John Schilling wrote

Because conventional powder guns absolutely cannot accelerate a projectile
to even one-quarter the necessary velocity.

I'd have thought a Yank would know about guns.

Let's say for argument the necessary velocity is 8.5 km/s.

Bull got around 3.6 km/s at (S)HARP. There are a few 13,000 fps (~4km/s)
powder guns around today, and I've heard of plans for a 5km/s one, though I
doubt it could be called "conventional".

A slight variation is the two-stage gasgun, which can reach about 8 km/s. I
believe the first was built in 1957. LLNL built one in 1972 and is still
using it. It's powered by gunpowder.

Experimental railguns have already achieved 30,000+ fps, well over orbital
velocity, but the power supplies are very expensive.

The US army has reportedly tested a battlefield 2.5kps "electromagnetic gun"
intended for tanks, but I don't know how that works. Another "e-m gun" was
developed (or at least funded) as part of the SDI project, to shoot down
sats and incoming warheads, but again I know no details.


Various types of gasgun using eg hydrogen as the working fluid can also
reach orbital velocity plus, at least in theory.

I designed one about 30 years ago, to launch 2-ton blocks of ice. Almost
everyone I spoke to then laughed at the idea, though I've seen it suggested
again since.


Because even an exotic gun
that hypothethicall could reach such velocities, would be launching a
"capsule" containing an astronaut mass of bloody hamburger uniformly
smeared across the back wall

For people you need about 300km of barrel length, at about 10g max. The only
really suitable place to build one is in Ecuador. Which is unfortunately
subject to earthquakes and volcanoes.

Exiting from the gun barrel is a bit of a hard problem, you change very
quickly from +10g to -(lots of) g's. Atmospheric heating isn't actually that
much of a problem, though aerodynamic forces are. As are noise and other
environmental considerations!



Guns may also be useful for getting the first few kps and a bit of altitude,
though that is a bit problematic as far as cost/benefit goes. They are
initially expensive, and the high acceleration and aerodynamic forces mean
that the second-stage projectiles need to be strongly built and thus heavy.
Alternatively the gun must be long, and even more expensive.

The military potential is also a problem, but that's a problem in any space
effort. The US Govt., and especially the US military, does not want anyone
(else?) in space.


--
Peter Fairbrother