Hello everyone, I am thinking of a thought experiment about a so-called "space tower" but don't have enough information to estimate the numbers. I understand the material strength needed for such high structures is huge, because of the weight of the structure. But what if the "tower" would be vacuumed and would have an open end extending to a near-vacuum height "above" atmosphere, while the bottom would be tightly closed (access via airlock)? Actually that would be more like a very long tube or gun barrel maybe, that could contain an electromagnetically propelled elevator (like a vertical maglev). 1. How tall should the tower be so that the pressure inside could be easily kept constant (vacuum) using pumps? I am not aiming for perfect vacuum but if (I suppose) the decrease in atmospheric pressure with height is not linear, a compromise could be found so that the tower is tall enough so that it could be easily kept vacuumed using pumps but also not too tall so that its weight would be kept as low as possible... 2. Because the tower is "full of vacuum" (pardon my expression) and would be displacing a huge mass of air (even if the upper atmosphere is much less dense) would the buoyancy force help reducing its weight and consequently would reduce the needed material strength? Does it matter that the top end is open? 3. If instead of having a foundation, the tower would be supported by huge pillars, would that make any difference regarding buoyancy? Thanks for any input!