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The much-older [[vactrain]] concept resembles a [[high-speed rail]] system without substantial [[Drag (physics)|air resistance]] by employing [[maglev|magnetically levitating]] trains in [[vacuum|evacuated]] (airless) or partly evacuated tubes. However, the difficulty of maintaining a vacuum over large distances has prevented this type of system from ever being built. By contrast, the Hyperloop alpha concept was to operate at approximately {{convert|1|mbar|Pa|spell=in|lk=on}} of pressure and requires the air for levitation.<ref name="nova20130813"/>
=== Initial design concept === ▼== Technical Challenges ==
Implementing a full-scale, operational hyperloop system faces numerous engineering and scientific hurdles that go beyond the basic concept. These challenges must be addressed for the system to be considered feasible, safe, and economically viable.<ref name="PremsagarKenworthyCritReview"/>
The hyperloop alpha concept envisioned operation by sending specially designed "capsules" or "pods" through a steel tube maintained at a partial vacuum. In Musk's original concept, each capsule would float on a {{convert|0.5-1.3|mm|in|2|order=flip|abbr=on}} layer of air provided under pressure to [[air caster|air-caster]] "skis", similar to how pucks are levitated above an air hockey table, while still allowing higher speeds than wheels can sustain. With [[rolling resistance]] eliminated and air resistance greatly reduced, the capsules can [[gliding flight|glide]] for the bulk of the journey. In the alpha design concept, an electrically driven [[ducted fan|inlet fan]] and [[axial compressor]] would be placed at the nose of the capsule to "actively transfer high-pressure air from the front to the rear of the vessel", resolving the problem of air pressure building in front of the vehicle, slowing it down (the [[Kantrowitz limit]]). A fraction of the air was to be shunted to the skis for additional pressure, augmenting that gain passively from lift due to their shape.<ref name="Alpha SpaceX" /> ▼=== Tube Integrity and Vacuum Maintenance ===
Creating and maintaining a near-vacuum environment over hundreds of kilometers presents significant challenges.
*'''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>
In the alpha-level concept, passenger-only pods were to be {{convert|2.23|m|order=flip|abbr=on}} in diameter and were projected to reach a top speed of {{convert|760|mph|km/h|abbr=on}} (Machto ~1.0maintain inaerodynamic theefficiency.<ref low-pressurename="Alpha tube)SpaceX" to/><sup> maintain(Section aerodynamic efficiency4. 4)</sup> The design proposed passengers experience a maximum inertial [[g-force|acceleration]] of 0.5 g, about 2 or 3 times that of a commercial airliner on takeoff and landing. <ref{{citation nameneeded|date= "AlphaSeptember SpaceX" /><sup> (Section 4.4)</sup>2021}}▼=== Aerodynamics and the Kantrowitz Limit ===
Even in a partial vacuum, air resistance becomes significant at the proposed high speeds.<ref name="LangConnollyReview2024"/>
*'''[[Kantrowitz limit]]:''' As a pod travels through the confined tube, air builds up in front of it. If the gap between the pod and the tube wall is too small relative to the pod's speed (high blockage ratio), the air cannot flow around the pod efficiently, causing the air to compress and potentially choke the flow near Mach 1. This dramatically increases drag and necessitates managing the air ahead of the pod.<ref name="Alpha SpaceX" /><sup>(Section 4.4)</sup><ref name="OpgenoordCaplanAIAA2017"/><ref name="LangConnollyReview2024" /> Musk's alpha design proposed an onboard compressor to actively transfer air from the front to the rear, though this adds complexity, weight, and energy consumption to the pod.<ref name="Alpha SpaceX" /><sup>(Section 4.4)</sup> Alternative solutions involve larger tube diameters (reducing blockage ratio) or operating at lower speeds.<ref name="openmdao"/><ref name="Chin2015"/>
*'''Air Management:''' The interaction of the high-speed pod with the residual air, especially if using air bearings or aerodynamic lift surfaces as initially proposed, is complex and requires precise control.<ref name="mechsite2019092" /><ref name="LangConnollyReview2024" />
=== Levitation and Propulsion ===
Efficiently levitating and propelling pods requires advanced, reliable systems.
