# Preliminary design of Propellant ducts

I'm trying to roughly estimate dimensions and masses of propellant and pressurant ducts for conventional upper- & transfer stages.

My basic approach follows Barlow's formula for cylindrical vessels (hoopstress), but that results in wall thicknesses that seem very small.

Example: A stainless steel pipe with a diameter of 30mm and a pressure of 12bar(~175psi) would only have a 0.1mm wall.

My usual sources for the preliminary design of propulsion systems (Huzel/Huang, Sutton/Biblarz, Humble/Henry/Larson) unfortunately don't cover that topic.

Therefore I'm asking if you could recommend some links, papers or books with reliable, "rocket-related" information or best practices for dimensioning pressurant and propellant ducts.

• What kind of safety factor are you using? Pressure vessels should have a very high SF. – Brian Lynch Dec 19 '15 at 18:13
• I use a design pressure of 2x Maximum Expected Operating Pressure. That seems to be the usual safety factor for pressure vessels during preliminary design and gives reasonable results for pressure tanks. – cl10k Dec 19 '15 at 19:29
• For pressure vessels you want something much higher, in the 5-10 SF range. The main reason is that failure modes are linked to things like cracks that will occur well before typical plastic failure. – Brian Lynch Dec 19 '15 at 19:31
• No!? I don't know anything about ducts (is that even the correct english term?), but at least for pressure tanks that would be a more than unusal safety factor. All topic related sources that I read so far, agree on a safety factor of 2x. Even commercial sellers like Airbus Defence & Space design with a burst pressure of 2x MEOP – cl10k Dec 19 '15 at 19:38
• Well you should trust those sources more than me, there may be a trade-off in terms of aerospace SF numbers vs. typical pressure vessel SF numbers. But understand that the reason you get 0.1 mm with SF=2 is also the reason why you normally use a higher SF -- 0.1 mm will carry the stress of the pressure but a small collision could cause a hole which would lead to catastrophic failure. Hence why you use a higher SF! – Brian Lynch Dec 19 '15 at 19:42