Why empty VLS cells usually mean a return to port

Can vertical-launch systems be reloaded while underway?

In theory some U.S. ships can, but in practice it is rarely exercised and most navies would need to reload in port.

Some U.S. ships have retained an ability to reload at sea, though the practice is neither routine nor common elsewhere. In a real campaign, a depleted VLS magazine therefore creates a logistics and basing problem. Missile capacity depends on the availability of secure ports and replenishment infrastructure as well as the number of installed cells.