Elon musk Hyperloop one

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Published Date:11-07-2017
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Hyperloop Alpha Intro The first several pages will attempt to describe the design in everyday language, keeping numbers to a minimum and avoiding formulas and jargon. I apologize in advance for my loose use of language and imperfect analogies. The second section is for those with a technical background. There are no doubt errors of various kinds and superior optimizations for elements of the system. Feedback would be most welcome – please send to hyperloopspacex.com or hyperloopteslamotors.com. I would like to thank my excellent compadres at both companies for their help in putting this together. Background When the California “high speed” rail was approved, I was quite disappointed, as I know many others were too. How could it be that the home of Silicon Valley and JPL – doing incredible things like indexing all the world’s knowledge and putting rovers on Mars – would build a bullet train that is both one of the most expensive per mile and one of the slowest in the world? Note, I am hedging my statement slightly by saying “one of”. The head of the California high speed rail project called me to complain that it wasn’t the very slowest bullet train nor the very most expensive per mile. The underlying motive for a statewide mass transit system is a good one. It would be great to have an alternative to flying or driving, but obviously only if it is actually better than flying or driving. The train in question would be both slower, more expensive to operate (if unsubsidized) and less safe by two orders of magnitude than flying, so why would anyone use it? If we are to make a massive investment in a new transportation system, then the return should by rights be equally massive. Compared to the alternatives, it should ideally be:  Safer  Faster  Lower cost  More convenient  Immune to weather  Sustainably self-powering  Resistant to Earthquakes  Not disruptive to those along the route Is there truly a new mode of transport – a fifth mode after planes, trains, cars and boats – that meets those criteria and is practical to implement? Many ideas for a system with most of those properties have been proposed and should be acknowledged, reaching as far back as Robert Goddard’s to proposals in recent decades by the Rand Corporation and ET3. Unfortunately, none of these have panned out. As things stand today, there is not even a short distance demonstration system operating in test pilot mode anywhere in the world, let alone something that is robust enough for public transit. They all possess, it would seem, one or more fatal flaws that prevent them from coming to fruition. Constraining the Problem The Hyperloop (or something similar) is, in my opinion, the right solution for the specific case of high traffic city pairs that are less than about 1500 km or 900 miles apart. Around that inflection point, I suspect that supersonic air travel ends up being faster and cheaper. With a high enough altitude and the right geometry, the sonic boom noise on the ground would be no louder than current airliners, so that isn’t a showstopper. Also, a quiet supersonic plane immediately solves every long distance city pair without the need for a vast new worldwide infrastructure. However, for a sub several hundred mile journey, having a supersonic plane is rather pointless, as you would spend almost all your time slowly ascending and descending and very little time at cruise speed. In order to go fast, you need to be at high altitude where the air density drops exponentially, as air at sea level becomes as thick as molasses (not literally, but you get the picture) as you approach sonic velocity. So What is Hyperloop Anyway? Short of figuring out real teleportation, which would of course be awesome (someone please do this), the only option for super fast travel is to build a tube over or under the ground that contains a special environment. This is where things get tricky. At one extreme of the potential solutions is some enlarged version of the old pneumatic tubes used to send mail and packages within and between buildings. You could, in principle, use very powerful fans to push air at high speed through a tube and propel people-sized pods all the way from LA to San Francisco. However, the friction of a 350 mile long column of air moving at anywhere near sonic velocity against the inside of the tube is so stupendously high that this is impossible for all practical purposes. Another extreme is the approach, advocated by Rand and ET3, of drawing a hard or near hard vacuum in the tube and then using an electromagnetic suspension. The problem with this approach is that it is incredibly hard to maintain a near vacuum in a room, let alone 700 miles (round trip) of large tube with dozens of station gateways and thousands of pods entering and exiting every day. All