Claiming that “substantial progress” is being made toward flying an unmanned, vertical takeoff and landing (VTOL), tailsitter aircraft from a naval ship, the U.S. Defense Advanced Research Projects Agency (Darpa) recently directed Northrop Grumman to build a second test vehicle. The contractor also announced that the so-called “Tern” program is gaining momentum.

Under a third phase of the Tern program, Northrop Grumman will build two full-scale flying-wing aircraft featuring twin, contra-rotating, nose-mounted propellers. If the Navy eventually adopts it, the VTOL aircraft would provide “organic, persistent, long-range reconnaissance, targeting and strike support” from small-deck ships such as frigates and destroyers. The name Tern was previously used as an acronym for Tactically Exploited Reconnaissance Node.

Darpa and the Office of Naval Research signed a memorandum of agreement in 2014 to share responsibility for the development and testing of a Tern demonstrator system. The Marine Corps Warfighting Laboratory is also supporting the program. Earlier plans called for flight tests as early as next year, but Darpa now expects that flight testing at sea will take place in late 2018.

The agency awarded Northrop Grumman an “other transaction agreement” to build a Tern demonstrator in December 2015. In a press release on November 28, the contractor said it recently surpassed two key program milestones. In mid-October, Northrop Grumman completed a critical design review (CDR) of the Tern’s GE Aviation turboshaft engine—a model that it declined to identify. According to Darpa, “This type of engine was chosen because it is mature and powers multiple helicopter platforms currently in use.” A CDR of the Tern’s vehicle management system followed; this provided approval of the hardware and software enabling the aircraft to launch and land vertically and transition to horizontal flight.

On November 17, Darpa said it will fund a second test vehicle. “Darpa has been thinking about building a second Tern test vehicle for well over a year,” said Dan Patt, the agency’s Tern program manager. “Adding the second technology demonstrator enhances the robustness of the flight demonstration program and enables military partners to work with us on maturation, including testing different payloads and experimenting with different approaches to operational usage.”

Since work on Tern’s third phase started earlier this year, the program has finished fabricating major airframe components, and plans call for completing assembly early next year, Darpa said. “Numerous modifications” of the GE engine have been tested. A software integration test station opened this summer to support software development; it uses high-fidelity simulation tools to test aircraft control software in all phases of flight.

Wind tunnel testing of a one-fifth scale model of the aircraft is about to begin at the NASA Ames’ National Full-Scale Aerodynamics Complex at Moffett Field, Calif., Darpa said.

The current program schedule calls for integrated propulsion system testing to begin “in the first part of 2017,” followed by ground-based testing in early 2018 and at-sea flight tests later that year. “We’re making substantial progress toward our scheduled flight tests, with much of the hardware already fabricated and software development and integration in full swing,” said Darpa Tactical Technology Office director Brad Tousley.