Types of Rocket Fuel: From Hydrazine Rocket Fuel to Green Propellants

Since the first orbital launch more than 60 years ago, humanity has made great progress in the development of near-Earth space and deep space, which today include the development of new rocket fuel types — renewable green propellants. Thousands of satellites revolve around the Earth today, flags of the USA and China flutter on the Moon, and the Perseverance rover sends videos with Red Planet landscapes, which seemed fantastic just a few years ago.

In the last decade alone, the number of orbital rocket launches has increased by more than 50% (from 74 in 2010 to 114 in 2020), and this trend will continue. Space traffic is becoming denser, and some scientists have already sounded the alarm. Indeed, with each rocket launch, tens of tons of fuel are burned, polluting the upper layers of the atmosphere with harmful emissions of soot and aluminium. And this is when the Earth is already choking on car exhaust and industrial emissions. So, are existing types of rocket fuel bad for the environment?

The world sooner or later would be on the verge of an ecological catastrophe if nothing changes, and only a transition to renewable energy and green propellants can save it. The leading countries of the world are joining forces to achieve carbon neutrality, and this, in particular, affects the space and rocket launch industry. This article will explain why rocket fuel is bad for the environment and what are the advantages of green propellants.

How do fuel characteristics affect rocket launches?

Today, almost three dozen different rockets and their modifications are in operation worldwide, about 20 more are being prepared for the first launches, and 50+ are retired.

Depending on the type of rocket (small, medium, heavy or superheavy), its launch weight can reach tens and even hundreds of tons. So, going beyond the atmosphere and accelerating to orbital speed requires a significantly predominant thrust of jet engines. This is where rocket fuel plays a key role.

Unlike a machine engine, a rocket engine does not rotate anything; it receives energy by burning fuel in combustion chambers. The resulting high-pressure gas is expelled in one direction, causing the rocket to fly at a certain acceleration in the other direction. The speed of gas emission is called thrust or engine force and is measured in Newtons, since this principle of engine operation is consistent with Newton’s third law, “Action equals reaction”. In other words, fuel is the main source of energy for the engine, determining its performance by 90%. So, when choosing a fuel type, it is important to consider some other aspects besides the pressing question — is rocket fuel toxic? These aspects include:

1. Specific impulse (ratio of engine thrust to fuel mass flow, measured in m/s). The higher the specific impulse, the less fuel is required to put the same load payload into orbit.
2. Fuel density. The higher it is, the smaller the volume required by the engine and fuel system.
3. Combustion temperature. For example, cryogenic fuel with a low combustion temperature requires special storage, the use of cold-resistant materials in the engine design, and the thermal insulation of the rocket. This complicates the launcher development and creates special requirements for the launchpad infrastructure. Similar high requirements for the design and engine manufacture of materials are imposed by fuel with a high combustion temperature.
4. Combustion stability. The instability of fuel combustion can lead to failure or explosion of engines.
5. Toxicity. For example, highly toxic hydrazine rocket fuel creates increased environmental and economic risks due to the carbon footprint and emergency situations (human health, flora and fauna suffer, soils, water bodies are contaminated, etc.).

Thus, a highly efficient rocket fuel must meet the following requirements: have a high specific impulse and density, be stable, safe to handle, non-toxic, and compatible with structural materials. And of course, it should have enough raw materials for production. Another important question — is rocket fuel renewable?

What type of fuel do rockets use?

There are three types of rocket fuel: solid, liquid and hybrid. Let’s take a closer look at what type of fuel rockets use.

Solid rocket fuels

Solid fuel is a mixture of substances that burn without access to oxygen. It is divided into homogeneous and mixed. The first is usually a solid solution of nitrocellulose in nitroglycerin. The mixed one consists of ammonium perchlorate, fine spherical powder of aluminium or magnesium and an organic polymer. In terms of environmental friendliness, solid fuel is considered the dirtiest. In addition, it is very expensive to manufacture, despite its high energy efficiency. Solid propellant engines were widely used in the side boosters of Space Shuttle, Titan, Delta, Ariane-5 rockets. Today, solid propellant engines are mainly used in the 3rd and 4th stages of rockets to bring satellites into geostationary orbits or planetary trajectories.

Hybrid rocket fuels

A hybrid fuel is a mixture of a solid and a liquid oxidizer. It cannot ensure high thrust, so hybrid fuel engines are rarely built. A polybutadiene/liquid nitrogen hybrid engine was used in SpaceShip One, which later became the prototype for Virgin Galactic’s VSS Unity air-launch system. In the summer of 2021, VSS Unity carried out the first-ever suborbital tourist flight.

The German private aerospace startup HyImpulse Technologies also plans to use a paraffin/liquid oxygen hybrid engine in its light rocket SL 1.

