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We could be saved by the square-cube law, or, The Power of Being Big. In essence, let's assume you've made your own fully-shielded space capsule for one, and that you need, for example, a pound of material for every square inch of its surface, and that you've made it into a sphere to economize on material. Wise choices, all of them. A 2m diameter capsule would come in at 19,500 lbs, or so, for that many square inches. Congratulations. You've just found out why we don't shield small spacecraft. Now, however, let's get an estimate of what it would cost to shield 200 people, providing them each with a volume-equivalent of your capsule, or 4.2ish cubic meters per person. Really packing them in. This is a 11.6-meter diameter sphere, with roughly 3,300 pounds of shielding per person. At 2000 people, packed in like sardines, we're at 25.2 meters in diameter and 1550 lbs of shielding per person. A 54-meter sphere packs 20,000 sardines, and requires a mere 717 lbs of shielding per person. A factor of 10-20 better. Obviously, we want to relax the space constraints a bit. However, the form factor of shielding in the larger craft is going to be much better suited to the reutilization of supplies as shielding. In the limiting case, of course, there is effectively no required shielding per occupant. Long before that, magnetic shielding schemes become a viable option, too. A more practical limit would be to consider a transport module made up of re-entry craft seating 1-10 persons, with the ablative heat-shields and supplies facing outward. In this case, assume we can get down to 5 times your cross-sectional area in shielding, and guess that that's about 2.5 square meters. That's 4000/lbs per person. So, still a factor of 5 better than the solo case. Additionally, this is a lot of arithmetic to ask of google and http://www.calculatorsoup.com/calculators/geometry-solids/sp..., so the numbers may be off. Also, the Dragon capsule checks in at 5.8 m^2 of heat shield per astronaut, which may be a practical limit. And I just made up the 1lb/in^2 shielding requirement. |