bit on one end to make it more cone-shaped.  This helps it slip onto the end of the load casing to swage the tape down flat.  



The case liner is loaded as if it were to be placed in the casing and fired immediately.  If you wish to glue the grains in, this is a good time to do so.  Or add some pyrogen between grains.  I often stick in disks of fuse paper between grains to facilitate ignition.

 

A sheet of plastic wrap is cut a little longer than the grain load.  

 

It's all wrapped up, and the ends twisted and pressed flat against the grains.

 

If this load were to be fired tomorrow, this might well be enough.  But this thin polyethylene is semi-permeable to atmospheric moisture, which will diffuse across the membrane and be absorbed by the propellant.  After a week, it will be very difficult to ignite.  After a month it will be useless.  A better seal is required.  



Hence the aluminum tape.  Cut three strips the length of the package, or a little longer.  Not shorter.



While I'm at it, I cut two short pieces, roughly square.  These will cover the ends.



The paper backing is peeled from one strip.  It is flattened and laid, sticky-side up, so that a second strip can be pressed down on its edge, sticking them together.  They should overlap by 1/4th to 1/3rd inch.  Be sure to press on the seam to make sure the strips are stuck the whole length.  It is easier to do this when the paper backing is still on the second strip, before it is peeled.

 

The second strip is peeled, and a third added.  Finally, the combined strips are wide enough to go around the load.  

 

Load is placed in the middle, and rolled up firmly and smoothly.  
 
 

Any overlap at the ends is pushed toward the middle.  One of the squares is peeled and pressed on the end,

 

The corners are folded down and squeezed round.  

 

To round it out a little more, the end is rolled on a flat surface.  



Now it's time to use that tool.  The swage is pressed on the end of the tube, forced down as far as it will go, and rotated a bit.  

 

This squeezes the last 1/3rd inch or so down very tightly, eliminating folds and creases that are likely to leak.  



Do the other end, and the load is ready to store.  Oh, but do label it.  I have not illustrated that here, but I do it religiously.  At least with the date and project number, so I can look it up in my notebook days, weeks, or months later and figure out what it is.  

Here is a real load out in the field, ready for use.  Click Here for the page reporting that launch, which includes an exciting movie of the opening of this load and its insertion into the motor casing.  



Seems a shame... I just made this package and here I am about to open it up.  But that's show biz!  

A sharp knife point is used to slit the tape, which peels back easily.  Hmmm.  Wonder if I put a strip of dental floss under the wrap, would it make a zip-open pack?  No, that's just silly.  And it might leak.  Besides using a knife is a manly thing.  

 

The motor load is now easily accessible, and ready to go into the motor casing.

 

These packages should be stored where they will not be banged against sharp things.  Any compromise to the foil package may leak, allowing moist air to enter.  As barometric pressure changes, air will be forced in and out of the tiniest hole, carrying moisture with it.  

Now some folks will say "Hey, isn't that foil tape EXPENSIVE?"  Well, yes, sort of.  But let's put it in perspective.

I just bought a 60-yard roll of Nashua 324 from Home Depot for $14.37.  
Each of these 3-grain packages uses about 1 yard, so one roll would make 60 packages.
$14.37 / 60 = $0.2395.  
Thus the tape for each of these packages costs about a quarter.  To me that is a bargain.  

Considering the effort and expense of making those propellant loads, the time, trouble, and gas money getting to a launch site, and how much I rely upon them once I'm there, the cost of a quarter to keep the propellant loads fresh and usable is a no-brainer.  Until someone can show me a better way, or a just-as-good way that costs less, I will continue with this method.

Jimmy Yawn
Recrystallized Rocketry