Launch loop

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 News. In 1982, Paul Birch published a series of papers in Journal of the British Interplanetary Society which described orbital rings 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, launch loops use electromagnetic suspension (EMS). ==Description==

Consider a large cannon on an island that shoots a shell into the high atmosphere. The shell will follow a roughly parabolic path for the initial flight, but drag will slow the shell and cause it to return to Earth in a much more vertical path. One could make the path purely ballistic by enclosing the predicted path in a tube and removing the air. Suspending such a tube would be a significant problem depending on the length of the path. However, one can use the shell to provide this lift force, at least temporarily. If the tube is not exactly along the flight path of the shell, but slightly below it, as the shell passes through the tube, it will be forced downward, thereby producing an upward force on the tube. To stay aloft, the system would require the shells to be fired continually. The launch loop is essentially a continuous version of this concept. Instead of a cannon firing a shell, a mass driver accelerates a cable into a similar trajectory. The cable is surrounded by an evacuated tube, which is held aloft by pushing down on the cable using electromagnets. When the cable falls back to Earth at the other end of the trajectory, it is captured by a second mass driver, bent through 180 degrees, and sent back up on the opposite trajectory. The result is a single loop that is continually travelling and keeping the tube aloft. To use the system as a space launcher, a launch loop would be about 2,000 km long and 80 km high. 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 cm (2 inches) in diameter, moving around the loop at 14 km/s (31,000 miles per hour). Keeping the system aloft requires a significant amount of lift, and the resulting path is much flatter than the natural ballistic path of the rotor. 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. ==Comparisons==

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 about 0.5% of what 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, 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 discrete bolts rather than a continuous rotor, as with the launch loop architecture. Knapman has also mathematically shown that the meander instability can be tamed. The skyhook is another launch system concept. A skyhook could be either rotating or non-rotating. The non-rotating skyhook hangs from a low Earth orbit down to just above the Earth's atmosphere (skyhook cable is not attached to Earth). The rotating skyhook changes this design to decrease the speed of the lower end; the entire cable rotates around its center of gravity. The advantage of this is an even greater velocity reduction for the launch vehicle flying to the bottom end of the rotating skyhook which makes for an even larger payload and a lower launch cost. The two disadvantages of this are: the greatly reduced time available for the arriving launch vehicle to hook up at the lower end of the rotating skyhook (approximately 3 to 5 seconds), and the lack of choice regarding the destination orbit. ==See also==