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[[Image:LaunchLoop.svg|thumb|right|300px|Launch Looploop. (with thanksNot to Keith Lofstrom-1985scale). The red marked line is the moving loop itself, blue lines are stationary cables.]]
A '''launch loop''' or '''Lofstrom loop''' is a published design for aan [[beltactive structure]] [[Maglev (mechanicaltransport)|beltmaglev]] based [[maglevcable transport]] system intended for [[orbital launch]] system that would be around 2000 2,000 km (1,240 mi) long and maintained at an altitude of up to 80  km (50  mi). ItA provideslaunch aloop potentialwould waybe ofheld [[non-rocketup spacelaunch]].at [[Spacecraft|Vehicles]]this weighingaltitude 5by metricmomentum tonsof wouldthe bebelt [[maglev|electromagneticallyas accelerated]]it oncirculates top ofaround the cablestructure. whichThis forms ancirculation, accelerationin trackeffect, fromtransfers whichthe theyweight wouldof bethe projectedstructure intoonto Eartha [[orbit]]pair orof magnetic bearings, one at each end, which evensupport beyondit.
 
Launch loops are intended to achieve [[non-rocket spacelaunch]] of [[Spacecraft|vehicles]] weighing 5 metric tons by [[Maglev (transport)|electromagnetically accelerating]] them so that they are projected into Earth [[orbit]] or even beyond. This would be achieved by the flat part of the cable which forms an acceleration track above the atmosphere.<ref>Indistinguishable from Magic- [[Robert L. Forward]], chapter 4.</ref>
The published cost estimates for a working launch loop are significantly lower than a [[space elevator]], with a greater launch capacity, lower payload costs and similar or greater payload masses; and unlike the space elevator no new materials need to be developed.
 
The system is designed to be suitable for launching humans for [[space tourism]], [[space exploration]] and [[space colonization]], and provides a relatively low [[g-force|3''g'' acceleration]].<ref name=launch1985/>
 
==History==
Launch loops were described by [[Keith Lofstrom]] in November 1981 Reader's Forum of the [[American Astronautical Society]] News Letter, and in the August 1982 [[L5 Society|L5]] News.
 
In 1982 [[Paul Birch (writer)|Paul Birch]] published a series of papers in ''[[Journal of the British Interplanetary Society]]'' which described [[orbital ring]]s and described a form which he called Partial Orbital Ring System (PORS).<ref>[http://www.paulbirch.net/OrbitalRings-II.zip [[Paul Birch (writer)|Paul Birch]] Orbital Rings-I 12]</ref>
 
The launch loop idea was worked on in more detail around 1983–1985 by Lofstrom.<ref name=launch1985/><ref>December 1983 Analog magazine</ref> It is a fleshed-out version of PORS specifically arranged to form a mag-lev acceleration track suitable for launching humans into space; but whereas the orbital ring used superconducting [[magnetic levitation]], launch loops use [[Electromagnetic suspension]] (EMS).
 
The system is designed to be suitable for launching humans for [[space tourism]], [[space exploration]] and [[space colonization]].
==Description==
[[Image:LaunchLoopRotor.pngsvg|thumb|right|Launch loop accelerator section (withreturn thankscable tonot Keith Lofstrom-2002shown)]]
A launch loop is proposed to be a structure around 2,000&nbsp;km long and 80&nbsp;km high. The loop runs along at 80&nbsp;km above the earth for 2000&nbsp;km then descends to earth before looping back on itself rising back to 80&nbsp;km above the earth to follow the reverse path then looping back to the starting point. The loop would be in the form of a tube, known as the ''sheath''. Floating within the sheath is another continuous tube, known as the ''rotor'' which is a sort of belt or chain. The rotor is an [[iron]] tube approximately 5&nbsp;cm (2&nbsp;inches) in diameter, moving around the loop at 14&nbsp;km/s (31,000 miles per hour).<ref name=launch1985/>
The launch loop was proposed in 1985 by [[Keith Lofstrom]]. It is essentially a hybrid of the [[orbital ring]] concept and the [[space fountain]]. It is an oval ring around 2000 km long, it has two base stations about 2000 km apart on Earth which can launch and catch a very fast moving [[iron]] belt called a "rotor" to and from high altitude.
 
Although the overall loop is very long, at around 4000 4,000&nbsp;km circumference, the beltrotor itself iswould be thin, around 5 &nbsp;cm diameter and the sheath is not much bigger. The rotor for the loop is made of iron and is in the shape of a pipe, and it is spaced from a sheath by [[magnetic bearing]]s. As well as holding the belt in place, the sheath also maintains a vacuum which avoids atmospheric friction.
 
