Pressure Vessel Stand

I actually wrote a full report about the design of this stand for my final work term report, but I hope to summarize that report here in a less formal way.

The hybrid rocket engines produced by Waterloo Rocketry during my time in the team all relied upon self-pressurizing nitrous oxide as the liquid oxidizer propellant. The oxidizer tank holds this pressurized nitrous oxide and feeds it to the combustion chamber during an engine burn.

In the early days of the team, this stand was very rudimentary. Take a look:

This stand served its purpose, but with a few big flaws that I wanted to address:

1. There is no blast shield, which presents a safety risk in the event of an explosion.

2. The water jacket (a jacket of warm water surrounding the tank to increase tank pressure during cold weather) is hastily designed and consists of many consumable components. It spilled water in an uncontrolled manner and it was opaque, preventing us from viewing the oxidizer tank.

3. Balancing the oxidizer tank on top of a load cell presents leveling errors that wouldn’t exist if the tank were instead hung from a tensile load cell.

4. Larger oxidizer tanks are not accommodated.

The final design is a bolted steel frame designed to accommodate all the various systems. The 1/4” polycarbonate windows offer very high impact strength (for stopping shrapnel in an explosion) and easy inspection since they’re transparent. Although I performed a stability analysis and found that it would take high windspeeds to have even the slightest chance of tipping the stand, paranoia dictated that I make leg extenders to totally rule out any chance of tipping under any circumstances. The leg extenders are easily removed, which allows the stand to have a small footprint when in storage.

I sized the stand to allow for modest expansions of the oxidizer tank in future versions. More on that later…

The water jacket is basically a separate design project in its own. The main outer shell of it is a 8” polycarbonate tube. There are 3D-printed hose adapters on the bottom and top of the jacket. The lower hose adapter has a smaller diameter than the upper one, since the jacket is bottom filling. The flow velocity is higher on the inlet due to the existing pressure head from the faucet, but the upper hose adapter passively drains since the top of the water jacket is open. Filling from the bottom makes sure that the heating is even and there isn’t a cold spot at the bottom of the tank. The outlet hose is also directed to a stormdrain, instead of just spilling all over the place.

Analysis

I performed a stress analysis to check for stability and structural soundness. I checked the top plate and bottom plates using FEA, the columns for buckling, bolts for shear, and the water jacket for hoop stress.

I also had to size the sealing O-Rings for the water jacket, to minimize leaking. Although some leaking is tolerable since there are no sensitive components at the base of the tank stand, leaks should obviously be mitigated. I learned to size the O-ring using the design guide written by Apple Rubber Products, and optimized the stretch and compression accordingly.

I performed a drag calculation by using the drag coefficient for an infinite square cylinder, and found that a conservative estimate for the minimum windspeed to tip the stand was 66 km/h. Therefore, the stand was not in danger of tipping from winds under normal test conditions.

Final Thoughts

Funnily enough, at the time that I made the stand I thought that there was no way the tank would get too much larger. The tank at the time was around 3 feet long, and I built the stand to accommodate a tank of up to 4 feet in length. Of course, the very next year the team increased the size to almost 7 feet in length. The unfortunate consequence is that my tank stand was sentenced to a premature death, but its spirit lived on since the team essentially just made a larger version of the same design.

This was the most substantial engineering project that I had taken on outside of a course or workplace setting. It’s really satisfying to take something from a concept through design, analysis, production, and testing.