Mass drivers

John Wheeler wrote:
> IMHO there is a fundamental problem with mass drivers, but
it's economic, not technical. Mass drivers represent a huge
investment, and they will have to transport a lot of cargo to bring
the cost per ton down. However, until the costs come down,
there won't be much demand for space transportation. So you
have a chicken-and-egg kind of problem.

From that permanent page:
The Mass Driver is made of a small variety of parts, all simple
and repeated in a modular way which expedites simple
replacement and maintenance using a small stock of spares in
case of a small failure. It is a lightweight device, which makes
stockpiles of spare parts relatively inexpensive; all of which are
attractive features of a device operating in a remote place like the
Moon.

I'm still thinking of the asteroid retrieval mission. Latch onto an
800 meter stony NEA, and drag it back. (missions like this have
been in the AIAA & such for years. The inventment is under $50
bllion, and the return after a decade is hundreds of billions.)
Send out the parts of the mass driver which aren't just rolled &
stamped metals and concrete & wires,and build up the track
length from local materials making up the bulk.

The question is that I don't know the initial mass needed at the
asteroid. How many (Shuttle-Z?) HLV shots?