Introduction: Simple Solid Rocket Engine
This Instructable is not intended to show a practical fuel application. The fuel I'll show here is one I play with a lot and use for demo's to show people the basics of rocket propulsion. This is an early fuel mixture that was used in early sounding rocket's but was quickly forgotten about when much more efficient fuels were found. This is a fairly safe fuel to make so I feel safe showing it here. This is intended as an introduction to rocketry and the fuel used therein. I hope you enjoy it and please feel free to comment if you want to know more.
If you are familiar with how a rocket works, feel free to skip to Step 1. Otherwise, fear not, just read on.
Here are the main types of rocket engines:
Solid Fuel Engines: These are what we're looking at, they are by far the simplest and easiest to get your head around. An example would be the Space Shuttle's boosters, the white rockets attached to the side of the orange fuel tank. This type can not be stopped once it's lit.
Liquid Fuel Engines: These are the most complicated; they work by combining and igniting two+ liquid fuels. The Saturn 5 that carried the Apollo astronauts used these. These can be stopped and started repeatedly as long as they have fuel.
Hybrid Engines: These are great, as of this writing I am beginning construction of a hybrid engine and will post when it's working, hopefully I can test in a month or so. These use a solid fuel that DOES NOT contain an oxidizer but rather a liquid oxidizer, either pure Oxygen or Nitrous Oxide is blown over the solid fuel to sustain the reaction. A Google search will turn up an article about a guy that works for Valve (yes, the game company) who made an acrylic and oxygen hybrid engine. These have the advantages of a solid rocket booster while being able to be throttled and even turn off and on.
Here's the very basics of how a rocket works:
A rocket works by burning a fuel to create exhaust, it's the release of this exhaust that generates the thrust that pushes the rocket skyward. Think of a balloon, if you inflate a balloon and don't tie it closed, when you let go the balloon will fly around the room as the air escapes. This is because the pressure inside the balloon is greater than the pressure outside. So the air wants to escape and there's only one way for it to go, out the bottom. You can think of a rocket the same way, the gasses given off as the fuel burns can reach a pressure more than 20 times the pressure of the outside air. This is denoted 20 atm. or 20 atmospheres. To give you an idea, the pressure inside a normal party balloon is around 1.01 atm. so only just a little above atmospheric pressure. The pressure in a small, suborbital rocket is at least 19 times that at a given second. I mention this both to give you an idea of how this stuff works and to explain the potential power of the chemicals we're working with.
Newton laid the groundwork for all of this in his early studies of motion, particularly the motion of the planets around the sun. I am going to assume that you have had no formal physics education from here on in so please understand that this goes much deeper then I have the time or wherewithal to explain. Newton observed, and proved, that every reaction has an equal and opposite reaction. So when the gasses are forced out of the nozzle, the rocket is pushed forward, just like our baloon.
If you have more questions, and I hope you do, please ask in the comments, I'm happy to answer what I know or point you in the right direction if I don't. If your really embarrassed to ask in the comments, I understand, just PM me.
Step 1: The Setup
Here is the list of items required for this project:
A double boiler: I don't own one so I improvised one as you can see above, just a large pot, half filled with water and another bowl floating in the water.
A small model rocket: you can get these at any hobby store, a Mosquito or Swift work well but any one like the one pictured above will work. They run around $5.00.
A scale: You'll need a scale that can at least measure to within a gram. The more accurate you can get the better.
Potassium Perchlorate: Order it off Amazon, you can get amazingly cool chemicals on Amazon. This will act as the oxidizer, it gives up oxygen when heated so the sugar can burn more efficiently. There are much more efficient oxidizers but this one is fairly stable so I though it was best for an introduction.
*A note on Potassium Perchlorate, this is a chemical used in fireworks, it sparks when struck hard. It's used to create those purple/white sparks in large fireworks, so be careful with it.
Sugar: Common household sugar. This is the fuel. When it reacts with the oxidizer, Potassium Perchlorate in this case, it creates very hot CO2, which is expelled at high velocities to create thrust as discussed on the intro.
Water: Tap water is fine but of course distilled water is the best due to it's lack of impurities. This is used to dissolve both the sugar and the Potassium Perchlorate.
Step 2: Fuel Mixture
The fuel we're making is often called "Candy Propellant" since it uses sugar as a main ingredient and smells like cotton candy when it burns. The flame produced should be purple and white with white smoke. Later I'll cover what probably went wrong if you see something else.
Rocket fuel mixtures are given as a ratio usually, this ration is a weight ration, not a volume ratio. So you will need a small scale such as the one pictured above. How much of the ingredients you use is up to you and depends on the size of the rocket you bought.
