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. 

The first major project that I led was the design, analysis, manufacturing, and testing of a custom pyrovalve from September 2021 until May 2022. Although we were unable to launch at Spaceport America Cup 2022, we learned a lot of lessons which helped us launch our first-ever rocket in 2023. 

Pyrovalve Design

The job of the pyrovalve was to act as a run valve, allowing high-pressure nitrous to flow downstream to the combustion chamber when we told it to. The pyrovalve design was chosen because it is simple, elegant, and less prone to failures than other more complicated valves. It works by igniting a small black powder charge to move a piston from a closed to open position. In the closed position (shown in the top left), the piston covers the inlet to the valve and holds the high-pressure nitrous liquid with O-rings. In the open positions (shown in the bottom left), the piston moves over so that the inlet is no longer covered and fluid can flow to the outlet, again being sealed by O-rings. There was an older iteration of the pyrovalve that lacked an offset inlet/outlet and we updated this to ensure that at least one O-ring was blocking the leakpath at all times during operation. 

The O-rings were sized using the Parker O-Ring Handbook and Buna-N was chosen as the material for its durability and compressibility. A vent hole was added to one end cap to allow air to escape. A groove allows for an e-match to be run into the black powder chamber. Each cap was secured using bolts, which were sized and arranged according to bolt strength calculations and our calculated estimate of the impact force of the actuated piston. The outer diameter of the inlet and outlet tube stubs were sized to fit inside a compression Swagelok fitting. The fluids system, including the pyrovalve, were designed in Solidworks. 

The pyrovalve was the key component of the fluids assembly and interfaced with the rest of the system through Swagelok compression fittings. The tee-valve shown below allowed for the tank to be filled, while the check valve ensured there was no backflow during the filling process. The bent pipe was needed to align the outlet of the pyrovalve to be coaxial with the combustion chamber.

During the manufacturing process, we ran into issues with using the CNC lathe to produce the cylindrical version of the design. To fix this, we updated the design to be more boxy, allowing for it do be produced much more easily on the CNC mill.   

Pyrovalve Analysis

We analyzed our pyrovalve during the initial design process using both hand calculations and computational methods. To get an estimate of the pressure drop (which we needed to know to determine the expected pressure at the injector), we first used the Darcy Equation shown below using some basic assumptions including no cavitation and initial pressure of 750 psi. From these calculations, we compared the results for different inner diameters and materials for the valve and decided on 3/8" aluminum with a pressure drop of around 100 psi. 

We then ran a Computational Fluid Dynamics (CFD) analysis with similar assumptions in ANSYS FLUENT to validate our hand calculations and it roughly agreed with our estimate. 

Pyrovalve Manufacturing

We manufactured the pyrovalve housing using the HSMWorks CAM software and a Haas Mini Mill. The piston was manufactured on an ST-20Y Haas Lathe. The feeds, speeds, and step sizes appropriate for aluminum were chosen. Bolt holes were hand-tapped. 

Pyrovalve Assembly

Next, we assembled the pyrovalve into the rest of the fluids system in preparation for testing. 

Pyrovalve Cold Flow and Static Fire Testing

We first tested the pyrovalve using a cold flow to see if there were any leaks and if the mass flow rate was as expected. After this was successful, we went for a full static fire test. The pyrovalve worked as designed and delivered high-pressure nitrous with around 100 psi of head loss from the oxidizer tank to the combustion chamber. One issue we found during testing was the e-match conducting to the pyrovalve housing instead of the other lead, thus never igniting. We fixed this by adding electrical insulation on the inside of the cap. Another issue we experienced later on in our test campaign was the end cap bolts and their threads blowing off. We realized that this was due to fatigue stress, which we failed to originally account for. We sized up the bolts and inspected the threads after each actuation.