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| Space Cable |
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TechnologyThe 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 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 also provide thrust; the combination acts as a linear electric motor. 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 22,000 miles, which is beyond the
capability of materials known today. Kevlar® is a Dupont
registered trademark. |
Tube LevitationThe tubes are held up by bolts travelling inside them at velocities around 3.3 km/sec. Each bolt exerts a levitation force perpendicular to its direction of travel, and this force holds up the nearest section of the tube. The bolt carries the 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 shape of the tubes is illustrated. The equations are given in a journal paper. The altitude at the centre is 140 km (87 miles). The range (distance covered on the earth’s surface) is 155 km. |
 Shape
of the tubes
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| Arrangement of magnets and coils for Inductrack |
In the last few years, very strong permanent magnets have
become available. They are made with Neodymium Iron Boron (NIB) and are
available commercially with strengths of 1.2 Tesla. More traditional
ferrite magnets are cheaper and have strengths of 0.4 Tesla. Ferrites
have the advantage that they do not conduct electricity. NIB is a
conductor and so suffers from eddy currents, which can cause losses in
a variable magnetic field. A reasonable compromise is to use NIB in the
bolts and ferrites in the tubes. The magnetic field experienced by the
magnets in the bolts can be kept nearly constant as they travel along
the tubes with stabilizing coils and circuits.
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.
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| Side
view (elevation) of ramp and ambit |
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| Overhead view (plan) of the ramp, ambit and accelerator pair |
The ambit radius is 560 metres. It uses superconducting magnets, which are highly efficient both magnetically and in energy consumption. The accelerator pair uses electromagnetic coils, effectively acting as the primary of a linear synchronous motor.
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.
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.
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The vehicles draw their main power directly from the
energy of
the travelling bolts, which provide both levitation and thrust. The
vehicles’ electromagnets energize coils in the bolts. The forces can be
adjusted according to the angle of inclination, which is very steep
near the ground and eventually flattens out near the top of the space
cable. The arrangement is illustrated in the figure. 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 launch small unmanned vehicles directly
into orbit or even to the Moon or planets.
For human travel, gentler acceleration is needed. This could be
achieved by making the Space Cable very long, but this is more
expensive.
It is more economical to use the Space Cable as a booster for an
upper-stage rocket, replacing the expensive fuel-hungry first-stage
rockets for manned flights. |
 Vehicle
ascending
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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 more than twice as long as 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 of 140 km 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 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 of 140 km.
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 for taking a tube down for servicing.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
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
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
Dynamically Supported Launcher, J. Knapman, Journal of the British Interplanetary Society, 58(3/4), 2005, pp 90-102 Launcher JBIS.pdf