In a significant accomplishment for space exploration, SpaceX, led by visionary entrepreneur Elon Musk, successfully completed the fourth test flight of its massive Starship on June 6. The towering 400-foot rocket, along with its Heavy booster, took off after 6 PM IST from the company’s Starbase launch site near Boca Chica Beach, South Texas. The mission aims to revolutionise space travel economics by reducing launch costs through economies of scale.
Challenges with carbon fibre
An interesting aspect of the Starship is its stainless steel construction, which is different from traditional materials. In a detailed discussion with Adam Burrows of Princeton University, Musk explained why stainless steel was chosen over carbon fibre. Initially, SpaceX experimented with advanced carbon fibre but faced significant challenges. Musk highlighted the material’s porous nature and flammability, especially troublesome given the pressurised cryogenic methane and hot oxygen gases used in the Starship, which required a lining that compromised carbon fibre’s mass efficiency.
The Starship’s large nine-metre diameter presented another challenge, requiring precise application of 60 to 220 carbon fibre layers, depending on the tank section. Any imperfections, like bubbles or leftover separator sheets, would jeopardise the entire structure.
Alternate materials
Faced with these obstacles, SpaceX explored alternative materials. The company had previously used an aluminium-lithium alloy in its Falcon 9 rockets but found it to be too expensive at around $40 per kilogram and difficult to weld.
The search led to stainless steel, particularly the 300 series, which Musk noted for its remarkable cryogenic properties. At liquid oxygen temperatures, stainless steel’s strength increases without becoming more brittle - a critical advantage since both fuel and oxygen onboard are cryogenically chilled. Moreover, stainless steel's ease of welding and affordability - costing roughly $4 per kilogram - made it an attractive option.
Advantages of stainless steel
Musk further explained stainless steel’s role in reducing the need for extensive heat shielding during reentry. While carbon fibre's strength diminishes beyond 200 degrees Celsius, stainless steel can withstand temperatures exceeding 1000 degrees Celsius. This resilience allows for a lighter heat shield, as its mass correlates with the temperature transmitted to the rocket’s hull.
Musk also pointed out that the choice of stainless steel for the construction of the Starship spacecraft negates the need for paint, which can add significant weight to the structure—especially considering its substantial nine-metre diameter. Further emphasising the material’s benefits, Musk revealed that the decision to utilise ultra-hard, cold-rolled stainless steel for the Starship was a pivotal factor in choosing the same material for the manufacturing of Tesla’s Cybertruck.
SpaceX's material innovation journey began with stainless steel 301, which, despite its merits, revealed issues at cryogenic temperatures. The company then transitioned to stainless steel 304 and ultimately developed its proprietary alloy, designated 30X, surpassing the performance of both 301 and 304.
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