Propulsion
Meet Our Propulsion Team!
Kyle Evangelista was the head of the propulsion sub-team and spent most of the year working on fuel grain design and ablative materials with Josh Watson. Cristian Finley, in addition to being team manager, reworked the old burn valve ignition system for the new injection system, which he also implemented. Evan Andress was responsible for both sets of O-rings in the combustion chamber, as well as being the team dad. Rory Duncanson and Ian Humprey worked on the nozzle and nozzle retention system.
Ox Tank
We chose a 7" oxidizer tank because it has less hoop stress than the 8" alternative, thus giving us a higher factor of safety. The 8' length allows us to store 100 lbs of nitrous oxide with about 10% ullage. This is about as long as you can make your ox tank for this width vehicle without causing it to be far too long when fully assembled.
Ox Tank Upper Head
The upper ox tank head is 9" long and made of a solid piece of aluminum billet. Aside from containing the oxidizer, it also couples to the forebody with a diameter change. It also surrounds much of the instrumentation for the oxidizer, like pressure sensors, relief valves, and temperature sensors.
Ox Tank Lower Head
The lower ox tank head is similar in function: it connects the aftbody with the ox tank tube and has shelf for ease of manufacturing and assembly. The lower face of the head is the location of the injection system. This injector/coupler/head is manufactured from the same piece of aluminum billet.
Chemical Equilibrium Analysis
The simulation of combustion at extreme temperatures and pressures was pretty much solved by NASA in the 60s. Unfortunately, they solved it in Fortran. They have a usable interface for their algorithm on their website, but I ended up using rocketCEA so that I could integrate it into my Python programs more easily. By the end, I was simulating with high fidelity a massive range of pressures and O/Fs for nitrous with and without contamination and a variety of ABS compositions. It was pretty epic.
Nozzle Geometry
The shape of a bell nozzle can be described by many terms - skewed parabola, quadratic bezier, fit point curve, fit spline, etc. - however, they all mean the same thing. They represent two points connected by a line that has a specific angle at the entrance and exit. We chose an 80% bell as a good trade-off between optimal flow and minimal weight just because a few research papers recommended it, determining the entrance and exit angle from a Newlands paper.
O-Rings
We initially planned to seal the oxidizer tank with O-rings because we would have a-weld free design. The reason we wanted a weld-free design is because welding makes the metal weaker due to temper loss. In the end we decided to weld because it has fewer failure modes, and it is a lot less work to avoid tapping holes and bolting and unbolting on-site. We are, however, using O-ring to seal the lower oxidizer tank head to the pre-combustion chamber and to seal the nozzle against the phenolic. This prevents a potential gas cycle. Evan also wrote a very detailed O-ring sizing guide, as well as a calculator that you can use to double check.
Fuel Grain
One of the main objectives of the propulsion team is to research and compare multiple fuels to determine which one performs the best. We design a fuel grain that burns quickly and produces a large amount of thrust in a short period of time. This year we are 3D printing our fuel grain out of ABS thermoplastic because we can design our own complex port geometry, allowing the grain to regress faster.
Ignition/Fill System
An important aspect of any propulsion system is the ignition and pre-heating of the fuel/oxidizer. Without an energetic ignition, the rocket could start too slowly off of the launch rail, which can lead to stability issues and poor performance. We used a fairly obscure method of igniting the rocket by combining our fill system (nylon tubing attached to DOT compression fittings) with our ignition system (APCP pellets taped to pyrodex pellets). The nylon tubing stayed attached to the DOT fittings until ignition of the APCP when they burned away, allowing the pressured Nitrous to flow to the combustion chamber.
Ablative
Ablative serves as a sacrificial substance that erodes in the high temperature of the combustion chamber, protecting the more critical phenolic tube that houses the fuel grain. The fuel also functions as an ablative, so the only areas where our homemade mixture of Epsom salt, epoxy, and Kevlar pulp is necessary is in the pre and post combustion chamber. To create the required tube shape we used a 3D printed mold, pouring the mixture in and allowing it to cure. Post-mission analysis showed that the ablative worked much better than anticipated, with the majority of the ablative emerging unscathed and only a small amount of the exposed surface burned away.
Nozzle Material
Our nozzle was machined out of isostatically-pressed graphite. The material has been reliable for us in the past (in the back corner of the room there is one nozzle-sized billet and one absolutely massive billet) and is known for its resistance to thermal shock. However, it is very thermally conductive, which makes retention a challenge, especially because you cannot bolt into it (too brittle). We had a thick aluminum retention ring around it, which melted quite a bit but remained mostly intact, and a steel retention coupler that seemed okay. Relying on a melty tube of aluminum seems, in hindsight, rather unwise. In the future, I would recommend a trying out a phenolic nozzle with an aluminum retention system. It would be lighter, and potentially less nasty as a machining operation. It would also be a little easier to handle, sense phenolic is less brittle and less dusty than graphite.