I didn't notice either that I'd calculated 30000 cm sq (so tired)
After my new designs (Page 5 is quite outdated) I have 3800 cm sq to cover with armour, and according to http://www.jindalstainless.com/toolkit/ ... lator.html 15mm HDPE at high end HDPE standard at directplastics - 96 g/cm^3 (checked the density on their data sheets) equates to 5.5kg rounded? One of us has done wrong calculation, and chances are it's me.