SNW #38: Comparison of a "traditional" Rocket Engine and the SpaceX Raptor

 Rocket engines are the heart of spaceflight, providing immense power required to lift a vehicle from the surface of the Earth. While the fundamental principle remains Newton´s Third Law of Motion (for every action, there is an equal opposite reaction), engines have evolved drastically over the years.

This week I would like to focus on the comparison of two rocket engines. Specifically a “traditional” rocket engine compared to the SpaceX Raptor engine used on the Starship. This means an open-cycle gas generator engine, compared to the revolutionary full-flow staged combustion Raptor engine.

This comparison will highlight how a rocket engine works and what are the main differences between the traditional way of making them and the approach SpaceX took to rethink the rocket engine.

THE TRADITIONAL ROCKET ENGINE: OPEN-CYCLE (GAS GENERATOR)

Many iconic engines throughout history, from the Saturn V´s F-1 engine to the Space Shuttle´s SSMEs, have relied on variations of the open-cycle design. The most common type of an open-cycle engine is the gas generator cycle.

It works in the following manner:

1. Liquid propellants (typically a fuel like Kerosene/RP-1 and an oxidizer like liquid oxygen/LOX) are stored in separate tanks.
2. To get the propellants into the high-pressure combustion chamber, powerful turbopumps are used. These pumps are driven by a turbine.
3. A small portion of the main propellant (around 2-5%) is diverted into a separate, much smaller combustion chamber called the “gas generator”. The propellants are burned here, creating hot, high pressure gas.
4. This hot gas is then routed through a turbine, which is in turn connected to the turbopumps, forcing the main propellants into the primary combustion chamber.
5. The main part of the propellants is injected into the main combustion chamber, where they ignite and burn at high pressure and temperature.
6. The superheated, high-pressure exhaust gasses are then directed through a carefully shaped nozzle (a de Laval nozzle). As the gas makes its way through the nozzle, its pressure drops in the thinner part, and its velocity dramatically increases, creating thrust.
7. Crucially, the exhaust from the gas generator is vented out externally, meaning this portion of the propellant does not contribute to the overall thrust of the engine, making the cycle less efficient.

The question you might be asking is: if you need the turbines to spin to get fuel moving and you need the gas generator to spin the turbines, how do you start the engine in the first place?

The answer is that usually, these engines have a system in place to spin the turbines externally. Once they are moving, the gas generator starts working and the engines are on.

As we can see, the fuel is diverted from the direct path to the main combustion chamber and a small part of it is being used to spin the gas generator.

THE SPACEX RAPTOR ENGINE: FULL FLOW STAGED COMBUSTION

The Raptor engine, powering SpaceX´s Starship, represents a significant leap in rocket engine technology. This cycle is far more engineeringly complex but offers substantial performance and reusability advantages.

It works in the following manner:

1. Raptor uses cryogenic liquid methane and liquid oxygen as fuel and oxidizer.
2. Unlike the single gas generator, the FFSC cycle uses two preburners
   1. A fuel rich preburner burns a large amount of methane with a small amount of oxygen, creating hot, fuel-rich gas.
   2. An oxidizer rich preburner does the same, but inverted. It burns a large amount of oxygen with a small amount of methane, creating oxidizer-rich gas.
3. Each preburner drives its own dedicated turbopump. The fuel-rich gas drives the oxidizer turbopump, and the oxidizer-rich gas drives the fuel turbopump. This means all the propellant flows through a turbine before entering the main chamber.
4. After driving their respective turbopumps, all the pre-burned, high-pressure, hot gases (both fuel-rich and oxidizer-rich) are injected into the main combustion chamber. They mix and undergo final combustion.
5. The nozzle shape is similar to all traditional engines, using a de Laval nozzle.

Because all of the fuel flows through the turbines and into the main combustion chamber, there is no wasted exhaust. This leads to significantly higher efficiency.

This cycle also allows the engine to run at much higher pressures (typically over 300 bar compared to other engines which run closer to 100-200 bar). Higher chamber pressure directly translates to higher thrust and better specific impulse.

It also makes the engine more easily reusable, due to the clean-burning methane and the way of the cycle. The “cooler” preburner exhaust also reduces thermal stress on the turbopumps.¨

As we can see, the FFSC cycle is far more advanced engineering wise, as the fuel separates and needs to meet again.

That´s it for this week, next week we are expecting these launches:

- A Starlink launch on Saturday *

- Another Starlink launch on Tuesday *

- Starship IFT-9 on Wednesday

- An Electron launch on Wednesday

- Another Starlink launch on Wednesday *

- A GPS III Falcon 9 launch on Friday *

- Another Starlink launch on Friday