= Segment Replacement =

== Segmentation ==

The rotor is not continuous.  It picks up speed as it descends from 80 km altitude to surface level - from 14,000 meters per second to 14,056 meters per second.  Since the mass flow rate must remain the same, the density must decrease by 0.4%.  That much strain would put way too much useless stress on the rotor - we can relieve it by lengthening the rotor at screw joints between segments, with alternating right and left hand twists.  A 5 meter rotor segment would have a gap that expands by 2 centimeters.   

Rotor segments will get damaged by meteorites, debris, or other.  Over time, we will need to "deconstruct" the loop and rebuild it with a replacement rotor, but in the short term we may need to replace badly damaged segments one at a time.

The rotor will be laminated iron, with thin laminations oriented to minimize drag in "DC" fields (such as the turnaround magnets), and maximize drag in the time-varying field of the magnet launch rails under the payloads.  The rotor should be stiff (difficult to bend) but not strong - if exposed to high stress, such as a release into water or atmosphere, it should fracture into high drag fragment.  Rotor segments should be designed to burn up rapidly if they get loose, creating a cloud of incandescent rust.

Rotors are coated with a thin interposer layer of iron carbide followed by a 40 nm layer of [[ https://en.wikipedia.org/wiki/Diamond-like_carbon | diamond ]].  Carbon has a lower atomic weight, so loose carbon atoms are less likely to lead to hypervelocity spalling cascades.  However, if the diamond is abraded away, iron can be released into the vacuum plenum and create more damage.  Large abrasions should be replaced.

== Inspection ==

The rotor will pass by a chain of high speed cameras for imaging as it passes into the east end turnaround.  It may be possible to orient CCD cameras so that the scan clock moves at the same rate as the rotor - for a 40 MHz clock, that is 350 micrometer precision.  Small anomalous areas may be scanned with electron beam imaging further down the line, and characterized.  There will also be an inspection station at the west end, which will transmit its data to east end much faster than the rotor gets there.

== Characterization Time ==

There will be about 20 seconds between the time a rotor segment passes the camera and it rounds the D magnet and passes back up the incline.  If we allow 5 seconds for decision making, that allows us to launch a repair robot with at 50 gees (480 m/s^2^) for rendezvous with the rotor 200 km westward of east station, about 35 seconds later.  Computers will maintain a detailed map of the rotor surface, updated every pass, including additional information from the west station scanner, and detailed maps of marred or damaged spots.

This is the case for an emergency repair - normally, we can plan this out over many loop cycles.  The repair must still be rapid.

== Repair Robots ==

A repair robot will be expendable - used once.  Long and skinny, just wide enough for two opposing magnet rails and a carrier with a replacement rotor segment.  The westbound sheath is wider to accomodate this robot - normally, no vehicles ride the outside of it. Unlike the reverse trip portrayed in R.G. Williscroft's novel [[ Foreword |"Slingshot"]], workers evacuating east station will leave in small [[EmergencyReentry | reentry capsules with parachutes]].

Launched from east station, it accelerates at high drag (480 m/s^2^) down the track, reading bar codes off the passing segments and under control of the main loop.  It drags up to near full rotor speed 30 seconds later,  about 200 kilometers west of east station,
then making a rendezvous and stop precisely over the damaged station.

== Repair sequence ==

We have about two minutes from start to finish, as the rotor travels 1800 kilometers towards west station at 14 kilometers per second.

The robot will clamp onto the segments on either side of the damaged segment, and high speed Torx drivers will unscrew, rescrew, then remove the screws holding the damaged segment.  It will be lifted out, the new segment inserted (screw, unscrew, rescrew) and the robot will prepare for jettison near west station.

== Jettison ==

Airlock ports will open near west station, and the robot will fly out westward, at 14 kilometers per second, faster than earth escape velocity (ll km/s).  For non-emergency repairs, we will try to do this around local midnight, which means the rotor escapes from the earth at about 8.7 km/s ( sqrt[14^2-11^2] ), subtracted from the earth's 29.8 km/s orbital speed, putting the segment in an elliptical orbit with a perhelion of ('''MoreLater''').  It will make thousands of orbits before it eventually runs into the earth again - hopefully it will delaminate, fragment, and burn up before it hits the surface.  This is a less serious problem than losing a large part of the rotor because of a break - far more mass may be lost into space.