it takes is one leaky seal or a small crack somewhere in the hundreds of miles of tube and the whole system stops working. However, a low pressure (vs. almost no pressure) system set to a level where standard commercial pumps could easily overcome an air leak and the transport pods could handle variable air density would be inherently robust. Unfortunately, this means that there is a non-trivial amount of air in the tube and leads us straight into another problem. Overcoming the Kantrowitz Limit Whenever you have a capsule or pod (I am using the words interchangeably) moving at high speed through a tube containing air, there is a minimum tube to pod area ratio below which you will choke the flow. What this means is that if the walls of the tube and the capsule are too close together, the capsule will behave like a syringe and eventually be forced to push the entire column of air in the system. Not good. Nature’s top speed law for a given tube to pod area ratio is known as the Kantrowitz limit. This is highly problematic, as it forces you to either go slowly or have a super huge diameter tube. Interestingly, there are usually two solutions to the Kantrowitz limit – one where you go slowly and one where you go really, really fast. The latter solution sounds mighty appealing at first, until you realize that going several thousand miles per hour means that you can’t tolerate even wide turns without painful g loads. For a journey from San Francisco to LA, you will also experience a rather intense speed up and slow down. And, when you get right down to it, going through transonic buffet in a tube is just fundamentally a dodgy prospect. Both for trip comfort and safety, it would be best to travel at high subsonic speeds for a 350 mile journey. For much longer journeys, such as LA to NY, it would be worth exploring super high speeds and this is probably technically feasible, but, as mentioned above, I believe the economics would probably favor a supersonic plane. The approach that I believe would overcome the Kantrowitz limit is to mount an electric compressor fan on the nose of the pod that actively transfers high pressure air from the front to the rear of the vessel. This is like having a pump in the head of the syringe actively relieving pressure. It would also simultaneously solve another problem, which is how to create a low friction suspension system when traveling at over 700 mph. Wheels don’t work very well at that sort of speed, but a cushion of air does. Air bearings, which use the same basic principle as an air hockey table, have been demonstrated to work at speeds of Mach 1.1 with very low friction. In this case, however, it is the pod that is producing the air cushion, rather than the tube, as it is important to make the tube as low cost and simple as possible. That then begs the next question of whether a battery can store enough energy to power a fan for the length of the journey with room to spare. Based on our calculations, this is no problem, so long as the energy used to accelerate the pod is not drawn from the battery pack. This is where the external linear electric motor comes in, which is simply a round induction motor (like the one in the Tesla Model S) rolled flat. This would accelerate the pod to high subsonic velocity and provide a periodic reboost roughly every 70 miles. The linear electric motor is needed for as little as 1% of the tube length, so is not particularly costly. Making the Economics Work The pods and linear motors are relatively minor expenses compared to the tube itself – several hundred million dollars at most, compared with several billion dollars for the tube. Even several billion is a low number when compared with several tens of billion proposed for the track of the California rail project. The key advantages of a tube vs. a railway track are that it can be built above the ground on pylons and it can be built in prefabricated sections that are dropped in place and joined with an orbital seam welder. By building it on pylons, you can almost entirely avoid the need to buy land by following alongside the mostly very straight California Interstate 5 highway, with only minor deviations when the highway makes a sharp turn. Even when the Hyperloop path deviates from the highway, it will cause minimal disruption to farmland roughly comparable to a tree or telephone pole, which farmers deal with all the time. A ground based high speed rail system by comparison needs up to a 100 ft wide swath of dedicated land to build up foundations for both directions, forcing people to travel for several miles just to get to the other side of their property. It is also noisy, with nothing to contain the sound, and needs unsightly protective fencing to prevent animals, people or vehicles from getting on to the track. Risk of derailment is also not to be taken lightly, as demonstrated by several recent fatal train accidents. Earthquakes and Expansion Joints A ground