Rocket Fuel Types: What fuels dreams of distant planets?

When mankind seriously decided to conquer space, it became clear that the capabilities of our terrestrial fuel, be it gasoline or even aviation kerosene, were not enough to tear a giant rocket away from the Earth and make it cross the Kármán line. 

Creating efficient space fuel was one of the major challenges of the early space age and remains so to this day. What did space truckers use to power the vehicles then? What do they use today, and what will they use tomorrow? In this article, you will learn what rocket fuel is made of and how it has evolved. 

The Physics of Freedom: How Exactly Does a Rocket Function to Break Earth’s Bonds?

Before we dive into the chemistry of space fuel, let’s understand the principle of rocket operation, which is based on Newton’s third law: action equals counteraction. In other words, to push something forward, it is necessary to push something back. A simple example: inflate a balloon and let it go. It will fly because the air rushes out of it downward, pushing the balloon upward. A rocket essentially does the same thing, only… without air.

Conventional engines (like in a car or airplane) take oxygen from the air so the fuel can burn. This process is called oxidation. But there is no air in space, so the rocket must carry with it both the specific burnable agent and an oxidizer that will allow the fuel to burn without air. These two components, the burning agent and oxidizer, are called a fuel pair or rocket propellant.

In the engine’s combustion chamber, the components ignite, producing red-hot gases (products of combustion) at high pressure. These gases are ejected downward from the rocket nozzle at tremendous speed (action), creating a jet thrust that pushes the rocket upward (counteraction). The faster and more massive the flow of gases, the greater the thrust and the faster the rocket’s acceleration.

The efficiency of propellants is characterized by specific impulse – the time in seconds that an engine producing 1 kilogram-force thrust can run, using up 1 kilogram of fuel. In essence, it is like “mileage per liter”, only for rockets.

The Alchemist’s Pantry: Exploring the Diverse Types of Rocket Fuels

Over decades of rocket science, engineers and chemists have developed many different types of spaceship fuel based on purpose, spacecraft size, required thrust, cost, safety, and even environmental considerations. All these propellants can be divided into 4 major categories: liquid, solid, hybrid, and mono.

The table below shows the key differences. 

Rocket Propellant Types: Key Characteristics & Uses

And now let’s learn about the most common propellants.

Hydrogen and Oxygen (LH2/LOX)

This pair of components is a classic liquid space fuel that gives record speeds for interplanetary flights. Its flame is almost invisible, but launching it generates giant clouds of vapor from the water produced during combustion – in essence, it is rocket propellant made of the simplest elements. However, the requirement to store superfluid hydrogen at -253°C causes many metals to become brittle, requiring unique alloys and tank and piping designs, such as those used in the J-2 Saturn V or RS-25 Space Shuttle engines.

RP-1 (Highly refined kerosene) 

Rocket Propellant-1 is an “elite” version of kerosene that is most often used in tandem with LOX. As the primary propellant for the first stages of many rockets (Falcon 9, Atlas, Soyuz), RP-1 is particularly valued for its high density. This allows for more compact fuel tanks, which reduces the aerodynamic drag of the rocket when passing through dense layers of the atmosphere. The characteristic thick orange exhaust and the mighty roar of the kerosene engines have become true symbols of space power.

Methane Rocket Fuel ( LCH4/LOX)

Liquid methane with oxygen is a promising propellant of the 21st century. It has been specifically chosen as the main SpaceX Starship fuel type. The key advantage of methane is the possibility of autogenous supercharging of tanks (using part of the vaporized fuel to create pressure), which simplifies the design of engines. In addition, methane, unlike kerosene, hardly smokes, which makes it a strategically important SpaceX fuel for the reuse of Starship Raptor and BE-4 engines installed in Blue Origin New Glenn and ULA Vulcan Centaur rockets.

Solid Rocket Fuel (Descendants of gunpowder)

Yes, yes, it was gunpowder that was the first rocket fuel. The Chinese used it in firecrackers and fire arrows back in the 11th century. Today, solid propellants are made of carefully selected and compressed chemical composites (most often aluminum as a burning agent + ammonium perchlorate as an oxidizer + HTPB polymer as a binder). Their combustion can be incredibly fast, providing huge momentum in seconds, and their insensitivity to external atmospheric pressure makes them particularly efficient on launch. This is why solid propellants are still relevant for the lateral boosters of launch vehicles.

Biofuel (Green liquid propellants)

Kerosene, ethanol, and methane rocket fuel with the prefix “bio” are a glimpse into a future where propellants will become more economical and environmentally friendly. Scientists are experimenting with recycling plastic waste (Skyrora Ecosene technology) or organics (manure, food waste, wood, algae, etc.). The main goal is to achieve high energy density and combustion stability comparable to traditional analogs, reducing harm to the Earth’s ecology and possibly paving the way for the creation of propellants for space colonies.

Is there a universal rocket fuel formula?

There is no single formula for rocket propellant because of the variety of tasks facing rockets. The choice is always a compromise between efficiency, cost, and safety. However, the basic principle is the same – a chemical combustion reaction that releases a large amount of energy to create jet thrust. Perhaps a universal engine will appear in the distant future, but for now, engineers are selecting optimal propellants for each spacecraft.

How much fuel does a rocket use?

VERY MUCH. This is one of the most impressive facts when you start learning about space fuel. For a rocket to get off the ground and reach the first space speed to enter orbit (about 28,000 km/h), 80-90% of its mass at launch must be propellant. For example, Saturn V, which delivered astronauts to the Moon, with a launch mass of 2.8 million kg, carried 2.3 million kg of propellant. And Falcon 9 with a launch mass of 549 tons carries from 450 to 500 tons.

This disproportion is explained by Tsiolkovsky’s equation: to achieve high speed, a rocket must carry many times more fuel compared to its “dry” mass. So, a rocket is, in fact, a giant fuel tank with engines and a small payload.

Spaceship fuel: A Vision of the Future

Understanding what rocket fuel is made of and how it works is key to our continued expansion into space, so the race for the perfect propellant continues unabated: scientists are searching for formulas that can deliver more speed, safety, and range. Each new successful compound is not just a step towards new horizons, it is fuel for our most audacious dreams. And even though this path is long and thorny, history proves: the power and perseverance of the human mind, like the Universe, knows no bounds.