Launch loop - AbsoluteAstronomy.com

A launch loop or Lofstrom loop is a proposed system for launching objects into space orbit using a moving cable-like system attached to the earth at two ends and suspended above the atmosphere in the middle. The design concept was published by Keith Lofstrom

Keith Lofstrom

Keith Lofstrom is an electrical engineer. He has a BSEE and MSEE from University of California, Berkeley.He is more widely known in the space advocacy community for a ground-based space launcher design, the Launch Loop,...

and describes an active structure

Active structure

An active structure is a mechanical structure with the ability to alter its configuration, form or properties in response to changes in the environment....

maglev cable transport

Cable transport

Cable transport refers to the broad class of transport modes that rely on vehicles pulled by cables, rather than having an internal power source...

system 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 a belt that 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

Non-rocket spacelaunch

Non-rocket space launch is a launch into space where some or all needed speed and altitude is provided by non-rocket means, rather than simply using conventional chemical rockets from the ground. A number of alternatives to rockets have been proposed...

of vehicles

Spacecraft

A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....

weighing 5 metric tons by electromagnetically accelerating them so that they are projected into Earth orbit

Orbit

In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

or even beyond. This would be achieved by the flat part of the cable which forms an acceleration track above the atmosphere.

The system is designed to be suitable for launching humans for space tourism

Space tourism

Space Tourism is space travel for recreational, leisure or business purposes. A number of startup companies have sprung up in recent years, hoping to create a space tourism industry...

, space exploration

Space exploration

Space exploration is the use of space technology to explore outer space. Physical exploration of space is conducted both by human spaceflights and by robotic spacecraft....

and space colonization

Space colonization

Space colonization is the concept of permanent human habitation outside of Earth. Although hypothetical at the present time, there are many proposals and speculations about the first space colony...

, and provides a relatively low 3g acceleration

G-force

The g-force associated with an object is its acceleration relative to free-fall. This acceleration experienced by an object is due to the vector sum of non-gravitational forces acting on an object free to move. The accelerations that are not produced by gravity are termed proper accelerations, and...

History

Launch loops were described by Keith Lofstrom

Keith Lofstrom

in November 1981 Reader's Forum of the American Astronautical Society

American Astronautical Society

Formed in 1954, the American Astronautical Society is an independent scientific and technical group in the United States dedicated to the advancement of space science and exploration. AAS supports NASA's Vision for Space Exploration and is a member of the Coalition for Space Exploration and the...

News Letter, and in the August 1982 L5

L5 Society

The L5 Society was founded in 1975 by Carolyn and Keith Henson to promote the space colony ideas of Dr Gerard K. O'Neill.The name comes from the and Lagrangian points in the Earth-Moon system proposed as locations for the huge rotating space habitats that Dr O'Neill envisioned...

News.

In 1982 Paul Birch

Paul Birch (writer)

Paul Birch is a British author, engineer and scientist, who has worked in radioastronomy and satellite communications, and now writes full time.He is a former Fellow of the British Interplanetary Society....

published a series of papers in Journal of the British Interplanetary Society
Journal of the British Interplanetary Society
The Journal of the British Interplanetary Society is a technical scientific journal, first published in 1934. JBIS is concerned with space science and space technology...

which described orbital ring

Orbital ring

An Orbital Ring is a concept for a space elevator that consists of a ring in low earth orbit that rotates at above orbital speed, that has fixed tethers hanging down to the ground.The structure is intended to be used for space launch....

s and described a form which he called Partial Orbital Ring System (PORS).

The launch loop idea was worked on in more detail around 1983–1985 by Lofstrom. 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

Magnetic levitation

Magnetic levitation, maglev, or magnetic suspension is a method by which an object is suspended with no support other than magnetic fields...

, launch loops use Electromagnetic suspension

Electromagnetic suspension

Electromagnetic Suspension is the magnetic levitation of an object achieved by constantly altering the strength of a magnetic field produced by electromagnets using a feedback loop...

(EMS).

Description

A launch loop is proposed to be a structure around 2,000 km long and 80 km high. The loop runs along at 80 km above the earth for 2000 km then descends to earth before looping back on itself rising back to 80 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

Iron

Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

tube approximately 5 cm (2 inches) in diameter, moving around the loop at 14 km/s (31,000 miles per hour).

Although the overall loop is very long, at around 4,000 km circumference, the rotor itself would be thin, around 5 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 reactive centrifugal force

Reactive centrifugal force

In classical mechanics, reactive centrifugal force is the reaction paired with centripetal force. A mass undergoing circular motion constantly accelerates toward the axis of rotation. This centripetal acceleration is caused by a force exerted on the mass by some other object. In accordance with...

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.

Launching payloads

To launch, vehicles are raised up on an 'elevator' cable that hangs down from the West station loading dock at 80 km, and placed on the track. The payload applies a magnetic field which generates eddy current

Eddy current

Eddy currents are electric currents induced in conductors when a conductor is exposed to a changing magnetic field; due to relative motion of the field source and conductor or due to variations of the field with time. This can cause a circulating flow of electrons, or current, within the body of...

s in the fast-moving rotor. This both lifts the payload away from the cable, as well as pulls the payload along with 3g (30 m/s²) acceleration. The payload then rides the rotor until it reaches the required orbital velocity

Orbital velocity

Orbital velocity can refer to the following:* The orbital speed of a body in a gravitational field.* The velocity of particles due to wave motion, in particular in wind waves....

