Having a vendor substitute a substandard alloy for a rocket component…can cost you everything. This lesson will be an in depth analysis of some of the most sophisticated aluminum alloys in use today by the aerospace industry including the 2195-T84 Airware alloy used by SpaceX in the Falcon 9.
Aluminum in the aerospace industry is almost always used with other metals to form an alloy. Sometimes a fraction of a percent of an added metal can dramatically change the properties of your alloy.
Most metals start to break down with exposure to oxygen and a combination of heat and oxygen is almost always destructive. When exposed to oxygen aluminum will form a layer of aluminum oxide, which is Al2O3.
Aluminum oxide is found in crystalline form in the mineral corundum. Corundum would be transparent but impurities in nature such as iron, titanium, chromium, vanadium and magnesium can cause these crystals to be red, blue, green or other colors which we call rubies, sapphires and emeralds.
This top layer of transparent aluminum oxide that forms over an aluminum alloy makes it very resistant to corrosion. In fact, the aerospace industry was very happy to find that transparent aluminum, as mentioned in the Star Trek IV film, The Voyage Home, is possible.
Aluminum Oxynitride, often called ALON (not Elon) is created by fusing together aluminum, oxygen and nitrogen under pressure with a laser to remove electrons and allow chemical bonding. The resultant material is a transparent metal. This makes for excellent windows and telescope shields for use in space as it is much stronger and lighter than glass or any other plastic based alternative. This material is also being used in the Tesla Cybertruck’s windows.
Aluminum alloys used in aerospace over the last 70 years include the alloys 2014, 2219, 7050 and 7055. The 7xxx alloys are made brittle by exposure to cryogenic liquids and would not be a good choice for this purpose. Aluminum alloys were used in the space shuttle Super Lightweight External Tank which held the hydrogen to be burned by the Space Shuttle Main Engines.
It is also used to construct the Space Launch System main tank. These tanks are also made from the aluminum alloy 2195 which replaced the 2219 aluminum-copper alloy used previously and saved over 2,725 kilograms on these tanks.
The Falcon 9 uses an alloy of aluminum and lithium in much of its structure. The fuel and oxidizer tanks on the falcon 9 are made with the top and bottom domes of the tanks being just aluminum, while the cylindrical portion of the tank is made from aluminum lithium alloy...
These alloys may be complicated… but adding elements can have a very significant effect on the alloys characteristics. Adding lithium can save mass, as the density of aluminum is 2.70 g/ccm while that of lithium is 0.534 g/ccm.
Lithium is in fact the lightest possible metal until we have metallic hydrogen, which will actually be a liquid metal and not a solid. and a small addition of these other elements can produce alloys that are very formable and machinable, while still being strong under extreme conditions.
The aluminum alloy 2195-T84 has a 5% higher modulus and greater tensile strength than older aluminum alloys, providing a high-strength, damage resistant and friction stir weldable alloy that is excellent for cryogenic applications like the supercooled RP-1 and liquid oxygen used by SpaceX in the Falcon 9.
Modulus is the engineering term used to describe the stiffness or elasticity of a material. It is equal the the stress applied divided by the resulting elastic strain. A stiffer material will have a higher modulus. If a material becomes too stiff it can become brittle.
Exposure to repeated heating and cooling, as Falcon 9 rockets are reused, can effect the tensile strength of an alloy.
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