Oh So Many Ways to Weld A Rocket

"Welders were created because engineers need heroes too"


I once saw a meme poking fun at two books titled "What They Teach You at Harvard Business School" and "What They Don't Teach You at Harvard Business School". The joke was that by definition, these two books combined must contain the complete sum of all human knowledge! Well if that's the case, then the chapter on welding techniques must be in the latter book, because as someone who studied finance in undergrad, I can personally attest that they do NOT teach you welding in business school...

When I transitioned from Wall Street to SpaceX, I knew that the sort of finance I did for investment banking is pretty different from corporate finance at an actual company, so I'm constantly trying to learn more and more about the operations the actual engineers and technicians on the floor are performing. As it turns out, apparently there's a bazillion different ways to weld metal together, each with its unique pros and cons for building a rocket. So I thought it'd be neat to dive into a few examples!

The practice of welding dates back several millennia to the Bronze and Iron Ages, yet for most of human history, if you wanted to join two pieces of metal together, pretty much your only option was forge welding, whereby a blacksmith heated the metal in a furnace until it approached its melting point, then manually hammered the red-hot pieces together into the desired shape. This process works fine if you're building a pretty simple tool like a sword or a plow, but as the Industrial Age began, modern factories needed more efficient and consistent methods of joining metal

The first real advancement in welding was the invention of arc welding in the late 1800s, which uses an electric current generated between a metal stick (the electrode) and the base metal to provide the heat necessary to melt the material. Arc welding, in its numerous variants and techniques, was crucial behind the production efforts of all the countries involved in the World Wars, and even by the onset of the Space Race, the Saturn V and much of the other hardware needed to get to the moon was still being constructed by skilled workers manually joining the components of the F-1 engines and propellant tanks with arc welding. No such thing as computer-aided design (CAD) back then, just expert craftsmen with unmatched dexterity! 

Perfect arc welds done on the Saturn V F-1 engine's gimbal mount (bottom left) and turbopump exhaust manifold

While conventional arc welding is simple and inexpensive, it inevitably comes with its drawbacks, mainly that certain materials like high-strength steel, titanium, and aluminum alloys may be susceptible to embrittlement due to reactions with the surrounding air. To protect the weld from ambient nitrogen and oxygen, welders began using inert shielding gases in the first half of the 20th century. Many techniques were developed, but one known as tungsten gas arc welding (GTAW), also known as tungsten inert gas (TIG) welding, is used particularly often in aerospace

Northrop was the first to perfect GTAW in 1941. It uses a torch containing both a non-consumable tungsten electrode and a constant source of shielding gas (often helium or argon) to simultaneously heat the metal enough to create the weld and protect the weld from the surrounding atmosphere. I found this pretty neat article about how GTAW was crucial to repairing a cracked hydrogen fuel line on Space Shuttle Atlantis that was discovered just before its flight on STS-112 in 2002. Because the cracks were minuscule (some weren't even visible to the naked eye and were only discovered with borescopes, X-rays, and ultrasonic scans), it took extreme coordination by the welders to ensure all the cracks were successfully penetrated without excessive heat deformation to the surrounding metal

A diagram of Gas Tungsten Arc Welding, and liftoff of STS-112 in October 2002

Of course, welding technology has continued to advance in more recent decades, with newer techniques employing more exotic methods of melting and joining metal while minimizing defects as much as possible. Rather than relying on electricity or combustion to generate heat, energy beam welding uses either lasers or high-velocity electrons to generate the heat needed to make extremely precise, high-penetration welds with minimal distortion to the surrounding metal. While these high-tech processes are fast and automated, they're unfortunately quite costly. Electron beam welding is also challenging because it can only be performed in a vacuum environment, otherwise air molecules would completely disrupt the beam of electrons! To that end, while any factory here on Earth must first invest in a vacuum chamber to take advantage of electron beam welding, in a cool technology demonstration, the Soviets used a handheld electron beam gun to make welding repairs to the Salyut 7 space station during a spacewalk in 1984!

Soviet cosmonaut Svetlana Savitskaya, the first woman to ever walk in space, using her electron beam gun to perform maintenance on Salyut 7

Finally, there's an entire class of solid-state welding methods that completely sidesteps the process of melting metals together and instead joins them via some other highly energetic process, such as intense vibrations or pressure. One relatively recent technique called friction stir welding is very popular nowadays for manufacturing the propellant tanks on rockets, due to its ability to rapidly make high strength welds across very large workpieces. Rather than melting the metal plates, a rapidly rotating tool generates just enough heat through friction to soften the metal, then mechanical pressure is applied to join the two "plasticized" pieces together, much like molding clay. There's an insightful article by Ars Technica that discusses the enormous friction stir weld tools that had to be installed at NASA's Michoud Assembly Facility in Louisiana for the Space Launch System

A liquid oxygen tank test article being constructed for SLS at Michoud using friction stir welding

Any other welding techniques you know of that are commonly used in aerospace? The only joining I do at SpaceX is the INNER JOIN of a SQL query (sorry, nerdy finance joke!), so would love to hear your thoughts!

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