, and 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

Rocket engine

A rocket engine, or simply "rocket", is a jet engineRocket Propulsion Elements; 7th edition- chapter 1 that uses only propellant mass for forming its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in accordance with Newton's third law...

("kick motor") or other means is needed to circularize the trajectory to the appropriate Earth orbit.

The eddy current technique is compact, lightweight and powerful, but inefficient. With each launch the rotor temperature increases by 80 kelvin

Kelvin

The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

s due to power dissipation. If launches are spaced too close together, the rotor temperature can approach 770 °C (1043 K), at which point

Curie point

In physics and materials science, the Curie temperature , or Curie point, is the temperature at which a ferromagnetic or a ferrimagnetic material becomes paramagnetic on heating; the effect is reversible. A magnet will lose its magnetism if heated above the Curie temperature...

the iron rotor loses its ferromagnetic

Ferromagnetism

Ferromagnetism is the basic mechanism by which certain materials form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished...

properties and rotor containment is lost.

Capacity and capabilities

Closed orbits with a perigee of 80 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 escape orbits

Escape velocity

In physics, escape velocity is the speed at which the kinetic energy plus the gravitational potential energy of an object is zero gravitational potential energy is negative since gravity is an attractive force and the potential is defined to be zero at infinity...

, gravity assist trajectories past the Moon

Moon

The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

, and 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

Geosynchronous orbit

A geosynchronous orbit is an orbit around the Earth with an orbital period that matches the Earth's sidereal rotation period...

insertion this would need to provide a delta-v

Delta-v

In astrodynamics a Δv or delta-v is a scalar which takes units of speed. It is a measure of the amount of "effort" that is needed to change from one trajectory to another by making an orbital maneuver....

of about 1.6 km/s, for LEO

Low Earth orbit

A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...

to circularise at 500 km would require a delta-v of just 120 m/s. Conventional rocket

Rocket

A rocket is a missile, spacecraft, aircraft or other vehicle which obtains thrust from a rocket engine. In all rockets, the exhaust is formed entirely from propellants carried within the rocket before use. Rocket engines work by action and reaction...

s 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

For a launch loop to be economically viable it would require customers with sufficiently large payload launch requirements.

Lofstrom estimates that an initial loop costing roughly $10 billion

1000000000 (number)

1,000,000,000 is the natural number following 999,999,999 and preceding 1,000,000,001.In scientific notation, it is written as 109....

with a 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 five-year payback period, the costs for accessing space with a launch loop could be as low as $3/kg.

Advantages of launch loops

Compared to space elevators, no new high-tensile strength materials have to be developed, since the structure resists Earth's gravity by supporting its own weight with the kinetic energy of the moving loop, and not by tensile strength.

Lofstrom's 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.

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. 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.

Launch loops are intended for human transportation, to give a safe 3g 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 are compatible with large-scale commercial space tourism

Space tourism

Space Tourism is space travel for recreational, leisure or business purposes. A number of startup companies have sprung up in recent years, hoping to create a space tourism industry...

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 joule

Joule

The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...

s or 1.5 petajoules) would be approaching the same total energy release as a nuclear bomb explosion (350 kilotons of TNT equivalent

TNT equivalent

TNT equivalent is a method of quantifying the energy released in explosions. The ton of TNT is a unit of energy equal to 4.184 gigajoules, which is approximately the amount of energy released in the detonation of one ton of TNT...

), 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 km altitude with minimal damage, such as parachutes.

Therefore for safety and astrodynamic

Astrodynamics

Orbital mechanics or astrodynamics is the application of ballistics and celestial mechanics to the practical problems concerning the motion of rockets and other spacecraft. The motion of these objects is usually calculated from Newton's laws of motion and Newton's law of universal gravitation. It...

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 two main points of instability are the turnaround sections and 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. However, an additional instability is present in that the cable/sheath/rotor may undergo meander

Meander

A meander in general is a bend in a sinuous watercourse. A meander is formed when the moving water in a stream erodes the outer banks and widens its valley. A stream of any volume may assume a meandering course, alternately eroding sediments from the outside of a bend and depositing them on the...

ing modes (similar to a Lariat chain

Lariat chain

A Lariat chain is a chain that hangs off, and is spun by a wheel. It is often used as a science exhibit or a toy.The original was created in 1986 by , as an Artist-in-Residence project at the Exploratorium in San Francisco....

) 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 Alexander Bolonkin 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; and the force and friction in the ground 28 km diameter turnaround sections are gigantic. In 2008, 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.

External links

www.launchloop.com
SpaceCable Another similar idea for launch assist/short range travel/recreational extremely high altitude trips
Space Elevator Stage 1: Through the Stratosphere, John Knapman, Keith Lofstrom, presentation at Microsoft conference center, August 2011.

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