based high speed rail system is susceptible to Earthquakes and needs frequent expansion joints to deal with thermal expansion/contraction and subtle, large scale land movement. By building a system on pylons, where the tube is not rigidly fixed at any point, you can dramatically mitigate Earthquake risk and avoid the need for expansion joints. Tucked away inside each pylon, you could place two adjustable lateral (XY) dampers and one vertical (Z) damper. These would absorb the small length changes between pylons due to thermal changes, as well as long form subtle height changes. As land slowly settles to a new position over time, the damper neutral position can be adjusted accordingly. A telescoping tube, similar to the boxy ones used to access airplanes at airports would be needed at the end stations to address the cumulative length change of the tube. Can it Really be Self-Powering? For the full explanation, please see the technical section, but the short answer is that by placing solar panels on top of the tube, the Hyperloop can generate far in excess of the energy needed to operate. This takes into account storing enough energy in battery packs to operate at night and for periods of extended cloudy weather. The energy could also be stored in the form of compressed air that then runs an electric fan in reverse to generate energy, as demonstrated by LightSail. Hyperloop Preliminary Design Study Technical Section 1. Abstract Existing conventional modes of transportation of people consists of four unique types: rail, road, water, and air. These modes of transport tend to be either relatively slow (i.e., road and water), expensive (i.e., air), or a combination of relatively slow and expensive (i.e., rail). Hyperloop is a new mode of transport that seeks to change this paradigm by being both fast and inexpensive for people and goods. Hyperloop is also unique in that it is an open design concept, similar to Linux. Feedback is desired from the community that can help advance the Hyperloop design and bring it from concept to reality. Hyperloop consists of a low pressure tube with capsules that are transported at both low and high speeds throughout the length of the tube. The capsules are supported on a cushion of air, featuring pressurized air and aerodynamic lift. The capsules are accelerated via a magnetic linear accelerator affixed at various stations on the low pressure tube with rotors contained in each capsule. Passengers may enter and exit Hyperloop at stations located either at the ends of the tube, or branches along the tube length. In this study, the initial route, preliminary design, and logistics of the Hyperloop transportation system have been derived. The system consists of capsules that travel between Los Angeles, California and San Francisco, California. The total trip time is approximately half an hour, with capsules departing as often as every 30 seconds from each terminal and carrying 28 people each. This gives a total of 7.4 million people each way that can be transported each year on Hyperloop. The total cost of Hyperloop in this analysis is under 6 billion USD. Amortizing this capital cost over 20 years and adding daily operational costs gives a total of about 20 USD (in current year dollars) plus operating costs per one-way ticket on the passenger Hyperloop. Useful feedback is welcomed on aspects of the Hyperloop design. E-mail feedback to hyperloopspacex.com or hyperloopteslamotors.com. 2. Table of Contents 1. Abstract .................................................................................. 6 2. Table of Contents ...................................................................... 6 3. Background .............................................................................. 8 4. Hyperloop Transportation System .................................................... 9 4.1. Capsule............................................................................ 11 4.1.1. Geometry .................................................................... 13 4.1.2. Interior ....................................................................... 15 4.1.3. Compressor .................................................................. 17 4.1.4. Suspension ................................................................... 20 4.1.5. Onboard Power ............................................................. 22 4.1.6. Propulsion ................................................................... 22 4.1.7. Cost ........................................................................... 23 4.2. Tube ............................................................................... 24 4.2.1. Geometry .................................................................... 25 4.2.2. Tube Construction .......................................................... 26 4.2.3. Pylons and Tunnels ......................................................... 27 4.2.4. Station Construction ....................................................... 