===Ability to stay aloft===
The loop starts off at ground level, and stationary. The rotor is spun up, turned by a [[linear motor]] powered by a several hundred megawatt power station. As the speed increases the central parts of the structure are arranged to push upwards into an approximate arch shape- carried there by the momentum of the rotor. When the cable reaches an altitude of around 80 kilometers the loop is restrained and shaped by cables that hang down to sea level. The rotor is spun up to a linear speed of 14 km/s taking almost 5 minutes to make a revolution. Using a 300 MW power generator, this would take about two months to reach full speed.
When at rest, the loop is at ground level. The rotor is then accelerated up to speed. As the rotor speed increases, it curves to form an arc. The sheath forces it to follow a curve steeper than the rotor's natural ballistic curve, which, in turn, exerts a [[reactive centrifugal force]] on the sheath, holding it aloft. The loop would be anchored to the ground to remain at a fixed height.
 
Once raised, the structure needsrequires somecontinuous power to deal with power dissipated inovercome the magnetic bearings and energy to deal with the imperfect vacuum in the sheath; overall this requires around 200 MW of energydissipated. Additional energy would be needed to power any vehicles that are launched.<ref name=launch1985/>
 
===Launching payloads===
To launch, vehicles are raised up on elevatorsan to'elevator' acable that hangs down from the West station loading dock at 80 &nbsp;km, and placed on the track. The payload then createsapplies a magnetic field which generates [[eddy current]]s in the fast-moving rotor,. whichThis both liftlifts the payload away from the cable, as well as pullingpulls the payload along with 3''g'' (30 m/s²) acceleration. The payload then rides the rotor until it reaches the required orbital velocity, and then leaves the track.<ref name=launch1985/>
 
If a stable or circular orbit is needed, once the payload reaches the highest part of its trajectory then an on-board [[rocket engine]] ("kick motor") or other means is needed to circularisecircularize the trajectory to the appropriate Earth orbit.<ref name=launch1985>[http://www.launchloop.com/LaunchLoop?action=AttachFile&do=get&target=launchloop.pdf PDF version of Lofstrom's 1985 launch loop publication (AIAA 1985conference)]</ref>
To launch, vehicles are raised up on elevators to a loading dock at 80 km, and placed on the track. The payload then creates a magnetic field which generates [[eddy current]]s in the fast-moving rotor, which both lift the payload away from the cable, as well as pulling the payload along with 3''g'' (30 m/s²) acceleration. The payload then rides the rotor until it reaches the required orbital velocity, and then leaves the track.
 
The eddy current technique is compact, lightweight and powerful, but inefficient. With each launch the rotor temperature increases by 80 [[kelvin]]s due to power dissipation. If launches are spaced too close together, the rotor temperature can approach 770 °C (1043 K), at which [[Curie point|point]] the iron rotor loses its [[ferromagnetism|ferromagnetic]] properties and rotor containment is lost.<ref name=launch1985/>
If a stable or circular orbit is needed, once the payload reaches the highest part of its trajectory then an on-board rocket engine ("kick motor") or other means is needed to circularise the trajectory to the appropriate Earth orbit.<ref>[http://www.launchloop.com/launchloop.pdf PDF version of Lofstrom's 1985 launch loop publication (AIAA 1985)]</ref>
 
===Capacity and capabilities===
The eddy current technique is compact, lightweight and powerful, but inefficient. With each launch the rotor temperature increases by 80 [[kelvin]]s due to power dissipation. If launches are spaced too close together, the rotor temperature can approach 770 °C (1043 K), at which point the iron rotor loses its [[ferromagnetism|ferromagnetic]] properties and rotor containment is lost.
Closed orbits with a perigee of 80 &nbsp;km quite quickly decay and re-enter, but ain launchaddition loopto wouldsuch beorbits, ina andlaunch ofloop by itself, notwould onlyalso be capable of directly reachinginjecting payloads such an orbit; but also of reachinginto [[escape velocity|escape orbits]], [[gravity assist]] trajectories past the [[moonMoon]], as well asand other non closed orbits such as close to the [[Trojan_points#L4_and_L5|Trojan points]].
 
