Airships (sometimes referred to as dirigibles, or more commonly, zeppelins) are an advanced type of aerostat, or lighter-than-air vehicle that utilizes rudders and propellers to provide maneuverability and thrust while airborne. They use large, internal ‘cells’ of lighter-than-air gas to stay aloft, typically helium; hydrogen was used in pre-WWII German zeppelins – only because of an American embargo on helium at the time – and was considered highly dangerous due to its flammability. Most modern airships are commonly powered by everything from solar panels (using nanotech embedded in the outer skin) and vent-style wind turbines, to thorium reactors (MSRs).