I have been wondering this. If always seems that when I see the layout of the components of a spacecraft always seems to consist of a central bus whereby the propellant tanks is in the center and the payload is attached on the sides. Seeing the issues of radiation, and the high cost of rad-hardened elections, I am wondering why the propellant tanks are not used more to shield the elections. For example, encase the sensitive elections into a small balder that is suspended in the center of the main propellant tank. Just wondering if its mostly a cost/complexity issue or if there is something more fundamentally wrong with this observation.