between 117 and 128 VLS cells. However, the final DDG-1000 design provides only 80 cells. Zumwalt uses MK.57 Peripheral Vertical Launching System (PVLS) cells which are larger than the Mk.41 cells found on most American destroyers. Each VLS cell can be quad packed with RIM-162 Evolved Sea Sparrow Missiles (ESSM). This gives a maximum theoretical load of 320 ESSM missiles. The ESSM is considered a point defense weapon not generally used for fleet area defense. The Zumwalt-class destroyer is not an Aegis system. It instead uses the class-unique Total Ship Computing Environment Infrastructure (TSCEI) integrated mission system. The Mk 57 PVLS is capable of accommodating all Standard missile types. It has not been publicly stated if the TSCE will be modified to support the Standard missile or the ballistic missile defense mission. Naval fire support role The design concept for the Zumwalt class developed from the "Land Attack Destroyer (DD 21)" development effort. A primary goal for DD 21 was to provide sea-based fire support for on-shore troops as part of the force mix that would replace the retiring s as mandated by Congress. There was considerable skepticism that the Zumwalt class could succeed in this role. The Zumwalt class was intended to provide naval surface fire support (NSFS) using the AGS and additional land attack using Tomahawk missiles from its PVLS launchers. As deployed, the Zumwalt class cannot provide NSFS since there only 90 rounds of ammunition are available that are compatible with the AGS. The Zumwalt class were re-purposed as surface attack vessels and are no longer intended for use as land attack destroyers. Tumblehome design stability The stability of the DDG-1000 hull design in heavy seas has been a matter of controversy. In April 2007, naval architect Ken Brower said, "As a ship pitches and heaves at sea, if you have tumblehome instead of a flare, you have no righting energy to make the ship come back up. On the DDG 1000, with the waves coming at you from behind, when a ship pitches down, it can lose transverse stability as the stern comes out of the water – and basically, roll over." The Navy had decided not to use a tumblehome hull in the CG(X) cruiser before the program was canceled, which may suggest that there were concerns regarding Zumwalts seakeeping abilities. However, the tumblehome hull proved seaworthy in a 1/4-scale test of the hull design