Columbia Space Initiative:

Introduction

I co-led the fluids subteam team on the Columbia Space Initiative Rocketry team from September 2021 until June 2023. Throughout this time, I was responsible for the fluid propulsion system of the hybrid rocket, including the oxidizer tank, piping, venting, valves, fittings, quick disconnects, pressure transducers, fill system, and ground support equipment.  The fluid system must fill an oxidizer tank with high-pressure nitrous oxide and then reliably provide it to the combustion chamber via the injector. Nitrous oxide is very sensitive to its surrounding environment, thus accurate pressure and weight readings are also essential. 

One of the major projects that I led was the design, analysis, manufacturing, and testing of an oxidizer tank. The success of the tank along with the rest of the fluids system led to the team's first ever rocket launch at the Spaceport America Cup in the summer of 2023. Through this experience, I learned about the design process from the cradle to the grave. 

Oxidizer Tank Design

The oxidizer tank featured two dual-purpose, aluminum bulkheads that formed a bolted piston seal with redundant O-rings. These bulkheads were inserted inside an aluminum cylinder to form the tank, with slight differences for the venting and injector sides. The bulkheads served to safely hold pressures of up to 1000 psi inside the tank and also interfaced with the rocket's superstructure via a ring structure as shown in the diagram below. This design was chosen because the oxidizer tank itself is structural as a loadpath for the thrust. 

The bulkhead O-rings were chosen according to the Parker O-ring Handbook. Basic dimensions were determined by the inner diameter of the aluminum tube (6 inches) and machining constraints (end mill lengths). The bulkheads were designed with a torispherical shape to best distribute pressure. The bulkheads also included port holes for a venting assembly, a pressure transducer, and a fill/run orifice.

The oxidizer tank was the central part of the fluids system as seen in the Piping and Instrumentation Diagram below. 

Oxidizer Tank Analysis

We analyzed the oxidizer tank through a variety of hand calculations, as shown below. The first factors we looked at were thin-walled pressure vessel stresses, such as hoop stress and axial stress. As we were using torispherical end caps and a cylindrical tube, these stresses were well below the ultimate tensile strength of aluminum and thus we had large factors of safety.  Next, we calculated bolted closure stresses to determine which types of bolts to use, how many bolts to use, how to space these bolts out, and bolt locations. These stresses included bolt shear, bolt tear-out, casing tensile, and bearing. All of these stresses were designed to have at least a factor of safety of 2. 

We also performed a basic Finite Element Analysis of our pressure vessel to ensure that our hand calculations were correct. 

Oxidizer Tank Manufacturing

We then moved on to manufacturing the bulkheads, which was challenging due to their complex geometry. We used the Fusion CAM software to generate programs for both the CNC Haas ST-20Y lathe and CNC Haas Mini Mill. As shown below, the lathe was used to create the structural interface of the bulkhead, while the mill was used to create the torispherical shape on the inside. 

We also had to machine the bolt holes in the aluminum tube, which proved difficult due to its size. We made sure that the hole lined up by using a radial indexer. 

Oxidizer Tank Assembly

With the bulkheads fully machined, we fully assembled the oxidizer tank by inserting our redundant Buna-N O-Rings in each bulkhead with Krytox lubricant and press fitting them into our tube. We then connected it to the rest of the piping assembly. This assembly included fittings, a custom servo-actuated ball valve, and a flexible hose to ensure that the fluids system would not take any load. 

Oxidizer Tank Hydrostatic and Static Fire Testing

Finally, it was time to pressure test our oxidizer tank. To do so, we performed a hydrostatic test by first filling the tank with water and making sure that no air remained since any gas present in the system can present danger. Then we started to pressurize the tank, but once we got past 1000 psi, we heard a soft pop and saw that our retaining bolts had sheared out of their holes. This was due to bearing stress, which we had originally failed to account for in our calculations. We redid the calculations, added more bolt holes, remade the bulkheads and tried again. This time we pressurized the water to 1.5 times the maximum operating pressure (1300 psi) and held it for 30 minutes with only a few psi lost. 

After the oxidizer tank passed the hydrostatic test, it was time to cold flow it. Once we verified that the tank could hold nitrous with no leakage, we attached the combustion chamber and went for a static fire. The oxidizer tank and the rest of the fluids system performed spectacularly well, supplying high-pressure nitrous to the combustion chamber and resulting in a successful fire!

2023 Launch Video

After multiple static fires, we successfully launched the rocket at the 2023 Spaceport America Cup. This video showcases our work throughout the year that led to the team's first-ever launch. 

From Cradle to Grave...

Due to our thrust not reaching the levels we hoped for, our rocket was not fully stable after leaving the launch rail. This led to it being canted over from the wind and ultimately a shorter apogee than intended. Our recovery system did not deploy due to this short apogee and thus we won the "Best Treecutters Award", although it really should be called the "Best Ditch Digger". The wreckage was pretty awesome and needless to say, we had to start from scratch for next year's rocket. 

Technical Guide to Fluids System

A guide document that I helped write going over the design, manufacture, and test processes for our fluids system. This document was used to educate new members and ease the knowledge transfer process.