Few X-planes have ever led directly to an operational aircraft. Many of the technologies they pioneered have found their way into production aircraft but often after a gap of many years, even decades—an interregnum known as the “valley of death.”

• Shepard to fly stealthy hybrid-electric unmanned aircraft
• Gremlins will ready air-recoverable UAV for operational use

In a bid to bridge the gap between technology demonstration and full-scale development, DARPA is exploring the concept of an “X Prime” program that would fit between an experimental X-plane and a Y-plane prototype and help transition capabilities from research to operation.

“There is this ‘valley of death’ we famously talk about where you are doing an X-plane and want to focus on the technology. You want to minimize the cost and maximize the number of things you can tackle, so you pick that pure technology piece,” said Michael Leahy, director of DARPA’s Tactical Technology Office (TTO).

“If you’re successful with that, there’s usually a gap because you should never build an acquisition program on a DARPA S&T [science and technology] effort. We may fail, and then you’re left holding the bag on those resources,” he told the American Institute of Aeronautics and Astronautics Aviation 2021 virtual forum on Aug. 5. “So how do you bridge that gap?”

DARPA’s X Prime concept calls for a program lasting a couple of years that would follow the X-plane and in which “you look at how can you still do some S&T and prepare the way for a Y-plane,” Leahy said.

Whereas an X-plane demonstrates high-risk, high-payoff technologies and incorporates off-the-shelf components to reduce cost and risk in other areas, a Y-plane supports a program of record and incorporates operational requirements and systems, including software and sustainment.

Leahy used the example of Boeing’s X-45A unmanned combat air vehicle demonstrator, an early 2000s program he led at DARPA that was technically successful but never led to an operational vehicle. “We’d like to learn lessons [from an X-plane] and make changes,” Leahy said. “Take [the X-45A]. It has the wing in the wrong damn place.” But plans for follow-on vehicles never reached fruition.

In DARPA’s vision, an X Prime program would tackle system-level integration risks and provide an interim capability to meet an urgent operational need. Such a program could also yield operational subsystems for a minimum viable product (MVP) that could be fielded.

The Defense Department’s advanced research agency is exploring the X Prime concept with two different projects. One is the Shepard program (for Series Hybrid-Electric Propulsion Aircraft Demonstrator) to develop an efficient hybrid-electric system and integrate it into an innovative aircraft design that includes essential operational considerations and mission system components.

“Shepard is taking a pure X-plane, the Great Horned Owl, which was a great demonstration of a capability for hybrid-electric propulsion but wasn’t something you can use in the battlespace,” Leahy said. “We’re going to wrap a new skin around it to give it the survivability that it needs. And we’re going to try to demonstrate we can do that quickly.”

Great Horned Owl was an early-2000s Intelligence Advanced Research Projects Activity (IARPA) program to develop an ultraquiet, high-efficiency unmanned aircraft with ducted-fan hybrid-electric propulsion. According to information first obtained by The War Zone, the project resulted in the XRQ-72A flying-wing UAV design developed by Northrop Grumman subsidiary Scaled Composites.

Shepard demonstrates one approach to X Prime programs that is still S&T-focused but with an emerging-mission push. “There are three things you can possibly control—cost, schedule and performance—and you can control maybe two of them,” Leahy said.

In this approach, “I’m going to fix the cost,” he said. “I’m going to fix the schedule, and say:  ‘Meet me on the range in 20 months to show me what you can do.’ And I’m going allow you to vary the performance in that MVP kind of way to get some initial operational capability.

“We need to get this concept of a minimum viable product,” he continues. “We’re never going do that exactly the same way as the software industry, but how to get things done quicker is important for us to figure out. It’s both on the industry side and being disciplined, and on the user side and not changing requirements. I have a fixed amount of money, a fixed time, and I can vary the performance if I need to.”

The second approach to X Prime is “more of an acquisition pull,” he said. This is to be demonstrated by DARPA’s Gremlins program. The S&T goal of Gremlins is to demonstrate the airborne recovery of an air-launched unmanned aircraft. If that is successful, DARPA plans to add mission systems to the Gremlins air vehicle in an X Prime phase and demonstrate an operational capability that could be fielded.

Led by Leidos company Dynetics, Gremlins has yet to demonstrate an airborne recovery because of aerodynamic interaction between the Kratos-developed X-61A air vehicle and the stabilized docking bullet deployed by the roll-on/roll-off recovery system onboard the Lockheed C-130 host aircraft.

Recovery attempts in November 2020 came within inches of success, but “sometimes inches might as well be miles,” Leahy said. Dynetics believes it has solved the problem, and more flight tests are planned this fall. “We’re going to take one more shot at it,” he added. “If we fail this time, then we will stop and reset, and say: ‘What do we need to do to attack that particular problem?’”

Leahy also highlighted other TTO X-plane programs, including LongShot, which is expected to demonstrate an air-launched UAV that can carry air-to-air missiles closer to their targets. “If I have fifth-gen fighters, but they can’t get close enough to the fight, what if I take an airborne UAV that has the survivability to take those weapons into the environment where they can be highly effective?” he said.

In February, General Atomics, Lockheed Martin and Northrop Grumman were awarded contracts for Phase 1 of LongShot. DARPA also has begun to award Phase 1 contracts for the CRANE program (for Control of Revolutionary Aircraft with Novel Effectors) to design an X-plane enabled by active flow control. Boeing’s Aurora Flight Sciences and Lockheed Martin are the first participants to be selected.

Editor's note: This article was updated to include more information about the first participants in Phase 1 of DARPA's CRANE program.