Geek Guide to Rocket Engines

The principle behind rocket engines is quite simple, but why do rocket engines look so complicated?

Has watching the achievements of SpaceX and Blue Origin given you a newfound interest in space exploration. Perhaps you are curious about how a rocket engine actually works, and the highly technical wikipedia article on rocket engines isn’t quite cutting it for you.

Is there perhaps a friendlier explanation with lots of colorful pictures? That is exactly what this article about. I’ll explain how rocket engines work without dumbing it down.

The same principle that moves a rocket forward, is what causes a balloon to move forward, when you let out air.

The core principle behind rocket engines has a lot in common with how a balloon letting out air propels itself forward. Gunpowder rockets work on the same principle and even squids propel themselves forward using the same mechanism.

But why, you might wonder are rocket engines so complicated then? The principle is so simple! It is true that you could build a simple rocket with very few parts. Complexity increases dramatically when you want high performance and low weight!

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A simple rocket which uses water as propellant.

The simplest rocket you can imagine is a solid fuel rocket like the ones used during new years celebrations. One step up are water rockets in which you pump in pressurized air so that water can be ejected at pressure to shoot the rocket into the air. Technically speaking we would call that a monopropellant rocket.

We can take this even one more step, and heat up the water inside the rocket to steam and then use the pressurized steam to propel the rocket. But already at this point complexity starts to add up. You need much stronger tanks to avoid blowing the whole thing up. You need some way of heating the water etc.

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Simplest form of liquid propellant rocket engine. A pressured gas such as Helium is used to push the fuel and oxidizer (liquid oxygen usually) into the combustion chamber, where it is burned to produce thrust.

The high powered rockets used to send people and satellites into space use liquid propellants which are combusted. These are called bipropellant rockets. Their propellant is made up of two parts:

  1. Fuel. Common fuels are Kerosene (RP1, kind of like airplane fuel), Hydrogen and liquid methane. The Falcon 9 Merlin rocket engines use Kerosene.
  2. Oxidizer. The oxidizer is used to burn the fuel in the reaction chamber. Typically the oxidizer is simply liquid oxygen (LOX).

The propellant combinations are often written as LOX/RP1, LOX/H2. The simplest way of building a bipropellant rocket engine, is to use a pressured gas tank to push the fuel and oxidier into the combustion chamber. This has similarities with how pressurized air is used to push out the water in a water bottle rocket.

This is in fact used in real rocket engines when you don’t need really high performance. E.g. SpaceX’s kestrel engine, which was used on the last stage of the rocket in vacuum was such a rocket engine. But the powerful engines needed to push a rocket off the ground needs far more performance.

That is when you need a turbopump rocket engine.

Turbopump Rocket Engine

The propellant needs to enter the reaction chamber fast so lots of mass can be emitted as hot burning gas quickly (this is what produce thrust). To do this a turbo pump is used. This is similar to the pumps used in jet engines, turbo chargers in cars as well as pumps for fire engines.

Note, turbo, has nothing to do with car engines. The word turbo is derived from turbine. Anything involving rotation and fluids is referred to as turbo-machinery. But in casual speech turbo is often used as short hand for turbo-charger. A turbopump has the word turbo in it because it involved rotation and fluids: a turbine driving a centrifugal pump.

You got a propeller-like thing called an impeller, which when rotating sucks fuel in at the center (caused by pressure drop) and throws fuel out to the sides using centrifugal force. So like a washing machine the liquid gets pushed out to the walls and pushed through multiple slits and collected by a tube on the outside which sends it into the reaction chamber.

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A centrifugal water pump, similar to the ones used on e.g. fire engines. The pump on a rocket engine is quite similar, but deals with fuel and liquid oxygen instead.

The second problem is of course how to drive this pump. On a fire engine you just use an internal combustion engine. But in a rocket engine we use a gas turbine. A turbine is something that rotates when a fluid (gas or liquid) passes through it. So windmills, hydro electric turbines and steam turbines in coal power plants are all examples of turbines.

Here is the funny thing: A small rocket engine is used to drive this turbine. A simple approach is to feed hydrogen peroxide into a catalyst. This will cause it to decompose into oxygen and produce hot steam quickly. The resulting steam will then drive the turbine driving the turbo pump.

In more advance designs they utilize the main rocket fuel in various ways to drive the turbine. There are many feedback systems one can create, e.g. sending part of the combusted fuel from the main chamber back to drive the turbine driving the fuel and oxidizer pumps.

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A gas-generator cycle rocket engine. Some of the fuel is burned in a smaller pre-burner chamber. The exhaust from this makes the turbine spin when it exits through it. The turbine drives the fuel and oxidizer pumps. The Merlin 1 Engine used on the Falcon 9 rocket, is a gas-generator cycle rocket engine.

It is gaining maximum efficiency which complicates the design of rocket engines. In early rocket engines such as the V2, they just used Ethanol (same as you drink) to as rocket fuel because Ethanol can be mixed with water. This made it possible avoid burning up the bell shaped rocket nozzle.

But you are always looking for higher performance. That means higher performance fuel such a Kerosene, but this cannot be mixed with water and anyway that would reduce efficiency.

Thus we need to add more complexity. As shown in the diagram above. They would channel the rocket fuel through pipes in the nozzle itself. The rocket fuel would then absorb heat and it would come pre-heated into the combustion chamber. Hence the heat isn’t lost, but you avoid overheating the nozzle and burning it up.

Combustion Chamber Injector

Fuel has to react with oxidizer (typically oxygen) to burn. You cannot make something burn without oxygen. E.g. a log of wood doesn’t burn on the inside, because there is no oxygen there. It only burns on the exterior surface where there is oxygen. Hence to get efficient combustion the oxidizer and fuel has to get mixed really well.

That is why a rocket engine needs a combusion chamber injector. You cannot just have a pipe for the oxidizer and fuel going into the chamber.

The simplest solution to achieving this mix is by using a pintle injector as found on a garden hose, or water spray can. If you twist it so it almost closes you get a nice cone shape with lots of little dropplets. Exactly what you want to get maxiumum mixing of fuel and oxidizer.

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Example of pintle injector, showing water exiting at different positions for the pintle.

So this is the bare bones, which hopefully got you interested in learning more about rockets and space exploration.

More Articles on Rocket Engines

I have written a number of other articles on space exploration if you are interested in more details.

Gas milage and horsepower on a rocket engine. In this article I discuss why the measure of fuel efficiency and horsepower has to be quite different for a rocket engine. It is also an friendly introduction to the math and physics behind rocket engines.

Why can sugar be used as rocket fuel? This is a more detailed explanation of what exactly makes something suitable as a rocket fuel. We will look at the parts of chemistry relevant for a rocket fuel/propellant maker.

How to choose a rocket engine and propellant. So you have learned what makes something a rocket fuel and oxidizer (propellant) but given all the choices, what decides the propellants used? Why do different rocket stages often use different fuels? Why are the engines different?

Most interesting rocket engines. A huge number of rocket engines exist. But which ones are most interesting and useful to learn about? I have made a pick of engines I think you should know about if you are curious about rocketry.

Geek dad, living in Oslo, Norway with passion for UX, Julia programming, science, teaching, reading and writing.

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