24mm Sorbitol motors:

Drysophila Project:

Booster Motor Assembly

Note: there are probably easier ways to make a flame resistant tube to deliver ignition from the booster at burn out to the top of the sustainer core.
Ken from Sugpro suggested teflon tubing.
I haven't had time to try this yet. (Sure would be easier than what follows ;-)

The following instructions are what worked for me, using what I had around the house.

Using the upper-stage/sustainer motor find a plastic straw that will fit in the nozzle and up the Bates grains' core.. Leave a little slack. For these 24mm D motors with 5mm nozzles a coffee stirrer worked well. Set the sustainer aside, as this straw will be built into the booster motor.

Find a dowel that fits in the straw. Cut a strip of foil that can wrap around the dowel. Cut the straw to a length that will reach out of the end of the booster motor and up to the top of the Bates grains in the sustainer motor. In the picture below I used a previously made unit to determine length rather than remeasure everything.

Wrap foil around the dowel and insert into the straw

Cut the foil slightly longer than the straw. The foil lining is needed to keep the straw from melting prematurely. (Some discussion on SugPro suggested the foil may not be needed. I still like to add it.)
Carefully withdraw the dowel leaving the foil in the straw.

Glue the ends of the foil to the ends of the straw. Let dry.

Locate a drill bit the same diameter as the straw and drill a hole in the center of the KapUts cap

Insert the straw into the hole in the cap. Glue it in place and align. I use 5 min epoxy. I also then folded the foil over the straw.

Using string that has been well coated with meal powder and dried, cut the fuse a bit long. Insert into straw taking care not to catch foil. What is meal powder? check out Wikipedia, they have an entry and United Nuclear gives a good writeup

Make sure fuse extends a bit out both ends and tag it in place with more meal powder slurry or NC. I made a mess on this one and it might not fit in nozzle so I later trimmed it up a bit.

I then made a long case that would serve to hold the cap/straw while pouring the propellant.

A coring dowel with washers is so that one can change the depth of the core and control when the sustainer ignites. I made my first at 0 second delay and the static worked well. The first flight was also 0 second delay and flew well. RocSim shows the optimum sustainer ignition delay to be 2 or 3 seconds. I will try a 1 sec delay next time. I'd rather keep this delay on the short side for safety in flight

After pouring let cool till solid

Purple arrow says pour epoxy several mm thick.

A regular Bates grain is inserted in the case followed by the booster grain we just built.
Because the booster grain is partially cored this is actually a "1 and 3/4 grain" motor See SIM data
The SIM prediction is a low E motor as an E29. The static data shows this to be a high D as a D25.

Below is a schematic showing the difference between the booster delay and a sustainer delay

The booster motor is placed in the booster section with thrust ring at the base. The booster assembly is carefully aligned with the sustainer that has a standard D16 sorbitol motor. The optimum delay is 7 seconds when flying these in the rocket shown. So D25-0 to a D16-7 are the motor designations here.

Based on one static and one flight this seems to be a good system for low power and maybe mid power rockets. Larger rockets can afford to have the weight of electronic ignition and they should for safety reasons. (This process might have some value? in larger rockets as a backup redundancy to the electronic 2nd stage ignition.) This bird is ready to Fly !

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