Rocket Fuel Basics


Note: This entry was initially written for Level 2 cadets at the 540 Golden Hawks Squadron. See other posts in this series.


Ready for some rocket science?

A transparent model rocket motor. Source: https://www.youtube.com/watch?v=_xvVJQSGHts

Let’s start with some basics.

A rocket is anything that moves with its own propellant. Propellant is anything that forces another thing to move using Newton’s third law of motion: for every action, there is an equal and opposite reaction.

A balloon that’s leaking air is a type of rocket: the air is acting as a propellant, and the reaction is the balloon moving in the opposite direction. Other types of rockets use more fancy propellants and better guidance systems, but the principle is the same.

Most rocket propellants are actually a combination of fuel and oxygen. There is a difference between fuel and propellant — fuel is anything that needs oxygen to burn, like wood, metal, and rubber. Rockets use this fuel, and add some oxygen to it, to create a propellant.

Rockets can use two different types of fuel: solid and liquid.

Source: https://cosmosmagazine.com/technology/how-rockets-work

A liquid rocket holds its liquid fuel and its liquid oxygen source (called oxidizer) in separate containers. Using pumps, it combines the two liquids in the combustion chamber, where it gives them a final spark to ignite them. A liquid rocket is also called an engine.

A solid rocket has the fuel mixed with the oxidizer the whole time. A small spark at any time will set the whole thing off. A solid rocket is also called a motor.

More on solid rockets

At its most basic, a solid rocket is literally just the propellant surrounded by a tube of some sort. Think of a hobby rocket. (That’s the gif at the top of the article.) You light off the end and it just burns.

Bigger solid rockets get more complicated. Take the Solid Rocket Boosters (SRBs) for the space shuttle.

The blueprints of an SRB. Source: https://link.springer.com/chapter/10.1007/978-1-4614-0983-0_4

The fuel is surrounded by “inhibitors” (pretty much a rubber to help control the burn after ignition) and insulation (protecting the casing from exploding due to the heat).

What do the SRBs burn as fuel? Aluminum — powdered very finely!

There’s also a chamber (aka perforation) in the centre. The initial ignition happens inside the chamber and burns outward.

The chamber in solid fuel rockets can have different shapes, which affect the thrust. A perfect circle burns slow at first then burns faster. A star burns fast then slow. A ring would burn at a constant pace. The SRBs use an 11 point star.

The different kinds of perforation shapes, a.k.a. “grains”. Source: https://www.youtube.com/watch?v=xDl_LO6nOnI

Compare the SRB diagram to a small model rocket motor diagram. Note how they both have a heat resistant nozzle, & the ignition happening within a chamber (at the top of the SRB and at the bottom for the model rocket).

The parts of a model rocket motor. Source is p. 76 here: https://estesrockets.com/wp-content/uploads/Catalogs/Estes_2015_Catalog.pdf

Once you ignite a solid rocket, there’s no going back. This simplicity makes them very cheap to design and build, but quite dangerous to use.

A Few Words on Liquid Rockets

Liquid rockets (typically burning liquid hydrogen) are much more complicated & expensive. What makes them complicated? The fact that you have to store the fuel and oxygen separately, and combine them using a series of tubes.

Liquids are also less dense than solids, which means you need more of them to get the same amount of thrust. And since most liquid fuels and oxidizers are gasses at room temperature, you have to spend a lot of energy cooling them down extremely (to negative 200 degrees celsius!), so they stay a liquid.

(When you see a spaceship about to launch giving off steam on the launch pad, know that’s just letting out some steam from the super-cold liquid propellant on board. )

The steam coming off the SpaceX ship. Source: https://gfycat.com/impressivemajorbluewhale

The benefit is that you can control the thrust after liftoff. Because the fuel needs the oxidizer to burn, you have have to shut off the valve leading from the oxidizer, to the professor. Since the oxidizer and fuel are stored separately, and not mixed, this is very easy to do.

A properly designed nozzle on a rocket makes the flames shoot out faster. You can tell whether a nozzle is properly aligned or note about diagram use. This is very helpful in moving the rocket faster. Since fire is shooting out of the nozzle, it must be very strong so that it won’t break. Liquid rockets can use their unburnt fuel to cool off the nozzle to prevent it from overheating and breaking.

The nozzle of a solid fuel rocket must be constructed differently than a liquid fuel rocket. Since there are no liquids on a rocket motor, the nozzle must be built of materials with much higher heat resistance, which are heavier and more expensive.

Summary

Rockets move based on Newton’s third law of aerodynamics. Solid rockets are cheap and relatively easy to build, but can’t be controlled after being ignited.

Liquid rockets separate the propellant and the fuel, and since gasses are in general less dense than solids, you have to spend a lot of money on getting more. On the other hand, liquid fuels are very easy to use, since you full control how much thrust you want.


The video at the bottom compares how different rockets use their fuel: Saturn V, the Space Shuttle, the SpaceX Falcon Heavy, and the Space Launch System (SLS). The blue is oxygen, & the red/orange are the fuels. Only the SRBs (on the Shuttle and Falcon) use solid fuel.

From left to right: the Saturn V, the Space Shuttle, the Falcon Heavy, and the Space Launch System. Full 9 minute video here: https://www.youtube.com/watch?v=su9EVeHqizY