Nagoya scientists make scramjet work at Mach 3–4, closing the gap for hypersonic engines
Researchers at Nagoya University have developed a device that allows a scramjet—a supersonic combustion ramjet engine—to ignite and sustain stable combustion at flight speeds of Mach 3 to 4. Scramjets were previously considered viable only at speeds above Mach 5, where the incoming air is hot enough to self-ignite fuel. At lower speeds, the air temperature drops significantly, making spontaneous ignition impossible without an external energy source.
This created a gap in propulsion technology: turbo-ramjet engines can operate up to roughly Mach 3–4, while scramjets only work from Mach 5 upward. The Nagoya team chose not to push the upper limit of turbo-ramjets but instead to lower the minimum operating speed of scramjets, effectively bridging the gap.
The key innovation is a rotating detonation torch (RD torch). Unlike conventional igniters, this device continuously injects hot products of detonation combustion into the scramjet's combustion chamber. In experiments, the team successfully ignited hydrogen fuel and maintained stable combustion while the main airflow through the chamber remained supersonic—a condition that had previously been extremely difficult to achieve at lower Mach numbers.
The RD torch produces a steady stream of high-temperature gas from a continuous detonation wave, which reliably triggers and sustains the combustion of hydrogen in the supersonic flow. The researchers demonstrated that the approach works under conditions simulating flight at Mach 3–4, precisely the range where scramjets had been unable to operate.
This breakthrough could eventually lead to hypersonic vehicles capable of seamless operation across a wide speed spectrum—from subsonic takeoff through transonic acceleration to hypersonic cruise—using a single engine type. By closing the performance gap between turbo-ramjets and scramjets, the technology brings practical hypersonic air transport one step closer to reality. The team plans to further refine the RD torch for integration into full-scale engine prototypes.
