Merlin vs Raptor: How SpaceX Rocket Engines Work

SpaceX’s Merlin and Raptor rocket engines both generate thrust by burning propellant and expelling exhaust at high speed, but they use different propellants and cycles: Merlin burns RP-1 kerosene with liquid oxygen in a gas-generator cycle, while Raptor burns liquid methane with liquid oxygen in a full-flow staged combustion cycle. In simple terms, Merlin is the workhorse that has powered the Falcon 9 and Falcon Heavy, while Raptor is the next-generation engine designed for Starship and Super Heavy.

Background: Why Two Engines?

SpaceX developed Merlin first because it needed a reliable, relatively simple engine to get to orbit and land boosters. RP-1 (a refined kerosene) is dense, cheap, and easy to handle, making it ideal for a reusable first stage. However, RP-1 leaves soot and requires complex turbopump machinery, and it has a lower specific impulse than hydrogen or methane.

Raptor was designed for Mars missions and full reusability. Liquid methane (CH₄) can be produced on Mars via the Sabatier reaction using local CO₂ and water, and it burns cleaner than RP-1, reducing engine maintenance. Methane also has a higher specific impulse than RP-1, and its low boiling point allows for easier pressurization and densification. The full-flow staged combustion cycle extracts more energy from the propellants, making Raptor more efficient than Merlin.

How Merlin Works

Merlin uses a gas-generator cycle. A small amount of RP-1 and liquid oxygen (LOX) is burned in a separate pre-burner to produce hot gas that drives the turbopumps. The turbine exhaust is then dumped overboard (not into the main combustion chamber), which is simpler but slightly less efficient.

The main propellants are pumped at high pressure into the main combustion chamber, where they ignite and expand through a converging-diverging nozzle to produce thrust. The engine is throttleable, allowing the Falcon 9 to land by reducing thrust to about 40% of maximum. Merlin’s thrust is about 845 kN (190,000 lbf) at sea level, with a specific impulse of ~282 seconds at sea level and ~311 seconds in vacuum.

How Raptor Works

Raptor uses a full-flow staged combustion cycle. Both the fuel-rich and oxidizer-rich pre-burners drive separate turbopumps, and all of the turbine exhaust is routed into the main combustion chamber. This means 100% of the propellant is burned in the main chamber, yielding higher efficiency and chamber pressure (up to 300 bar in later versions).

The engine burns liquid methane and liquid oxygen. The fuel-rich pre-burner drives the methane turbopump, and the oxygen-rich pre-burner drives the LOX turbopump. The two hot gas streams are injected into the main chamber, where they mix and burn. Raptor produces about 2.3 MN (520,000 lbf) of thrust at sea level, with a specific impulse of ~330 seconds at sea level and ~380 seconds in vacuum. It is also throttleable to about 40% for landing.

Key Differences: Propellants and Cycle

• Propellants: Merlin uses RP-1 (kerosene) + LOX; Raptor uses liquid methane (CH₄) + LOX.
• Cycle: Merlin uses a gas-generator (open cycle); Raptor uses full-flow staged combustion (closed cycle).
• Efficiency: Raptor has higher specific impulse and chamber pressure, making it more efficient.
• Reusability: Raptor is designed for many flights with minimal maintenance; Merlin has been reused up to 20 times but requires more refurbishment.
• Thrust: Raptor is about 2.7 times more powerful than Merlin at sea level.

Common Problems and Misconceptions

A common misconception is that Raptor is simply a bigger Merlin. In reality, the engine cycles are fundamentally different. Another misconception is that methane is used because it’s “cleaner” in an environmental sense—it’s actually chosen for performance and in-situ resource utilization on Mars.

Merlin’s main issue is coking: RP-1 leaves carbon deposits in the engine, which can clog coolant channels and require cleaning between flights. Raptor’s main challenge is the extreme pressure and temperature of the full-flow cycle, which stresses seals and materials. Both engines have experienced failures during development, but SpaceX iterates rapidly, and both have achieved successful flights.

How to Choose Between Them (for a Mission)

If you’re launching a satellite to low Earth orbit (LEO) or geostationary transfer orbit (GTO), Merlin on the Falcon 9 is a proven, cost-effective choice. It has a high thrust-to-weight ratio and is well-suited for booster recovery. For deep-space missions, crewed flights, or Mars colonization, Raptor’s higher efficiency and use of methane make it the better option, especially because methane can be produced on Mars.

For a small payload, you might choose a smaller engine like the Kestrel (used on Falcon 1), but that’s retired. In general, the choice depends on payload mass, destination, and reusability requirements.

Costs and Safety

Exact engine costs are not publicly disclosed, but RP-1 is cheaper than methane per kilogram, though methane is often produced from natural gas. However, the cost of the engine itself dominates. Merlin is likely cheaper to manufacture due to its simpler cycle, but Raptor’s higher performance may justify its cost for heavy missions.

Safety-wise, both engines use cryogenic propellants (LOX at -183°C, methane at -162°C) and are highly pressurized. RP-1 is toxic and carcinogenic, while methane is non-toxic but highly flammable. SpaceX follows strict ground safety protocols, and both engines have flight termination systems for launch aborts.

Frequently Asked Questions

Why doesn’t SpaceX use hydrogen like the Space Shuttle?

Hydrogen gives high specific impulse but is very low density, requiring huge tanks, and it causes hydrogen embrittlement in metals. Methane offers a good balance: higher density than hydrogen, higher performance than RP-1, and easier to handle.

Can Raptor be used on Falcon 9?

No. Raptor is physically larger and more powerful, and the Falcon 9’s structure, fuel tanks, and plumbing are designed for Merlin. Retrofitting would require a complete redesign.

How many times can a Merlin engine be reused?

SpaceX has reused Falcon 9 boosters up to 20 times, but the engines are inspected and refurbished between flights. The Merlin itself is designed for multiple flights, but the exact number depends on wear and tear.

What is the thrust of Raptor compared to the Saturn V’s F-1?

The F-1 produced about 6.7 MN (1.5 million lbf) at sea level, which is roughly three times the thrust of a single Raptor. However, Super Heavy uses 33 Raptors, giving a total thrust of about 76 MN (17 million lbf), which is more than double the Saturn V’s first stage.

Is Raptor more efficient than Merlin?

Yes, Raptor has a higher specific impulse and a higher thrust-to-weight ratio, meaning it uses propellant more efficiently. This is due to the full-flow staged combustion cycle and the use of methane.

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