Why Do Rocket Engines Look so Complex?

The principle behind rocket engines is simple, so why do rocket engines have such a complex array of tubes and joints? Why are they considered so difficult to make?

Space exploration has become hot again ever since SpaceX, led by billionaire Elon Musk, achieved massive success with its Falcon 9 rocket. In June 2022, it was reported that SpaceX launches 10 rockets for every one by its main competitor.

Thus, now is a great time to get interested in space exploration and rockets. A lot is happening. The enormous Starship by SpaceX is evolving and competitors such as Rocket Lab are also developing innovative new rocket designs such as the Neutron rocket.

To understand rocket engines, you could read Wikipedia, but they will tell you that a rocket engine is (gobbledygook for non-experts) :

A rocket engine uses stored rocket propellants as the reaction mass for forming a high-speed propulsive jet of fluid, usually high-temperature gas. Rocket engines are reaction engines, producing thrust by ejecting mass rearward, in accordance with Newton's third law.

Is there perhaps a friendlier explanation with plenty of colorful pictures? That is precisely what this article is about. I will explain how rocket engines work without dumbing it down. It will help you understand why something that starts out simple in principle ends up being very complex.

Basic Principle of a Rocket Engine

A rocket engine in its most basic form behave much like a balloon letting out air. Black powder rockets motors work on the same principle and even squids propel themselves forward using the same mechanism.

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

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!

The simplest rocket you can imagine is a solid fuel rocket, like the ones used during new year's 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.

A simple rocket which uses water as propellant.

We can step it up a notch and increase engine performance by heating the water inside the rocket engine until steam is formed. Steam creates a lot more pressure than you can achieve by merely pumping air into a water rocket bottle. There are in fact companies such as ARCA, which uses a steam rocket called LAS as their first stage in a multi-stage rocket.

Already at this point, complexity starts to add up. You need much stronger tanks to avoid blowing the whole thing up, and you need materials which can handle higher temperatures. You also need some way of heating the water.

Yet, steam-based rockets are not powerful enough to get you far. What drives a rocket forward is called a propellant. We have just looked at air and water propellant. The kick each kilo of propellant gives a rocket engine is of vital importance. If a kilogram of water does not give enough kick to lift a kilogram of water, then you would not get anywhere. That is why rocket engine designers are always looking for ways of getting more kick out of every kilogram of propellant.

How a Basic Bipropellant Rocket Engine Works

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:

• Fuel — Common fuels are Kerosene (RP1, kind of like airplane fuel), Hydrogen and liquid methane. The Falcon 9 Merlin rocket engines use Kerosene.

• Oxidizer — The oxidizer is used to burn the fuel in the reaction chamber. Typically, the oxidizer is simply liquid oxygen (LOX).

We often see abbreviations such as LOX/RP1 and LOX/H₂ to denote propellant. LOX/RP1 simply means that liquid oxygen (LOX) is used as oxidizer in combination with Kerosene (RP1) as fuel.

The simplest way of building a bipropellant rocket engine, is to use a pressured gas tank to push the fuel and oxidizer into the combustion chamber (where fuel and oxidizers are mixed to react). This approach has similarities with how pressurized air is used to push out water in a water bottle rocket.

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.

While sounding simplistic, this pressurized-gas approach is used in real rocket engines, which don't need high performance. For example, SpaceX’s kestrel engine, was previously used on the last stage of the Falcon 9 rocket. This is a stage used in vacuum when a multi-stage rocket has shed most of its mass and no longer require as powerful engine.

But the powerful engines needed to push a rocket off Earth's planetary surface demand far more performance, than an engine use to adjust the position of a spacecraft already in orbit. In these cases, we need powerful turbo-pump rocket engines.

Turbopump Rocket Engine

The rocket propellant needs to enter the combustion chamber fast, so lots of mass can be emitted as hot burning gas quickly (this is what produce thrust). To accomplish this feat, a turbopump is used. A turbopump is similar to the pumps used in:

• Jet engines

• Turbochargers in cars

• Water pumps used by 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 used as shorthand for turbocharger. A turbopump has the word turbo in it because it involved rotation and fluids: a turbine driving a centrifugal pump.

Inside the turbopump 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.

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, hydroelectric turbines and steam turbines in coal power plants are all examples of turbines.

Here is the funny factoid:

A small rocket engine is used to drive the turbine inside the main rocket engine. A simple approach is to feed hydrogen peroxide into a catalyst. The catalyst will cause the hydrogen peroxide to decompose into oxygen and produce hot steam quickly. The resulting steam will then drive the turbine which again drives the turbopump.

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, for instance sending part of the combusted fuel from the main chamber back to drive the turbine driving the fuel and oxidizer pumps.

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 act as rocket fuel because Ethanol can be mixed with water. Mixing with water made it easier to cool the bell shaped rocket nozzle. If the exhaust from the engine is too hot we risk melting the nozzle.