Based on publicly available information, it’s become increasingly clear that there are at least two (and likely three) hypersonic aircraft in active development within the secretive confines of American R&D facilities. If (or maybe more appropriately, when) such a platform enters service, it promises to upend the hypersonic arms race that America has been consistently characterized as losing to near-peers in China and, to a lesser extent, Russia.

Dr. Chris Combs on Twitter: https://twitter.com/DrChrisCombs

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Further Reading:
Hypersonic Arms Race: https://www.sandboxx.us/blog/is-america-really-losing-the-hypersonic-arms-race/
X-20 Dyna-SOAR: https://www.sandboxx.us/blog/x-20-dyna-soar-americas-hypersonic-space-bomber/
Hypersonic Missile Problems: https://www.sandboxx.us/blog/high-speed-hype-the-problems-with-hypersonic-missiles/
Darkstar and the SR-72: https://www.sandboxx.us/blog/is-there-a-real-secret-aircraft-behind-lockheed-and-top-guns-darkstar/
Interview w/ Hermeus' Skyler Shuford (Video): https://youtu.be/vaz7wAHzOR4
Darkstar China story: https://www.sandboxx.us/blog/top-gun-maverick-producer-says-darkstar-looked-so-real-china-moved-spy-satellite-to-see-it%EF%BF%BC/
Russian Hypersonic Hoax: https://www.sandboxx.us/blog/why-calling-russias-kinzhal-a-hypersonic-missile-is-a-stretch/

Citations:
X-15 Neil Armstrong: https://theaviationgeekclub.com/the-story-of-neil-armstrongs-x-15-test-flight-that-bounced-off-the-atmosphere/#:~:text=On%20Apr.,angle%20of%20attack%20during%20pullout.
Neil Armstrong NASA: https://www.nasa.gov/centers/glenn/about/bios/neilabio.html
Richard Hallion quote: https://www.defensemedianetwork.com/stories/what-might-have-been-x-20-dyna-soar/
Hypersonic missile costs: https://www.bloomberg.com/news/articles/2021-11-12/hypersonic-sticker-shock-u-s-weapons-may-run-106-million-each
Russian and Chinese hypersonics in service: https://www.sandboxx.us/blog/here-are-all-hypersonic-weapons-russia-and-china-have-in-service/
Combined cycle engines: https://www.colorado.edu/faculty/kantha/sites/default/files/attached-files/russell.pdf
Mayhem contracting docs: https://www.thedrive.com/the-war-zone/mayhem-hypersonic-strike-recon-jet-contract-awarded-to-leidos
Leidos contract award: https://www.leidos.com/insights/leidos-awarded-334m-air-breathing-hypersonic-system-contract
Scramjet propulsion: https://upcommons.upc.edu/bitstream/handle/2099.1/20295/Technical%20Report.pdf
X-43A: https://ntrs.nasa.gov/api/citations/20080017221/downloads/20080017221.pdf
X-51 Waverider: https://www.google.com/search?q=x-51+nasa&oq=x-51+nasa&aqs=chrome..69i57j0i390l4.2672j0j9&sourceid=chrome&ie=UTF-8
Turbofan function: https://www.grc.nasa.gov/www/k-12/airplane/aturbf.html
Hermeus Darkhorse: https://www.hermeus.com/darkhorse
Hermeus Air Force contract: https://www.aflcmc.af.mil/news/article-display/article/2720331/air-force-awards-contract-to-hermeus/
Raytheon Hermeus investment: https://www.rtx.com/news/news-center/2022/05/12/raytheon-technologies-venture-capital-group-invests-in-hermeus
Hermeus F-100 announcement: https://www.hermeus.com/press-release-f100-engine
Hermeus turbojet/ramjet transition: https://www.sandboxx.us/blog/hypersonic-firm-hermeus-proves-their-mach-5-jet-engine-works/
Archive of LM's SR-72 website: https://web.archive.org/web/20180119145223/https://www.lockheedmartin.com/us/news/features/2015/sr-72.html
PopSci SR-72 Cover Story: https://www.popsci.com/inside-americas-next-spyplane/
Aviation Week/Pop Mech report on SR-72 demonstrator: http://aviationweek.com/defense/amid-sr-72-rumors-skunk-works-ramps-hypersonics
O'Banion Quote: https://jalopnik.com/lockheed-martin-may-be-closer-to-completing-the-hyperso-1821894586
Follow up by Bloomberg: https://www.bloomberg.com/news/articles/2018-01-16/america-s-fastest-spy-plane-may-be-back-and-hypersonic#xj4y7vzkg