31 4.2.5. Cost ........................................................................... 32 4.3. Propulsion ........................................................................ 32 4.3.1. Capsule Components (Rotor) ............................................. 35 4.3.2. Tube Components (Stator) ................................................ 36 4.3.3. Energy Storage Components .............................................. 37 4.3.4. Cost ........................................................................... 37 4.3.5. Propulsion for Passenger Plus Vehicle System ......................... 38 4.4. Route .............................................................................. 38 4.4.1. Route Optimization ........................................................ 40 4.4.1.1. Los Angeles/Grapevine - South ........................................ 43 4.4.1.2. Los Angeles/Grapevine – North ........................................ 45 4.4.1.2. I-5 .......................................................................... 47 4.4.1.3. I-580/San Francisco Bay................................................. 48 4.4.3. Station Locations ........................................................... 50 4.5. Safety and Reliability ........................................................... 52 4.5.1. Onboard Passenger Emergency ........................................... 52 4.5.2. Power Outage ............................................................... 53 4.5.2. Capsule Depressurization ................................................. 53 4.5.3. Capsule Stranded in Tube ................................................. 54 4.5.4. Structural Integrity of the Tube in Jeopardy ........................... 54 4.5.5. Earthquakes ................................................................. 54 4.5.6. Human Related Incidents ................................................. 54 4.5.7. Reliability.................................................................... 55 4.6. Cost ................................................................................ 55 6. Conclusions ............................................................................ 56 7. Future Work ........................................................................... 57 3. Background The corridor between San Francisco, California and Los Angeles, California is one of the most often traveled corridors in the American West. The current practical modes of transport for passengers between these two major population centers include: 1. Road (inexpensive, slow, usually not environmentally sound) 2. Air (expensive, fast, not environmentally sound) 3. Rail (expensive, slow, often environmentally sound) A new mode of transport is needed that has benefits of the current modes without the negative aspects of each. This new high speed transportation system has the following requirements: 1. Ready when the passenger is ready to travel (road) 2. Inexpensive (road) 3. Fast (air) 4. Environmentally friendly (rail/road via electric cars) The current contender for a new transportation system between southern and northern California is the “California High Speed Rail.” The parameters outlining this system include: 1. Currently 68.4 billion USD proposed cost 2. Average speed of 164 mph (264 kph) between San Francisco and Los Angeles 3. Travel time of 2 hours and 38 minutes between San Francisco and Los Angeles a. Compare with 1 hour and 15 minutes by air b. Compare with 5 hours and 30 minutes by car 4. Average one-way ticket price of 105 one-way (reference) a. Compare with 158 round trip by air for September 2013 b. Compare with 115 round trip by road (4/gallon with 30 mpg vehicle) A new high speed mode of transport is desired between Los Angeles and San Francisco; however, the proposed California High Speed Rail does not reduce current trip times or reduce costs relative to existing modes of transport. This preliminary design study proposes a new mode of high speed transport that reduces both the travel time and travel cost between Los Angeles and San Francisco. Options are also included to increase the transportation system to other major population centers across California. It is also worth noting the energy cost of this system is less than any currently existing mode of transport (Figure 1). The only system that comes close to matching the low energy requirements of Hyperloop is the fully electric Tesla Model S. Figure 1. Energy cost per passenger for a journey between Los Angeles and San Francisco for various modes of transport. 