To access circular orbits using a launch loop a relatively small 'kick motor' would need to be launched with the payload which would fire at [[apogee]] and would circularise the orbit. For [[geosynchronous orbit|GEO]] insertion this would need to provide a [[delta-v]] of about 1.6 &nbsp;km/s, for [[Low Earth orbit|LEO]] to circularise at 500 &nbsp;km would require a delta-v of just 120 m/s. Conventional [[rocket]]s require delta-vs of roughly 10 and 14&nbsp;km/s to reach LEO and GEO respectively.<ref name=launch1985/>
==Capacity and capabilities==
Closed orbits with a perigee of 80 km quite quickly decay and re-enter, but a launch loop would be, in and of itself, not only capable of directly reaching such an orbit; but also of reaching [[escape velocity|escape orbits]], [[gravity assist]] trajectories past the [[moon]] as well as other non closed orbits such as close to the Trojan points.
 
Launch loops in Lofstrom's design are placed close to the equator<ref name=launch1985/> and can only directly access equatorial orbits. However other orbital planes might be reached via high altitude plane changes, lunar perturbations or aerodynamic techniques.
To access circular orbits using a launch loop a relatively small 'kick motor' would need to be launched with the payload which would fire at [[apogee]] and would circularise the orbit. For [[geosynchronous orbit|GEO]] insertion this would need to provide a [[delta-v]] of about 1.6 km/s, for [[LEO]] to circularise at 500 km would require a delta-v of just 120 m/s.
 
Launch rate capacity of a launch loop is ultimately limited by the temperature and cooling rate of the rotor to 80 per hour, but that would require a 17 [[gigawatt|GW]] power station; a more modest 500 MW power station is sufficient for 35 launches per day.<ref name=launch1985/>
Launch loops in Lofstrom's design are placed close to the equator and can only directly access equatorial orbits. However other orbital planes might be reached via high altitude plane changes, lunar perturbations or aerodynamic techniques.
 
===Economics===
Launch rate capacity of a launch loop is ultimately limited by the temperature and cooling rate of the rotor to 80 per hour, but that would require a 17 [[gigawatt|GW]] power station; a more modest 500 MW power station is sufficient for 35 launches per day.
Clearly, forFor a launch loop to be wortheconomically buildingviable it would require customers with sufficiently large payload launch requirements for it to be the cheapest option; however the system costs do not seem terribly out of line with other launch options.
 
Lofstrom estimates that an initial loop costing roughly $10 [[1000000000 (number)|billion]] with a 1 one-year payback could launch 40,000 metric tons per year, and cut launch costs to $300/kg, or for $30 billion, with a larger power generation capacity, the loop would be capable of launching 6 million metric tons per year, and given a 5 five-year payback period, the costs for accessing space with a launch loop could be as low as $3/kg.<ref name=isdc>[http://www.launchloop.com/LaunchLoop?action=AttachFile&do=view&target=isdc2002loop.pdf Launch Loop slides for the ISDC2002 conference]</ref>
==Economics==
 
==Comparisons==
Clearly, for a launch loop to be worth building it would require customers with sufficiently large payload launch requirements for it to be the cheapest option; however the system costs do not seem terribly out of line with other launch options.
==Difficulties=Advantages of launch loops===
LaunchLofstrom's launch loops launch are expected to launch at high rates (many launches per hour, independent of weather), and are not inherently polluting. (rocketsRockets create pollution such as nitrates in their exhausts due to high exhaust temperature, and can create greenhouse gases depending on propellant choices),. but launchLaunch loops as a form of electric propulsion can be clean, and can be run on geothermal, nuclear, wind, solar or any other power source, even intermittent ones, as the system has a huge built-in power storage capacity.
 
Unlike space elevators which would have to travel through the [[Van Allen belts]] over several days, launch loop passengers can be launched to low earth orbit, which is below the belts, or through them in a few hours. inThis would be a similar orbitsituation to that faced by the Apollo astronauts, who had radiation doses 200 times lower than the space elevator would give.<ref>[http://space.newscientist.com/article/dn10520-space-elevators-first-floor-deadly-radiation.html New scientist: First floor deadly radiation]</ref>
Lofstrom estimates that an initial loop costing roughly $10 billion with a 1 year payback could launch 40,000 metric tons per year, and cut launch costs to $300/kg, or for $30 billion, with a larger power generation capacity, the loop would be capable of launching 6 million metric tons per year, and given a 5 year payback period, the costs for accessing space with a launch loop could be as low as $3/kg.<ref>[http://www.launchloop.com/isdc2002loop.pdf Launch Loop slides for the ISDC2002 conference]</ref>
 
