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A '''launch loop''' or '''Lofstrom loop''' is a published design for an [[active structure]] [[Maglev (transport)|maglev]] [[cable transport]] system intended for [[orbital launch]] that would be around 2,000 km (1,240 mi) long and maintained at an altitude of up to 80 km (50 mi). A launch loop would be held up at this altitude by momentum of the belt as it circulates around the structure. This circulation, in effect, transfers the weight of the structure onto a pair of magnetic bearings, one at each end, which support it.
 
Launch loops are intended to achieve [[non-rocket spacelaunch]] of [[SpacecraftStatek kosmiczny|vehicles]] weighing 5 metric tons by [[Maglev (transport)|electromagnetically accelerating]] them so that they are projected into Earth [[orbitOrbita|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 system is designed to be suitable for launching humans for [[spaceTurysta kosmiczny|Space tourism]], [[spaceEksploracja kosmosu|Space exploration]] and [[spaceKolonizacja kosmosu|Space colonization]], and provides a relatively low [[g-force|3''g'' acceleration]].<ref name=launch1985/>
 
==History==
==Description==
[[Image:LaunchLoopRotor.svg|thumb|right|Launch loop accelerator section (return cable not shown)]]
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 [[ironŻelazo|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/>
 
Although the overall loop is very long, at around 4,000&nbsp;km circumference, the rotor itself would be thin, around 5&nbsp;cm diameter and the sheath is not much bigger.
 
===Ability to stay aloft===
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 [[reactiveSiła odśrodkowa|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 requires continuous power to overcome the energy dissipated. Additional energy would be needed to power any vehicles that are launched.<ref name=launch1985/>
 
===Launching payloads===
To launch, vehicles are raised up on an 'elevator' cable that hangs down from the West station loading dock at 80&nbsp;km, and placed on the track. The payload applies a magnetic field which generates [[eddyPrąd wirowy|Eddy current]]s in the fast-moving rotor. This both lifts the payload away from the cable, as well as pulls the payload along with 3''g'' (30 m/s²) acceleration. The payload then rides the rotor until it reaches the required orbital velocity, and 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 [[rocketSilnik rakietowy|Rocket engine]] ("kick motor") or other means is needed to circularize the trajectory to the appropriate Earth orbit.<ref name=launch1985>[http://launchloop.com/LaunchLoop?action=AttachFile&do=get&target=launchloop.pdf PDF version of Lofstrom's 1985 launch loop publication (AIAA conference)]</ref>
 
The eddy current technique is compact, lightweight and powerful, but inefficient. With each launch the rotor temperature increases by 80 [[kelvinKelwin|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 [[ferromagnetismFerromagnetyzm|ferromagnetic]] properties and rotor containment is lost.<ref name=launch1985/>
 
===Capacity and capabilities===
Closed orbits with a perigee of 80&nbsp;km quite quickly decay and re-enter, but in addition to such orbits, a launch loop by itself would also be capable of directly injecting payloads into [[escapePrędkość velocityucieczki|escape orbits]], [[gravityAsysta grawitacyjna|Gravity assist]] trajectories past the [[Księżyc|Moon]], and 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 [[geosynchronousOrbita orbitgeosynchroniczna|GEO]] insertion this would need to provide a [[deltaDelta-v|Delta-v]] of about 1.6&nbsp;km/s, for [[LowNiska Earthorbita orbitokołoziemska|LEO]] to circularise at 500&nbsp;km would require a delta-v of just 120 m/s. Conventional [[rocketRakieta|Rocket]]s require delta-vs of roughly 10 and 14&nbsp;km/s to reach LEO and GEO respectively.<ref name=launch1985/>
 
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.
Launch loops would be quiet in operation, and would not cause any sound pollution, unlike rockets.
 
Finally, their low payload costs are compatible with large-scale commercial [[spaceTurysta kosmiczny|Space tourism]] and even [[space colonisation]].
 
===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> [[jouleDżul|Joule]]s or 1.5 petajoules) would be approaching the same total ''energy'' release as a [[nuclear bomb]] explosion (350 kilotons of [[Równoważnik trotylowy wybuchu jądrowego|TNT equivalent]]), although not emitting nuclear radiation.
 
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 would be deliberately dumped at preselected 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.
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/>
 
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 [[meanderMeander (geografia)|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.
 
===Competing and similar designs===
* [[Non-rocket spacelaunch]]
* [[Orbital ring]]
* [[Kolejka górska|Roller coaster]]/[[Mass driver]]
* [[Winda kosmiczna|Space elevator]]
* [[Space gun]]
* [[Space fountain]]
* [[Turysta kosmiczny|Space tourism]]
* [[cyclotronCyklotron|Cyclotron]] - the magnetic fields necessary to deflect the loop are similar to a cyclotron
* [[Belt (mechanical)]]
* [[Siła odśrodkowa|Reactive centrifugal force]] - the force that would hold up the loop against gravity
* [[Cable transport]]
* [[Tether propulsion]]
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