The motor will use solid fuel composed of fertilizer and sugar. The maximum theoretical specific impulse is in the 200-220 seconds range. The fertilizer is ammonium nitrate and the sugar is sucrose (table sugar). Intensive study is needed for final formulation of the propellant. The problems are ignition, phase stabilization and the casting process. It is the most critical part of the whole project. If this part is impossible the whole project will fail.
A fiberglass casing with graphite nozzle should provide a good mass fraction.
I decided there is no need to hide fuel details. The materials are legal in most countries, and the technical process intended is copied entirely from ammonium nitrate plants. Since both materials and information are publicly available for anyone, I will describe them here. This site is a documentation of team progress so it’d better be complete.
The fuel will contain ammonium nitrate mixed with 8-10% potassium nitrate for phase stabilization. The exact percentage will be determined by testing the volume change of actual grains after exposing it to temperature variation cycles. Then sugar is added (15% for the start), and everything is dissolved in water. A coffee filter will be used to remove undissolved impurities. Then a vacuum pump and a vacuum distillation set are used to drive the water off at a temperature bellow 90 Celsius degrees. That’s because at 90 degrees Celsius the ammonium nitrates decomposes. The outcome is a liquid with high concentration. It will be applied in layers and dried with a hot air gun.
Turns out I need special vacuum grease. Tests are further delayed.
Turns out the vacuum pump is no good. To be replaced.
I got a vacuum pump from a broken refrigerator for 25$, from a company servicing such equipment. Later I found this much cheaper alternative: https://www.avs.org/AVS/files/10/1043c4c6-597a-498f-a45d-7f1698d8c382.pdf. If tests are successful with the refrigerator pump I will try the syringe method too.
Failed another test. The pump is good and so is the vacuum distillation set. The hose is too soft (obviously).
Recent tests indicate the fuel might sustain combustion. Further tests are needed for measuring the burning rate. The production process is tedious.
Observation: With heating the pressure rises. The vacuum pump must be able to compensate. Also a manometer and a thermometer are needed for continuous monitoring.
The fuel burns alright under proper conditions. Working with it in a safe manner requires a lot of testing though.