Lightweight Design Concept for a Vacuum-tube Jet
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KeywordsLightweight Design Test Track Short Development Time Levitation System Friction Brake
It took just half a year for the students at Technical University of Munich to build a prototype for the Hyperloop competition. They claim the hybrid lightweight design can achieve speeds of up to 350 km/h. A position-dependent prioritised weight optimisation strategy helped meet the challenges of the extremely short development time and a tight budget.
It normally takes around six hours to drive from San Francisco to Los Angeles — when traffic conditions are favourable. Flying is faster, but security screening and check-in as well as the journeys to and from airports eat up most of the time advantage.
In his alpha design , he set out what such a transportation system would have to look like to outperform rival systems in terms of cost, safety and speed. Key components include the simple steel tubes with a diameter of 2.23 m (passenger version; cargo version: 3.30 m) mounted on columns.
This elevation offers four advantages: it reduces the ground footprint required and means height differences can be easily compensated; damping elements in the column could also absorb any expansion and contraction processes caused by temperature; and it offers increased protection against earthquakes.
In the tubes, air pressure of 100 Pa (0.001 bar); generated by standard pumps together with a compressor in the capsule ensure the planned top speed can be reached with an acceptable level of energy input. The airflow from the compressor also helps the air bearings function, on which the passenger capsules glide at cruising speed.
Linear motors working on the rotor/stator principle would pave the way to accelerate the capsules from the outside, with the “rotor” located on board the vehicle. It could be implemented as a simple metal sheet to keep weight down. The stator would be situated in the tube and would supply the power. As Musk sees it, solar panels fitted to the tubes, as in Figure 1, should deliver more power than the overall transportation system consumes.
The Hyperloop Competition
The Munich team aimed to carry off the prize for the best technical Hyperloop pod with a safe prototype that is as scalable as possible. As proposed in Elon Musk’s alpha study, the students from TU Munich integrated a compressor to channel any remaining air in the tube to the back.
For economic reasons, the alpha concept requires fatigue strength commensurate with the service life of aircraft components. The strains also resemble those experienced in aircraft construction owing to changes between the (near) vacuum and normal pressure. At the same time the structural mass was to be kept as low as possible to minimise the input energy required for the levitation system and to make payload capacity as high as possible.
Outer Shell and Structure
The external shape of the Munich Hyperloop pods resembles a droplet. The front is dominated by the compressor intended to channel any residual air to the rear at high speed.
The prototype achieves the high stiffness needed to avoid bending and torsional vibration through an internal structure of strutted aluminium profiles and an outer shell of CFRP. The alternatives of a carbon monocoque or a self-supporting aluminium “body” were rejected as unfeasible within the available development time.
At the centre of the outer shell are intermediate modulus carbon fibres that sponsor Becker Carbon made into two shell parts. The material for the moulds (Rakutool MB 0600) was provided by sponsors Esterlössl and Rampf. For cost reasons, the team from Munich resorted to a trick: the outer shell is built from two identical CFRP parts; practically halving the mould construction costs.
The compressor and its cover were fitted into a recess cut into the front. Another special feature came at the rear. As the carbon fibre shell blocks radio waves, a window made of basalt fabric, which allows them to pass through, was inserted for the communications box provided by SpaceX.
The fact that the shell is unexposed to aerodynamic forces meant the team could focus on maximising the overall design rigidity. The CFRP construction with sandwich reinforcement outperformed all other conceivable alternatives like aluminium or fibreglass. A detailed discussion of the materials to be used can be found in .
At high speeds, even 100 Pa give rise to considerable air resistance.
Virtual Reality instead of Windows
As the pod does not need any windows within the dark steel tube, the Hyperloop team’s design provides for a flat, ergonomically formed seat and a virtual reality headset. Here, the on-board computer could provide wide-ranging programs or display a simulation of the landscape around the pod.
Passengers face backwards not just to save space. If it had to brake in an emergency, the pod would subject passengers to a deceleration force of up to 1.3 g — a little bit more than an emergency braking car. Travelling backwards makes it more tolerable for the human body.
The seat in the prototype is made of CFRP and was taken from a recumbent bicycle. The seat is mounted on a component support made of a CFRP sandwich. Like an aircraft cabin floor, it seals off the “cabin” at the bottom. It also contains all the control elements. Unlike a real Hyperloop pod, the prototype lacks a pressurised cabin.
Key to the hyperloop concept is minimal air pressure to reduce air friction as far as possible. Elon Musk proposes a residual air pressure of 100 Pa (0.001 bar) as economically viable since it can be be achieved using standard pumps and with an acceptable level of energy input. However, at the high speeds envisaged, the air pressure building up in front of the pod is so considerable, even at 0.001 bar, that drag increases significantly (Kantrowitz Limit).
The compressor comes from a German Air Force Alpha Jet, is made of titanium and aluminium and reaches speeds of up to 17,500 rpm. The compressor mounting comprises two aluminium trays, each weighing 14 kg. Sponsor Toolcraft milled the complex mountings made of reliable 7075 aluminium from a chunk with a total weight of 140 kg provided by Gleich Aluminium.
The shaft is also made of 7075 aluminium. Wherever possible, the students chose the same material pairings to avoid teething troubles caused by varying rates of thermal expansion. The use of titanium was ruled out owing to significantly higher production costs.
A water-cooled electric motor especially enhanced for the Hyperloop pod drives the compressor. It weighs 5.3 kg and delivers nominal output of 30 kW and nominal torque of 20 Nm at a maximum speed of 17,500 rpm. An additive process was used to produce the motor cowling from ABS polymer.
The students used prepreg carbon fibre mats to design the air intake and the front part of the air channel (air duct). Rakutool WB 0700 was used as the material for the moulds. The higher thermal load was the key factor dictating the use of high-quality material and more expensive moulds. As it is here that the air is compressed, the duct had to demonstrate high thermal stability.
