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 procedure 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 Jonathan Carter made for me.  (It was a lot prettier before I started abusing it!)

Only two materials are needed, plastic wrap and Nashua 324 aluminum-foil duct tape.  

 

Plastic wrap can be any kind.  Saran Wrap, Handi-Wrap, used shopping bags.... and it doesn't really need to be plastic.  Aluminum foil works very well, and I'm sure plain paper or wax paper would work OK too.  The only function of this first wrap is to keep the Nashua 324 from contacting the case liner. 

Here I am using Reynold's Plastic Wrap, which it good for this demo because it is colored, thus easier to see on the photos.  It also "matches" the color of the casing!  Martha Stewart (R) would be proud!  The primary purpose of this first wrap layer is to prevent the foil tape from adhering to the case liner tube when we seal it up. 

Without the plastic wrap or something, the foil tape would stick directly to the case liner.  I made that mistake the first time, and spent the better part of an hour at the launch peeling and scraping off the foil tape.  So this pre-wrap is quite a bargain in terms of time saved later.

Here's the stuff that took an hour to remove:



Nashua 322 on the right is the same thing, just not quite as wide as 324A.  It could be used, but would require four strips rather than three. 

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 great care to ensure that there are no such defects in our finished package, or all this work could be for naught.  

Bad news:  a special tool is required.  Good news is that it is cheap and easy to make.  Take a 1-1/2 inch female slip-fit PVC fitting, and file it out a bit on one end to make it more cone-shaped.  This will be used in the final operation of sealing our package to swage the tape down flat on each end, helping to make that airtight seal we so desperately need.



The case liner is loaded up with grains 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.  I've started doing that, using a little epoxy to hold them in place.  That way the case liner helps support the grains under acceleration, they should not bang around as much in the casing.
I often add some pyrogen between grains, like disks of fuse paper between grains to facilitate ignition. Some folks place O-rings between the grains to hold them apart a little ways.  Whatever you do to assemble your load before inserting into the case for firing, now is the time. 

 

Now that the load is assembled and ready to fly, I cut a sheet of plastic wrap a little longer than the case liner.  

 

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

 

If this load were to be fired tomorrow or the next day, this might much sealing might 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, the load will be difficult to ignite.  After a month it could be useless.  For long storage, plastic wrap is not adequate, 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. 

When I first started using this tape, I spent a lot of time trying to get the paper backing separated from the tape.  It was very hard to get anything between the tape and the backing.  Then I remembered something mom taught me:  Bend a corner over to make a small-radius curve.  This usually pulls the backing away immediately.  Works best if you roll so the backing is on the outside of the bend, the tape on the inside.  I like this tape much better now that I recall this trick.

The tape is flattened and laid out, 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 where the two strips join to make sure they are well-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 special 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 last night 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 such a manly thing.  

 

Thanks to the pretty polyethylene, the load slips out easily, ready to go into the matching motor casing. 

 

These packages may be impervious to moisture, but they are not immune to puncture.  They should be stored where they will not be banged against sharp things. 
Any compromise to the foil package may allow leakage.  And little holes can let in a lot of moisture - 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 324A 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, spending a quarter to keep the propellant fresh is a no-brainer. 

Until someone shows me another way that is easier, better, and/or cheaper, I will continue with this one.

Jimmy Yawn
Recrystallized Rocketry
jyawn@sfcc.net
rev. 12/26/07