Launch loop

A launch loop or Lofstrom loop is a design for a belt based maglev orbital launch system that would be around 2,000 km 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, in effect it transfers the weight of the structure onto magnetic bearings at each end which support it.

Launch loops are intended to provide a way for non-rocket spacelaunch of vehicles weighing 5 metric tons by 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.

The published cost estimates for a working launch loop are significantly lower than a space elevator, but additionally with a greater launch capacity, lower payload costs and similar or greater payload masses. 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.

History

The launch loop was proposed somewhere around 1983-1985 by Keith Lofstrom [According to [http://wiki.launchloop.com//index.cgi?WikiPedia] there was a description in December 1983 Analog magazine] . It is essentially a hybrid of the orbital ring concept and the space fountain arranged to form a mag-lev acceleration track suitable for launching humans into space.

Description

A launch loop would be a structure around 2,000 km long, it has two "deflector" base stations 2,000 km apart on Earth which have a diameter of 28 km and can launch, catch and turn around a very fast moving iron belt called a "rotor" to and from high altitude.

Although the overall loop is very long, at around 4,000 km circumference, the belt itself is thin, around 5 cm diameter and the sheath is not much bigger. The rotor for the loop is made of ferromagnetic iron and is in the shape of a pipe, and it is spaced from a sheath by magnetic bearings. As well as holding the belt in place, the sheath also maintains a vacuum which avoids atmospheric friction.

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.

Once raised, the structure needs some power to deal with power dissipated in the magnetic bearings and to deal with losses due to the imperfect vacuum in the sheath; overall this requires around 200 MW. Additional energy would be needed to power any vehicles that are launched.

Launching payloads

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 currents 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.

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. [http://www.launchloop.com/launchloop.pdf PDF version of Lofstrom's 1985 launch loop publication (AIAA 1985)] ]

The eddy current technique is compact, lightweight and powerful, but inefficient. With each launch the rotor temperature increases by 80 kelvins 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 ferromagnetic properties and rotor containment is lost.

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 orbits, gravity assist trajectories past the moon as well as other non closed orbits such as close to the 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 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. Conventional rockets require delta-vs of roughly 10 and 14 km/s to reach LEO and GEO respectively.

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.

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 GW power station; a more modest 500 MW power station is sufficient for 35 launches per day.

Economics

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.

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. [http://www.launchloop.com/isdc2002loop.pdf Launch Loop slides for the ISDC2002 conference] ]

Comparisons

Advantages of launch loops

Launch loops 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. Launch 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 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. This would be a similar situation to that faced by the Apollo astronauts, who had radiation doses 200 times lower than the space elevator would give. [ [http://space.newscientist.com/article/dn10520-space-elevators-first-floor-deadly-radiation.html New scientist: First floor deadly radiation] ]

Unlike space elevators which are subjected 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. Since debris does not persist, it only has one chance to impact the structure. Whereas the collapse period of space elevators is expected to be of the order of years, damage or collapse of loops in this way is expected to be rare.

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.

Launch loops would be quiet in operation, and would not cause any sound pollution, unlike rockets.

Finally, their low payload costs is compatible with large-scale commercial 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 (1.5×10¹⁵ joules or 1.5 petajoules) would be approaching the same total "energy" release as a nuclear bomb explosion (350 kilotons of 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 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.

Therefore for safety and astrodynamic reasons, launch loops are intended to be installed over an ocean near the equator, well away from habitation.

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.

The instabilities are primarily in the turnaround sections as well as the cable.

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. 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.

The cable sections also share this potential issue, although the forces are much lower, but an additional instability is present in that the cable/sheath/rotor can undergoing snaking vibration 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

In works by Bolonkin [Bolonkin A.A., Non-Rocket Space Launch and Flight, Elsevier, 2006, 488 pgs.] [Paper IAC-2-IAA-1.3.03 by A.Bolonkin at the World Space Congress – 2002, 10-12 October, Houston, TX, USA.] [Journal of the British Interplanetary Society, Vol. 56, 2003, No.9/10 , pp.314-327] 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 he claims that they could wedge in the tube; and the force and friction in the ground 28 km diameter turnaround sections are gigantic. In Bolonkin 2008 [Bolonkin A.A., New Concepts, Ideas, and Innovations in Aerospace, Technology and Human Science, NOVA, 2008, 400 pgs.] a simple rotated close-loop cable is proposed, which can launch the space apparatus which, it is claimed, is suitable for current technology.

Another project, the Space cable, is a smaller design intended for launch assist for conventional rockets, and suborbital tourism. [ [http://www.spacecable.org.uk/Stability%20IAC.pdf Space Cable] ]

References

ee also

* Megascale engineering
* Non-rocket launch
* Orbital ring
* Roller coaster/Launch track
* Space elevator
* Space fountain
* Space tourism
* cyclotron - the magnetic fields necessary to deflect the loop are similar to a cyclotron
* Belt (mechanical)

External links

* [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