24mm Sorbitol motors:
Drysophila Project: Grain Construction

When I first tried to make 24 mm sorbitol motors I poured directly into the case. This gave inconsistent results with sporadic case burn through and pressure spikes due to flame front finding its way up the side of the propellant. The solution to these problems came in the form of autoclavable (ie heat resistant) plastic test tube caps. These are K-16 caps available from Bellco. They measure 38mm long, 19.5mm O.D. 18mm I.D. so give a slightly progressive "Bates" grain. The propellant is poured in these caps. They are made into bates grains and delay grains and then inserted into the paper/clay casings. This method along with switching to silicate for the paper case glue has solved the problem so far.

For the Bates grain a 1 mm hole is drilled in the center of the end of the cap. The drilled hole is too small for any significant amount of propellant to leak out yet allows the needle on the end of the coring dowel to pass through thus allowing the wooden dowel coring rod to be centered as the propellant cools. The minimum order is a case of 1000 caps with a minimum total order of $50. So I bought a case !

This is enough for 500 D16 motors so I better get busy :-)

A casting rack is made by gluing some paper casing tube sections to a board. They are made snug so the cap does not sink in. They are long enough to allow for the extended needle. This rack actually has 8 tubes Two are shown along with the caps and dowel. Also note the pink plumbing washer that helps center the coring rod at the top while the needle in the bottom hole centers the bottom of the dowel.

For propellant info; see Richard Nakka's site Read Richard's safety page: Wear gloves and eye protection and a safe method of heating. I prefer a double boiler oil/paraffin. Keep the heat source shielded and cooler than the ignition temp of the propellant. Keep the oil covered/contained so propellant can not fall into it. Also avoid any spilling of oil onto heat source. Some other sugar sites mention other types of heaters. For me I'm sticking to the double boiler.
Here is an idea; Take a bag of chocolate chips and melt without stirring. If the chocolate burns you have hot spots. A double boiler won't burn the chocolate.

The melted propellant is poured into the yellow cap making sure to eliminate any bubbles and make complete contact to the inside of the cap. The needle end of the coring rod is inserted and fish around center bottom till the tiny hole is found. Push the dowel to the bottom. scrap any excess propellant overflow at the top and add the pink washer to center the top of the dowel. Turn this dowel every few minutes so it doesn't stick and remove after 3 or 4 turnings. Just as soon as the propellant is set enough to not slump.

I use a coring rod that is smaller (4mm) than my usual (5.5mm) desired core so I drill the core out larger which allows me to also drill through the cap end. The drill bit doesn't get too gummed up if I use incrementally larger drill bits.

This is not a classic Bates grain in that it is a bit long and one end is inhibited. Nakka's SRM.xls motor simulation file can be made to account for one end inhibited. To see how this is done go to the first of my SIM pages..... CLICK here.


The last grain was the thrust Bates grain. The following is the "delay" grain. But as you will see this grain also provides some thrust as well. The Nakka SRM.xls sim file can also be made to account for this added "partial" grain's thrust.

The difference in construction is the cap end is not drilled. The coring dowel only goes deep enough to allow the remaining propellant to act as a delay grain. I also add a plumbing washer that stays with the grain. The washer may not be needed by why mess with success.

After the propellant is poured, the washer is installed and the delay grain coring rod inserted. The depth will determine the delay.

Image below shows a poured grain setting up with the dowel set to give a 6 second delay. The black top washer fits tight to set the delay position. The pink washer is loose and will remain with the grain while it cools after the rod is removed. (Why? keeps the moisture out.)

The rod is twisted every few minutes and removed after a few twistings

The grain can then cool for several hours before the pink washer is removed. I don't have trouble with short term moisture once it cools

The sketch below (top drawing) shows the size ratios to scale. The second grain shows how the epoxy end plug is drilled 3mm to provide a touch hole for the ejection charge.

Click here to go to the motor assembly page.
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