The Transportation Security Administration is dropping nearly half a billion dollars on compact, next-generation body scanners to better detect concealed explosives, such as the newfangled underwear bomb recently seized in Yemen. The imaging machines, like other upgraded systems, hide a passenger’s nude body from TSA officers by displaying generic representations of appendages with suspect items flagged. Two five-year contracts, each worth $245 million, have been awarded to L-3 Communications Corp. and American Science and Engineering Inc., according to government databases.