Lockheed pushing $1 billion Mach 6 airbreather

https://www.flightglobal.com/news/articles/lockheed-pushing-1-billion-mach-6-airbreather-423198/

Lockheed Martin's unmanned SR-72 aircraft concept has surfaced again with renewed vigour, with company leadership now pushing a reusable, air-breathing hypersonic vehicle as an "affordable" way to validate a new propulsion concept for achieving speeds within the atmosphere between Mach 6.0 to Mach 20.