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@@ -48,7 +48,7 @@ A neat compromise is an atmosphere at a reasonably safe total pressure of perhap
The pressure difference will put some strain on the walls, but planes manage this with extremely thin skin (for mass reasons), so this shouldn't be an enormous problem, especially since for thermal reasons (detailed [below](#thermal-management)) there will be no windows, let alone openable windows. There will also need to be airlocks between the main cube and ambient-pressure sections such as the terminals for external rail/road/foot/ship access (including freight), which is a larger problem, because cycling speed is limited by human comfort (avoiding ear damage). It's been surprisingly difficult to find good data on what rate of pressure change is dangerous, but one reference is aircraft, which should apparently do at most [300 ft/min](https://www.ncbi.nlm.nih.gov/books/NBK219009) of descent (so about twenty minutes for 0.75 atm to 1 atm). Extrapolating somewhat from the figures in [this](/assets/misc/ryan2018.pdf), it may only be possible to go down to 95%–98% of outside pressure without unreasonably delaying entry and exit. This might also be necessary to keep emergency evacuation practical.
The cube certainly also needs active ventilation. We can estimate the air handling requirements based on the existing standards ([Approved Document F](https://assets.publishing.service.gov.uk/media/62a761edd3bf7f03667c667e/ADF2_revised.pdf#page=18)) of 10L/s/person (or 1L/s/m<sup>2</sup> of floor area). In this case, the floor area constraint would be binding, assuming 5m per floor for 600 total floors. The system has to move 5.4e9 L/s, or 5.4e6 m<sup>3</sup>/s, across all floors, or 9000m<sup>3</sup>/s for one floor, which has external faces of 3000m (horizontally) by 5m (vertically). Assuming that all flow is in one direction (e.g. east/west) and that the same volume of air is exhausted as taken in, we can calculate the necessary cross-sectional area of ducting from velocity. The velocity used in ducts is constrained by, apparently, noise, with branches in residential systems at up to [3m/s](https://www.engineeringtoolbox.com/sizing-ducts-d_207.html) according to a website. At that low speed a full fifth of the cross-section is in use for ducting, but we can use higher speeds for the central parts; 20m/s for all of it drops the area requirements to 450m<sup>2</sup> per floor, or 3%. Some of the system will of course need lower speeds, so the real figure will be between these, but I think this is enough to demonstrate rough feasibility.
We can estimate the requirements of the active ventilation based on the existing standards ([Approved Document F](https://assets.publishing.service.gov.uk/media/62a761edd3bf7f03667c667e/ADF2_revised.pdf#page=18)) of 10L/s/person (or 1L/s/m<sup>2</sup> of floor area). In this case, the floor area constraint would be binding, assuming 5m per floor for 600 total floors. The system has to move 5.4e9 L/s, or 5.4e6 m<sup>3</sup>/s, across all floors, or 9000m<sup>3</sup>/s for one floor, which has external faces of 3000m (horizontally) by 5m (vertically). Assuming that all flow is in one direction (e.g. east/west) and that the same volume of air is exhausted as taken in, we can calculate the necessary cross-sectional area of ducting from velocity. The velocity used in ducts is constrained by, apparently, noise, with branches in residential systems at up to [3m/s](https://www.engineeringtoolbox.com/sizing-ducts-d_207.html) according to a website. At that low speed a full fifth of the cross-section is in use for ducting, but we can use higher speeds for the central parts; 20m/s for all of it drops the area requirements to 450m<sup>2</sup> per floor, or 3%. Some of the system will of course need lower speeds, so the real figure will be between these, but I think this is enough to demonstrate rough feasibility.
Obviously, the cube's density increases the risk of airborne pathogens, all else equal: however, most buildings today don't have very good ventilation[^9] due to [skill issues](https://asteriskmag.com/issues/05/lies-damned-lies-and-manometer-readings), outdated building stock and high retrofit costs, so a ground-up design with modern HVAC provision should be able to do better than average. We could also incorporate [far-UVC](https://www.worksinprogress.news/p/flipping-the-switch-on-far-uvc) technologies, which have seen little adoption despite being obviously correct to install. Biosafety can also be improved by minimizing leaks and ensuring isolation between the HVAC systems for different sections, which is again much easier in a newly built structure.