Choosing the right material for a rocket engine can be a bit of a challenge, especially when big players like NASA & SpaceX are using exotic materials such as Narloy-Z and Niobium alloys. We’d like to give you a run down of what we did for the Gizzard-1 Engine to perform analysis.
You can download a copy of our spread sheet on our documents page.
In order to survive the extreme thermal environment of the combustion chamber and nozzle, the engine wall materials had to balance the conduction of heat avoid reaching their maximum temperature with the thermal stresses caused by the large temperature delta between the chamber and the coolant.
To give our engine the safest possible margins, we attempted to select materials so that we could get as close as possible to a working pure regenerative cooling condition before adding film cooling.
We compared a variety of different materials by using different heat flux for a range of ΔTs, and then comparing the material properties per ΔT using the following equation:
in which, P coolant & P gas are pressures at their respective locations, R is the radius of curvature, t is thickness of the wall, E is young’s modulus, a is thermal coefficient of expansion, q is heatflux, υ is poisson’s ratio, and k is thermal conductivity.
We compared the performance of around 30 promising materials when under the thermal conditions of our engine and at various temperatures. All of the material properties come from a variety of sources and the specifics can be found on our material selection sheet.
Stress Analysis of Wall Materials Under Engine Conditions at 800°F Wall Temp from OTRA’s Material selection sheet.
From our analysis, we found that outside of exotic materials such as niobium and molybdenum alloys, the best performers were all copper alloys. High strength, high conductivity alloys such as beryllium copper performed very well, as did pure copper. After the copper alloys, the only other economical materials that came close were basic carbon steels such as SAE 1018, 1040, and 1060. No stainless steel came close to working, even in the relatively low thermal conditions of the nozzle exit. The nickel based superalloys, such as inconel, likewise performed badly and at best were about on par with regular steel but at a massively increased price. The performance of nickel based superalloys improved dramatically with decreased wall thicknesses to around half a millimeter, allowing the coolant to require significantly less thermal capacity & increasing stress safety factors. Unfortunately, the manufacturability of such a system is complicated and not cost effective.
From the data above, we selected pure copper and carbon steel as our materials of choice. Copper was relatively economical and has both very high thermal conductivity and resistance to oxidation. In theory copper could withstand the stresses without film cooling if enough fluid is run through the regenerative channel to cool it. The main difficulties would come with machining and brazing the parts together. Low and medium carbon steel on the other hand is cheap and very easy to machine and weld. This allowed us to make several steel engines for testing purposes. The primary drawback is that without very thin walls, film cooling a necessity in order for the steel to survive the thermal stresses. In order to resist corrosion in the high oxidation environment, the steel engines also required copper and nickel plating.
Hope this helps,
-OTRA Propulsion Team