Block III through SSN-791 (8 boats) make up the Third Block or "Flight" and began construction in 2009. Block III subs feature a revised bow with a Large Aperture Bow (LAB) sonar array, as well as technology from Ohio-class SSGNs (2 VLS tubes each containing 6 missiles). The horseshoe-shaped LAB sonar array replaces the spherical main sonar array which has been used on all U.S. Navy SSNs since 1960. The LAB sonar array is water-backed—as opposed to earlier sonar arrays which were air-backed—and consists of a passive array and a medium-frequency active array. Compared to earlier Virginia-class submarines about 40% of the bow has been redesigned. South Dakota (SSN-790) will be equipped with a new propulsor, possibly the Hybrid Multi-Material Rotor (HMMR), developed by Defense Advanced Research Projects Agency (DARPA). The Hybrid Multi-Material Rotor program is an attempt to improve the design and manufacturing process of submarine propellers with an aim of reducing the cost and weight of the propeller/rotor as well as improving overall acoustic performance. Block IV Block IV involved 10 boats. In 2013, execution of this 10-submarine contract was put in doubt by budget sequestration in 2013. The most costly shipbuilding contract in history was awarded on 28 April 2014 as prime contractor General Dynamics Electric Boat took on a $17.6 billion contract for ten Block IV Virginia-class attack submarines. The main improvement over the Block III is the reduction of major maintenance periods from four to three, increasing each boat's total lifetime deployments by one. The long-lead-time materials contract for SSN-792 was awarded on 17 April 2012, with SSN-793 and SSN-794 following on 28 December 2012. The U.S. Navy has awarded General Dynamics Electric Boat a $208.6 million contract modification for the second fiscal year (FY) 14 Virginia-class submarine, SSN-793, and two FY 15 submarines, SSN-794 and SSN-795. With this modification, the overall contract is worth $595 million. Block IV consists of 10 submarines. Block V Block V involves 10 boats and may incorporate the Virginia Payload Module (VPM), which would give guided-missile capability when the SSGNs are retired from service. The Virginia Payload Module will be included on Block V submarines starting with the second boat, SSN-803. The Block V boats with VPM are expected to triple the capacity of shore targets for each boat. Construction on the first two boats of this block was expected to begin in 2019 but was pushed back to