4. Hyperloop Transportation System Hyperloop (Figure 2 through Figure 3) is a proposed transportation system for traveling between Los Angeles, California, and San Francisco, California in 35 minutes. The Hyperloop consists of several distinct components, including: 1. Capsule: a. Sealed capsules carrying 28 passengers each that travel along the interior of the tube depart on average every 2 minutes from Los Angeles or San Francisco (up to every 30 seconds during peak usage hours). b. A larger system has also been sized that allows transport of 3 full size automobiles with passengers to travel in the capsule. c. The capsules are separated within the tube by approximately 23 miles (37 km) on average during operation. d. The capsules are supported via air bearings that operate using a compressed air reservoir and aerodynamic lift. 2. Tube: a. The tube is made of steel. Two tubes will be welded together in a side by side configuration to allow the capsules to travel both directions. b. Pylons are placed every 100 ft (30 m) to support the tube. c. Solar arrays will cover the top of the tubes in order to provide power to the system. 3. Propulsion: a. Linear accelerators are constructed along the length of the tube at various locations to accelerate the capsules. b. Stators are located on the capsules to transfer momentum to the capsules via the linear accelerators. 4. Route: a. There will be a station at Los Angeles and San Francisco. Several stations along the way will be possible with splits in the tube. b. The majority of the route will follow I-5 and the tube will be constructed in the median. Los San Angeles, Francisco, CA CA Figure 2. Hyperloop conceptual diagram. Figure 3. Hyperloop tube stretching from Los Angeles to San Francisco. In addition to these aspects of the Hyperloop, safety and cost will also be addressed in this study. The Hyperloop is sized to allow expansion as the network becomes increasingly popular. The capacity would be 840 passengers per hour which more than sufficient to transport all of the 6 million passengers traveling between Los Angeles and San Francisco areas per year. In addition, this accounts for 70% of those travelers to use the Hyperloop during rush hour. The lower cost of traveling on Hyperloop is likely to result in increased demand, in which case the time between capsule departures could be significantly shortened. 4.1. Capsule Two versions of the Hyperloop capsules are being considered: a passenger only version and a passenger plus vehicle version. Hyperloop Passenger Capsule Assuming an average departure time of 2 minutes between capsules, a minimum of 28 passengers per capsule are required to meet 840 passengers per hour. It is possible to further increase the Hyperloop capacity by reducing the time between departures. The current baseline requires up to 40 capsules in activity during rush hour, 6 of which are at the terminals for loading and unloading of the passengers in approximately 5 minutes. Hyperloop Passenger Plus Vehicle Capsule The passenger plus vehicle version of the Hyperloop will depart as often as the passenger only version, but will accommodate 3 vehicles in addition to the passengers. All subsystems discussed in the following sections are featured on both capsules. For travel at high speeds, the greatest power requirement is normally to overcome air resistance. Aerodynamic drag increases with the square of speed, and thus the power requirement increases with the cube of speed. For example, to travel twice as fast a vehicle must overcome four times the aerodynamic resistance, and input eight times the power. Just as aircraft climb to high altitudes to travel through less dense air, Hyperloop encloses the capsules in a reduce pressure tube. The pressure of air in Hyperloop is about 1/6 the pressure of the atmosphere on Mars. This is an operating pressure of 100 Pascals, which reduces the drag force of the air by 1,000 times relative to sea level conditions and would be equivalent to flying above 150,000 feet altitude. A hard vacuum is avoided as vacuums are expensive and difficult to maintain compared with low pressure solutions. Despite the low pressure, aerodynamic challenges must still be addressed. These include managing the formation of shock waves when the speed of the capsule approaches the speed of sound, and the air resistance increases sharply. Close to the cities where more turns must be navigated, capsules travel at a lower speed. This reduces the accelerations felt by the passengers, and also reduces power requirements for the capsule. The capsules travel at 760 mph (1,220 kph, Mach 0.91 at 68 ºF or 20 ºC). The proposed capsule geometry houses several distinct systems to reside within the outer mold line (Figure 4). Compressor Inlet Compressor Batteries fan motor Firewall/ Seating Suspension sound bulkhead Air storage (2 x 14) Figure 4. Hyperloop passenger capsule subsystem notional locations (not to scale). 