How Rocket Engines Work, Part 1: The Basics

I’ve been talking about this for a while now, but here it it: how rocket engines work.

Now, this is something that I’ve been procrastinating on, since rocket engines are quite literally rocket science (and aren’t that simple).

However, I think I should try to cover it.

First, the basics. A rocket engine uses Newton’s Third Law to move things forward. It consumes a thing (a propellant), which is thrown out the back to create motion the other way (forwards). It’s like throwing a heavy object forwards while on roller skates – you move back because you moved the object forwards. “Every action has an equal and opposite reaction” – Isaac Newton.

Another factor to consider is the nozzle.

There is technically a nozzle on every single rocket in the world, as it is defined as the exit point of the exhaust from the rocket. Good nozzles, however, are vital for making sure that a rocket is efficient.

A simple rocket nozzle.

The main point of most nozzles is to speed up the propellant by making the channel smaller, kind of like putting your finger over part of a garden hose to make water shoot out further. Rocket fuel burning makes a lot of pressure – the nozzle takes this pressure and changes it into speed.

The other role of a nozzle is to expand the exhaust gases. The means allowing the plume to expand and push off the nozzle, so that the rocket gets more thrust in return.

Different nozzles are used in different situations. Air pressure is high in the lower atmosphere and lower in the upper atmosphere, so the expansion rate of exhaust gases changes too; thus, nozzles that expand less are used near the surface, since the exhaust cannot expand as much with so much pressure outside, and nozzles that expand more are used in outer space, since the exhaust can expand more. This is seen with real life examples:

RL-10 with cutaway.JPG
The RL-10 engine, used in outer space, seen with a big engine bell.
SpaceX Testing Merlin 1D Engine In Texas.jpg
SpaceX’s Merlin 1D engine, used as a first stage engine, with a smaller engine bell compared the the entire engine.

Good, now we can investigate rocket fuel.

Right, so how does something like this work?

Space Shuttle Atlantis lifting off for it’s mission, STS-132. Photo Credits: Wikimedia Commons.

The Space Shuttle is one of the coolest vessels to have ever flown. Not only does it look majestic, but it covers 3 big types of rocket engines:

  1. Solid Rockets
  2. Liquid (Bipropellant) Engines
  3. Monopropellent Engines

This may sound really complicated right now, but trust me, it will make more sense soon.

Solid Rockets:
This is usually the first example of a rocket that people give when thinking about them.  The most evident applications of these are fireworks. You light the fuse, the rocket shoots up. Easy. But how do they work?

As the name implies, solid rockets use solid fuel. All rockets need fuel to burn, and oxidizer to allow the fuel to burn. A good analogy is a campfire. The wood is the fuel in the situation, and the air around is the oxidizer. You need both fuel and air to make a fire. Take away any one of those – no more fire.

Since there is no air in space, you need to bring your own oxidizer to space if you want your engines to work there. Furthermore, the concentration of oxygen (the actual oxidizing part of air) in the air is only around 21%. In order for rockets to produce as much flame as they do, they need a more pure oxidizer.

Other than that, a solid rocket is really simple. You have some stuff that burns, and somewhere for the flame to go out of. The flame goes out the exhaust nozzle after the fuel burns, and the rocket goes forward. There is no plumbing to worry about or other moving parts in a basic solid rocket.

The Space Shuttle has 2 Solid Rocket Boosters (SRBs) mounted on the side of its big orange fuel tank. These are a lot like scaled up firework rockets. Once they are ignited, they basically cannot be stopped and keep burning until they run out of fuel. This makes it tricky to incorporate solid rockets sometimes, because you need to calculate exactly how long each booster will burn according to your needs. Being unable to control SRBs is one of their biggest downsides.

Two Space Shuttle SRBs on the Crawler transporter.jpg

Whereas fireworks use black powder as fuel, SRBs use aluminum as fuel and ammonium perchlorate as an oxidizer, which is all held together by some “rocket glue” to prevent the powder from falling out or separating.

The center of the engine is made hollow, so that when it’s time to ignite the engines, a flame is shot down the middle, which causes the propellant to burn.

The SRBs on the Space Shuttle are made in segments, which are stacked on top of each other to make a finished booster. You can see here that the middle of the booster is hollow, as mentioned above. The segments are welded together and secured with rubber O-rings.

Solid rockets are simple and reliable. That’s why they’re used for all kinds of things, including side boosters, as well as missiles. On the other hand, they’re not very efficient, nor can they be throttled. For a more efficient engine, look to the next section.

Liquid Engines:
The Space Shuttle’s main engines, the RS-25 SSME, produce more thrust combined than 6 Boeing 747s put together, and burn for around 8 minutes to go to space. These engines are very powerful, and their thrust can be changed, unlike the SRBs. These are example of Liquid Bipropellant Engines.

A rocket engine firing. A blue flame is projecting from a bell-shaped nozzle with several pipes wrapped around it. The top of the nozzle is attached to a complex collection of plumbing, with the whole assembly covered in steam and hanging from a ceiling-mounted attachment point. Various pieces of transient hardware are visible in the background.

As the name also implies, instead of using solid fuel, liquid engines use liquid fuel, kind of like how a car uses gasoline.

Unlike a solid rocket, liquid engines are much more complicated, required the fuel to be injected into a combustion chamber, rather than simply letting a stack of solid fuel burn.

Liquid engines are by far the most powerful and most efficient engines covered here, and are often the primary stage on modern-day rockets.

The main drawback of liquid engines is their complexity – they’re hard to make, expensive and heavy. There are many designs for liquid engines, all of which combine the fuel and oxidizer into a combustion chamber. I’ll cover rocket engine plumbing in part 2.

Monopropellant Engines:
Now that we’ve gotten to space and gotten rid of the powerful main engines, we need a way to control our orientation (which way we point) and make small adjustments. That’s where monopropellant engines come in. They use only 1 propellant, which is why they’re called monopropellant. You may be asking, “How does this happen? Don’t you need a fuel and an oxidizer for fire?”, and while you do need fuel and oxygen for fire, a monopropellant works off of a different type of reaction.

Monopropellant engines use an unstable compound, that decomposes (and in turn expands like if it was a fire) and produces thrust. The substance being loaded into the spacecraft above (MESSENGER, a Mercury-visiting probe) is hydrazine, or N2H4. When passed over a catalyst, it decomposes into nitrogen and hydrogen gas, producing heat in the process.

Monopropellant engines, like solid rockets, are simple, but can also be controlled, which makes them very useful for small adjustments. Often, they are used for RCS (Reaction Control System) thrusters. The Space Shuttle has an RCS, but it uses two propellants rather than one because of the extra power needed.

SpaceX’s Falcon 9 Rocket uses “cold gas thrusters” as an RCS. It takes nitrogen, heats it up, and fires it out in jets, which technically makes it a monopropellant.

Like SRBs, monopropellant engines are inefficient, which is why you don’t see them as the first stage of a rocket. They’re also really weak, so they’re only good use is for small bursts of thrust like for changing orientation in space.

In conclusion, rocket engines come in many varieties, with each having its own strengths and weaknesses. Each type has its own properties and applications. Next time, I will be focusing on liquid engines and the many ways that they can be built. If you want me to elaborate on anything of cover something I haven’t already, leave a comment below or on this page.
Otherwise, thanks for reading!

Stay tuned and stay sciency,



1 Comment

  1. A nice learn about rocket engine… hope we can find more powerful and efficiency Rocket engine to help human discovery more far away…


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