Space Cable









Space Cable Home

Space Tourism

Communications and Science
Gateway to the Planets and Beyond
Technology

Engineering Challenges

Prospects, Economics and the Environment
Contacts

    

Technology

The space cable consists of several pairs of evacuated hollow tubes held up by so-called bolts moving fast inside them. Permanent magnets provide the levitation, so that the bolts do not touch the tubes and there is no friction. Permanent magnets are used in some prototype magnetic-levitation trains and also in pumps and other machines, where magnetic bearings minimize friction. Having a vacuum in the tubes avoids air resistance, an idea that is also used to some extent in a prototype train, the Swissmetro.

Each end of the space cable is on the ground (or possibly at sea), where surface stations turn the bolts around and send them back along the tubes. Although this could be done with permanent magnets, superconducting magnets are more cost effective here. Superconducting magnets are another technology used in trains, including the Shanghai Airport railway, opened in 2004; they are readily available commercially and have many other applications.

When vehicles travel on the space cable, they are levitated magnetically from the passing bolts, which can also provide thrust; the combination acts as a linear electric motor. This is suitable for manned vehicles. An electric coil gun mounted on the space cable can accelerate small payloads directly to interplanetary space.

There is significant tension in the tubes because of the resolution of forces within them. Kevlar® is the preferred material to provide strength, as it is widely available and reasonably priced. There is no need to consider exotic materials such as carbon nanotubes. They are needed for the space elevator, an idea that was one of the inspirations for the space cable. The space elevator has to support its own weight over a length of 36,000 km, which is beyond the capability of materials known today.


Kevlar® is a Dupont registered trademark.




Tube Levitation

The tubes are held up by bolts travelling inside them. In a 300-metre high rescue ladder, their speed is 186 metres/sec. In a version that launches a manned vehicle directly to orbit, the speed is around 10.9 km/sec. Each bolt exerts a levitation force perpendicular to its direction of travel, and this force partly holds up the nearest section of the tube. The bolt carries the partial weight of this section of tube, and so it behaves as though it were heavier by this amount. At the top, the bolts hold the entire weight of the tubes and anything supported from them, but at the sides they only support part of the weight; the rest of the weight is held by tension in the tubes. The tension is greatest at the top, and so a strong material such as Kevlar® is needed to sustain it.

The bolts do not lose momentum due to levitation, because the levitation force is perpendicular to their motion. They lose momentum due to gravity, but they regain it when descending. They lose momentum when providing thrust to a vehicle. To allow for this, the surface stations increase the bolts' speed just before a launch takes place. This temporarily increases the tension in the tubes somewhat.

The shape of the tubes is illustrated. The equations are given in a journal paper. The tube lengths vary from 635 metres in the rescue ladder to as much as 1050 km.
Levitation forces in a tube
Levitation forces in the tubes

Stable Levitation

The permanent-magnet levitation method used for some prototype trains is known as Inductrack. The magnets on the train are arranged in a particularly efficient configuration – the Halbach array – which concentrates all the magnetic field on one side. The track contains coils. As the train passes over the coils, the magnets induce currents in them that cause stable repulsion.

Inductrack Halbach Array
Arrangement of permanent and electro-magnets in bolts and tubes; the arrow shows direction of travel

The losses in the coils are acceptable for a train, but they are too high for the space cable. A more sophisticated scheme is needed, pioneered by Richard Post, the same person who designed Inductrack. This scheme was designed for magnetic bearings in flywheels and is used in pumps and other equipment. It has been adapted to the space cable by having permanent magnets in the tubes and electromagnets in the bolts. The ferrite cores of the bolts' electromagnets are attracted to the permanent magnets in the tubes, with the electromagnetic coils providing stability. In this design, the bolts include electronic circuits to control the stabilization forces precisely. The estimate is that the space cable will then consume about 3 megawatts of power to maintain stability. This consumption can easily be made up. Solar panels held above the clouds can generate hundreds of megawatts of power.

The strongest permanent magnets available are made with Neodymium Iron Boron (NIB) and are available commercially with strengths of 1.2 Tesla, at least three times the strength of ferrite magnets with consequent saving of weight.

Electromagnets in the tops of the bolts are used for vehicle thrust and levitation. They are only activated when the bearer comes by. The magnetc field is conveyed from the bolts to the bearer by small passive ferrites in the sides of the tubes adjacent to these electromagnets. The ferrites run the length of each tube in the form of teeth designed to transmit the magnetic field in the required direction without leakage along the tubes. This design has been verified using the software package Finite Element Method Magnetics (FEMM 4.2).


