|
|
|
|
| Space Cable |
|
|
|
|
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 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 LevitationThe 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. |
 Levitation
forces
in
the
tubes
|
 |
| Arrangement
of
permanent
and
electro-magnets
in
bolts
and
tubes;
the arrow shows direction of
travel |
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).
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.|
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
on
bearer
|
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.
 |
 |
|||
| Side view (elevation) of ramp and ambit | Overhead view (plan) of the ramp, ambit and accelerator pair |
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.
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.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