HAWTHORNE, Calif., July 25 — The Raptor engine family, developed and manufactured by SpaceX, represents a milestone in rocket propulsion: it is the third full-flow staged combustion engine in history and the first to power a vehicle in flight. Before Raptor, no engine of this type had ever flown. Only two designs had reached test stands — the Soviet RD-270 project in the 1960s and the Aerojet Rocketdyne Integrated Powerhead Demonstrator in the mid-2000s.
Strategic context and technical lineage
The achievement places SpaceX in a category by itself, but the engineering lineage is geopolitically telling. Other staged combustion engines — the RS-25 that powered the Space Shuttle, the Russian RD-180, and the RD-191 — all use simpler forms of the cycle. The full-flow staged combustion cycle, by contrast, uses twin-shaft staged combustion with both oxidizer-rich and fuel-rich preburners.
That design allows the full flow of both propellants through the turbines without dumping any unburnt propellant overboard. The Raptor differs fundamentally from its predecessor, the Merlin engine, which uses an open-cycle gas generator system.
In the full-flow cycle, the energy from the preburners is spread across the entire fuel flow, yielding cooler turbine exhaust than other closed cycles. That contributes to long engine life. Open-cycle engines, by contrast, operate their turbines at maximum survivable temperature.
An oxygen-rich turbine powers the oxygen turbopump, and a fuel-rich turbine powers the methane turbopump. Both oxidizer and fuel streams are converted to gas phase before entering the combustion chamber.
The Raptor 2 version uses coaxial swirl injectors instead of the Merlin’s pintle injectors. The engine operates at an oxidizer-to-fuel ratio of approximately 3.8 to 1.
Reusability and the Starship programme
SpaceX President Gwynne Shotwell said the engine would deliver long life and more benign turbine environments. The Raptor is designed for reuse with minimal maintenance, aiming for extreme reliability to support airline-level safety required for point-to-point Earth transportation. The engine is central to SpaceX’s super-heavy-lift Starship rocket, powering both the Super Heavy booster and the Starship second stage.
Starship missions include lifting payloads to Earth orbit and are planned for missions to the Moon and Mars. Raptor uses cryogenic liquid methane and liquid oxygen as propellants, both stored as subcooled liquids to increase density and performance.
Methalox burns cleanly, reducing carbon buildup. The use of subcooled propellants also reduces cavitation risk at turbopump inlets, enhancing reliability. Those design choices are all aimed at the kind of repeated, rapid reuse that SpaceX has outlined for Starship.
In the full-flow cycle, the cooler turbine exhaust is a direct advantage for longevity. The record shows that before Raptor, no full-flow staged combustion engine had flown; only the Soviet RD-270 and the Integrated Powerhead Demonstrator had been tested on the ground.
Now, with Raptor powering a vehicle in flight, the technology has moved from paper and test stands to operational reality.
Sources
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