In 1948, the French air ministry issued a specification that read like science fiction: a point-defense interceptor capable of climbing to 60,000 feet in under three minutes and intercepting Soviet bombers at Mach 1.8. The problem was that no jet engine of the era could do it. Turbojets throttled up too slowly and starved for oxygen above 50,000 feet. Rockets had the thrust but burned through propellant in under two minutes.

SNCASO's answer was the SO.9000 Trident — a knife-shaped aircraft with a rocket motor in the tail and two small turbojets mounted on the wingtips. The turbojets got it off the runway and provided cruise power. The SEPR 481 tri-chamber liquid-fuel rocket, burning nitric acid and furaline, lit off for the climb and dash. Each of its three chambers could be fired independently, giving the pilot a crude form of throttle control on a rocket engine — a genuinely novel piece of engineering.

First flight was 2 March 1953 at Melun-Villaroche, with test pilot Jacques Guignard. The follow-on SO.9050 Trident II flew on 19 July 1955 and, on 3 May 1957, pilot Charles Goujon took it to Mach 1.96 at 60,000 feet — roughly 1,300 mph. It could climb to 65,000 feet in under three minutes, faster than anything in NATO service. The French air force ordered six pre-production Trident IIs.

Then it fell apart. The reasons were mundane and cumulative:

• Propellant handling was miserable. Red fuming nitric acid ate through ground equipment, dissolved seals, and required men in rubber suits to load. Turnaround took hours.
• Charles Goujon died on 21 May 1957 when a Trident II's rocket exploded during ground testing — a burn-through in one of the SEPR chambers.
• Dassault's Mirage III flew in 1956 on a single Atar turbojet with an optional SEPR rocket pack. It couldn't match the Trident's climb, but it could do everything else adequately, land at any airfield, and didn't require a hazmat crew.
• The interceptor mission itself was being overtaken by surface-to-air missiles — the Bloodhound and Nike Hercules were arriving in numbers.

The program was cancelled in 1958. The airframes went to museums. The Trident had been technically superior in its narrow envelope and operationally impossible everywhere else.

Why revisit it now? Every reason the Trident failed has been engineered away:

• Hydrogen peroxide and hybrid rocket motors (as flown by Virgin Galactic and various sounding rockets) give you throttleable, storable, non-toxic rocket thrust without the nitric acid nightmare. Ground crews don't need rubber suits.
• Modern turbofans — even small ones like the Williams FJ44 or the Pratt F135's core technology — deliver far better thrust-to-weight than the 1950s Turbomeca Marboré that pushed the Trident.
• The mission is back. Hypersonic glide vehicles and high-altitude cruise missiles are exactly the threat the Trident was designed to catch: fast, high, and requiring an interceptor that can climb faster than a conventional fighter. The US Air Force's stalled "NGAD" program and DARPA's "Glide Breaker" both circle the same problem the Trident solved 70 years ago.
• Composite airframes handle sustained supersonic heating that killed titanium-poor 1950s designs.

A mixed-power interceptor — turbofan cruise, hybrid rocket burst — could reach 80,000 feet in two minutes and loiter on jets for an hour. The Trident's architecture was correct. It just needed 2020s materials, propellants, and mission demand. All three now exist.