Relevant Files
Project
Details
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Date: Fall 2024 - Spring 2025
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Organization: Columbia Space Initiative
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Objective: Design and manufacture a passive nitrogen quick disconnect for a hybrid rocket to allow for a full abort window.
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Key elements: Check valve, probe, housing, clip
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Role: Project owner
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Achievements:
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Designed, modeled, and machined parts.
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Validated design through testing.
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Launched vehicle; QD disconnected and re-sealed, constituting a full success.
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High-Pressure Nitrogen Quick Disconnect System
Notes
When I transferred to Columbia, I knew from the get-go that I would be joining the Columbia Space Initiative, specifically the rocketry team. My first year in the club would also be my first time designing parts that would affect an entire team rather than myself: a challenge I was excited to take on. I was able to take ownership of the nitrogen quick disconnect (QD) system, a part which had failed last year resulting in a large loss of performance. My objective was to make a part as simple and reliable as possible to connect the nitrogen tanks on the ground to the composite over-wrapped pressure vessel (COPV) on the vehicle. This part needed to allow two way flow in and out to both pressurize the COPV and maintain abort capability until immediately after launch, where it would disconnect and re-seal, not allowing gaseous nitrogen (GN2) to escape the vehicle.
Early on in the development cycle, I decided on a passive design as opposed to an active one. This meant that rather than the QD being actuated by a process on the ground, it would use the force of the rocket itself to actuate. This would ensure reliable separation, as well as enabling a full abort capability, with the ability to de-pressurize up until the rocket left the ground. Once this decision was made, many designs were created and modeled attempting to fill this requirement. This was the first time in the team’s short history that a passive QD was explored, thus, there was very little combined experience in designing a device like this. Because of that fact, I went through several unique designs and review meetings before I was satisfied with the final product. Examples of some of these early designs can be found below, with descriptions attached.
The final design consisted of four main parts: A commercial-off-the-shelf (COTS) check valve, a probe, a housing, and a clip. This system was partially inspired by the designs of Half Cat rocketry, but modified for our purposes. Its operation functioned as follows: A check valve with compression fittings on either side would be mounted on the vehicle and connected to the COPV, with the poppet sealing against flow. A housing with a flange at the end would then be compression fit onto the other end, leaving a smooth edge for a probe to be inserted into. The probe would then be inserted into the housing, pressed up against the flange of the housing, and pushing the check valve’s poppet up, allowing for two way flow. This probe would be sealed with a set of two hard o-rings, each supported by a backup ring to resist the extremely high pressure. The probe would also have an NPT connection for connection to the GN2 supply on the ground. The flanges of the housing and the probe would be held by a clip. This clip would be placed onto the flanges, held loosely by a small amount of friction. As the GN2 supply was opened, however, over 4000PSI of pressure would press against the clip. Considering the area, this equated to a force of about 200lbs to the edges of the clip, which greatly increased the frictional resistance as well, ensuring the clip would hold. The clip would be tied down to the ground, either by ground anchors in testing, or to the rail during launch. When the rocket lifted off, the force of the vehicle would easily overcome the frictional forces on the clip, and pull it off. The pressure in the line would then burst the probe and housing apart. With nothing to push the poppet up, the check valve would re-seal, and the vehicle would not vent GN2 during ascent.
The housing and probe of the QD were machined out of aluminum, while the clip was made out of nylon using Formlabs’ selective laser sintering 3D printing method. Both hand calculations for stress, along with finite element analysis were used to validate the ability for the QD to hold pressure, as well as the ability for the clip to resist the forces placed on it. During the machining process, I learned both how to use computer aided machining to generate g-code for the CNC machines, as well as how to operate the machines themselves. While the part did not contain complex geometry, the small scale of the part did present difficulty. Due to the long and thin tools needed to make the parts, there were issues with chatter, especially with the probe and its o-ring grooves. This resulted in less than optimal tolerance, which was unacceptable with high pressures. To cut at such a small radius, our lathe needed to spin faster than allowed, thus, we made several modifications to reduce these effects. For one, I changed how the part was machined, prioritizing removing outside material first while it was most supported on the inside. Next, I shortened the length of the probe considerably, with the poppet being instead pushed by a secondary piece of pipe. This pipe would not need to be tightly toleranced, and was cut down to a smaller radius to ensure clearance even if concentricity was not perfect.
We tested the QD during a wet dress rehearsal. We used a pneumatic cylinder attached to a length of cord tied to the QD clip to simulate the force of the rocket. Following the tests, we were confident in the design, and the part was taken to launch. All the hours designing, re-designing, machining, and re-machining proved fruitful, as the launch was fully successful. The QD encountered no issues in its operation, and successfully sealed after launch. While I had engineering experience going into this project, it was an entirely different process designing a high value with actual applications for a team as opposed to a personal project. I not only learned so much about design and manufacturing as a result of this project, but also about the structure of professional engineering teams, and how to work as a part of one.
Media Gallery
The left video showcases a machining simulation of the probe section. First the NPT connection on the left was drilled. The stock was then flipped, and the probe hole was drilled to the end of the first o-ring groove after facing. Next, the stock was turned down to match the diameter of the probe, and the first o-ring grooves were cut. Then, the probe hole was drilled all the way through, with the second grooves being cut afterwards. This was done to stabilize the part as much as possible while machining, balancing a clean and concentric through hole with precise grooves and a concentric probe. Lastly, the material around the NPT connection was cut, and the piece was parted. Due to the small diameter and tight spacing, a grooving tool was used for most of the cuts.