Launch Loop Frequently Asked Questions
Q1: The rotor is a big lump of magnetic, at least somewhat conductive metal which passes repeatedly through a very large magnetic field which lifts it and curves it, and it does this at extremely high speeds (14 km/s). This can be expected to generate at least some eddy currents, as eddy currents are roughly proportional to speed. Would the energy loss and rotor heating due to this effect be prohibitive to a launch loop? How hot does it get?
A1: Eddy currents occur when there are changes in the magnetic field that pass through a conductor. Provided the lift magnetic field is sufficiently uniform parallel to the motion of the rotor; when the field first penetrates the rotor this sets up eddy currents which then creates a potential difference across the conductor which resists further flow and the currents cease. Thus there is very little heating produced at all.
Eddy currents are reasonably beneficial in the lifting section, because they oppose changes to the magnetic field between the plates and the magnets, thus making the changes when the gap changes slower and easing control issues and damping the lift effect.
Q2: If the rotor is constructed so that eddy currents can't occur, how can acceleration of the payload be achieved?
A2: The rotor will be constructed to permit eddy currents, as eddy currents help damp out the rotor dynamics, as well as help the acceleration of the payload.
- Eddy currents are also used to accelerate and levitate the payload; the payload generates a large magnetic field that causes an eddy current in the iron rotor, which pulls the payload along with an acceleration of 3g a few centimetres above the rotor's sheath. Q3: Do the catenary-like tensile cables give sufficient lateral stability to the structure? The tensile cables give a static pull on the structure sufficient to curve it down to the horizontal, as well as help keep it pointed in the right direction. Most of the lateral stability is provided by the lifting coils and the motion of rotor, vibration prevention is provided by active elements in the sheath. Q4: Is there an issue with the vehicle balancing on top of the rotor like that? The centre of mass of the vehicle is maintained near the cable, and the vehicle attitude can be controlled with attitude jets and/or momentum wheels. Q5: if the rotor was to escape containment, can the structure survive the fall from 80km? It should be possible for the cable to survive the fall with the rotor gone. Q6: How much energy is stored in the rotor at full speed? How big an explosion would containment loss be? A6: the rotor is designed to not be lost. The magnetic levitation is massively redundant, sections can fail completely without any problems as neighbouring levitation sections just work slightly harder to compensate. However the energy in the rotor is huge, equivalent to a several hundred kiloton bomb. The energy release would cause devastation for up to a few miles, which on the scale of the launch loop is fairly small. Because the launch loop would be operated in the ocean, this would be of little consequence, other than to replace any damaged sections. Because the rotor is travelling very fast, there would be some tiny risk to anything in line with the loop, but most fragments would impact the ocean or the atmosphere or leave the Earth's gravity entirely. There would be little risk to spacecraft from orbital space debris issues, as the fragments would be on orbits that intersect the atmosphere and so would all soon re-enter. Q7: during startup in particular, as well as shutdown, will the cable go through resonances that may potentially damage it? The rotor is actively stabilised, so will not go through resonances, as it is damped at all times. Q8: Why is there a loading dock at 80km altitude, rather than at the ground? A8: from the 1985 paper: "The long elevators to the stations are supported by pulleys from the anchor cables. The vehicles are brought up these cables rather than up the west incline to simplify the spacing controllers on the incline. Other benefits of this approach are minimized incline weight, shorter upward transit times, and less likely hood of sheath damage." Q9: What's the difference between the $10 billion version and the $30 billion version? Could there be an upgrade? A9: They're mostly the same. The cheaper version has a quicker payback on investment, so the launch costs are set at $300/kg, and a much smaller power generation capacity, which limits the launch rate. (After the first year, the costs should go down significantly, once the hardware is paid off.) It also needs less powerful linear accelerators due to the lack of available power for them. The $10 billion version might be upgradeable, but probably there would need to be shut down for a few months. Q10: Would the rotor fry due to runaway erosion due to the extremely high temperatures reached in the near vacuum of the sheath? A10: the rotor would be coated with a coating that prevents significant erosion, possibly boron. Q11: Is the rotor suspension mechanism extraordinarily unstable? A11: Electromagnet suspension requires electronic systems to stabilise it. With an electronic system active, provided the system doesn't break, it is completely stable. In case of failures the rotor suspension mechanism has at least 10 redundancy; it needs at least 10 consecutive units to fail for the rotor to crash. Barring common mode failures (which have to be designed out of any system), such a failure is essentially impossible. Most normal systems have a redundancy ratio of 2 or 3. If the system did fail there would be a big explosion, but most of the expensive parts of the loop would be expected to survive; as would anybody launching at the time. Q12: The rotor goes faster than orbital velocity, does this give a net antigravity effect and lift the structure? A12: Unfortunately, not to any significant degree. To look at this another way, the loop is pulled downwards by gravity in more or less the same direction along its whole length, and this net force must be resisted by the bearings.
