Caravan airships are unarmed vessels designed purely for the transport of goods. They dominate the stratosphere as the most common airship type by far. Caravan airships usually have no armour aside from the few millimetres of duralumin the envelope and deck provide. Though their engine type and capabilities are very varied, the Wright R-1820 Cyclone is somewhat more common due to its extremely efficient altitude high-altitude turbocharger. Size and shape may also vary, but generally all these airships carry from 3 to 12 tons of cargo. Caravan airships may be escorted by other armed airships or travel alone. Sometimes they may function as logistics trains, carrying fuel and ammunition for larger combat airships.