Using CFRP was the easiest way of producing the geometrically elaborate, multi-curved shape of the front part of the air duct. In contrast, the rear, straight part of the air channel is made of simple aluminium sheeting. Simple bent components made of 5083 aluminium were used to divert the forces into the structure. Rubber bearings dampen compressor vibration.
The planned costs for a project to build a high-speed rail connection between San Francisco and Los Angeles are almost 70 billion dollars. This figure was excessive for Elon Musk, founder of Paypal, Tesla and SpaceX. As an alternative he proposed constructing two steel tubes with very little air pressure along existing highways, in which capsules travelling at almost the speed of sound would cover the distance in half an hour.
In July 2015 SpaceX launched a competition to promote the Hyperloop idea. Students from around the world were invited to design prototype capsules. More than 700 student teams followed SpaceX’s call. After the design proposal submissions had been inspected, the 200 best teams were allowed to present their ideas at A&M University, Texas, at the end of January 2016. 30 of them were then invited to implement their ideas and demonstrate the outcomes on a test track near Los Angeles at the end of January 2017. After more than 100 individual inspections three pods made it to the final round of the competition, the test runs in the evacuated tube of the test track. Here the WARR Hyperloop pod was not only the fastest but also the only one to complete the full distance.
Undercarriage and Guide System
The undercarriage of the Munich Hyperloop pod is basically a laser-welded structure made of aluminium. To save weight, the students produced the longitudinal guide as a milled hollow part from 7075 aluminium; reinforced with a bonded aluminium plate for extra rigidity. Although the original design advocated screw connections here, but greater strength could be achieved at the same weight with an adhesive bond.
At rest and low speeds, the Munich Hyperloop pod uses wheels made of 6082 aluminium. The Austrian company Asma coated the aluminium wheels with a polyurethane elastomer tread. The wheels have already been successfully tested up to a speed of ≤ 350 km/h on a test facility in Garching (near Munich) belonging to TÜV SÜD.
The guide system surface comprises 6101 aluminium with excellent conductivity. A guiding rail made of 6161 aluminium is situated in the centre. The Hyperloop team chose guiding wheels made of somewhat softer 6060 aluminium to prevent damage to the guiding rail. They are held in position in a mounting made of 7075 aluminium by springs. If the pod threatens to wander off track, the spring pressure keeps it back in place.
Low-speed manoeuvring System
Whereas an acceleration sled propells the pod over the first two hundred metres of the test track, the future Hyperloop will accelerate and brake primarily through the use of linear motors. Nevertheless, the students had to fit an additional system for manoeuvring at low speeds — the “Low-speed manoeuvring system”. It is propelled by an electric scooter wheel with a hub motor. Two of these motors drive the two rear wheels of the pod. This simple drive system allows speeds of up to 25 km/h.
While the future Hyperloop is intended to ride on air bearings, the Munich students’ prototype uses a passive levitation system. From speeds of around 100 km/h permanent magnets induce a magnetic field in the aluminium guide system, enabling the pod to levitate.
The nyodymium-iron-boron magnets (material: ND45) are affixed to the aluminium plates on the underside of the pod, such that they hover just a few millimetres above the trackway. They weigh 35 kg in total.
All the vehicles taking part in the competition have to have two redundant braking systems. The TU Munich team chose a conventional design taken from automotive engineering with a pneumatically operated friction brake and an electromagnetic eddy current brake.
The team developed the pneumatic friction brake components themselves. The key elements are made of aluminium and were produced in the MakerSpace workshop on the Garching campus. The brake pads came from the automotive sector. In tests, the system easily performed emergency braking from speeds of up to 350 km/h.
As with ICE trains, however, the main braking system for high speeds is to be provided by non-wearing eddy current brakes. The braking system developed by the Munich Hyperloop team in collaboration with Knorr-Bremse is the prototype’s heaviest single component. In the course of development, sponsor Knorr-Bremse succeeded in cutting the weight from 2 × 90 kg to 2 × 70 kg.
The high weight is mainly due to the iron cores in a copper coil. MayTec profiles reinforced with aluminium sheets to distribute peak loads were again used to transfer the forces to the structure.
IT Infrastructure and Power Supply
Most of the power in the Munich Hyperloop pod is consumed by the compressor. Navigation and control components also require a modest amount. In addition, the battery system must always hold reserve power for the electromagnets in the eddy current brakes.
Accordingly, the Munich team installed two lithium-ion rechargeable batteries with a total capacity of 18 Ah in their prototype. The high-voltage unit delivers 500 V for the compressor and eddy current brakes. A low-voltage unit supplies 12, 24 and 48 V for the other components. The battery housing was produced from POM polymer. It operates at normal pressure to avoid elaborate functional testing in vacuum conditions.
During the run, more than 60 sensors generated a significant flow of data, which was processed by four micro-controllers; communicating with each other over a CAN bus. The aluminium housings provide an electromagnetic shield and mechanical protection for the electronics.
▸ Maximum speed: ≤ 350 km/h
▸ Gross weight: 600 kg
▸ Payload: 100 kg
▸ Dimensions: 4.2 × 1 × 1.1 m (LxWxH)
▸ Compressor output: 30 kW
Two challenges had a significant impact on the work of the team: a tight budget, despite many sponsors, and the extremely short development time. For this reason, not all options of lightweight design could be exploited to the maximum.
The Munich designers gave an demonstration of the quality of their work in the final competition on 29 January 2017 in Hawthorne, California. The pod from Munich was not only the fastest capsule (“Winner over all, fastest pod“) but also the only one to complete the full distance honored with the „Best Performance in flight“ award.