Unlike space elevators which are subjectsubjected to the risks of space debris and meteorites along their whole length, launch loops are to be situated at an altitude where orbits are unstable due to air drag,. andSince debris does not persist, thus debrisit only has one chance to impact the structure. Whereas lossthe collapse period of space elevators mayis notexpected to be unusualof the order of years, lossdamage or collapse of loops in this way is expected to be rare. In addition, launch loops themselves are not a significant source of space debris, even in an accident. All debris generated has a perigee that intersects the atmosphere or is at escape velocity.
==Difficulties of launch loops==
A running loop would have an extremely large amount of energy in the form of linear momentum. While the magnetic suspension system would be highly redundant, with failures of small sections having essentially no effect at all; if a major failure did occur the energy in the loop (1.5×10<sup>15</sup> [[joule]]s or 1.5 petajoules) would be approaching the same total energy release as a small [[nuclear bomb]] explosion (350 kilotons of [[TNT equivalent]]) but would not emit ionising radiation.
 
Launch loops are intended for human transportation, andto give a safe 3''g'' acceleration which the vast majority of people would be capable of tolerating well,<ref name=launch1985/> and would be a much faster way of reaching space than Spacespace Elevatorselevators.
While this is a large amount of energy, it is unlikely that this would destroy very much of the structure due to its very large size, and because the energy release would be spread-out over several minutes. Steps might need to be taken to lower the cable down from 80 km altitude with minimal damage, such as parachutes.
 
Launch loops would be quiet in operation, and would not cause any sound pollution, unlike rockets.
For safety and [[astrodynamics|astrodynamic]] reasons, launch loops would be located over an ocean near the equator, well away from habitation.
 
Finally, their low payload costs isare compatible with large-scale commercial [[space tourism]] and even [[space colonisation]].
The published design of a launch loop requires electronic control of the magnetic levitation to minimise power dissipation and to stabilise the otherwise under-damped cable.
 
==Advantages=Difficulties of launch loops===
A running loop would have an extremely large amount of energy in the form of linear momentum. While the magnetic suspension system would be highly redundant, with failures of small sections having essentially no effect at all;, if a major failure did occur the energy in the loop (1.5×10<sup>15</sup> [[joule]]s or 1.5 petajoules) would be approaching the same total ''energy'' release as a small [[nuclear bomb]] explosion (350 kilotons of [[TNT equivalent]]), but wouldalthough not emitemitting ionisingnuclear radiation.
Launch loops launch are expected to launch at high rates (many launches per hour, independent of weather), and are not inherently polluting (rockets create pollution such as nitrates in their exhausts due to high exhaust temperature, and can create greenhouse gases depending on propellant choices), but launch loops can be clean, and can be run on geothermal, nuclear, wind, solar or any other power source, even intermittent ones, as the system has a huge built-in power storage capacity.
 
While this is a large amount of energy, it is unlikely that this would destroy very much of the structure due to its very large size, and because most of the energy release would be spread-outdeliberately overdumped severalat minutespreselected places when the failure is detected. Steps might need to be taken to lower the cable down from 80 &nbsp;km altitude with minimal damage, such as parachutes.
Unlike space elevators which would have to travel through the [[Van Allen belts]] over several days, launch loop passengers can be launched to low earth orbit, which is below the belts, or through them in a few hours in a similar orbit to the Apollo astronauts, who had radiation doses 200 times lower than the space elevator would give.<ref>[http://space.newscientist.com/article/dn10520-space-elevators-first-floor-deadly-radiation.html New scientist: First floor deadly radiation]</ref>
 
ForTherefore for safety and [[astrodynamics|astrodynamic]] reasons, launch loops wouldare intended to be locatedinstalled over an ocean near the equator, well away from habitation.
Unlike space elevators which are subject to the risks of space debris and meteorites along their whole length, launch loops are to be situated at an altitude where orbits are unstable due to air drag, and debris does not persist, thus debris only has one chance to impact the structure. Whereas loss of space elevators may not be unusual, loss of loops in this way is expected to be rare.
 
The published design of a launch loop requires electronic control of the magnetic levitation to minimise power dissipation and to stabilise the otherwise under-damped cable.
Launch loops are intended for human transportation, and give a safe 3''g'' acceleration which the vast majority of people would be capable of tolerating well, and would be a much faster way of reaching space than Space Elevators.
 
The two main points of instability are the turnaround sections and the cable.
Launch loops would be quiet in operation, and would not cause any sound pollution, unlike rockets.
 
