Hypersonic flight above Mach 5 has been demonstrated by NASA’s X-43A (Mach 9.6 in 2004) and by successful DARPA/Air Force HAWC air-breathing cruise-missile tests in 2021 and 2023, yet reliable, affordable operational weapons remain undeveloped.

Scramjets breathe atmospheric oxygen and can sustain high-speed cruise efficiently once underway, but cannot start from rest and must keep a flame stable in supersonic airflow; rockets carry their own oxidizer, enabling launch from standstill or space but at the cost of greater weight.

Many proposed hypersonic cruise missiles therefore combine a rocket booster for initial acceleration with a scramjet for the sustained cruise phase, with the choice of propulsion determined by mission requirements.

In November 2004, a small unmanned aircraft called the X-43A, built by NASA, flew at roughly Mach 9.6, nearly ten times the speed of sound. A rocket first carried it to the right height and speed, and then, for a few precious seconds, an engine that breathed the thin air around it took charge. Those seconds answered a question engineers had wrestled with for years: could such an engine truly work in flight? It could.

Military laboratories soon followed. In 2021, DARPA and the United States Air Force announced a successful flight of the Hypersonic Air-breathing Weapon Concept, or HAWC, an experimental cruise missile developed by Raytheon. Two years later, in 2023, a second HAWC design, this one from........

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