[ExI] question re "honkin' big cannon" space launch

[Jeff Davis]

Sometime back I was chatting with spike about various nerd-boy (or
girl) topics, when I touched on the ancient(Verne/Wells?) idea of
space launch by cannon.  Spike, (in a manner far too conventional and
dismissive for one of his imagination and flagrant sense of humor)
naysayed the notion, mentioning some problem (hypersonic acoustic
shock, or some such techno-jargonaut obscurantism).  I was not
impressed.  I figured if the nose cone was pointy enough, then the
acceleration of the surrounding air molecules could be kept low enough
to get around those difficulties to which spike and his overqualified
"sources" were referring.

Fast forward, oh, eight or nine years, to the latest reprise of the
space launch topic, and Paul Fernhout mentions "sky hooks" and a
"space pier".  I asked Paul offlist about the "space pier", thinking
he was joking, but no, he wasn't, and he provided me with a link(which
I have since lost).

On that website, I found a crude drawing depicting a horizontal
launch/acceleration platform several tens of miles in length held
aloft some tens of miles up by legs/struts.   Could have been fifty
miles up, I don't remember exactly, but what I do remember is that
there was no mention of no mention of "hypersonic acoustic shock" or
whatever and it wasn't 100, 110, or 150 miles up, you know, up there
in viable-orbit-land, which is to say thoroughly "out of the
atmosphere".   There was, however, mention of post-"launch"
rocket-mediated orbit "widening" to get it out of the upper reaches of
the atmosphere.

Which got me to thinking again about the gun launch idea.  And so my question.

Let's get specific.  What are the limitations -- boundaries if you
will -- of the gun launch -- or similar
"emerges-from-launch-tube-at-orbital-or-greater velocity" launch
schemes?  And what features -- such as the pointy nose cone --  can
help?   In particular, what's the altitude lower bound?  Yeah, that's
what I want to know: lowest possible altitude?

Thoughts?

Best, Jeff Davis

"When I am working on a problem I never think about beauty. I only
think about how to solve the problem. But when I have finished, if the
solution is not beautiful, I know it is wrong."
                      - Buckminster Fuller