A nice write up by the folks at Copenhagen Suborbitals in regards to pressurizing your rocket tanks with nitrogen or helium. Helium is preferred, as outlined but cost comes into play and for most amateur projects is probably fine, nitrogen does have the tendency to dissovle into the liquid oxygen on the other hand.
We did three tests in all to illuminate this issue. For all three tests we aimed at having the same amount of fuel and oxydizer in the tanks, and pressurising to the same pressure level.
For the first test we pressurised the tanks using helium and did a 7 second main stage burn.
For the second test we pressurised the tanks using nitrogen and did a 7 second main stage burn.
For the third test we again pressurised the tanks using nitrogen. After the tanks were pressurised, we left the system on hold for 5 minutes to quantify the loss of pressure/gas over time. Gas or pressure is lost or reduced by cooling in the tank as well as the gas dissolving into the liquid oxygen.
… the tank pressures are between 3% and 5.4% below the pressures obtained using helium. Why is this so? The partial answer can be deducted from the top center graph. This graph shows the duty cycle of the DPR (Dynamic Pressure Regulation) valves. Both the LOX tank and the alcohol tank have a DPR valve, the graph shows the average of the two valves for the three tests, as plotting six graphs would be too confusing. It is clear to see that during the two tests using nitrogen, the two valves are opened to a much larger degree. This is to be anticipated, as nitrogen has a much larger ”viscosity” than helium.
What to take from it? Nitrogen can be used for test purposes but it does alter your engines performance and needs to be accounted for. For more accurate and tedious testing, use Helium (ideally always use 🙂 ), oh and if you are going after that altitude goal with your rocket, use helium, it is much lighter!