4.1.1. Geometry In order to optimize the capsule speed and performance, the frontal area has been minimized for size while maintaining passenger comfort (Figure 5 and Figure 6). Figure 5. Hyperloop passenger transport capsule conceptual design sketch. Figure 6. Hyperloop passenger transport capsule conceptual design rendering. The vehicle is streamlined to reduce drag and features a compressor at the leading face to ingest oncoming air for levitation and to a lesser extent propulsion. Aerodynamic simulations have demonstrated the validity of this ‘compressor within a tube’ concept (Figure 7). Figure 7. Streamlines for capsule traveling at high subsonic velocities inside Hyperloop. Hyperloop Passenger Capsule The maximum width is 4.43 ft (1.35 m) and maximum height is 6.11 ft (1.10 2 2 m). With rounded corners, this is equivalent to a 15 ft (1.4 m ) frontal area, not including any propulsion or suspension components. The aerodynamic power requirements at 700 mph (1,130 kph) is around only 134 hp (100 kW) with a drag force of only 72 lb (320 N), or about the same f force as the weight of one oversized checked bag at the airport. The doors on each side will open in a gullwing (or possibly sliding) manner to allow easy access during loading and unloading. The luggage compartment will be at the front or rear of the capsule. The overall structure weight is expected to be near 6,800 lb (3,100 kg) including the luggage compartments and door mechanism. The overall cost of the structure including manufacturing is targeted to be no more than 245,000. Hyperloop Passenger Plus Vehicle Capsule The passenger plus vehicle version of the Hyperloop capsule has an increased 2 2 frontal area of 43 ft (4.0 m ), not including any propulsion or suspension components. This accounts for enough width to fit a vehicle as large as the Tesla Model X. The aerodynamic power requirement at 700 mph (1,130 kph) is around only 382 hp (285 kW) with a drag force of 205 lb (910 N). The doors on each side will f open in a gullwing (or possibly sliding) manner to allow accommodate loading of vehicles, passengers, or freight. The overall structure weight is expected to be near 7,700 lb (3,500 kg) including the luggage compartments and door mechanism. The overall cost of the structure including manufacturing is targeted to be no more than 275,000. 4.1.2. Interior The interior of the capsule is specifically designed with passenger safety and comfort in mind. The seats conform well to the body to maintain comfort during the high speed accelerations experienced during travel. Beautiful landscape will be displayed in the cabin and each passenger will have access their own personal entertainment system. Hyperloop Passenger Capsule The Hyperloop passenger capsule (Figure 8 and Figure 9) overall interior weight is expected to be near 5,500 lb (2,500 kg) including the seats, restraint systems, interior and door panels, luggage compartments, and entertainment displays. The overall cost of the interior components is targeted to be no more than 255,000. Figure 8. Hyperloop passenger capsule version with doors open at the station. Figure 9. Hyperloop passenger capsule version cutaway with passengers onboard. Hyperloop Passenger Plus Vehicle Capsule The Hyperloop passenger plus vehicle capsule overall interior weight is expected to be near 6,000 lb (2,700 kg) including the seats, restraint systems, interior and door panels, luggage compartments, and entertainment displays. The overall cost of the interior components is targeted to be no more than 185,000. 4.1.3. Compressor One important feature of the capsule is the onboard compressor, which serves two purposes. This system allows the capsule to traverse the relatively narrow tube without choking flow that travels between the capsule and the tube walls (resulting in a build-up of air mass in front of the capsule and increasing the drag) by compressing air that is bypassed through the capsule. It also supplies air to air bearings that support the weight of the capsule throughout the journey. The air processing occurs as follows (Figure 10 and Figure 11) (note mass counting is tracked in Section 4.1.4): Hyperloop Passenger Capsule 1. Tube air is compressed with a compression ratio of 20:1 via an axial compressor. 2. Up to 60% of this air is bypassed: a. The air travels via a narrow tube near bottom of the capsule to the tail. b. A nozzle at the tail expands the flow generating thrust to mitigate some of the small amounts of aerodynamic and bearing drag. 3. Up to 0.44 lb/s (0.2 kg/s) of air is cooled and compressed an additional 5.2:1 for the passenger version with additional cooling afterward. a. This air is stored in onboard composite overwrap pressure vessels. b. The stored air is eventually consumed by the air bearings to maintain distance between the capsule and tube walls. 