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We make our own solid rocket propellant from basic components and build custom motors from scratch.  After much research and safety implementations, we have successfully built several successful solid rocket motors from a variety of common goods with very low cost.  There have been many experimental projects like ours, the technology is usually called 'Sugar Rocketry' or 'R Candy Motors' if you'd like to do a Google search.  Our favorite links will be at the bottom also.  High power rocketry is a hobby, where rocket kits and prebuilt motors can be purchased.  Check out the NAR and TRA websites for the professional stuff.  Navic has studied propulsion for many years, and focused on rocket propulsion systems specifically so starting with that knowledge helps.  KoD picked up the science quickly and maintain a strong focus on the hardware systems which can be found later on this page also.  Lets get to the fun stuff!

Rocket Science Basics:
A rocket is a propulsion system that carries fuel and oxidizer inside the machine.  In contrast, your car engine or a jet engine carries its fuel and uses the air from the atmosphere as the oxidizer.  Rockets can work in space where there is no oxygen, so it must be carried on-board for combustion.  Solid propellant is a mixture of the fuel and oxygen into a hardened form, and liquid propellant stores the fuel and oxidizer in separate tanks.  The mechanisms that create thrust by controlling the combustion of these propellants are categorized into 'motors' and 'engines'.  Motors means there are no movable parts, and typically solid propellant systems use motors.  Engines containing movable parts to carry the separate fuel and oxidizer from the tanks to the engine via pumps, so liquid propellant systems use engines.  Solid rocket motors are unique in the fact that they are typically the highest impulse motors that exist for space travel.  Most solid rocket motors ignite and expels massive amounts of thrust without the ability to stop until the fuel is gone, which means the motor hardware needs to be able to support this fast, powerful stress.  Because the fuel and oxidizer is contained in the system, the rapid burning of that mixture decreases the overall weight of the system which means the thrust generated will propel the system faster as burn time increases.

A simple cutaway of a motor:
Generally the combustion chamber converges 30-45 degrees and the nozzle diverges 12-15 degrees.  The throat is straight and typically a lot shorter than that picture shows.  Pressure is explained in Bernoulli's principle if you'd like more information.  Basically the propellant's combustion is fed through the chamber and throat to create thrust, and the nozzle controls the exhaust to maintain thrust efficiency.  As you can imagine, the high pressure, heat and speed of the flow through the chamber, throat and nozzle applies massive amounts of stress that can cause erosion.  If the structure can't withstand the forces, the motor will explode or just basically not work.

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KoD and Navic's Rocketry Program:
Sugar is a great fuel because it's chemically C₁₂H₂₂O_11.  Potassium Nitrate KNO₃ is a great oxidizer.  Bonding the two ingredients with heat and then igniting it creates great combustion and the exhaust is basically carbon dioxide and water molecules.  Sugar and Potassium Nitrate are cheap too!  A binder is a substance that helps bond the fuel and oxidizer together.  Karo syrup is good for that.  We combine the ingredients with a bit of water, heat it up while stirring to get the bond just right.  This takes a lot of patience and basically the cooking is a form of art.  Mastering propellant cooking will take several batches, but once you have the method down you can start changing the ratio of fuel and oxidizer to find the best mixture.  The ingredient measurements are not set in stone.  Our favorite is 100g KNO3, 50g Sugar, 18g Karo syrup, and 1/4 cup water.  Here is a video of a few test burns of our fuel mixtures:



PVC is great for the casing.  We grind up kitty litter and hammer about 1/2 inch or so into the end of the PVC pipe using a wooden dowel that matches the inner diameter of the PVC.  Packing it as tight as possible, it will serve as the top cap.  The freshly cooked propellant is poured into the case and then left to dry in which it turns rock hard.  We drill through the center of the propellant up to the kitty litter cap.  This is the necessary for proper surface area burning.  The motor part is built with the next larger diameter sized PVC pipe.  We have a 12 degree plastic cone in which we hot glue the motor PVC base to.  Using Durham's Rock Hard Water Putty, we fill it to the top of the cone, place a metal washer over the cone to strengthen the throat area.  A plastic straw cut to length is placed over the tip of the cone and on top of the throat washer and then the water putty is added to about 1/2 inch from the end of the PVC pipe.  After it dries for 24 hours we remove the hot glue and plastic nozzle cone.  Carefully cutting the 30 degree convergence for the combustion chamber with a knife then sanding it down to perfection, we have the complete motor.

