In 1964 — the same year the SR-71 Blackbird made its first flight — the CIA quietly asked American aerospace firms a simple question: what comes after Mach 3? The Blackbird's titanium airframe was already known to be vulnerable to Soviet SA-5 surface-to-air missiles, and satellite reconnaissance couldn't yet provide the on-demand coverage the agency wanted. The answer, developed over the next five years under two related programs, was one of the most audacious aircraft ever seriously designed: Project Isinglass (McDonnell, 1964–1965) and its successor Project Rheinberry (McDonnell, 1965–1968).

Isinglass was the smaller of the two — a rocket-powered reconnaissance glider air-launched from a B-58 Hustler, boosting to roughly Mach 4–5 and 100,000 feet. The CIA looked at it, decided it wasn't enough of a leap, and told McDonnell to go bigger. What came back was Rheinberry.

Rheinberry was a manned, single-pilot, rocket-powered reconnaissance vehicle launched from atop a modified B-52. Its design specs, declassified in 2013 by the CIA:

• Cruise speed: Mach 20 (roughly 13,000 mph)
• Altitude: 150,000 feet — twice the SR-71's ceiling
• Range: 3,700+ nautical miles
• Propulsion: Pratt & Whitney XLR-129 staged-combustion rocket engine, 250,000 lbf, using LOX/LH2
• Airframe: Ablative-cooled Rene 41 nickel superalloy leading edges, transpiration cooling on the nose
• Reentry profile: Skip-glide, similar to Sänger's silbervogel — pull up out of the atmosphere between passes to shed heat

This was not a paper airplane. McDonnell built full-scale mockups. Pratt & Whitney actually built and test-fired the XLR-129 engine at 250,000 pounds of thrust — it remained, for decades, the highest-performance staged-combustion hydrogen engine ever tested in the West. (The RD-0120 that powered the Soviet Energia rocket used similar principles.) The heat-shield materials were tested in arc-jet facilities at Ames.

Why it died: Three things happened between 1966 and 1968. First, the KH-9 HEXAGON satellite program matured, promising near-real-time film-return imagery from orbit. Second, the CIA's National Reconnaissance Office (NRO) argued — correctly — that satellites couldn't be shot down by SAMs. Third, Rheinberry's projected cost hit $5 billion in 1968 dollars (roughly $45 billion today), and Vietnam was eating the defense budget. President Johnson cancelled Rheinberry in early 1968. The XLR-129 program limped along until 1970, when its technology was rolled into what became the Space Shuttle Main Engine.

Why it could work now: Every technical objection to Rheinberry has been retired. Transpiration cooling is standard in modern rocket engines. Additive manufacturing lets us print single-piece Inconel combustion chambers that would have taken McDonnell 18 months to hand-fabricate. The XLR-129's staged-combustion principles are now routine — SpaceX's Raptor and Blue Origin's BE-4 both use closed-cycle designs. Autonomous flight control eliminates the pilot and the life-support mass penalty. And the reconnaissance mission, thought dead in 1968, is very much alive again: satellites have predictable orbits, and modern anti-satellite weapons make LEO reconnaissance vulnerable in ways it wasn't in 1970. A hypersonic air-launched glider that shows up unpredictably, images a target, and is gone in twelve minutes is exactly what the DoD's current "Mayhem" and "Blackswift" programs are trying to build — 55 years after Rheinberry solved most of the problem on paper.