The US Air Force is currently operating in secret an unmanned stealthy flying wing built by Northrop Grumman and tailored for the same stealthy strategic reconnaissance role that the Quartz and Tier III would have filled. Informally dubbed “RQ-180” because it was referred to by this label in a December 2013 issue of Aviation Week and Space Technology (although that may not be its true designation), it first flew on August 3, 2010 and nine service test vehicles were built before production began in 2016-2017. The “RQ-180” draws upon Northrop Grumman’s SensorCraft design studies conducted under contract from the Air Force Research Laboratory, and it is apparent from Northrop Grumman financial records that NG won a secret USAF competition initiated in 2007 for a stealthy strategic reconnaissance drone drawing heavily upon the requirements laid out for the SensorCraft program. Northrop Grumman’s experience with development and flight testing of the “RQ-180” probably helped the company with design of the flying wing bomber proposal that won the LRSB competition in September 2015 and became the B-21 Raider.

Although the US Air Force has yet to publicly unveil the “RQ-180”, ground-based sightings of the drone flying at high altitudes in 2020 and 2021 show that the USAF may lift the cloak of secrecy surrounding this UAV program sooner of later. In short, the Northrop Grumman “RQ-180” fills the penetrating ISR niche left vacant by the SR-71’s retirement in the 1990s, and notwithstanding the fact that rumors in the early 1990s about the Air Force deploying a hypersonic follow-on to the SR-71 turned to be untrue due to the “donuts-on-a-rope” contrails being dismissed as being produced by airliners and the fact that scramjet and turbine-based combined cycle engine technology was in its infancy in the 1980s and 1990s, Lockheed did work on designs for a hypersonic spyplane for the USAF in the late 1970s and 1980s, including the methane-fueled Mach 5 Penetrator.

References:

Merlin, P.W., 2023. Dreamland: The Secret History of Area 51. Atglen, PA: Schiffer Publishing. (available for purchase at Amazon)

https://www.forbes.com/sites/davidaxe/2021/09/07/americas-new-stealth-d…

https://aviationweek.com/defense-space/aircraft-propulsion/possible-pho…

https://premium.globalsecurity.org/intell/systems/rq-180.htm