HAWTHORNE, Calif., July 17 — In the high-stakes arena of rocketry, where every kilogram of thrust and every degree of turbine temperature is fought over, a quiet revolution has been unfolding inside SpaceX’s manufacturing halls. The Raptor engine, the powerplant for the company’s Starship system, represents a fundamental break with the past — not just for SpaceX, but for the entire history of liquid-fueled rocketry. Against this backdrop, the engine’s technical lineage is worth examining.
Raptor is the third rocket engine ever designed with a full-flow staged combustion fuel cycle, and the first such engine to actually power a vehicle in flight. Before Raptor, no full-flow staged combustion engine had ever flown.
Only two designs had even reached test stands: the Soviet RD-270 project in the 1960s, and the Aerojet Rocketdyne Integrated Powerhead Demonstrator in the mid-2000s. Both remained experimental. Raptor made the leap from laboratory to launch pad.
The engine uses cryogenic liquid methane and liquid oxygen — a propellant combination known as methalox — to power both the Super Heavy booster and the Starship second stage. Starship’s missions include lifting payloads to Earth orbit, and the vehicle is planned for missions to the Moon and Mars.
The engines are designed for reuse with little maintenance. Gwynne Shotwell has claimed the engine would deliver long life and more benign turbine environments.
A cycle that changes everything
The full-flow staged combustion cycle is the key innovation. Unlike the open-cycle gas generator system used by Raptor’s predecessor, the Merlin engine, full-flow staged combustion uses both oxidizer-rich and fuel-rich preburners. This allows the full flow of both propellants to pass through turbines without dumping any unburnt propellant overboard.
The result is a system that spreads preburner energy across the entire fuel flow, keeping turbine exhaust cooler than other closed cycles. This cooler operation contributes directly to long engine life.
In the Raptor design, an oxygen-rich turbine powers an oxygen turbopump, and a fuel-rich turbine powers a methane turbopump. Both propellant streams are fully converted to gas before entering the combustion chamber, which speeds mixing and reduces chamber size and mass. Torch igniters are used in the preburners.
The Raptor 2 variant has no main igniter in the combustion chamber because preburner exhaust temperatures are already high enough, eliminating the need for the dedicated igniter fluid that the Merlin required. Raptor 2 also uses coaxial swirl injectors instead of Merlin’s pintle injectors.
The engine uses deep cryogenic propellants cooled near their freezing points, which increases density and performance while reducing the risk of cavitation. The oxidizer-to-fuel ratio is approximately 3.8 to 1. Methalox burns cleanly, reducing carbon buildup — a practical advantage for engines intended to be reused many times.
What is at stake
In the corridors of the aerospace industry, the Raptor’s success is watched closely. The RS-25 engine that powered the Space Shuttle used a simpler staged combustion cycle, as did Russian engines like the RD-180 and RD-191. But none of those engines attempted the full-flow architecture.
Raptor is designed for extreme reliability, aiming to support airline-level safety for point-to-point Earth transportation — a goal that would require engines to fly hundreds or thousands of times without failure. The wider picture is one of ambition.
Starship is intended to be the most powerful launch vehicle ever built, and the Raptor is the engine that must make that possible — not just once, but repeatedly, with minimal refurbishment. Whether the engine can deliver on that promise will determine the pace of SpaceX’s plans for the Moon, Mars, and beyond.
