Cut the candybar into 10g chunks, and toss those chunks into the freezer for a few hours. (this makes it brittle enough to grind well)



While that is freezing we can start our engine plug, and potassium nitrate grind.



Lets start with the engine plug.



Measure out about 2 table spoons of kitty litter into your grinder, and grind into a fine powder (due to varying hardness and clumping, there may be bigger chunks, in the powder this is ok, just try to get it atleast 60% fine powder.



*Note* Some people have mentioned that these clay/dust catlitter plugs are blowing out, i did not have this problem, but some say if you use a pipe de-burring brush inside the pipe it may scratch the pvc up enough to give the litter a little more to grab onto.



Now take your 2.5"long pvc tube and place it standing up on the anvil and pouring 1 teaspoon of ground kitty litter into it.



Slide The round topped carriage bolt head first into the pvc pipe and compact the kitty litter using the hammer (hive it about 10 good whacks, so it really compacts together into a solid plug. (see 2nd photo)



This plug we will in the end drill the rocket nozzle (unlike in our tests we drilled the other end, and found some trouble with it)



Hold the PVC tube up to the light to see how thick it is. you want a plug of 1.5cm thick (remember the sides will be higher due to the bolt's head shape) if its thinner add some more kitty litter and compact using the hammer again. Mark the depth of the plug on the tube using the marker. this will be handy later for drilling/flaring the nozzle