The US Navy's quieter way to spy, the Ion Tiger, just bested its own 2009 flight record with a key assist from liquid hydrogen. The unmanned aerial vehicle had previously relied on 5000-psi compressed hydrogen for fuel, but for its latest flight test the Naval Research team swapped that out for a new cryogenic tank and delivery system that relies on the liquid stuff; a choice made for the element's increased density. With that one significant change in place, the craft was able to outperform its last endurance run of 26 hours and two minutes by almost double, lasting 48...