The legendary competition between the branches of the U.S. armed forces has taken on a sustainable twist. Take portable solar power, for example. The Marines just introduced a portable solar power system this spring, and a few weeks later the Air Force kicked in with a portable solar system of its own. Now the U.S. Army has entered the fray with a portable battery recharging kit called the Rucksack Enhanced Portable Power System (REPPS), which features a 62-watt solar panel "blanket" tucked into a backpack. The system was just deployed in Afghanistan this summer.