*'''[[Maglev]] Systems:''' Most current hyperloop concepts rely on magnetic levitation (maglev) rather than the air bearings proposed in the alpha design, due to challenges with air bearing stability and efficiency at speed.<ref name="PremsagarKenworthyCritReview" /> Implementing stable maglev over long distances, particularly on elevated pylons subject to movement or vibration, is challenging and expensive.<ref name="PremsagarKenworthyCritReview" /> Power requirements for levitation and propulsion, especially during acceleration, are significant.<ref name="EDS power consumption"/><ref name="ESC Hyperloop"/>
*'''Linear Motors:''' Linear electric motors embedded in the track or tube are typically proposed for propulsion.<ref name="Alpha SpaceX" /><sup>(Section 4.5)</sup> These require precise alignment with the pod and substantial power infrastructure along the entire route.<ref name="PremsagarKenworthyCritReview" />
=== Thermal Expansion ===
Long steel tubes exposed to varying ambient temperatures will expand and contract significantly. A several hundred kilometer steel tube could change length by hundreds of meters between temperature extremes.<ref>{{cite web|title=Thermal Expansion - Linear Expansion Coefficients|url=https://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html|website=Engineering Toolbox|access-date=8 April 2025}} Note: Calculation based on ~12 µm/(m·°C) for steel and a large temperature range.</ref> The alpha design proposed slip joints consisting of telescoping tubes with multiple seals to accommodate this, allowing axial movement while maintaining the vacuum seal.<ref name="Alpha SpaceX" /><sup>(Section 4.2)</sup> Designing reliable, long-lasting expansion joints capable of maintaining a vacuum seal under these conditions is a critical challenge.<ref name="PremsagarKenworthyCritReview" />
=== Alignment and Stability ===
Maintaining the precise alignment required for high-speed travel in a tube (tolerances likely in millimeters), especially over long distances and potentially across seismic zones, is a major challenge.<ref name="PremsagarKenworthyCritReview" /> Even minor misalignments or track irregularities could cause significant instability, vibration, or unsafe conditions at speeds approaching [[Mach 1]].<ref name="verge20130816"/> Elevated sections on pylons are particularly susceptible to ground movement, wind forces, and thermal effects impacting alignment.<ref name="PremsagarKenworthyCritReview" /><ref name="Alpha SpaceX" /><sup>(Section 4.1)</sup>
=== Safety and Emergency Systems ===
Ensuring passenger safety within a sealed, low-pressure environment presents unique challenges.<ref name="PremsagarKenworthyCritReview" />
*'''Decompression:''' A breach in the tube wall could lead to rapid, potentially catastrophic, decompression and expose pods to extreme aerodynamic forces. Systems for rapidly detecting breaches and safely stopping or diverting pods are crucial.<ref name="PremsagarKenworthyCritReview" /><ref name="Mercury-2016-09-16"/> The alpha design included emergency brakes on the pods and suggested pressure sensors along the tube.<ref name="Alpha SpaceX" /><sup>(Section 4.6)</sup>
*'''Emergency Evacuation:''' Evacuating passengers from a pod stopped within the tube (e.g., due to power failure or malfunction) potentially miles from a station is complex. Pods would need emergency oxygen supplies. Procedures might involve rescue vehicles within the tube, repressurization of sections, or potentially parallel emergency access tunnels, which would significantly increase cost.<ref name="PremsagarKenworthyCritReview" />
*'''Emergency Braking:''' Safely decelerating a pod from very high speeds requires robust braking systems that can function reliably within the low-pressure environment, potentially independent of the main propulsion and levitation systems.<ref name="Alpha SpaceX" /><sup>(Section 4.6)</sup>
=== Switching ===
Efficiently switching pods between different lines or diverting them to stations without significantly slowing down the main line traffic, and doing so within the vacuum environment, requires innovative and reliable mechanisms. Solutions involving mechanical switches within low-pressure sections or non-mechanical switching using magnetic fields have been proposed and demonstrated on a small scale by companies like Hardt Hyperloop.<ref name="eit.europa.eu"/>
▲=== Initial design concept ===
▲The hyperloop alpha concept envisioned operation by sending specially designed "capsules" or "pods" through a steel tube maintained at a partial vacuum. In Musk's original concept, each capsule would float on a {{convert|0.5-1.3|mm|in|2|order=flip|abbr=on}} layer of air provided under pressure to [[air caster|air-caster]] "skis", similar to how pucks are levitated above an air hockey table, while still allowing higher speeds than wheels can sustain. With [[rolling resistance]] eliminated and air resistance greatly reduced, the capsules can [[gliding flight|glide]] for the bulk of the journey. In the alpha design concept, an electrically driven [[ducted fan|inlet fan]] and [[axial compressor]] would be placed at the nose of the capsule to "actively transfer high-pressure air from the front to the rear of the vessel", resolving the problem of air pressure building in front of the vehicle, slowing it down (the [[Kantrowitz limit]]). A fraction of the air was to be shunted to the skis for additional pressure, augmenting that gain passively from lift due to their shape.<ref name="Alpha SpaceX" />
▲In the alpha-level concept, passenger-only pods were to be {{convert|2.23|m|order=flip|abbr=on}} in diameter and were projected to reach a top speed of {{convert|760|mph|km/h|abbr=on}} (Mach ~1.0 in the low-pressure tube) to maintain aerodynamic efficiency. The design proposed passengers experience a maximum inertial [[g-force|acceleration]] of 0.5 g, about 2 or 3 times that of a commercial airliner on takeoff and landing.<ref name="Alpha SpaceX" /><sup> (Section 4.4)</sup>
===Proposed routes===
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