Starting loop or Lofstrom loop – published project of cable transport system, intended to bring cargo into orbit. The project is based on a looped cord (loop), continuously moving at a huge speed (12-14 km / s) inside the vacuum pipe. The cord does not touch the walls of the pipe, they are separated by a magnetic suspension. In general, this device is a grandiose structure with a length of about 2000 km, and the loop itself must rise to a height of 80 km and stay on it due to the moment of inertia of the rotating cord. Rotation carries the weight of the entire structure on a pair of magnetic bearings that support it, one at each end. 

The starting loop is designed for non-rocket launches of space vehicles with a mass of up to 5 tons, using electromagnetic acceleration, both to orbit around the Earth, and beyond. Overclocking is carried out on a flat part of the cable, which is outside the dense atmosphere.

The published estimate of the costs for a workable starting loop was significantly lower than for a space elevator, while the proposed system has a higher launch rate, lower cost and the same or even higher payload (payload) than the equivalent. In contrast to the space elevator, it will not require any development of new materials.

The system is designed to ensure the launch of space tourists, and also aims to space exploration and space colonization, providing a relatively soft level of overload equal to 3g.

Description

The starting loop is a construction about 2000 km in size. The loop itself rises from the surface of the Earth to an altitude of 80 km, passes at this altitude of 2000 km, again descends to the surface of the Earth, unfolds and then repeats all the way back to the starting point. The loop has the form of a tube, hollow inside and called a shell. Inside the shell, another solid tube is suspended, called a rotor, which is a cord or chain. The rotor is made of iron and has a diameter of about 5 cm. It moves along a circle inside the loop at a speed of 14 km / s.

Although the loop is very long, about 4000 km, the rotor itself is quite thin, about 5 cm in diameter, and the shell is not much larger. The rotor is made of ferromagnetic iron in the form of a cord or tube with longitudinal compensators every meter or so. The rotor is separated from the envelope servo stabilizing magnetic bearings. The sheath is sealed, with the maintenance of vacuum, to minimize the resistance exerted on the rotor.

At rest, the loop will be at ground level. Then the rotor will begin to accelerate with a linear motor, which will consume several hundred megawatts of power. With an increase in rotor speed will be bent and acquire the shape of an arc. The shell will force it to take the shape of the curve steeper than the ballistic curve. In turn, the rotor will transmit the centrifugal force to the shell, keeping it in the air. The loop will take the desired shape and will be limited to a maximum height of ≈80 km due to cable attachment to the ground. When using a 300 MW generator, it takes about two months to reach full speed. After full promotion, the rotor will perform one revolution in about five minutes.

After raising the structure, it will require constant replenishment of its energy in order to compensate for energy dissipation in magnetic bearings, to stabilize the structure, and to compensate for losses due to imperfections in the vacuum envelope. All this will require about 200 MW of power, not counting the additional energy for launching space vehicles.

Cargo launch

To start up, the vehicle is lifted on the "lift cable", which hangs from the western loading station from an altitude of 80 km, and is placed on the rails of the accelerating section. The booster block creates a magnetic field, thanks to which eddy currents appear in a rapidly moving rotor. They also raise the payload over the cable and push it forward with 3g acceleration (30 m / s ²). The payload is accelerated by the rotor until it reaches the required orbital speed, after which it leaves the accelerating section.

If a stable or circular orbit is needed, then when the payload of the topmost point of the trajectory is reached, the onboard rocket engine ("accelerator") or another means necessary for directing the trajectory to the corresponding orbit around the Earth should be loaded with the useful load.

The eddy current method is compact, light and powerful, but inefficient. At each start-up due to power dissipation, the temperature of the rotor rises by 80 degrees. If run loads are too close to each other, the rotor temperature may be close to 770 ° C (1043 K), after which the rotor material loses ferromagnetic properties, and its tightness is compromised.

Starting Features and Performance

Orbits with a perigee of 80 km are unstable due to aerodynamic braking (the spacecraft is rapidly losing altitude and falling to Earth), but in addition to such orbits, the launch loop itself is able to directly throw useful cargo into orbit that requires a speed higher than the escape velocity, to make a gravitational maneuver around the Moon and to get to other distant orbits, including those nearest to the Trojan points.

To ensure launching into circular orbits from the launch loop, a relatively small accelerating engine will be needed, which will be activated at the apogee and correct the orbit. To hit the geosynchronous orbit (GSO), it will be necessary to increase the speed by about 1.6 km / s, and to achieve a low orbit (NO) at an altitude of 500 km, an addition to the speed of only 120 m / s will be required. Conventional rockets require additives to a speed of approximately 10 and 14 km / s andto achieve DK GSO, respectively.