4. An onboard water tank is used for cooling of the air. a. Water is pumped at 0.30 lb/s (0.14 kg/s) through two intercoolers (639 lb or 290 kg total mass of coolant). b. The steam is stored onboard until reaching the station. c. Water and steam tanks are changed automatically at each stop. 5. The compressor is powered by a 436 hp (325 kW) onboard electric motor: a. The motor has an estimated mass of 372 lb (169 kg), which includes power electronics. b. An estimated 3,400 lb (1,500 kg) of batteries provides 45 minutes of onboard compressor power, which is more than sufficient for the travel time with added reserve backup power. c. Onboard batteries are changed at each stop and charged at the stations. Axial compressor Air Out Nozzle expander p ≈ 2.1 kPa P ≈ 276 kW in T ≈ 857 K 𝑚 ≈ 0.29 kg/s Air In p ≈ 99 Pa Air Out 𝑚 ≈ 0.2 kg/s T ≈ 292 K Air Cooled F ≈ 170 N thrust 𝑚 ≈ 0.49 kg/s T  300 K P ≈ 58 kW thrust P ≈ 52 kW in Intercooler Intercooler Air Water Reservoir p ≈ 11 kPa p ≈ 101 kPa T ≈ 400 K T ≈ 293 K 𝑚 ≈ 290 kg Air Out p ≈ 11 kPa T ≈ 557 K Steam Steam Out Water In 𝑚 ≈ 0.14 kg/s 𝐻𝑂 ℓ 2 Figure 10. Compressor schematic for passenger capsule. Hyperloop Passenger Plus Vehicle Capsule 1. Tube air is compressed with a compression ratio of 20:1 via an axial compressor. 2. Up to 85% of this air is bypassed: a. The air travels via a narrow tube near bottom of the capsule to the tail. b. A nozzle at the tail expands the flow generating thrust to mitigate some of the small amounts of aerodynamic and bearing drag. 3. Up to 0.44 lb/s (0.2 kg/s) of air is cooled and compressed an additional 6.2:1 for the passenger plus vehicle version with additional cooling afterward. a. This air is stored in onboard composite overwrap pressure vessels. b. The stored air is eventually consumed by the air bearings to maintain distance between the capsule and tube walls. 4. An onboard water tank is used for cooling of the air. a. Water is pumped at 0.86 lb/s (0.39 kg/s) through two intercoolers (1,800 lb or 818 kg total mass of coolant). b. The steam is stored onboard until reaching the station. c. Water and steam tanks are changed automatically at each stop. 5. The compressor is powered by a 1,160 hp (865 kW) onboard electric motor: a. The motor has an estimated mass of 606 lb (275 kg), which includes power electronics. b. An estimated 8,900 lb (4,000 kg) of batteries provides 45 minutes of onboard compressor power, which is more than sufficient for the travel time with added reserve backup power. c. Onboard batteries are changed at each stop and charged at the stations. Axial compressor Air Out Nozzle expander p ≈ 2.1 kPa P ≈ 808 kW in T ≈ 857 K 𝑚 ≈ 1.23 kg/s Air In p ≈ 99 Pa Air Out 𝑚 ≈ 0.2 kg/s T ≈ 292 K Air Cooled F ≈ 72 N thrust 𝑚 ≈ 1.43 kg/s T  300 K P ≈ 247 kW thrust P ≈ 60 kW in Intercooler Intercooler Air Water Reservoir p ≈ 13.4 kPa p ≈ 101 kPa T ≈ 400 K T ≈ 293 K 𝑚 ≈ 818 kg Air Out p ≈ 13.4 kPa T ≈ 59 K Steam Steam Out Water In 𝑚 ≈ 0.39 kg/s 𝐻𝑂 ℓ 2 Figure 11. Compressor schematic for passenger plus vehicle capsule. 4.1.4. Suspension Suspending the capsule within the tube presents a substantial technical challenge due to transonic cruising velocities. Conventional wheel and axle systems become impractical at high speed due frictional losses and dynamic instability. A viable technical solution is magnetic levitation; however the cost associated with material and construction is prohibitive. An alternative to these conventional options is an air bearing suspension. Air bearings offer stability and extremely low drag at a feasible cost by exploiting the ambient atmosphere in the tube. Figure 12: Schematic of air bearing skis that support the capsule. Externally pressurized and aerodynamic air bearings are well suited for the Hyperloop due to exceptionally high stiffness, which is required to maintain stability at high speeds. When the gap height between a ski and the tube wall is reduced, the flow field in the gap exhibits a highly non-linear reaction resulting in large restoring pressures. The increased pressure pushes the ski away from the wall, allowing it to return to its nominal ride height. While a stiff air bearing suspension is superb for reliability and safety, it could create considerable discomfort for passengers onboard. To account for this, each ski is integrated into an independent mechanical suspension, ensuring a smooth ride for passengers. The capsule may also include traditional deployable wheels similar to aircraft landing gear for ease of movement at speeds under 100 mph (160 kph) and as a component of the overall safety system. Hyperloop Passenger Capsule Hyperloop capsules will float above the tube’s surface on an array of 28 air bearing skis that are geometrically conformed to the tube walls. The skis, each 4.9 ft (1.5 meters) in length and 3.0 ft (0.9 meters) in width, support the weight of the capsule by floating on a pressurized cushion of air 0.020 to 0.050 in. (0.5 to 1.3 mm) off the ground. Peak pressures beneath the skis need only reach 1.4 psi (9.4 kPa) to support the passenger capsule (9% of sea level atmospheric pressure). The skis depend on two mechanisms to pressurize the thin air film: external pressurization and aerodynamics. The aerodynamic method of generating pressure under the air bearings becomes appreciable at moderate to high capsule speeds. As the capsule accelerates up to cruising speed, the front tip of each ski is elevated relative