The ice gun

Courtesy ToughSF on this 31st day of April is presented here, the Ice Gun, claimed by Andrew Nowicki. In all its 1997 university \public_html glory. Boy I wish I still had my sbcglobal directory... but anyway.

This is what they call a Megastructure and how. 1000 km through an ice sheet, to lift cargo into LEO without the Vernean 100 m/s² from SpinLaunch and others like that. More like those bridges and loops.

On Earth, our only permanent icesheet of this size is the Antarctic one. Also it is worried that Antarctica won't always have its sheet, although there I call shenanigans; even if you believe CO₂ is a problem - I don't - it seems mainly a problem for the northern hemisphere. The extra water raised there will fall on the south and make Antarctic (and southern Andean) ice thicker. Someone who cares, who isn't me, might see this as a factor in how come our seas haven't risen already.

More serious is that the ice cannot be bored just anywhere; only where it is stagnant i.e. not subject to glacial movement. Given the possible tunnels the poles impose some hefty constraints on when is good to fire cargo into the ecliptic, rather than orthogonal to the Solar System into nothing. And then there's the whole summer/winter thing to consider. Why not just blast Orion? Maybe the nuclear fuel is problematic to mine and/or import.

Also, stress in the ice contributing to gradual collapse of the tunnel. Also also Stagnation Temperature.

If Vernean G is accepted, the gun can be shorter, like "only" 100 km. [UPDATE 5/8: 225 km concave seems the sweet-spot.] Of course then you're competing with SpinLaunch leaving absolutely no point in this structure much less in Antarctica. Treat this as proof-of-concept.

I'd consider the ice-gun mainly for the icy asteroids and for Callisto. In lower gravity the glaciers should run slower and be less subject to collapse; although, still at risk from the heat of any exiting spacecraft.

This gun would be wonderful for Ganymede as an underground structure; that's where the first colony will be. For this Medici moon I'd build for escape-velocity / v_∞ as to make the trajectory intersect Callisto; I do not expect Ganymede to host a human-rated orbital station, nor for regular shipments inbound to Europa etc. Another complication (lengthening the barrel) is that long-term Ganymedeans will be, er, twinks. Not as resistant to the hard G as is a good Venerean.