The turnaround sections are potentially unstable, since movement of the rotor away from the magnets gives reduced magnetic attraction, whereas movements closer gives increased attraction. In either case, instability occurs.<ref name=launch1985/> This problem is routinely solved with existing servo control systems that vary the strength of the magnets. Although servo reliability is a potential issue, at the high speed of the rotor, very many consecutive sections would need to fail for the rotor containment to be lost.<ref name=launch1985/>
Finally, their low payload costs is compatible with large-scale commercial [[space tourism]] and even [[space colonisation]].
 
The cable sections also share this potential issue, although the forces are much lower.<ref name=launch1985/> However, an additional instability is present in that the cable/sheath/rotor may undergo [[meander]]ing modes (similar to a [[Lariat chain]]) that grow in amplitude without limit. Lofstrom believes that this instability also can be controlled in real time by servo mechanisms, although this has never been attempted.
==References==
 
<references/>
===Competing and similar designs===
In works by [http://russianamericanbusiness.org/web_CURRENT/articles/352/1/Alexander-Bolonkin Alexander Bolonkin] <ref>Bolonkin, A.A., Non-Rocket Space Launch and Flight, Elsevier, 2006, 488 pgs.</ref><ref>Paper IAC-2-IAA-1.3.03 by A. Bolonkin at the World Space Congress – 2002, 10–12 October, Houston, TX, USA.</ref><ref>Journal of the British Interplanetary Society, Vol. 56, 2003, No.9/10 , pp.314-327</ref> it is suggested that Lofstrom's project has many non-solved problems and that it is very far from a current technology. For example, the Lofstrom project has expansion joints between 1.5 meter iron plates. Their speeds (under gravitation, friction) can be different and Bolonkin claims that they could wedge in the tube;{{Citation needed|date=December 2009}} and the force and friction in the ground 28&nbsp;km diameter turnaround sections are gigantic. In 2008<ref>Bolonkin A.A., New Concepts, Ideas, and Innovations in Aerospace, Technology and Human Science, NOVA, 2008, 400 pgs.</ref>, Bolonkin proposed a simple rotated close-loop cable to launch the space apparatus in a way suitable for current technology.
 
Another project, the [[space cable]], is a smaller design by [[John Knapman]] that is intended for launch assist for conventional rockets and suborbital tourism. The space cable design uses [[electrodynamic levitation]] rather than [[electromagnetic levitation]] and discrete bolts rather than a continuous rotor, as with the launch loop architecture. John Knapman has also mathematically shown that the meander instability can be tamed.<ref>[http://www.spacecable.org.uk/Stability%20IAC.pdf Space Cable]</ref>
 
==See also==
{{Commons category|Space elevators}}
{{Portal| Spaceflight }}
* [[Megascale engineering]]
* [[Non-rocket spacelaunch]]
* [[Orbital ring]]
* [[Roller coaster]]/[[LaunchMass trackdriver]]
* [[Space elevator]]
* [[Space gun]]
* [[Space fountain]]
* [[Space tourism]]
* [[cyclotron]] - the magnetic fields necessary to deflect the loop are similar to a cyclotron
* [[Belt (mechanical)]]
* [[Reactive centrifugal force]] - the force that would hold up the loop against gravity
* [[Cable transport]]
* [[Tether propulsion]]
* [[Magnetic levitation]]
* [[StarTram]]
 
==References==
{{Reflist|2}}
 
==External links==
{{Commons category}}
*[http://www.launchloop.com/ www.launchloop.com]
*[http://www.spacecable.org.uk/Stability%20IAC.pdf SpaceCable] Another similar idea for launch assist/short range travel/recreational extremely high altitude trips
*[http://server-sky.com/slides/SEplatform/ Space Elevator Stage 1: Through the Stratosphere], John Chapman, Keith Lofstrom, presentation at Microsoft conference center, August 2011.
 
{{Non-rocket spacelaunch}}
*[http://www.spacecable.org.uk/Stability%20IAC.pdf SpaceCable] Another similar idea for launch assist
 
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[[:Category:Human_spaceflightHuman spaceflight]]
 
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[[:Category:Maglev]]
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[[:Category:Single_stage_to_orbitSingle-stage-to-orbit]]
[[:Category:Spacecraft propulsion]]
[[:Category:Magnetic propulsion devices]]
[[:Category:Hypothetical technology]]
[[:Category:Vertical transport devices]]
 
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