The U.S. Navy is preparing to replace what some describe as the “deadliest plane in the world”—the E-6 Mercury. Designed to act as a flying communications link between the U.S. government and military forces worldwide, the E-6 Mercury is tasked with relaying orders to Ohio-class nuclear missile submarines on patrol, including orders to launch missiles. Unlike the current plane, which is based on a commercial airliner, the new plane would be based on the C-130J Super Hercules military transport aircraft.

The Navy, according to Defense News, is preparing to release a requirements list for the new plane, dubbed E-XX. The prefix “E” stands for electronic, a broad category that includes electronic attack, propaganda distribution, and other roles that involve transmitting on the electromagnetic spectrum. The service wants three test aircraft relatively quickly, with a contract for nine more by 2025.

The twelve new aircraft would replace the 16 E-6s currently in service.

The E-XX is the next generation TACAMO aircraft, an acronym that stands for “take charge and move out.” The planes are designed to relay Emergency Action Messages (EAMs) to submarines at sea. The submarines, armed with nuclear-tipped ballistic missiles, are tasked to disappear into the oceans and evade detection. In the event of a surprise nuclear attack on the United States, the submarines can surface to launch a devastating counterattack. By providing this capability, they deter adversaries from launching an attack in the first place.

TACAMO planes act as a flying message relay service, authenticating and then relaying EAMs between the National Command Authority, anyone who has the authorization to give orders to U.S. nuclear forces, and submarines at sea. In peacetime that is the President of the United States, but if the President is killed, incapacitated, or goes missing during an attack, that authority could devolve to someone else in the Cabinet or to military control. TACAMO planes ensure that the National Command Authority can command nuclear forces even in the midst of a nuclear war.

The 16-strong TACAMO fleet is divided into two squadrons, VQ-3 “Ironmen” and VQ-4 “Shadows,” all controlled by the 1st Strategic Communications Wing located at Tinker Air Force Base, Oklahoma. E-6s also operate from alert facilities at Travis Air Force Base, California and Naval Air Station Patuxent River, Maryland, where they can fly nearer to ballistic missile submarines on patrol.

One TACAMO plane is usually in the air at all times, and it’s not uncommon for up to six to be in the air at once. Despite alarmist reports on social media, the number of TACAMO planes in the air does not necessarily indicate any sort of imminent, nuclear-related crisis.

The older E-6 Mercury is based on the Boeing 707, one of many military planes based on the 1960s-era civilian jetliner. The Navy wants the new E-XX TACAMO to be based on the C-130J Super Hercules instead. Lockheed Martin’s entry for the E-XX competition is the EC-130J TACAMO, a plane it is co-developing with Raytheon and Northrop Grumman. It is not clear which other defense contractors might throw their hats in the ring, though one obvious possibility is Boeing.

TACAMO aircraft are some of the most important in the entire U.S. military arsenal, and their job as an essential link in nuclear command and control means they really do live up to their hype as the “deadliest planes in the world.” No matter how powerful a surprise attack is on the U.S. homeland, TACAMO aircraft ensure that surviving nuclear submarines can and will unleash a counterattack. In doing so, the planes also fulfill a greater mission: nuclear deterrence and keeping the peace.