Stabilization and Cross Winds

The support tubes are small versions of the space cable itself. There are nine support tubes to each pair of main tubes. They reach a height above 400 metres. As part of the ramp, they are the first stage in turning bolts from their steep incoming angle (82º) to the gentler angle of the gantry, and they are the last stage in turning outgoing bolts to the necessary angle. They also cope with the deflections caused by varying cross winds.

A system called active curvature control has been devised that takes advantage of the natural tendency of the cable to bend with the wind. When the control mechanism adjusts and limits the cable's bending so that the centrifugal force of the passing bolts balances the wind force. At the surface station, the support tubes absorb the lateral forces by limiting the deflection caused by winds and causing the the bolts to come back into line.

Sophisticated electronic controls are needed to achieve the correct balance. The greatest wind pressure comes from the jetstream. Between the altitudes of 20,000 and 40,000 feet (6–12 km) the wind can exert a force equivalent to 8 kg (more properly a force of 80 Newtons) per metre, assuming a tube diameter of 5 cm. This is stronger than a hurricane.

Fortunately, the control mechanisms do not have to provide the power to move the cable; it is only necessary to limit its movement. In fact, there is even the potential for a small amount of power to be generated this way, although it is unlikely to be a useful amount. Bending is achieved by adjusting the relative forces exerted by the stabilizing electromagnets in each bolt. They double up to stabilize the spacing between the bolts and the tubes as well as adjusting the curvature of the tubes.

Vehicle Levitation and Propulsion

Vehicles weighing up to 90 tonnes are carried by a bearer and released at or near the top. They draw their main power directly from the energy of the travelling bolts, which provide both levitation and thrust. The bearer is 150 metres long to give enough room to extract the required momentum from the bolts. Its electromagnets energize coils in the bolts. The forces can be adjusted according to the angle of inclination, which is steep near the ground and eventually flattens out near the top of the space cable. The acceleration is limited to about 6g (six times the earth's gravity), which is acceptable for human spaceflight. At this acceleration, it takes 520 km to reach 7.9 km/sec, which is enough to get into orbit if moving from west to east. This version of the space cable is therefore 1050 km long. A smaller version can replace the expensive first stage of a rocket. It can also be used as a test bed for hypersonic flight, including the testing of scramjets. Scramjets only operate above five times the speed of sound and are therefore extremely difficult to test at present.

Tourist vehicles (gondolas) will travel quite slowly – on the order of 200 km/hour – so as to prolong the experience. Vehicles will be capable of independent flight if necessary, being able to glide back to the surface in the event of an emergency.

The Space Cable can support an electric coil gun at high altitudes, greatly reducing the problem of air resistance. A coil gun can accelerate small payloads (up to a tonne) directly to orbit or to interplanetary space if mounted on such a long, high platform. The energy is stored in supercapacitors and delivered to coils as the payload passes. A cradle carries the payload until it is released. The cradle is about 20 metres long; its main element is a length of permanent NIB magnets bound with Reinforced Carbon Composite (RCC as used on the Space Shuttle's wings) and Kevlar. It is feasible to remove the electric coil gun when using the bearer for launching a large vehicle. Thus a single structure can support both modes.

Vehicle ascending
Vehicle ascending on bearer

Vacuum

There are several units for measuring vacuum, one of which is the millibar. Atmospheric pressure at sea level is 1000 millibars. A reasonable pressure inside the tube is about 10-8 millibar (a hundred billionth), which is well within the state of the art for modern vacuum pumps. Pumps will be on the ground, and the effect of the long tubes is that the pumps won’t be able to bring the pressure down to the desired level in a whole tube.

Fortunately, the bolts themselves drag the residual air along with them. During start-up, the pumps on the ground will be able to lower the pressure throughout the tubes to less than a tenth of a millibar. The bolts can then get through and clear the rest of the air. After start-up, normal running is very efficient; the overall losses in the space cable due to internal air resistance come to about 1.5 kilowatts.


Surface Stations

At each end of the space cable, the surface stations turn the bolts around. They can also accelerate or decelerate them as the need varies while vehicles are travelling. As shown in the side view, the ramp consists of a tunnel (down to 100 metres depth), a gantry and support tubes. They take incoming bolts from the main tubes and turn them to the horizontal. After that, the bolts normally pass to the ambit, which is a circular track. They then return via the ramp to the main tubes. For a size comparison, the Eiffel Tower and the Toronto CN Tower are drawn in.


Side view

Plan view


Side view (elevation) of ramp and ambit
Overhead view (plan) of the ramp, ambit and accelerator pair

The overhead view (plan) shows that there is an accelerator pair in the middle of the ambit. Its job is to take bolts out of circulation during shutdown and put them into circulation during start-up. Small adjustments to the bolts’ speed during normal operation can be made in the tunnel. The ambit radius in a 15-km high version of the space cable is 26 metres, but in the 140-km-high 1050-km-long version that can launch manned spacecraft directly to orbit, the radius is 3.2 km. For the 300-metre-high rescue platform, the surface station sits on the back of a truck. The tubes are wound round a drum of 1.5 metres radius that takes the place of the ambit.

