I have developed a pretty simple technique for rolling accurately sized KNO3/Sugar propellant grains. It is quite a bit faster than casting the grain in a mold and allows a lot of flexibility in sizing and coring technique. These are intended to work in the reusable rocket motors I developed that can be found by Clicking Here.
 
 
Rolling Propellant Grains
February 17, 2002
 
 
Project Goal:
 
Create a simple and fast technique for creating propellant grains
 
Project Parameters:

Construction Results:

Not much construction actually. Basically just used masking tape to fasten a piece of 400 grit wet/dry sandpaper to a flat board. I taped all four edges. Then I made to rails at the height of the intended grain diameter. I started with wood dowels, then found I needed another .040" so I added a strip of styrene. The strips are the white vertical lines in the photo above. Anything will work it seems. I taped these rails in place as well, although now that I know it works, I could probably break down and buy some screws and do it right!

I then took a scrap piece of wood and taped more 400 grit to it (held in gloved hand in the picture). This is what you hold when rolling the grain.

This photo is a bit confusing because it already has a nicely rolled grain in it, but hang on and I will explain how to form it as well.

 

After heating the KNO3 and Sugar untill melted, spoon some out into your hand- NOT! This stuff is very hot and will give you a wicked burn if it touches your flesh! I usually spoon a bunch onto the sandpaper covered board, then roll it up with my GLOVED hand. Note: these are very thick leather gloves! When you start moving it around, you get a feel for when it becomes the consistency of a dough ball. It can be worked easily at this point and is ready for our rolling.

 

To get the rolling started, shape it into a short little log and set it in the middle of the sandpaper.

 

Now just roll the scrap board (with sandpaper on bottom) across the propellant a few times. As it starts to cool it will get harder. Give it a final roll as it hardens and make sure it is straight. The scrap board should be riding on the rails on the side so that the diameter is accurate.

 

End result is a "stick" of propellant ready to break to length. For my first test I coated each stick with epoxy to inhibit burning on the outer surface. I also coated the outer surface with high temperature lubricant for good measure before inserting into the casing. So far I have made the following grains:

 

Update 3/9/02:
After much testing, the best inhibitor solution seems to be to wrap the grain in a few layers of glue saturated brown kraft paper. Diluted wood glue works pretty well for this but if the paper is too wet, it will desolve the outside of the sugar grain. This sometimes defeats the purpose of the inhibitor since the outside can start to burn if any gaps develop. Now I am using 5 min Epoxy for the glue and it works very well. After a burn, the remaining paper tube is intact with evidence of even burning inside. The motor casing seems to be cooler as well due to the insulating qualities of the paper. I use the same 5 min epoxy/kraft paper wrap for my delay grains as well as it seems to provide enough compression strength to allow me to seal without o-rings.

Ah Dextrose, keeps grains their "whitest white"! This is what they look like before wrapping

After the grain in epoxy/Kraft paper, I have found it helpful to give it a short wrap of wax paper. This helps provide a smooth even surface and insures that the end doesn't unwrap. Tape holds it all together while the epoxy dries.

Here are a batch of  wrapped grains after the epoxy hardens and the waxed paper is removed.

 

Rolling the grain under an Xacto knife makes for a clean cut. Usually just snaps off clean after cutting through the wrap.

 

Static Test Results:

Finished adjusting my "light duty" load cell for these motors and got some pretty good results.

(Click on image to see full size)

This is the Offset Core grain on the static test stand. Pretty long burn for a small sugar motor!

 

(Click on image to see full size)

This is the C-slot grain test fire. Some strange oscillations are appearing but I think it is from the test stand. The new motor mount stands pretty high and it may be bouncing around a bit. Burn time of about .400 seconds.

 

(Click on image to see video)

This is an AVI video of the C-slot firing on the test stand.

 

Conclusion:

While I am using these grains in 1/2 inch diameter motors, I would imagine this could be scaled up to about an inch or so in diameter. Might get a little tricky handling such a large mass of hot propellant but I will let you know how it goes when I try!

The best thing about this technique is that it enables a lot of flexibility in experimenting. I can cut the grain to any length, and with any type of core before loading.

Return To Home

Click here to mail me feedback!