My calculations for the volume of a 15m x 100m cylinder would be 17,671 m³ (π * (Diameter/2)² * Length). I think you calculated it with 15m as its radius.
For an atmosphere similar to the Earth's at sea level, you would only need to bring a single Starship with liquid nitrogen (~12k kg) or ammonia (~22k kg, which you could use the ammonia breaking process) to fill it up the volume. Starship can hold 100 tons in its cargo configuration. With a 15m radius you would still only need ~49k kg of liquid nitrogen or ~80k kg of liquid ammonia, which is doable in a single-ish Starship flight (maybe two with the ammonia if you couldn't use all of the cargo capacity).
This seems extremely doable on smaller sizes, and pretty doable for larger sized lava tubes (albeit expensive).
For an atmosphere similar to the Earth's at sea level, you would only need to bring a single Starship with liquid nitrogen (~12k kg) or ammonia (~22k kg, which you could use the ammonia breaking process) to fill it up the volume. Starship can hold 100 tons in its cargo configuration. With a 15m radius you would still only need ~49k kg of liquid nitrogen or ~80k kg of liquid ammonia, which is doable in a single-ish Starship flight (maybe two with the ammonia if you couldn't use all of the cargo capacity).
This seems extremely doable on smaller sizes, and pretty doable for larger sized lava tubes (albeit expensive).