"The Intelligence Surveillance and Reconnaissance (ISR) role for UAVs [unmanned aerial vehicles] is dependent upon the ability of the UAV to do its mission without the adversary being able to counter it. For many such ISR applications, the acoustic signature of the UAV alerts the adversary to the UAV’s presence and can interfere with the mission," IARPA explained in a public notice regarding the Great Horned Owl effort in 2011. "Battery powered UAVs are very quiet but lack endurance and payload capability. Better, more efficient, quiet power sources and propulsion techniques are needed to build next generation UAVs for ISR mission applications." The resulting XRQ-72A has a general planform reminiscent of other Northrop Grumman designs, including that of the B-21 Raider stealth bomber. It also has some broad external visual similarities to flying wing unmanned aircraft that other companies have developed over the years, including designs from Lockheed Martin.

USAF via FOIA A rendering of the XRQ-72A, along with various dimensional and other details.

The drone has a 30-foot wingspan, is 11.2-feet long measured from the nose to the ends of the wingtips, and is four feet tall when including the vertical wingtip stabilizers, according to the schematics we obtained via FOIA. Lockheed Martin's Skunk Works' X-44A, which The War Zone was first to report on and which the Air Force did not publicly acknowledge until 2018, also has a wingspan of around 30 feet. The X-44A represented an important "missing link" in the chronology of that company's advanced flying wing drone developments, including the RQ-170 Sentinel stealth drone. It's not clear how heavy the XRQ-72A is, but another AFRL document we received said that the Great Horned Owl program's requirements called for an air vehicle weighing between 300 to 400 pounds. The drone also appears to have at least some stealthy features, including its general shape and shrouded exhausts, and is made out of composite materials. Its most important feature, of course, is its hybrid propulsion system. A pair of fuel-powered generators inside the central fuselage produce the electricity that powers four ducted fan propulsors mounted on top of the aircraft's flying-wing fuselage. This is in some ways visually reminiscent of the general configuration of the Boeing X-48 series of experimental unmanned blended wing body demonstrators, but those aircraft used small turbojet engines.

USAF via FOIA An internal schematic and other more detailed views of the rendering of the XRQ-72A.

The schematics also show the XRQ-72A equipped with a typical ball-type sensor turret, which could include various types of video cameras. IARPA made no specific mention of what sensors it might be interested in any Great Horned Owl demonstrator being able to carry in its original 2011 announcement. It's not clear from the Air Force documents how the drone takes off or lands. There no landing gear present in the schematics, which could point to runway-independent operation using a catapult or rocket-assisted zero-length launch system. The Great Horned Owl program was initially focused simply on exploring technologies that could enable the development of a very quiet and efficient unmanned intelligence, surveillance, and reconnaissance drone. Hybrid propulsion concepts, which have been and continue to be of great interest to both military and commercial aviation communities, offer a potential path to significant improvements in fuel efficiency, reliability, and reduced acoustic signature over traditional aircraft propulsion concepts. AFRL is still experimenting with these ideas today, which you can read about in much greater detail in this past War Zone piece.

IARPA The Great Horned Owl program logo.

The XRQ-72A's exact capabilities are unclear. AFRL said that it expected the drone to have a "low acoustic signature at a speed of 50 KEAS [Knots Equivalent Air Speed]," according to the documents we obtained via FOIA. IARPA's stated goals in 2011 were a high-efficiency design that could be as quiet as 50 Phons at speeds up to 100 knots. A Phon is a unit of sound measurement equivalent to 1 decibel at a frequency of 1 kilohertz. Sound can be hard to measure. A typical conversation, where people are speaking without raised voices, or the hum of a dishwasher are typically described as being around 50 decibels loud, though that dissipates the further away you get from the source. A typical jet engine on a commercial airliner still registers around 130 decibels or more during takeoff, even at a distance of 100 feet. In 2012, IARPA did issue contracts to various companies to craft various ducted fan designs, as well as other propulsion system components as part of Phase 1 of the Great Horned Owl program. CRG and Xcelaero supplied hub-driven propulsors, while a firm called Theta Tech developed a hubless, or rim-driven design. As their name implies, hub-driven propulsors have a centrally mounted propeller inside the propulsor. A hubless design has fan blades mounted on the inside of a ring that rotates within the body of the propulsor. The schematic of the Northrop Grumman XRQ-72A shows hub-driven propulsors.

IARPA A factsheet on various Great Horned Own Phase 1 requirements and developments.