SpaceX Raptor

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Raptor is a family of rocket engines which are developed and manufactured by SpaceX for use in Starship. This rocket engine marks significant step in rocket engineering. It is designed with full-flow staged combustion fuel cycle and use of methane as a fuel, known as Methalox. SpaceX announced the development of the Raptor engine in 2012, with the first full-scale engine testing in September 2016. These advanced, reusable methane-oxygen engines are the primary power source for the SpaceX Starship system. 

The engine is powered by cryogenic liquid methane and liquid oxygen, known as Methalox. Refined liquid methane as well as LNG are typically used as a fuel for rockets in combination with oxygen. For example it is used in the TQ-12, BE-4, Raptor, YF-215, and Aeon engines.

A Raptor 1 rocket engine, SpaceX's factory in Hawthorne, California; source: Wiki

The Raptor engine is a full-flow staged combustion cycle engine, which allows for higher efficiency and performance compared to traditional rocket engines. The Raptor engine has for example more than twice the thrust of Merlin 1D, used in Falcon 9. 

The SpaceX Raptor engine uses a full-flow staged combustion (FFSC) cycle, which is a complex and highly efficient system that pre-burns both liquid methane and liquid oxygen in separate, fuel-rich and oxidizer-rich pre-burners to drive turbines. Unlike open-cycle engines, all propellant flows through these pre-burners and turbines, resulting in full gasification and complete mixing in the main combustion chamber for maximum thrust and reliability. 

Liquid methane and liquid oxygen are partially burned in two separate pre-burners: 

  • A fuel-rich pre-burner produces hot, fuel-rich gas. 
  • An oxidizer-rich pre-burner produces hot, oxidizer-rich gas. 

The fuel-rich gas drives the turbopump for the liquid methane.  The oxidizer-rich gas drives the turbopump for the liquid oxygen. Then both streams of hot gas, completely filling the turbines and pre-processors, are then directed to the main combustion chamber. In main combustion chamber, the two completely gasified propellant streams mix and fully combust in the main chamber, generating the high thrust required for Starship. 

Raptor burns a mixture of liquid methane and liquid oxygen, producing at sea-level approximately 230 tons of thrust per engine. The vacuum-optimized version of the Raptor, Raptor Vacuum (RVac), is designed to operate in the vacuum up to 250 tons of thrust.

There is advantage of methane/liquid oxygen combination over kerosene/liquid combination (kerolox) of producing small exhaust molecules, reducing for example of deposition of soot on engine components. Liquid methane has a lower specific impulse than liquid hydrogen, but it is easier to store due to its higher boiling point and density, as well as its lack of hydrogen embrittlement.

Source: SpaceX

The lower molecular weight of the exhaust leads to higher specific impulse by converting more thermal energy into kinetic energy available for propulsion. However, methane, despite having a higher molecular weight than liquid hydrogen, is advantageous because its greater density and warmer operating temperature result in smaller, lighter tanks and can be more efficiently stored and used in rocket engines for certain applications, even with a slightly lower theoretical specific impulse. 

Liquid methane has a temperature range (91–112 K) nearly compatible with liquid oxygen (54–90 K). The fuel currently sees use in operational launch vehicles such as Zhuque-2, Vulcan and New Glenn as well as in-development launchers such as Starship, Neutron, Terran R, Nova, and Long March 9.

SpaceX rocket engine version development, source: Wiki

Each version of the engine has a corresponding Raptor Vacuum version, known as RVac. Those vacuum-optimized versions of SpaceX's Raptor engine have a significantly larger nozzle for higher efficiency in space compared to the sea-level Raptor engine. The vacuum-optimized Raptor targets a specific impulse of ≈380 s (3,700 m/s).

Raptor 1 had 185 tons of force, while Raptor 2 increased this to 230 tons. The latest Raptor 3 engine boosts thrust to 280 tons (ton-force), is lighter than its predecessors, and incorporates advanced manufacturing techniques, such as metal 3D printing, for a more streamlined and robust design. 

The Raptor series by SpaceX, source: Google

SpaceX's Raptor engine is rapidly evolving from its 2019 Raptor 1 version to the current Raptor 3 version (to the date of 2024), becoming significantly lighter, more powerful, and more efficient with increased chamber pressure and a streamlined design. I have understood that the future of the Raptor engine involves more optimization for greater thrust and of course reliability, with aim to exceed 3,000 kN, to reduce mass, and enhance reusability to lower the cost of space exploration for projects like Starship and Mars colonization. I can only look forward. 

Notes:
  • Specific Impulse (Isp): A measure of a rocket engine's efficiency, essentially its "miles per gallon". A higher specific impulse means the engine gets more thrust for a given amount of propellant, allowing a rocket to achieve a greater change in momentum for the same amount of fuel. 
  • Exhaust Molecular Weight: The average mass of the molecules in the rocket's exhaust. 
  • Kinetic Energy: The energy of motion. 
  • Thrust: The force that propels the rocket forward. 
  • Tons of force refers to a unit of force equal to the weight of one ton due to standard gravity, which is the gravitational force exerted on a mass of one ton on the surface of the Earth. A metric ton-force is 1,000 kilograms-force. 

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