The Ratio we want is as follows:
So if you get one gram of sugar, you want 3 grams of Potassium Perchlorate, for a total weight of 4 grams. Of course the more accurate you are, the more efficient your fuel will be and you'll get a more impressive burn. It is worth noting that Potassium Perchlorate is denser then sugar, so even measuring out three time as much, will only give you a little more Potassium Perchlorate than sugar by volume. So trust your scale not your eyes in this case.
Step 3: Creating Our Fuel
Alright, now it get's interesting. I have the pictures above in order of the steps to follow.
Please read carefully, at least if you like your house in it's current, unburned-down state.
1) Heat your boiler so the water is.... well.... boiling.
2) Add the sugar you've measured out.
3) Add some water (this is where distilled water would be used if you have it), this is a bit tough to pin down, you want just enough water so that the sugar completely dissolves but you want to use as little water as possible, since your going to be drying it later.
4) Let the sugar dissolve, it'll look like the second picture. (I added some Karo Corn Syrup in that picture, but I've done it without and it works fine, the Karo just makes it a little easier to pack the fuel in later, so if you have it, great, if not you can use maple syrup, or just ignore it and it'll work fine.)
5) Add the potassium perchlorate, now your in a tricky situation, what you have is very flammable and if it get's lit, your going to have to douse it with a lot of water or a fire extinguisher. I chose Potassium Perchlorate because it's the most stable ingredient you can use here. But it is still rocket fuel so be careful.
6) Stir it in till it looks like the third picture, this will take a while.
At this point you just keep stirring, it will take a while, you want to get most of the water back out, we just used it to dissolve the sugar so it mixed more evenly, and heating it allowed the sugar and potassium perchlorate to both dissolve in the water more easily. Now we need to steam out most of the water till your left with a very dry dough. Once you have that, turn off the heat and pour/scrape your mixture into a clean bowl.
Congrats, you have effectively recreated fuel used in the early days of rocketry, I'm talking pre-NASA. But still, it's impressive and a good first step. This fuel was replaced by Sugar and Potassium Nitrate (more volatile but used in some fireworks as the propellant even today) and later by Potassium Nitrate and Sucrose (requires other ingredients and equipment and I don't even know what it's used for any more).
Step 4: Packing the Fuel
A normal SRB (Solid Rocket Booster) has a cone shaped, hollowed out area at the base, this facilitates a quicker, easier and more complete burn. To get this I just used the nosecone that came with the model rocket, just take it off, turn it around and plug the end of the cardboard tube as shown in the second picture.
Now you need to pack the fuel into the tube. I used a Popsicle stick, yes it's low tech but it works so what the heck.
In order to remove the nosecone I had to twist like I was unscrewing a lid, the fuel will want to stick to it so you'll want to clean the nosecone off before you put it back on the top of your rocket.
Now you have something that looks like the last picture. And now we get to have some fun.
Step 5: "Let's Light This Candle!" -Al Shepard
Now comes the moment of truth, I was very lazy and just stuck the nose of the rocket in the ground as you can see above. Now, as I mentioned before, I chose Potassium Perchlorate because it's one of the more stable fuels you can use. So you're going to have to work to light it, I just used my pocket torch but a kitchen lighter and a little patience will get it. You may get a few false starts depending on how much water is left and how well it's mixed. But when it all goes you'll get a good hot burn.
I'm sorry my pictures aren't all of the same chamber, but all of these are of the same fuel mixture we've been talking about so at least you can see that. The flame in the picture looks red but I can promise that in person, it's more purple than red.
Step 6: The End
Well I want to thank you for reading this. I know it's certainly not practical, but if space has grabbed you like it's grabbed me, this is still a cool experiment, this fuel actually sent up our early sounding rockets and you just made it at home and lit it. That to me was a lot of fun. As mentioned earlier I'm working on a Hybrid Rocket Engine at the time of this writing and will be testing it on the family farm in a month or so. I'll try and be sure to get a lot of photos and videos and make sure they get posted here. If your interested in that or have any questions just PM me and I'll try and reply quickly.
What went wrong?
I got a lot of black smoke and not much flame: There was something is the mixture that shouldn't have been, you either used too much Karo or Syrup or something else contaminated your fuel.
When I try and light it it will just spark occasionally but never catch:Your fuel is either still wet or was not mixed well enough. If you think it's just still wet, let it sit somewhere dry, away from things that can catch fire and away from things that can start them. When you do light it, be VERY careful, I've never had this problem but some people report that it can burn violently and dangerously if you pack it in then let it dry too much.
If you have other issues, just pm me.
Thanks again and God bless.