Mating the motor to the propellant case is done by sanding the propellant outer case until it just fits into the motor case.  The 1/4 inch is just enough room to fit 2 or 4 metal screws of the right length to bind the cases together.  Hot glue is then placed evenly around the top to help seal them together.  Ignition is via a sparkler (high tech, lol).  This method of ignition at the bottom of the propellant grain is not the preferred one, but it's easy and cheap.  A typical motor will survive 4 high power burns until the chamber and throat are eroded away to the point of thrust degradation.  The propellant tube can be rinsed clean and reused as well.

Typically a 9 inch long, 1 inch diameter motor and propellant case with propellant costs about $4 in supplies.  The burning rate has not been measured correctly, but estimation is a G powered hobby rocket motor.  The fun in changing aspects like ingredients, ratios, propellant tube length, diameter, core diameter and nozzle geometry is rewarding because each change brings new results, sometimes explosive failures and sometimes highly powerful and efficient rocket motors!



We started out using metal nozzles from hardware stores which worked okay.  The main goal was to make motors that would fit into Estes rocket kits, so most were 1 inch diameter.  To get a thrust reading we created a test stand out of a metal bowel and a camping toast maker that hung from a metal broom handle.  We strapped a fish scale in between but since it moved a lot and the exhaust gases covered the scale, no good measurements were taken.




KoD and the Mini K-450:
Inverse Engineering is the most successful from-scratch rocketry expert we've found so far.  KoD bought his book and made a smaller version of the motor.  Here is his tutorial video followed by the Mini K-450 testing, which is really cool.  The failed ground test scale footage was slowed down and we saw 220lbs recorded before the explosion.

The videos pretty much cover everything except the ignition process.  We used sparklers as mentioned above, but the best method for a solid propellant motor is ignition from the top of the propellant core (opposite the nozzle end).  There are many different ways to ignite from the top, but most methods use nichrome wire because passing current through it generates a lot of heat.  Nichrome wire is what makes a toaster and it's also inside hair blow-dryers.  If you find these at thrift stores you'll have quite the supply of igniter's for cheap.  Find a way to get the nichrome wire to touch the propellant while maintaining a complete seal.  Apply current to the nichrome wire to ignite the propellant.  We tried this method a couple times with sample propellant and PVC casings but took no pictures or videos.  One method that seemed the best was to forget the kitty litter end cap, cook the propellant and get it into the PVC case leaving about 1/2 inch at the top.  Let it cool, drill the center burn hole through the propellant and then drill two small holes just big enough to slide the nichrome wire or copper wire through right above where the propellant stops 1/2 inch down from the top of the case.  We soldered thick gauge wire to the ends of the nichrome, leaving the nichrome part the diameter of the inside of the PVC case, this way plastic coated copper wire was actually through both ends of the PVC rather than the nichrome, which would get hotter than the copper.  Using epoxy and some hot glue after, we made sure the copper wire was sealed into those two holes we made.  Use a bunch of epoxy for a small motor or some hydraulic cement for larger motors to fill in the PVC end cap and fasten it on top of the propellant case, just above where the copper wires go through it.  Keep the whole thing upside down while the cap dries because at this point the propellant is dry as well as the nichrome igniter wires.  If epoxy gets into the propellant who knows what will happen!  Keeping it upside down to dry is the best way to ensure the propellant is as clean as possible.  Make sure the copper wires are long enough to give you safe distance from the motor, connect them to a power source and in a couple seconds the motor should be ignited!

Why top ignition is better than bottom ignition has to do with pressure build up.  Basically starting the burn from the top of the propellant down the burn hole into the combustion chamber allows pressure build up faster than burning from the bottom of the propellant burn hole into the chamber, thus wasting less propellant before thrust is achieved.  If you notice the Mini K450 videos, the motor is ignited and smoking way before the thrust starts - this is wasted propellant and energy from the bottom ignition system.



Nose Cone Ejection:
KoD explains a method of pushing out the nose cone for the parachute to eject to begin the recovery process:






CATOs:
CATO stands for catastrophe at take off.  Basically an explosion at ignition or soon after.  It's bad.  Here's a small collection of our CATOs:









Hints and Advice:

• SAFETY FIRST!
  
• An electric wok is great for cooking propellant
• Use a wooden stirrer, the propellant doesn't stick too much
• For larger motors > 1 3/4" use hydraulic cement for chamber/nozzle
• Change one thing at a time when experimenting (scientific method)
  

Links: