A ballistic missile submarine on patrol wants to stay deep and silent. That makes it hard to talk to: ordinary radio waves barely penetrate seawater. The US Navy’s answer, since the 1960s, has been an aircraft that unreels a wire antenna several miles long and transmits on very low frequency (VLF). The programme is called TACAMO, for “Take Charge And Move Out”, and today it is flown by the Boeing E-6B Mercury.

Why a Wire, and Why So Long

Radio waves lose strength rapidly in salt water, and the higher the frequency, the faster they fade. VLF signals, with wavelengths of many kilometres, penetrate far enough for a submarine to receive them just below the surface while trailing a receiving antenna. But a VLF transmitting antenna has to be enormous to radiate efficiently. On the ground, VLF stations use towers hundreds of metres tall. In the air, the only practical answer is a long wire. According to the Navy’s programme description, the E-6 can deploy a long trailing wire antenna of about 28,000 feet and a short trailing wire of about 5,000 feet; the two act together as a dipole.

Flying in Circles

A wire streaming horizontally behind an aircraft radiates poorly for this purpose. What the E-6B needs is a wire hanging as close to vertical as possible. So once the antenna is out, with a drogue on the end to stabilise it, the crew flies a tight, steep orbit at slow speed, close to the stall, typically above 20,000 feet. As the aircraft circles, the end of the wire drops and hangs almost straight down beneath it. As The War Zone has described, the E-6B can repeat these orbits for hours while it transmits. Navy engineers proved the effect of this orbital manoeuvre on the radiated power decades ago.

VLF has a cost: the data rate is extremely low. It is suited to short, critical messages, such as Emergency Action Messages ordering a launch, not conversation.

From Hercules to 707 to Hercules Again

TACAMO began in 1962 as an interim system on converted C-130s, the EC-130G and EC-130Q, intended to fill the gap until a land-based extremely low frequency system could be built. When that ground system was judged not survivable enough, TACAMO became permanent. In the late 1980s the Navy chose the E-6A, a Boeing 707 derivative, to replace the Hercules; the upgraded E-6B also took over the Air Force’s airborne command post role, able to stay airborne for up to 72 hours with air refuelling. Now the mission is moving back to the C-130 family, in the shape of the E-130J Phoenix II.

What the Popular Clip Gets Right

The explanation usually given is right: the E-6B trails a wire about five miles long to send very low frequency messages to submarines underwater, and it flies tight circles so that the wire hangs nearly vertically.

Sources: Federation of American Scientists, E-6A/B Mercury programme description (US Navy Vision documents); Naval Air Development Center TACAMO history (1984), via the Navy Radio history archive; Tyler Rogoway, “Here’s Why an E-6B Doomsday Plane Was Flying Tight Circles Off the Jersey Shore Today”, The War Zone (13 December 2019).