Ox Tank (2023)

For the 2022-23 rocketry season, I led the design and manufacturing of two end caps for our nitrous oxide tank. These components were incredibly complex and our constraints demanded they fulfill several roles.

  1. Contain at least 1200 PSI of pressure for a factor of safety of 1.5
  2. Interface with both the combustion chamber (bottom) and payload (top) through a bulkhead interface. Since the bulkheads need to withstand the rapid acceleration the rocket endures (up to 3000N) it was an incredibly important part of the design.
  3. Interface with the airframe to ensure it’s adequately retained for flight.
Solidworks render of 2022-23 oxidizer tank. We had to make some rapid design changes later on, as I will discuss.

Because of the COVID-19 pandemic, the Columbia Space Initiative Rocketry team members who manufactured the oxidizer tank for the 2021-2022 rocket, which was 4.5″ in diameter, had already graduated. Thus, we were essentially starting from scratch.

The first iteration of the end caps was manufactured over a 4-week period on a Haas ST-20Y and Haas 3-axis mill after using Fusion 360 to CAM it.

After manufacturing concluded, my team and I began our hydrostatic testing regime. This involved filling the tank completely with water, then using a water pump to test our assembly up to the required PSI.

Unfortunately, hydrostatic testing was a complete failure. Our oxidizer tank failed at 600 PSI, less than the 750 PSI needed for a nominal launch and nowhere near the factor of safety needed to be approved for launch by ESRA.

Fluids subteam lead, Tycho Bogdanowitch, and the failed ox tank.

After returning from the failed test, the team and I began to investigate the source of the failure. The first was bearing stress, which is a force induced by the head of the bolts on the outer ox tank tube. We assumed, based on the hoop stress calculations, that bearing stress would be a non-factor, however, after further calculations, we determined that our current design had a factor of safety of 0.6.

With the failed test occurring in early April 2023, we had almost no time left to remanufacture an entire ox tank in time for a late April static fire. Within a week, a fellow fluids subteam member and I remanufactured both end caps with updated parameters. The improvements are as follows:

  1. Increased holes from 8 to 16.
  2. Increased hole depth from 0.25″ to 0.43″
  3. Increased ox tank thickness from 0.125″ to 0.1875″
  4. Increased airframe holes from 4 to 8

We verified these new parameters by calculating the new bearing stress, which was sufficient such that we had a factor of safety of 2.2 (roughly 1700 PSI). After I finalized both the top and bottom-end cap CAD, we started manufacturing again. Due to the increased hole depth and a lack of 6.5″ stock, the entire part required brand-new CAD and CAM.

The new end caps were designed and manufactured within a week, which as you can imagine required several late nights for myself and the team. However, I applied the lessons we learned from the previous end caps to produce near-perfect parts.

The next challenge was drilling precise 0.25″ holes in our ox tank. Due to error propagation with measuring calipers, it was nearly impossible to manually drill 16 holes as we did with 8 holes. Instead, we stretched the abilities of our lathe to use it as a rotary index.

While this was incredibly dangerous and definitely not in the design parameters of the lathe, it was also extremely effective. The result was zero-tolerance holes that aligned perfectly with those on the end caps.

Fortunately, our second hydrostatic test was successful, and the ox tank was validated to 1200 PSI. The next step was a cold flow, where we attempt to fill the oxidizer tank with nitrous oxide. We had two successful cold flows, one of which I recorded below. Since we were using an outdated valve with a larger volume, the flow was longer than optimal but a resounding success nonetheless.

Full fluids stack assembled at Nevis Labs for a cold flow. From the top down: venting solenoid, pressure transducer, nitrous oxide tank, check valve and fill line, pyro valve, flexible hose, and combustion chamber.
First cold flow! Tank was pressurized to about 600 PSI with ~20 lbs of N2O.
The end caps are validated at cryogenic temperature!

The 2023 oxidizer tank was further validated in cold flows and static fires, and launched in June 2023 on PleaseGoUp. We also successfully recovered the tank, which sustained some damage from the ballistic reentry!