Liquid rocket fuels

And finally, the most common today is a liquid fuel, divided into several subtypes. Below, you will see what type of fuel most of the rockets use.

1. Hydrazine rocket fuel (hydrazine/nitrogen tetroxide). Hydrazine rocket fuel is also called hyperhead fuel, which is considered very toxic. However, due to its high combustion stability, high density, energy efficiency, lightness and long storage life, it does not yet have an equivalent alternative for repetitive engines used in orbital manoeuvring. Used in Space Shuttle, Delta 2, Titan 2, Proton, etc.
2. RP1/LOX is kerosene/liquid oxygen. The most popular and cheapest fuel with a superbly developed infrastructure. It has an average environmental friendliness and density. This fuel pair is used in many modern rockets, particularly in the SpaceX Falcon 9, Soyuz, in the first stage of Atlas 5.
3. LH2/LOX is liquid hydrogen/liquid oxygen. It belongs to the cryogenic fuel class. Liquid hydrogen is highly environmentally friendly and gives a specific impulse about 30-40% higher than most other rocket fuels. But due to low density and low storage temperature, it is practically ineffective in the first stages of medium and heavy rockets. Rocket tanks filled with liquid hydrogen should be ten times the volume of kerosene tanks with equal weight. However, the LH2/LOX has found use in the upper stages, where thrust priority is reduced, and mass value is increased

So, now that you know what rockets use for fuel, let’s discuss the impact of space launches on the environment.

Is rocket fuel bad for the environment?

So is rocket fuel toxic or not? Definitely, yes. The solid propellant is the most toxic, emitting alumina, hydrogen chloride, nitrous oxide, soot, and carbon dioxide. Kerosene and hydrazine fuel engines emit CO2, nitrogen oxides, sulfur compounds and water vapour, which also leads to air pollution and ozone depletion due to the greenhouse effect.

But is the damage from a rocket launch so big? According to statistics, in recent years, the space industry has been producing about 25,000 tons of CO2 per year, which is about 40,000 times less than the entire aviation industry (which accounts for approximately 2% of climate pollution). But the problem is that rockets affect the upper layers of the atmosphere, which increases the environmental impact.

Of course, rocket launches do harm, but the transition to green propellants or renewable rocket fuel, as part of the global trend towards achieving carbon neutrality, is inevitable. Let’s find out what types of green rocket fuel can replace fossils.

What is renewable rocket fuel?

Humanity receives around 80 per cent of all energy from fossil fuels: coal, oil and natural gas. Its extraction and use account for three-quarters of emissions. But this is not the only problem. Unfortunately, fossil fuels are non-renewable resources and will have to wait millions of years for new coal, oil and natural gas deposits to form, which is unrealistic. The only available solution to replace fossil fuels today is a renewable fuel, usually obtained from biomass, which is why it is also called biofuel. But is rocket fuel renewable? Sure! More importantly, there are green propellants, too.

Green rocket fuel types

Cryogenic fuel liquid methane + liquid oxygen is considered the most promising for rocket engines. It is very cheap, and in terms of characteristics, it occupies an intermediate position between RP1/LOX and LH2/LOX. This fuel pair will power the BE-4 engines of the new generation of Blue Origin’s New Shepard and New Glenn rockets, as well as the Raptor engines of the SpaceX Starship, designed to colonize the Moon and Mars. By the way, methane can be partially extracted on Mars, which will reduce the cost of delivering the fuel needed to return. And on Earth, methane is now being extracted from wood waste. That is a good answer to a pressing question – is rocket fuel renewable?

A unique type of green rocket fuel is offered by the British aerospace company Orbex Space. Its lightweight Prime rocket will fly LOX/bioLPG, a mixture of liquid oxygen and biopropane. Biopropane for Orbex is supplied by another British company, Calor, which receives gas as a by-product from biodiesel production. According to Orbex Space, their rocket will be 86% less toxic at launch than a similarly sized fossil-fuelled rocket.

Another British startup, Skyrora Ltd, is building an XL light orbital rocket with hydrogen peroxide/Ecosene propellant engines. Ecosene is a more productive analogue of RP-1, which is obtained by processing plastic waste. Skyrora XL engines produce approximately 40% fewer emissions, carbon dioxide, carbon monoxide, soot and sulphur than RP1/LOX engines.

But the American startup bluShift Aerospace is developing an ultra-light rocket Stardust on solid biofuel obtained from agricultural waste. According to the company, their biofuels are completely non-toxic, carbon-neutral and can be purchased from farms across America.

These are just a few examples of how humanity is trying to save the planet. We see how automotive companies follow Tesla in the mass production of electric cars, aircraft manufacturers are seriously thinking about low-emission aircraft projects, and industrial giants, albeit slowly, are switching to wind power. The future is inevitable for green propellant fuels if, of course, we want to have a future.