Machining parts for test tanks

17 Jun 2021 by Finn in


(Edit 19 Aug 2022)

Before we build the flight-ready tanks for the Poseidon Rocket, we’re going to manufacture test tanks to see how they’re going to perform and hold up to the expected pressures.
You can find the concept and design of these tanks in a previous article: Poseidon Tanks: Concept and Design.

As discussed previously, the tanks must have a way to connect to valves and fittings. For this purpose we’re embedding a machined aluminum adapter into the ends of the tank.
So before laminating the tanks, we have to manufacture these adapters.


This adapter has to transfer the loads of working pressure to the valves and fittings, this as evenly as possible to prevent stress peaks on the tank bulkhead.

For this purpose, the top is shaped conical to take on the axial loads of the tank pressure and prevent tear out of the adapter.
To attach the mentioned valves and fittings, a BSP G1/4 thread is cut into the end of the adapter, as our valves and nozzle use this thread.
I’m going to use a high strength aluminium alloy – in this case AA7075 – which allows me to design the part rather thin and gives me enough confidence that the threads won’t strip out over time.
The parts are relatively small and have a very low batch size anyway, so the extra cost for a better alloy is negligible.

On the threaded side the adapter is able to recieve a 20mm wrench.


I looked into outsourcing the manufacturing to a CNC machining company, but the manufacturing cost for just a few parts is hillariously expensive for our purposes (After all, this student project is funded just by us).

The only lathes I had access to were the ones in our university’s student workshop, which just opened for vaccinated students.
Thanks to the helpful and dedicated shop supervisor, I managed to manually turn 4 adapters in under 13 hours.

Starting turning the inside of the conical tops


The aluminium stock in shape of a round, 35mm diameter bar of AA7075, is just long enough for 2 adapters.

  1. I turned down the cylindrical part of the adapter, leaving the whole conical section at 32mm outer diameter.
  2. Next was the drilling of the axial clearance hole. First with a centering drill, then going up the diamaters. Before the hole was tapped with a G1/4 tap, I used a boring bar to expand the hole to the 11.45mm inner diameter.
  3. Then, the outer diameters of the cylindrical sections were grooved in.
  4. I turned the outer conical section by rotating the tool sled to 37°
  5. After repeating this for the other half of the stock bar (2 adapters in one bar), the bar was sawed in half. This left 2 adapters where only the inner conical sections are left to be worked on.
  6. After clamping in the adpter on the cylindrical section, i turned to the conical back side, until the adpater had a sharp outer edge.
  7. Rotating the tool sled to 65°, i turned the first inside angle up to the edge.
  8. As the last step, i rotated the sled to 45° and took the angle to a 23mm diameter.

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