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*'''Sealing:''' The tube must be effectively sealed against air leaks along its entire length, including at joints, stations, and airlocks. Even small leaks could compromise the low-pressure environment, increasing drag and requiring continuous pumping.<ref name="PremsagarKenworthyCritReview" />
*'''Structural Integrity:''' The tube must withstand the substantial pressure difference between the low-pressure interior and the atmospheric exterior (approximately 1 atm or 101.3 kPa, equating to roughly 10 tonnes of force per square meter at near vacuum<ref>{{cite web |title=Pressure Force Calculation |url=https://www.omnicalculator.com/physics/pressure-force |access-date=2025-04-08 |website=Omni Calculator}} Note: Calculation based on atmospheric pressure ≈ 101.3 kPa.</ref>). The alpha design specified steel tubes with a wall thickness of around 20-25 mm (0.8-1.0 in).<ref name="Alpha SpaceX" /><sup>(Section 4.1, 4.2)</sup> A simple calculation for the compressive hoop stress in a 2.5 m diameter, 25 mm thick steel tube under this pressure yields approximately 5.1 MPa, which is significantly below the yield strength of typical steel (often 250 MPa or higher). However, the primary failure mode for such structures under external pressure is not yielding but [[Buckling|buckling instability]]. While theoretical calculations suggest a perfect cylinder of these dimensions could resist buckling from atmospheric pressure, real-world imperfections significantly reduce buckling strength. Therefore, robust engineering design, likely incorporating stiffening rings (as mentioned in the alpha design<ref name="Alpha SpaceX" /><sup>(Section 4.2)</sup>) to increase stability, is essential rather than relying solely on basic material strength. The design must also account for ground settlement, seismic activity, and potential impacts.<ref name="PremsagarKenworthyCritReview" />
*'''Pumping Systems:''' A large network of powerful and reliable vacuum pumps would be needed initially to evacuate the tube and continuously thereafter to remove any ingressing air from leaks and [[Outgassing|material outgassing]].<ref name="Alpha SpaceX" /><sup>(Section 4.3)</sup> The energy required is substantial.<ref name="PremsagarKenworthyCritReview" /> Calculating the precise energy is complex, but estimates can illustrate the scale. For the initial evacuation of a hypothetical 600 km route with a 2.5 m diameter tube down to 100 Pascals (Musk's target), the theoretical minimum energy required (assuming isothermal removal) is on the order of 600,000 kWh.<ref>Calculation: Volume ≈ π*(1.25m)²*600,000m ≈ 2.95 million m³. Work ≈ V*P_atm*ln(P_atm/P_final) ≈ (2.95e6 m³) * (101300 Pa) * ln(101300/100) ≈ 2.07e12 J. Energy ≈ 2.07e12 J / (3.6e6 J/kWh) ≈ 575,000 kWh.</ref> However, real-world pump system efficiencies are far lower than theoretical ideals (potentially 10-50% overall efficiency for such a system), meaning the actual energy consumed for initial pump-down could be significantly higher, likely several million kWh. Critically, energy is also required ''continuously'' to counteract gas influx from leaks through seals and welds along the tube length, and from outgassing of the tube's inner walls. This steady-state pumping load depends heavily on the chosen materials, construction quality, and seal technology, but represents a continuous operational energy cost.<ref name="PremsagarKenworthyCritReview" /> The alpha design suggested pumps roughly every 5 miles (8 km), indicating the anticipated need for distributed, ongoing power consumption for vacuum maintenance.<ref name="Alpha SpaceX" /><sup>(Section 4.3)</sup>
=== Aerodynamics and the Kantrowitz Limit ===
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