Much of the overall internal arrangement was continued in the subsequent "Thresher"- and "Sturgeon"-class submarines. The "Skipjack"s' five compartments were called the Torpedo Room, Operations Compartment, Reactor Compartment, Auxiliary Machinery Space (AMS), and Engine Room. With the addition of a missile compartment, the arrangement of the first 41 US nuclear-powered ballistic missile submarines (SSBNs) was similar. The design was primarily single-hull, with a double hull around the torpedo room and AMS for ballast tanks. The design was improved on the "Thresher"s, the one-off , and subsequent attack submarines by relocating the torpedo room into the operations compartment via angled midships torpedo tubes to make room for a large sonar sphere in the bow. The "George Washington" class, the first SSBNs, were derived from the "Skipjack"s, with rebuilt from the incomplete first . The hull of "Scorpion" was laid down twice, as the original hull was redesigned to become the "George Washington". Also, the material for building was diverted into building , which delayed "Scamp"s progress.