Diff for "VelocityShear"

Velocity Shear

The rotor moves at 14000 meters per second. The track moves at 0 meters per second. They are separated by 1cm of vacuum, with spacing actively controlled by a magnetic field. Sounds like a formula for disaster, right?

One way to characterize the small spacing versus high velocity problem is as a ratio of these two numbers, space/velocity. For the launch loop, this is 1cm/14000m/s or 720 nanoseconds. That seems like a tiny number.

However, the platter of a 3.5 inch hard drive is moving at 14 meters per second, and the spacing between the head and the platter is 10 nanometers. A running hard drive can be subjected to many gees due to unpredictable impact forces, or unexpected power removal. 10nm divided by 14m/s is 720 picoseconds, a thousand times smaller. The head spacing on a hard drive is maintained by airflow, this is called a "flying head". However, there is no other active control, beyond the servo that moves across the surface, and removes it very rapidly when the disk shuts down. If power is removed unexpectedly, the energy stored in the spinning platter is used to write buffered data, then move the head to a safe track. A disk drive manages all of this for years of operation without breaking the very fragile head or magnetic platter surface.

In reality, the problem is NOT relative velocities, it is relative energies. The issue in a launch loop is "hypervelocity spalling cascades". A sufficiently large particle either rotor or track at 14000 meters per second will have enough energy to kick loose more mass, which may impact the opposing surface and kick loose more.

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