A successful Japanese trial of a ramjet engine designed for Mach‑5 aircraft

Technical feasibility of Mach‑5 flight

• Commenters stress that an engine alone doesn’t make 2‑hour transpacific flights feasible; materials, thermal management, and airframe design are major unsolved issues.
• Hypersonic air‑breathing propulsion (ramjet/scramjet) is described as much harder than rocket-based hypersonic flight: internal shockwaves and heat loads inside the duct are key constraints.
• Some think hypersonic air‑breathing is “nowhere near” solved; others point to rumored programs (e.g., SR‑72, Meteor missile) as evidence the puzzle may be cracked for military-scale craft.
• Noise and structural vibration are expected to be significant for passengers, even at high altitude, due to transmission through the fuselage.

Passenger travel vs. missiles

• Many argue the obvious first application is hypersonic missiles, not passenger jets, given complexity, heat, and startup-speed requirements.
• Ramjets need high initial speeds; practical designs likely require separate propulsion (e.g., rockets or conventional jets) to get to ignition speed.
• Several posts suggest the “Mach‑5 passenger jet” narrative is partly cover or aspirational; the real driver is seen as cruise-missile tech and advanced fighters/interceptors.

Economics and market

• Concorde is repeatedly cited: technically impressive but economically marginal, with limited routes, high fuel burn, and sonic boom restrictions.
• Some say today’s world—more ultra‑wealthy, more long‑haul Asia–Pacific traffic—could support a boutique supersonic/hypersonic market, especially for business jets and key executives.
• Others argue there are no economies of scale; certification, maintenance, training, and separate fleets make commercial viability unlikely, even with high ticket prices.

Alternatives: suborbital and existing air travel

• Suborbital hops are proposed as more sensible for extreme speed: shorter global travel times, avoidance of sustained hypersonic heating, and a brief weightless experience.
• Discussion highlights that door‑to‑door time is dominated by ground logistics and airport security; some see this as the real bottleneck, others say trusted-traveler programs already minimize this for the target clientele.

Environment, safety, and politics

• Environmental impact is a major concern; some see hypersonic passenger travel as incompatible with climate goals.
• Several view this primarily as part of a broader hypersonic weapons race and missile defense ecosystem, not a civil aviation revolution.