Castelion, a startup trying to build a hypersonic weapon for the Pentagon, tested its system for the first time, it told Reuters on Monday, as a growing group of small hypersonic arms makers challenge large defense contractors with less expensive, rapidly produced products. The United States and China are engaged in an arms race to develop the most lethal hypersonic weapons, which travel in the upper atmosphere at more than five times the speed of sound and are designed to evade traditional defenses. Private U.S. company Stratolaunch on Saturday said it had successfully completed the first powered flight of its Talon-A reusable hypersonic test vehicle.