The ramp uses superconducting magnets. Recently, there has been progress on superconductors that only need to be cooled to the temperature of liquid nitrogen. Bismuth strontium copper oxide (BSCCO) has achieved excellent results. However, it is very expensive compared with the more conventional niobium titanium (NbTi) that works with liquid helium. The economics are still unclear.


Erection and Start-up

At least two tubes side by side are needed to be self supporting, so that bolts can make the round trip repeatedly. To erect the space cable, the method is first to raise one pair of tubes and start bolts travelling inside them. After that, more pairs of tubes can be raised using so-called crawlers, which are devices placed along the whole length that pull a tube alongside those already erected. The first tube pair supports it until the second pair is fully up. Then it can start to take bolts and become self supporting.

The erected tube is longer than the distance covered on the ground, so expansion joints are needed, and several stages using shorter tubes will be necessary to raise one to the full height at the centre. First, a pair of short tubes must be raised to a height at which the bolts can support them. The method proposed is to use an inflatable tube full of helium gas to raise the first pair of main tubes to a central height of 10 km. The surface stations then start to send bolts through. When enough bolts are travelling at the necessary speed, the helium tube can be deflated. At this stage, ballast is needed to hold down the angle of the main tubes at the surface stations to about 15º.

As the ballast is removed, the surface stations must adjust the number of bolts in the tubes. It takes more bolts per metre to support a tube when it is low than when it is high, assuming the speeds are about the same. To support a tube at a central height of 10 km, the bolts (each up to 10 kg in mass) need to be 1.7 metres apart, and the spacing increases to 5 metres as the tube is raised to the ultimate height. As the short tubes are raised, their length is extended. When they reach their fullest extent, the crawlers are used to draw a longer pair of tubes into place, and these take over the support. The crawlers then remove the shorter tubes; they will be reused later by adding extensions to make them full size. If necessary, this process can be repeated. In this way a pair of tubes eventually reaches the full altitude, and the other tubes can be drawn along it with the crawlers.

The reverse process can be used for decommissioning or in part for taking a tube down for servicing.

Earlier Work

Robert Forward describes the Space Fountain, an idea that emerged in about 1980. The Space Fountain retards rising bolts (called "pellets") to hold up the tube and accelerates the falling ones using linear electric motors. This avoids tension in the tube but at the expense of a great deal of power, even in the idle state.

Keith Lofstrom proposes the Launch Loop, a continuous belt that rises from the surface of an ocean by travelling faster than orbital velocity (14 km/sec) and is 2000 km long.

Links and References

Swissmetro: www.swissmetro.com

Inductrack: www.llnl.gov/str/Post.html

Space Elevator: www.spaceelevator.com

Launch Loop: www.launchloop.com

Space Fountain: R. L. Forward, Indistinguishable From Magic, Baen Publishing Enterprises, Riverdale, NY, USA, 1995, pp. 59-89 "Beanstalks"

The following published papers give technical and mathematical details of many aspects of the space cable:

Space Elevator Stage I, J. Knapman, 62nd International Astronautical Congress, Cape Town, South Africa, 3-7 October, 2011 Space Elevator Stage I.pdf

Space Elevator Stage I: Through the Stratosphere, J. Knapman and K. Lofstrom, Space Elevator Conference, Redmond, Washington, USA, 12-14 August, 2011 Through Stratosphere.pdf

Diverse Configurations of the Space Cable, J, Knapman, 61st International Astronautical Congress, Prague, Czech Republic, 27 September-1 October 2010 Diverse.pdf

The Space Cable: Capability and Stability, J. Knapman, Journal of the British Interplanetary Society, Vol. 62, No. 6, 2009, pp. 202-210 Stability JBIS.pdf

Improving Stability of the Space Cable, J. Knapman, 59th International Astronautical Congress, Glasgow, Scotland, 29 September–3 October 2008 Stability08 IAC.pdf

Stability of the Space Cable, J. Knapman, 57th International Astronautical Congress, Valencia, Spain, 2-6 October 2006 Stability IAC.pdf
Also in Acta Astronautica, Vol. 65, pp.123-130, 2009

High Altitude Electromagnetic Launcher Feasibility, J. Knapman, 41st AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Tucson, Arizona, USA, 10-13 July 2005 Launcher Feasibility AIAA.pdf
Also in AIAA Journal of Propulsion and Power, Vol. 22, pp.757-763, 2006

Dynamically Supported Launcher, J. Knapman, Journal of the British Interplanetary Society, 58(3/4), 2005, pp 90-102 Launcher JBIS.pdf