Hey /sci/ I just mocked up a basic design for a O’Neil cylinder with some modifications, this cylinder is made from magnesium taken from mercury, and launched into sun mercury L1 where the light is 7 times brighter. The cylinder is 19 km long and 3.8 km wide, 228 km^2 area, the hull is 9 meters thick and can handle 3 meters of dirt or 5 meters of water respectively on top of it. Internal illumination is provided for via diffusively reflecting “sun disks” that at each stage fractionally reflect part of a culminated central light beam, the surface level of light is 40% of earths ( the light saturation point of nearly all plants) atmospheric pressure is 0.5 bar 40% o2. Total mass 4-5 billion tonnes. A 1.15 giga watt sized (on mercury it’ll be 7 times higher) solar farm would be able to launch 29 billion tonnes of mass each year assuming the use of a railgun and 8.8 kJ per kg energy requirement. There is a non rotating mirror ring of 11.4 km^2 focusing light for the day light, and a 0.7km^2 ring a few hundred meters above for a moonlight for simulation of night 1/40,000th day light (10x brighter than a full moon). The cylinder rotates at 0.8g that allows a 20-30% safety factor. And neither of the sunlight or moonlight rings need move (which was a key limitation in the original design) , and because due to the high solar flux The mirror ring needn’t be obscenely large compared to earth l1. Radiation shielding is about 20 tonnes per square meter and although radiation flux is 7 times higher at mercury it is nearly entirely slow moving lowly penetrating solar protons and less deeply penetrating Galactic cosmic rays as at earths or Mar’s orbit due to mercury being closer within the suns own magnetic field ( total absorbed radiation per annum would be equivalent or less to here on earth) . Total absorbed radiation would be around 18% of incident flux provided the hull was 95%+ reflective giving a average day time temperature of 20-26 Celsius. I’d love some feed back.