|Linear accelerator concept for capsule acceleration and deceleration between 300 and 760 mph (480 and 1,220 km/h). Click to enlarge.|
Tesla Motors and SpaceX CEO Elon Musk released the preliminary design study for what he calls “The Hyperloop”—a new high-speed electric transportation system targeted for the the specific case of high-traffic city pairs (e.g., San Francisco and Los Angeles) that are less than about 1500 km or 900 miles apart. (For longer distances, Musk suggests, quiet supersonic air travel would be faster and cheaper.)
Hyperloop—which is an open-source concept, user feedback is welcome—consists of paired partially-evacuated tubes (0.015 psi, 100 Pa), with passenger capsules (or pods) 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. Stators are located on the capsules to transfer momentum to the capsules via the linear accelerators.
|Hyperloop concept sketch. Click to enlarge.|
To overcome the limiting problem of air building up in front of the traveling pod, Hyperloop proposes mounting 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. The mechanism would also create an air cushion that would create a low-friction suspension system.
Passengers may enter and exit Hyperloop at stations located either at the ends of the tube, or branches along the tube length. The capsules may be passenger-only, or, for more flexibility (and at a higher system cost), passenger and automobile combinations.
|“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...would build a bullet train that is both one of the most expensive per mile and one of the slowest in the world?...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.”|
For the study, the Hyperloop design team used a system servicing San Fransicso and Los Angeles. Total trip time is projected to be approximately half an hour, with capsules departing as often as every 30 seconds from each terminal and carrying 28 people each, thereby supporting a total of 7.4 million people each way per year. The total cost of Hyperloop in this analysis is under US$6 billion.
The two steel tubes would be welded together in a side-by-side configuration to allow the capsules to travel both directions. Pylons placed every 100 ft (30 m) support the tube pair. Solar arrays will cover the top of the tubes in order to provide power to the system. For the design study, the team proposed that the majority of the route would follow the I-5 and that the tube would be constructed in the median.
The compressor. Air processing in the Hyperloop passenger capsule is proposed as follows:
An axial compressor compresses tube air with a compression ratio of 20:1. Up to 60% of this air is bypassed, travelling via a narrow tube near the bottom of the capsule to the tail. A nozzle at the tail then expands the flow, generating thrust to mitigate some of the aerodynamic and bearing drag.
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. This air is stored in onboard composite overwrap pressure vessels. The stored air is eventually consumed by the air bearings to maintain distance between the capsule and tube walls.
An onboard water tank is used for cooling of the air. Water is pumped at 0.30 lb/s (0.14 kg/s) through two intercoolers (639 lb or 290 kg total mass of coolant). The steam is stored onboard until reaching the station. Water and steam tanks are changed automatically at each stop.
The compressor is powered by a 436 hp (325 kW) onboard electric motor, with an estimated mass of 372 lb (169 kg), which includes power electronics. 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.
Propulsion. The propulsion system must:
Accelerate the capsule from 0 to 300 mph (480 km/h) for relatively low speed travel in urban areas.
Maintain the capsule at 300 mph (480 km/h) as necessary, including during ascents over the mountains surrounding Los Angeles and San Francisco.
To accelerate the capsule from 300 to 760 mph (480 to 1,220 km/h) at 1g at the beginning of the long coasting section along the I-5 corridor.
To decelerate the capsule back to 300 mph (480 km/h) at the end of the I-5 corridor.
The designers project that the Hyperloop as a whole will consume an average of 28,000 hp (21 MW). This includes the power needed to make up for propulsion motor efficiency (including elevation changes); aerodynamic drag; charging the batteries to power on-board compressors; and vacuum pumps to keep the tube evacuated. A solar array covering the entire Hyperloop is large enough to provide an annual average of 76,000 hp (57 MW), significantly more than the Hyperloop requires, according to the plan.
The power architecture includes a battery array at each accelerator, allowing the solar array to provide only the average power needed to run the system. Power from the grid is needed when solar power is not available.
|Cross-section of rotor and stator. Click to enlarge.|
Linear induction motor. The Hyperloop uses a linear induction motor to accelerate and decelerate the capsule. The moving motor element (rotor) is located on the vehicle for weight savings and power requirements, while the tube will incorporate the stationary motor element (stator).
Each linear accelerator has two 65 MVA inverters, one to accelerate the outgoing capsule, and one to capture the energy from the incoming capsule. Inexpensive semiconductor switches allow the central inverters to energize only the section of track occupied by a capsule, improving the power factor seen by the inverters.
The rotor is an aluminum blade 49 ft (15 m) long, 1.5 ft (0.45 m) tall, and 2 in. (50 mm) thick. Current flows mainly in the outer 0.4 in. (10 mm) of this blade, allowing it to be hollow to decrease weight and cost. The gap between the rotor and the stator is 0.8 in. (20 mm) on each side. A combination of the capsule control system and electromagnetic centering forces allows the capsule to safely enter, stay within, and exit such a precise gap.
The stator is mounted to the bottom of the tube over the entire 2.5 miles (4.0 km) it takes to accelerate and decelerate between 300 and 760 mph (480 and 1,220 km). It is approximately 1.6 ft (0.5 m) wide (including the air gap) and 4.0 in. (10 cm) tall, and weighs 530 lb/ft (800 kg/m).
Laid out symmetrically on each side of the rotor, its electrical configuration is 3-phase, 1 slot per pole per phase, with a variable linear pitch (1.3 ft or 0.4 m maximum). The number of turns per slot also varies along the length of the stator, allowing the inverter to operate at nearly constant phase voltage, which simplifies the power electronics design, according to the team. The two halves of the stator require bracing to resist the magnetic forces of 20 lbf/ft (30 N/m) that try to bring them together.
Energy storage allows the linear accelerator only to draw its average power of 8,000 hp (6 MW) (rather than the peak power of 70,000 hp or 52 MW) from its solar array. The storage element is proposed to be built out of the same lithium ion cells available in the Tesla Model S. With proper construction and controls, the battery could be directly connected to the HVDC bus, eliminating the need for an additional DC/DC converter to connect it to the propulsion system, the team suggests.
A high speed transportation system known as Hyperloop has been developed in this document. The work has detailed two version of the Hyperloop: a passenger only version and a passenger plus vehicle version. Hyperloop could transport people, vehicles, and freight between Los Angeles and San Francisco in 35 minutes. Transporting 7.4 million people each way and amortizing the cost of $6 billion over 20 years gives a ticket price of $20 for a one-way trip for the passenger version of Hyperloop. The passenger plus vehicle version of the Hyperloop is less than 9% of the cost of the proposed passenger only high speed rail system between Los Angeles and San Francisco.
An additional passenger plus transport version of the Hyperloop has been created that is only 25% higher in cost than the passenger only version. This version would be capable of transport passengers, vehicles, freight, etc. The passenger plus vehicle version of the Hyperloop is less than 11% of the cost of the proposed passenger only high speed rail system between Los Angeles and San Francisco. Additional technological developments and further optimization could likely reduce this price.—“Hyperloop Alpha”
Future work. Among the recognized additional required work on the design is:
More expansion on the control mechanism for Hyperloop capsules, including attitude thruster or control moment gyros.
Detailed station designs with loading and unloading of both passenger and passenger plus vehicle versions of the Hyperloop capsules.
Trades comparing the costs and benefits of Hyperloop with more conventional magnetic levitation systems.
Sub-scale testing based on a further optimized design to demonstrate the physics of Hyperloop.