My first experience with rocket propulsion technology

I got my first real experience with rocket propulsion technology when I was doing my bachelor thesis. I decided to choose a topic that I was passionate about and that would allow me to do experimental work. When I approached my would-be mentor with the idea he panicked a bit because he is not an experimental physicist. But he soon figured out that I am very independent and skilled in experimental work. For my research I had to build my own rocket engine and measurement system (figure 1).

I searched for literature, other rocket enthusiasts and parts online and I soon had a nice database to work from. I discovered that most amateur rocketeers use a mix of potassium nitrate and sugar (sucrose, dextrose, eritritol etc.). Because it is relatively cheap and available I opted for the mix of potassium nitrate and dextrose. After a few tries and tests I got the hang of making the fuel. I built the first few test “engines” with PVC piping and concrete nozzles. Once I got a feel for what the thrust and pressure levels are and how the whole thing behaves I bought a graphite rod and machined some nozzles with different exit angles (figure 2). After that I constructed a test stand to which I could mount the engine and do the measurements (figure 3).

I did a few measurements on the engine with two of the machined nozzles and compared the measurements with theoretical predictions (figures 4, 5 and 6). The equations are listed in the bachelor thesis (link bellow). The thesis is written in my native language (Slovenian) but you can easily recognize out all the equations. I also used a program which a fellow amateur rocketeer created (link here) called SRM 2014 (Solid Rocket Motor). The authors name is Richard Nakka.

As you can see from figures 4, 5 and 6 the measured thrust, pressure and temperature don’t exactly match with predictions. The reasons are many but the biggest contributors are moister in the fuel and slow/uneven ignition of the propellant.

Even though the results did not quite meet the expectations I still learned a lot from doing the research/experiments and it further deepened my interest for rocket propulsion.

Figure 1. Measurement system with mounted rocket engine.
Figure 2. Machined graphite rocket nozzles: a) exit angle 0°, b) exit angle 15°. Both nozzles have the same intake angle 45°.
Figure 3. Schematic of measurement stand with mounted rocket engine: 1) rocket engine, 2) nozzle, 3) measurement head for temperature and pressure, 4) pressure sensor, 5) force plate, 6) temperature sensor (K-type), 7) pressure sensor connection pipe, 8) wall for force plate, 9) test stand base, 10) rocket engine mountings, 11) extra weight.
Figure 4. Theoretically predicted thrust vs. time curve for both nozzles and measured force vs. time (dotted curve) for both nozzles.
Figure 5. Theoretically predicted chamber pressure vs. time curve and measured chamber pressure vs. time curve (dotted curve) for both nozzles.
Figure 6. Measured chamber temperature vs. time for both nozzles. The theoretical chamber temperature is equal to the flame temperature which is around 1200°C for the potassium nitrate – dextrose fuel mixture.

About the Author: Marko