Mastering The Cosmos: A Fascinating Dive into the Mechanics of Liquid Rocket Engines

With the increased innovation in rocket technology by SpaceX and other private companies, rocket engines seem to be the technology that will power the future.

But what exactly powers these magnificent machines?

What is the science behind the seemingly magical propulsion system that sends rockets soaring into space?

There are 2 main types of rocket engines: Liquid Rocket Engines and solid Rocket Engines.

Solid rocket engines use a solid propellant that consists of a fuel and oxidizer mixture, while liquid rocket engines store their fuel and oxidizer in liquid forms separately.

The main difference between a solid and liquid rocket engine is a solid rocket engine cannot be easily stopped once ignited while a liquid rocket engine can be stopped and started again with a valve control system.

Since almost all engines these days are liquid rocket engines, we will be focusing on liquid rocket engines in this thread.

Basic Scientific Principles of Rocket Engines

diagram of the forces on a rocket
Credit: NASA

Newton’s Third Law

Rocket engines operate on a fundamental principle of physics, specifically, Newton’s Third Law of Motion. This law states:

“For every action, there is an equal and opposite reaction.”

In the context of a rocket engine, this means that as fuel is expelled from the engine downwards at high speed (action), the rocket is propelled upwards (reaction) with equal force.

thrust equation for a liquid rocket engine
Credit: NASA

Thrust: The Driving Force

Thrust, essentially, is what propels rockets into space.

A simple equation governs thrust:

Thrust = Mass Flow Rate x Exit Velocity + (Exit Pressure – Ambient Pressure) x Exit Area

Here’s what those terms mean:

– Mass Flow Rate of Exhaust Gasses: This is about how much mass or weight of gasses is coming out of the rocket engine every second.

It’s like how fast water flows out of a hose; in this case, it’s how fast gasses flow out of the rocket.

– Exhaust Velocity: This is about how fast the gasses are moving as they come out of the rocket engine. The higher the exhaust velocity, the faster the gasses are shooting out the back of the rocket.

– Exit Pressure: This is the pressure of the stuff (fuel and gasses) at the nozzle of the rocket, where it leaves the rocket.

Ambient Pressure: This is the pressure of the air or environment around the rocket. It’s like the normal air pressure we feel here on Earth.

– Exit Area: This is the size of the opening where the stuff (fuel and gasses) comes out of the rocket.

The first part of this equation signifies that increasing either mass flow rate (amount of propellant used per second) or exit velocity will increase thrust.

The second part takes into account external factors like atmospheric pressure which can affect thrust. In a space where the ambient pressure is close to zero, this part becomes negligible.

What this means is that in rocket engine innovation, scientists and engineers work to make rockets better by focusing on two main things: either improving the mass flow rate or the fuel exit velocity.

Therefore, these improvements help rockets go higher, faster, carry more weight, and allow us to explore deeper into space.

The Anatomy of Liquid Rocket Engines

diagram of a liquid rocket engine
Credit: Wikipedia

Rocket engines, while varying in design and complexity, generally consist of 6 key components: fuel and oxidizer tanks, pumps, an injector, a combustion chamber, a Converging-Diverging Nozzle, and a cooling system.

1. Fuel and Oxidizer Tanks

The fuel and oxidizer are held in separate, high-pressure tanks, controlled by an intricate valve system to control how much of each substance goes into the rocket engine, ensuring a controlled and efficient combustion process.

2. Pumps

diagram of the atlas 5 pump system
Credit: SpaceFlight 101

The pumps serve to transfer the fuel and oxidizer from their tanks to the combustion chamber. There are different ways to pump the propellant into the combustion chamber called cycle types.

We can compare rocket engine cycle types to internal combustion engine types.

Car engine types include 2-stroke, 2-cylinder, 4-stroke, 4-cylinder, supercharged, turbocharged, etc. Rocket engine cycles include Oxidizer-Rich, Fuel-Rich, Full Staged Combustion, etc.

They all operate under the same basic principles but employ different techniques to reach their power and/or efficiency goals.

injector plate of the Saturn V rocket
Credit: Steve Jurvetson

3. Injector

The injector system of a rocket is a crucial component that plays a vital role in mixing and delivering the propellants to the combustion chamber for efficient and controlled combustion.

Typically located at the entrance of the combustion chamber, the injector system consists of a series of precisely designed and arranged nozzles or injectors.

These injectors disperse the propellants in a specific pattern, creating a highly turbulent and atomized mixture that promotes rapid and thorough mixing of the fuel and oxidizer, which helps to get the most thrust out of the fuel.

4. Combustion Chamber

Inside the combustion chamber, the fuel and the oxidizer are carefully injected, mixed, and ignited. As these substances react chemically, they undergo a rapid combustion process, releasing an intense and controlled combustion.

This combustion generates an extremely hot and pressurized gas mixture that is forced through the Converging Diverging nozzle and produces the powerful thrust needed to propel the rocket forward into space.

diagram of a converging diverging nozzle
Credit: Spirax Sarco

5. Converging Diverging (CD) Nozzle

The converging-diverging nozzle is a special shape in a rocket engine that efficiently accelerates and expands hot gasses.

It helps convert thermal energy into high-speed exhaust gasses, providing a powerful forward thrust and increasing the efficiency of the engine.

While the CD Nozzle is the most common, the Aerospike Nozzle is another nozzle that offers some unique advantages over the CD Nozzle.

6. Cooling System

The temperatures reached within the combustion chamber and nozzle of a rocket engine far exceed the melting point of most metals and need some form of active or passive cooling to keep them from melting.

Like the pump system, there are many different types of cooling systems for rockets: radioactive cooling, ablative cooling, regenerative cooling, etc.

The main goal for all these cooling systems is to manage and dissipate the extreme heat generated during rocket engine combustion to prevent damage and maintain the engine’s structural integrity.

Fuel Selection in Rocket Engines

Selecting the right fuel for rocket engines is a critical process that involves careful consideration of various factors.

The ideal fuel should possess high energy content, ensuring more thrust per unit mass, and stability to prevent unwanted reactions during storage and handling.

The choice of fuel also depends on the mission requirements, such as the desired thrust level, specific impulse, and the environment in which the rocket will operate.

The most common oxidizer is liquid oxygen (LOX) and the most common fuels are Liquid hydrogen (LH2) and Rocket-grade kerosene (RP-1).

Conclusion

While this article only covers the very basics of rockets, there are tons of more factors that go into rocket engine design. A rocket engine embodies a symphony of physics and engineering.

It’s an intricate dance between combustion chemistry, fluid dynamics, thermodynamics, and structural mechanics – all working together harmoniously to achieve one goal: to overcome Earth’s gravity and reach beyond our atmosphere.

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