Introduction: Candybar (Milkyway) Rocket Engine

About: Im a bit of a geek of all trades. Of late most of my free energy has going into Making sure our hacker/makerspace is awesome! Come check us out!
Quelab (My local Hackerspace in Albuquerque NM),  Had a project theme of "Hacking Chocolate" for the month of February.  I'm allergic to eating chocolate, so I wanted to think of a use that was a bit more exciting than just watching others eat the stuff. 

So I decided to experiment with making a chocolate (candybar) powered rocket engine.  A quick googling showed me that I was remembering right that people do make Sugar and Potassium Nitrate rockets. (And even pixi stick rockets), so I decided to see if i could do it too but using a candybar! (you know how many calories there are in them Right?

Figuring sticking with a space theme, and avoiding complex compounds such as peanuts.  That a Milkyway candybar was just the ticket.  (turns out that was a lucky decision,  more on that later)

Step 1: Materials,

You will need the following Tools

- A small scale
- A hammer
- A grinder (mortar and pestle like i show here, or a coffee grinder you don't plan to use for food again)
- An anvil or chunk of steel or other hard object to pound on
- A Domed head carriage bolt (I believe the head size of mine was 3/4" and it was about 4" long)
- A drill, with, 7mm drill bit. and a counter sink drill bit, (or a  larger 1/2" bit will do too. 
- A permanent marker.

You Will need the following materials.
- Some clay kitty litter (a few ounces of clean clay based kitty litter will do) this will make the fire proof plugs at both ends of the engine
- A MilkyWay Candybar,  this will provide the fuel
- Some Potassium Nitrate (salt peter) (as pure as you can get) Oxidizer, 
- some thin wall PVC pipe (about the same internal diameter as the carriage bolt head), cut it into 2.5" lengths
- some cannon fuse,  or model rocket engine igniter.

Step 2: Getting Started.

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

Step 3: This Part Is a Grind.

With The candybar still in the freezer chilling,  measure out 25g of Potassium Nitrate into the grinder.

Grind it into a powdered sugar like consistency. (if using a electric grinder do it in small short grinds. to avoid getting hot)

Set it aside in sealed box.

Once the candybar has been frozen for a while,  go ahead and get it out and quickly grind it into a fine powder  (its tough to get it super fine,  due to the caramel,  may need to re freeze and grind some more.  the better the grind the better the rocket.

Once its well powdered, mix the powdered candybar and the powdered Potassium Nitrate , (if using a hand grinder feel free to grind the two together (Carefully minding heat and sparks)if not must mix well with a spoon)

Step 4: Now to Do Some Powder Testing. (optional)

I first tried Milkyway,  but later one of my co-hackers picked up the project and tried just plain milk chocolate Hershey's bar, which we found to not work as well at all (nearly not at all),  turns out the higher chocolate concentration was forming too much of a charred carbon/ash and was insulating the oxidizer and keeping if from getting hot enough to work)

I had done some test burns of the Milkyway, and Oxidizer,  but we also wound up doing test burns with several types of chocolate bars. to see if any worked better (they did not,  i suspect the malted nuget is helping it act more like pure sugar)

To do the test,  we would make a pile and light it with a fuse (or a torch and see how well it burned.  worth doing with your powder too before you go through the work of making the rocket.  If its popping and spitting too much you may not have it finely enough ground or not well enough mixed. 

Step 5: Time to Fill the Engine

Similar to the step of making the plug, we are going to add a little fuel/oxidizer and then tamp and then add some and tamp...

Add a full teaspoon of powder mix into the pvc tube and then insert the bolt and tamp it down, with 5 or so blows of the hammer, Remove bolt and add another spoon, and tamp.. keep going until you run out of fuel, or until you are about 2cm from the top of the tube after tamping. This will be for the top plug.

mark the fuel level, and then fill the remaining space with powdered kitty litter, and tamp it down, (may need to fill it up a few times,   each time it will compact a little less) once its not packing much more you can quit.

Step 6: Drilling Out the Throat and Nozzle

Sadly i don't have any great photos of this process, so  please bare with my crude drawing.  

Using a small drill bit,  3-4mm drill a pilot hole through the bottom plug (this would be the first layer of kitty litter you put in the tube and has been the bottom of the engine since the start.   It would be best to drill this with a drill press to make the hole nice and verticle. drilling in until you all most meet the other plug.

Be sure to remove the bit and clear out any fuel stuck in it from time to time,  Also dump out any loose powder we want this to be a very clean hole. keeping things slow and cool. as you go.

Now re-drill it with a bigger bit, i used about a 6mm (I think 7mm would be even better, you can experiment with different sizes)
(this hole up inside the rocket is called the throat.)

And then lastly you need to create a nozzle (or a cone) by grinding away some of the plug around the throat hole,  i used a 1/2" drillbit and just held it in my hand and twirled it to flare out the nozzle.

Step 7: Woosh!

It was ignited by a small bit of cannon fuse, I'm pretty sure we could use a standard esties rocket engine igniter, but we didn't have any on hand.

Might be a little anticlimactic, since we didn't actually put it in a rocket and launch it,  or even strap a stick to it and make a big bottle rocket.  but we currently have some burn restrictions here (drought), so we are keeping it on the ground for now.  That will come in the future.

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