Lofstrom launch loop structure is close to the equator, and can provide the launch only in equatorial orbit. However, there may be achieved and other orbital plane by means of a turn at the height of the lunar perturbation or by aerodynamic methods.

The maximum rate launcher launches the loop is about 80 per hour, and is ultimately limited temperature and time of cooling of the rotor, but this would require a power of about 17 GW. A more modest 500 MW capacity will be sufficient for 35 launches per day.

Economy

In order for the starting loop to be economically feasible, the appearance of customers with sufficiently large requirements for load capacity of launches is required.

Lofstrom estimated that the initial cost of the loop, is about $ 10 billion, paid off in one year, will need to run the 40000 tons per year, while the cost to launch to orbit will be about $ 300 / kg. If the initial investment to increase to $ 30 billion(to build a more powerful loop), the loop will run 6 million tons of cargo per year and, subject to a five-year payback period, cost of space may be less than $ 3 / kg [5].

Benefits

It is expected that the starting loop can ensure a high rate of starts (several starts per hour, regardless of the weather), and this system will not practically pollute the environment. At missile launch formed pollution in the form of nitrates due tohigh exhaust gas temperatures, and depending on the type of fuel may be emitted greenhouse gases. The starting loop, as a kind of electric power plant, is environmentally friendly; it can operate from any source of energy: geothermal, nuclear, solar, wind or any other, even non-permanent type, as the system has huge built-in energy storage.

Unlike the space elevator, which must pass through the radiation belt for several days, the passengers of the launch loop can be launched into a low Earth orbit below the radiation belt, or to pass through it in a few hours. This situation is similar to that encountered by the astronauts of Apollo, for which the radiation dose is 200 times lower than the space elevator can provide.

In contrast to the space elevator, which is at risk of collision with space debris and meteorites along its entire length, the starting loop is located at altitudes where the orbits are unstable due to air resistance. Space debris there for a long time is not preserved, the chance of collision with the installation is quite small. While the period of existence of the space elevator is of the order of several years, damage or destruction of the starting loop can happen relatively rarely. In addition, the starting loop itself is not a significant source of space debris, even in the event of an accident. All its possible fragments will have a perigee intersecting with the atmosphere, or their velocities will be lower than the first cosmic one.

The starting loop is aimed at transporting people, because in it the maximum acceleration of 3g is safe, the vast majority of people are able to withstand it. In addition, it provides a much faster way to reach outer space than a space elevator.

The starting loop will work silently; unlike missiles it will not have any noise impact.

Finally, the low cost of putting payloads into orbit makes it suitable for large-scale commercial space tourism and even for the colonization of space.

Difficulties

A twisted loop will store a huge amount of energy in the form of an impulse. Since the magnetic suspension system will have a large redundancy, a failure in a small area will not affect the system's operability. But if there will be a significant structural failure, there will be release of all stored energy (1.5 × 1015 joules or 1.5 petajoules), which is equivalent to the explosion of the atomic bomb, with capacity of 350 kilotons of TNT (but without the radiation emission). Although this is a huge amount of energy, it is unlikely that there will be the destruction of the entire structure due to its very large size, and also because when a fault is detected most of the energy will be directed to a special place. It may be necessary to take measures to reduce the cable from a height of 80 km with minimal damage, for example, to provide parachutes. Therefore, to ensure safety and for astronomical reasons, the starting loop will need to be installed above the ocean near the equator, away from populated areas.

The published design of the starting loop requires electronic control of the magnetic levitation to minimize dissipated power and stabilize the attenuation of the cable caused by other causes. The instability will arise first of all in the turning sections, and also in the cable.

Rotary sections are potentially unstable, since the motion of the rotor in the direction away from the magnets leads to a decrease in the magnetic attraction, while the movement toward the magnets creates an increase in attraction. In any case, instability arises. This problem is solved by using servo control systems to manage the power of the magnets. Although the reliability of the servo drives at high rotor speed is the subject of investigation, a lot of consecutive servo sections will be lost in order to contain the rotor in the event of a system failure.

Cable sections will also share this potential fate, although the forces here are much smaller. However, there is another potential instability consists in that the cable / shell / rotor can undergo meandering (as Lariat chain), moreover, the amplitude of oscillation of the process may increase without constraints (resonance). Lofstrom believes that this instability can also be controlled in real time by means of servos, but nobody did it.

To maintain the vacuum in the system at an acceptable level, it needs a plurality of evenly distributed along the length of the vacuum pumps (ie, at an altitude of 80 kilometers, too) is constantly working on the rollout, to compensate for the leakage.

It is difficult to obtain the required electrical power in the middle of the ocean.

Alexander Bolonkin noted a number of technical issues in the Lofstrom project. In particular, at junctions between 1.5-meter steel plates, jamming is possible, and frictional forces at a turning radius of 28 km are also great.

 Source Wikipedia [RUS]