Fabrication and Hydrostatic Evaluation of a Meter-Scale Aluminum Propellant-Tank Demonstrator by Hybrid Wire Arc Additive and Subtractive Manufacturing

Building the next generation of spacecraft requires materials that are both incredibly strong and light enough to fly. For decades, engineers have relied on traditional methods to shape metal, but these techniques often struggle when creating the large, complex curves needed for fuel tanks. A newer approach, known as wire arc additive manufacturing, offers a different path. Instead of cutting a block of metal down to size, this method builds an object up, layer by layer, using a robotic arm that acts like a highly precise welding torch. It melts a thin wire of metal and deposits it in a specific pattern, fusing it into a solid structure. While this technique holds great promise for creating massive parts quickly, it faces a significant hurdle: the intense heat required to melt the metal causes the material to warp and distort as it cools. Over the course of building a large object, these small warps can add up, leading to a final shape that is too crooked or weak to be useful.

To solve this problem, researchers at Shimizu Corporation and the Japan Aerospace Exploration Agency set out to combine the speed of this welding-based building method with the precision of traditional machining. They developed a hybrid system where a robot does not just add metal, but also removes it. By integrating a milling tool and a laser scanner into the same robotic cell, the system can measure the height of each new layer as it is being built and adjust the robot's path in real time. If the metal piles up too high or warps, the robot can pause the welding, grind away the excess material to create a perfectly flat surface, and then continue building. This process ensures that the final object maintains its intended shape, even when the heat tries to pull it out of alignment. The team put this system to the test by constructing a full-scale model of a propellant tank, the kind of vessel that would hold fuel for a rocket, and then subjected it to extreme pressure to see if it could hold.

The researchers constructed a demonstrator tank that stood 1.1 meters tall, with a wide section measuring one meter across and a narrower section half a meter across. The walls of this tank were built from aluminum alloy wire, deposited in layers to reach a thickness of five millimeters. To keep the structure rigid during the long process of adding layer after layer, the team introduced ring-shaped stiffeners inside the tank, which were manually welded in place before the robot resumed its work. A critical part of their success was the use of a laser profiler that scanned the surface of the tank ten times every second while the robot was welding. This allowed the system to constantly check the height of the metal being added and automatically adjust the robot's movements to keep the layers uniform. The result was a seamless metal shell that maintained a consistent wall thickness, despite the challenges of building such a large object with high heat.

Once the tank was complete, the team needed to verify that it was strong enough to function as a real fuel container. They sealed the top with a metal cover and filled the tank with water to perform a hydrostatic pressure test. This method is safe and effective for testing strength because water cannot be compressed like air, meaning the tank will not explode violently if it fails. They slowly increased the pressure inside the tank, watching closely at how the metal stretched and deformed. The tank held firm until the internal pressure reached 1.7 megapascals, at which point it finally burst. However, the rupture occurred at the seam where the manually welded cover met the robot-built tank, not in the main body of the tank itself. This indicated that the welded joint was the weakest point, rather than the metal walls created by the additive process.

The true measure of the tank's performance came from analyzing the moment the metal began to permanently stretch, a point known as the yield pressure. By measuring the strain on the tank's surface with sensitive gauges, the researchers found that the cylindrical section of the tank began to yield at a pressure of 1.30 megapascals, while the curved bottom section yielded at 1.38 megapascals. These numbers were remarkably close to what the team had predicted using computer simulations based on the known strength of the aluminum wire they used. The simulations had estimated a yield pressure of roughly 1.37 megapascals for the cylinder and 1.38 megapascals for the dome. The close match between the physical test and the computer model confirmed that the metal produced by the robotic welding process was just as strong as the raw material used to make the wire.

The study also examined whether the internal ring stiffeners made a significant difference in the tank's ability to resist pressure. The computer models suggested that adding these rings improved the pressure resistance by only about five percent, a relatively small gain. The experimental data supported this, showing that the tank performed well even without relying heavily on the stiffeners for its primary strength. The researchers noted that the slight differences between their predictions and the actual test results likely stemmed from simplifications in their computer models, such as not accounting for the exact shape of the manually welded cover or minor variations in wall thickness. Nevertheless, the core finding remained clear: the hybrid manufacturing system successfully produced a large, complex aluminum tank that met theoretical strength requirements.

This work demonstrates that it is possible to build meter-scale aerospace components using a combination of additive and subtractive techniques without sacrificing structural integrity. The key to this success was the ability to correct errors as they happened, preventing the accumulation of defects that usually plagues large-scale welding projects. While the team identified areas for improvement, such as enhancing the quality of the metal at the points where the welding process was restarted, the overall experiment proved the viability of the approach. The tank held up under pressure exactly as the laws of physics predicted, suggesting that this hybrid method could become a reliable way to manufacture the large, lightweight structures needed for future space exploration.