Hybrid Rocket Experimental Rigs

Key themes and skills developed:

·        Finding, interpreting and applying design/scientific literature

·        Developing and conducting validation experiments

·        Techniques for collecting reliable data

·        Expanding understanding of Arduino-C programming and ancillary hardware incorporation

·        Learning about rocket technologies and manufacturing processes

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I don’t think I need to elaborate on why working on rocket motors is of interest to me, they are rockets, after all. I had wanted to attempt rockets as soon as I got my lathe in 2014, but what these ambitions ripe for realization was my discovery of relatively inexpensive disposable Oxygen cylinders. They each provide 1L of pure Oxygen at 1600psi initial fill pressure. A short history of my endeavors pertinent to rockets goes something like this: in 2013 I experimented with developing cheap solid rocket motors using Potassium Nitrate and a variety of fuels. After acquiring the lathe in 2014, I was able to make these somewhat successfully, but they proved very expensive to scale up – then, online sale of KNO3 was banned in the UK. By 2015, I had encountered many project archives online documenting powerful and compact hybrid rocket motors that used seemingly benign fuels such as PVC plastic or plain old candle wax – this seemed well within my grasp. I purchased my first Oxygen cylinders in 2016 and put together a crude rocket system that I mounted to my bicycle (don’t worry, I have a much more thorough appreciation for safety these days). This turned heads but didn’t produce tangible thrust. Later in 2016, I designed a more considered rocket motor testing apparatus, with two motors: one would use paraffin wax, the other ethanol. This system was relatively complex, featuring wired control of ignition and valve actuation using a pneumatic system. The paraffin wax motor destroyed its ethanol competitor, producing thrust that maxed out my primitive thrust measuring system (and simultaneously fatally overheating it). I was in awe of how quickly it created heat and was inspired to pursue the technology further. In 2017, after returning from a study abroad program having taken a circuits class and loading up on bargain electronics components, I set to designing a more scientifically considered rocket motor. In the summer of 2017 I designed and manufactured the most recent design for a paraffin wax hybrid rocket, along with a sturdy rig that would permit me to electronically measure and record thrust data. In the winter of 2018, I completed all of the data acquisition and launch sequence hardware. All that remains is to conduct final calibration tests and produce igniters. I plan to test fire this motor for the first time on my next trip home. It should be said now, if it isn’t obvious, that I haven’t fired any rocket into the sky, I just do ground tests (so far).

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Hybrid rockets with no liquid fuels or pumps are relatively simple devices. Their performance is characterized by their geometries and refinement of operating parameters like oxidizer pressure and maximum permissible temperatures. I scoured the internet for educational resources that would help me design a motor with a target thrust of 350N. I encountered textbooks, project archives and informative sites (like those produced by NASA Glenn and Richard Nakka). I learned a lot about the important governing equations for rocket design, rocket technologies, and also some of the challenges faced by my professional counterparts in industry. I derived parameters that informed the geometries within the nozzle, the shape of the wax fuel core, combustion chamber length and regulator requirements. As with all my designs, there is an emphasis on DFMA. I had to size components and slightly compromise on geometries as my lathe and other tooling would permit (for example, the nozzle is made of hand blended conical sections, rather than being continuous). The resultant physical design was a balanced result of considering materials availability, manufacturing capabilities, and budgets.

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I started work on the data acquisition and firing sequence portions of the project before having ever worked with Arduino. However, I returned in 2018 much more experienced in this domain. I incorporated failsafe mechanisms into the physical hardware and the code operating the rocket. For example, each electronic igniter (which I make from scratch) has a solder fuse built in, which melts when they ignite. I programmed the Arduino to monitor whether this fuse had broken within 2.5 seconds of the igniter heater element being turned on – a time parameter I determined to be the threshold for plausible ignition or not, from experiments. I built in other robustness measures. For example, it tests that indicating switches on the valves are correctly triggered before burning up an igniter in vain, it also tests that fuses are unbroken before firing igniters. Thrust data is measured by recording the displacement of a linear plunger potentiometer (in a voltage divider arrangement). The rolling cart that the rocket is attached to the static part of the rig may only roll backwards by compressing a precision die-spring with a known stiffness coefficient. I conducted experiments using my lathe as a rigid testing mechanism to determine the relationship between voltage measured by the Arduino and pot travel. The first plunger pot that I used proved to have terrible linearity, so I bought a more reputable one, which was a great deal better (see images in gallery). I combine spring data and travel data to create thrust data after the collected data are smoothed and then processed in MATLAB. Pot voltage and a timestamp are printed to a text file that exists on an SD card shield wired up to the main Arduino board. I explored the use of curve-fitting and noisy-data smoothing tools in MATLAB to create the post-processing scripts. They produce awesome looking plots and summarize the quality of curve fits.

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I plan on making revision III of ‘the rocket’ after testing concludes on the current iteration, so watch this space!