Future Watch: Tern


(Photo by Tom Grey)

By Gil Graff

Tern is a joint Defense Advanced Research Projects Agency (DARPA) and Office of Naval Research (ONR) program to develop and demonstrate unmanned air vehicle technologies  capable of small-deck takeoff and landing, autonomy, and sustained operations at very long distances from their host ships. The program is named after the family of sea birds noteworthy for their ability to fly for long periods while migrating thousands of miles.

The program, initiated in 2014 and expected to be complete  in 2018, aims to provide situational awareness at distances well beyond the ranges achievable with current ship-based sensors and radars. In many situations, destroyers (DDGs) and littoral combat ships (LCSs) must operate without  support from land- or carrier-based aircraft. Today, small-deck aviation resources, such as those aboard DDGs and LCSs, are limited to rotary-wing and very small fixed-wing aircraft. Helicopters, while capable of launch and recovery from these platforms, have limited endurance and a much smaller mission radius when compared with carrier-based, fixed-wing aircraft. With a rotary-wing sustainable mission radius of less than 200 nautical miles, significant portions of the global land mass currently are inaccessible by air vehicles employed from small-deck platforms.

The Tern program will develop and demonstrate technologies  for a new class of medium-sized aircraft that would provide significantly enhanced Navy intelligence, surveillance, and reconnaissance (ISR) capabilities by using a fixed-wing, long-range/ endurance solution that would operate from DDG- and LCS-class ships. For ISR missions, the program goal is to enable a persistent orbit at 600 nautical miles with 500 pounds or more of payload. The objective system must be capable of executing autonomous  launch and recovery with minimal manpower  and be compatible  with small-deck aviation flight operations. Compatibility requirements include the ability to transit between the hangar and flight deck, to store and maintain the air vehicle, and to store spares and support equipment.

Tern builds on the Navy’s past technology investments in automated launch and recovery and autonomous  flight control, and on DARPA’s work in air vehicle configuration advancements. The program will push beyond the limits of the vertical takeoff and landing (VTOL) aircraft that operate from today’s ships. Current VTOL aircraft are inefficient cruise vehicles and consequently have limited flight endurance. Long-endurance and long-range fixed-wing aircraft have been demonstrated but require long runways for takeoff and landing. The challenge is to incorporate, in a single configuration, efficient flight, as well as shipboard VTOL. Previous studies have shown that the path to achieving both objectives is a vehicle that includes large wings for endurance, a large propeller swept area, and a propulsion system that provides control  authority when operating around the ship deck.

To fit within ship hangars, the vehicle must have a very short fuselage with significant folding ability. Unlike helicopters, which use rotor-based controls, and conventional  airplanes, which use relatively small propellers, the Tern vehicle wing-and-vertical- based controls must operate in the complex air flows in the large rotor wake. The transitions between launch and cruise flight and between cruise and recovery require control  at large angles of attack. The large angle of attack aerodynamics is similarly difficult to predict. The complex aerodynamics unique to Tern vehicles result in major design, simulation and test challenges. Precision landing with near-zero roll in heavy sea states is particularly challenging on small deck ships, which can have significant pitch, roll, yaw, and heave motions. Onboard power must provide high-thrust operation for takeoff or landing and very efficient low-throttle operation for long endurance at cruising speeds.

The complexity  of aerodynamics and the wind-over-deck environment are not easily replicated in subscale laboratory experiments. Full-scale flight testing at sea, therefore, is required to demonstrate risk reduction. In 2018, the Tern program will culminate in launch, recovery, and flight testing of a full-scale demonstrator  vehicle consistent with a tactical vehicle in size, mass, aerodynamics, propulsion, and autonomous  controls. Although not a production prototype, the Tern vehicle will demonstrate the characteristics required to evaluate future applications of this new class of aircraft as an innovative solution to current and future Navy ISR mission requirements.

About the Author:

Gil Graff is a program officer with the Office of Naval Research and deputy program manager of Tern.