Since 2020, and together with the RRS’s Xavier, we’ve been trying to determine if we could use seamless aluminum air tanks as effective propellant tanks for rockets. These tanks are designed for use with air suspension systems, and feature ports on both ends, as well as a number of secondary 1/4NPT ports, depending on the variant.

These 1/4NPT ports are inset, allowing for a thinner tank wall with full thread NPT. The NPT ports at the end vary, but often come in 3/8NPT, allowing it to port out to 1/2” tubing. This makes thermocouple dip tube fitting more constrictive, so welded dip tubes may be necessary.

Their operating pressure rating is only about 200psi, but this is merely due to an artificial limitation to common pneumatics pressures. A limited series of pressure tests to yield with two samples suggest that each tank is made with the same wall thickness, a little around 0.058” assuming 6061-T6 material properties. The narrower tanks were able to withstand higher pressures than the larger, wider tanks. Our own sample, a 6.625” OD tank, was hydrotested to 620psi before it started to balloon. A 2.875” diameter tank, destined for the RRS’s LR-S1, was hydrotested by Xavier to around 1500psi. Both of these yield pressures correspond to this wall thickness, give or take. With that we can draw up a table of expected proof pressures, as well as safe operating pressures with a 1.5 safety factor. These are extrapolated quite roughly to a range of common seamless air tank diameters.

Expected

Proof Pressure

1500 psi

925 psi

735 psi

620 psi

Nominal

Diameter

2.875”

4.5”

5.625”

6.625”

Expected

MEOP (1.5 SF)

1000 psi

615 psi

490 psi

413 psi

Each of these MEOP limits can be worked with, though the 6.625” tank leaves less room to breath in, so a motor may wish to stay at or below 400psi during operation. These tanks usually has larger capacities, and two are being used on a long term test vehicle project at Honkawa Rocketry.

Xavier took this even further however, by placing a custom order with the manufacturer to make his tanks without the 1/4NPT ports, leaving just the end ports. This reduces the leak paths, which have to be plugged on the ported tanks. Using this knowledge, we worked with UC Berkeley STAR to test these tanks in hot fire exercises with LOX, with which they were successful in their first ever LOX liquid bipropellant rocket test in April 2023.

These tests haven’t been widely advertised until more recently, particularly with STAR’s hotfire test as the trial of fire. Now with more test data, we can start to consider these tanks as a viable or even preferable alternative to the many custom welded tanks used by Universities.

It is still important that groups properly test these tanks upon receipt, and test samples before making large orders. They can be acquired readily and cheaply from amazon or your performance automotive shops near you, and custom tanks without extra ports have proven to be readily attainable for the discerning rocketeer. The 3/8NPT remains a limitation for effective flow rate, but it should be no issue for rockets of a moderate size and budget. Be prepared to repair the NPT ports as needed however, as they are likely not immaculate enough to avoid leaks out of the box.