Have you ever wondered what propels rockets out of the atmosphere and into the stars? Maybe you’ve seen the trail they leave behind as they streak through the sky, burning up their fuel. This article will go over the most common types of rocket fuels and how they’re used, so the next time you see a video of a launch you’ll have an idea of the processes behind it.
The basic principle behind rockets is called the conservation of momentum. An object’s momentum is the amount of mass it has multiplied by the velocity it’s traveling at. Mass is the amount of material an object is made out of, and velocity is both the speed and direction the object is going.
Momentum in a system is conserved, which means the amount of momentum entering the system will equal the amount leaving it plus anything inside the system. It’s basically saying that the mass and velocity has to go somewhere, it doesn’t poof out of existence.
So how does this principle let people send rockets into space? By burning rocket fuel, which is essentially a large mass of something that can catch on fire. You can think of rocket fuel as mass stored in the system, where the system is the rocket. When the fuel is burned, it pushes mass out the back of the rocket, leaving the system.
Now you have a large mass traveling quickly, pushing towards the ground. In order to maintain balance, the rest of the system moves up into the air to counter this. It’s a downward momentum and an upward momentum acting in opposite directions to cancel each other out. Momentum is conserved.
You might have noticed that the mass of rocket fuels isn’t anywhere near the total mass of the rocket, so how can the momentum generated by the fuel be enough to move something so massive? It’s because burning the rocket fuel converts mass into energy, although that’s a bit outside the scope of a brief article.
Different types of rocket fuels generate different amounts of momentum, and some are better suited to some applications than others. Many rockets will have multiple stages, sometimes using different fuels depending on the purpose of the stage.
A lot of rocket fuels require planning and careful attention to handle, since they’re designed to be highly flammable by definition. Some types of fuel are easier to work with than others and require less infrastructure. Here’s some details on some of the most commonly used rocket fuels:
Liquid hydrogen is one of the most popular rocket fuels used by companies like NASA because it has the lowest mass possible for a rocket fuel. Hydrogen is the lightest molecule, the very first element on the periodic table, so it’s not possible for it to get any lighter. What makes it a great rocket fuel is how intense it burns when combined with oxygen – it’s incredibly efficient, so it can lift a lot of mass relative to its own mass.
The complications with hydrogen and oxygen fuels are that they need to be kept as liquids, and they only stay in liquid forms at very high pressures and/or very cold temperatures. At atmospheric pressure, hydrogen gas liquefies at -423 degrees Fahrenheit (-252 degrees Celsius), and oxygen gas liquefies at -297 degrees Fahrenheit (-182 degrees Celsius). In order to be stored and transported, they both have to be kept in expertly crafted, highly insulated pressure vessels to keep them at high pressures.
Liquid hydrogen also has a bad habit of leaking if not planned for since the molecules are so small they can sometimes slip through very tiny gaps, including pores in welds. Liquid oxygen is extremely volatile, and will combust violently if given the opportunity. They both require expert handling to use.
RP-1 (Refined Petroleum-1) is another common type of rocket fuel. It’s a type of kerosene similar to the fuel used in jets. It’s not as efficient as liquid hydrogen, but it has some solid advantages that make it the better choice in some situations.
For example, RP-1 is a lot safer to handle than liquid hydrogen, since it’s liquid at room temperature and atmospheric pressure and therefore doesn’t have the same stringent requirements to use. It’s cheaper to manufacture, transport, and store, although if you really need to get the most mass up into space with the least amount of fuel, liquid hydrogen is the way to go.
Liquid methane is gaining attention as the next popular type of rocket fuel, and for good reason. It’s got several key advantages. It’s more efficient, cheaper to produce, and more environmentally friendly than RP-1. The biggest drawback to using liquid methane is, like hydrogen, it requires extremely cold temperatures and/or high pressures to be in a liquid state, and so requires specialized equipment to store and transport it, including pressure vessels.
Solid rocket fuel is an expansive category including just about anything used to propel rockets into the air, as long as it’s in a solid form when it’s lit. Almost all amateur rockets use solid rocket fuel, like gunpowder or “rocket candy”, which got its name from using types of sugar as an ingredient.
Larger rockets sometimes use solid fuels, although they’re more commonly reserved for boosters and stabilizers than the main propelling force doing most of the heavy lifting.
Biofuel is a new type of rocket fuel geared towards small launches and lighter payloads. It burns cleaner than RP-1, and as the technology develops, it will become more refined and cheaper to use. The first commercial rocket that uses biofuel was launched in 2021 by bluShift Aerospace.
Rocket fuels often need to be stored in custom high pressure vessels designed for aerospace applications, which is one of Rexarc’s specialties. Learn more about our ASME standard pressure vessels here, or feel free to reach out to us with any questions about aerospace applications and the types of vessels we design and fabricate.
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