Why Can Sugar be Used as Rocket Fuel?

A discussion of what allows a compound to be used as a fuel

Normally, I would not consider sugar to be particularly flammable or dangerous, but one of the scenes in the movie The Martian starring Matt Damon back in 2015 got me thinking.

In the scene, one astronaut makes a bomb using only sugar and liquid oxygen. Not what you typically think of as bomb making material. Curious, I investigated further and realized sugar is even used in amateur rockets. In fact, all sorts of things are used as fuels, ground up PVC tubes and gummy bears. The Myth Busters tried out the latter.

Mixture of sugar and potassium nitrate burning in a home made rocket engine

It may seem as if it is totally random what you might use as rocket fuel. However, there is a pattern to these accounts. For instance, you cannot use gummy bears as they are. They have to be dried and made into powder.

It is all about maximizing surface area

Why is it so important for fuel to be in powder form? Imagine throwing a cloud of powder in the air and somehow being able to ignite it. Around every powder particle, there will be some air which allows combustion to take place. Remember, nothing can burn without oxygen, or more specifically an oxidizer. If that powder was compacted into a solid block, however, combustion could take place only at the surface area of that block. That means far less would burn simultaneously. The key, in other words, is to expose as much surface area of the fuel as possible to an oxidizer.

One way of doing this is to mix a fuel like sugar with an oxidizer in solid form. But this is a general principle. That is what a carburetor in an internal combustion engine does. It mixes a liquid fuel like gasoline with air. It attempts to turn the liquid fuel into a mist of droplets, which maximizes the surface area of the fuel exposed to the surrounding air.

How liquid fuel is turned into a mist of droplets inside a carburetor in a car

The same happens in the combustion chamber of a liquid fueled rocket engine. Some sort of injector like a pintle injector for instance is used to spray the fuel into the combustion chamber, so it enters as a mist of fuel droplets. Then as much of the fuel as possible can react with the oxygen injected into the same chamber.

That is why a PVC tube in its original form won't work well as rocket fuel. It has to be ground up to increase surface area. Furthermore, with a solid, the individual particles in a pile won't easily find oxygen to react with. That is why we need to mix it with an oxidizer in solid form.

In fact, this is how gunpowder works. It is really just a mix of fuel in the form of coal and sulfur powder mixed with potassium nitrate (KNO₃), which works as an oxidizer.

Gunpowder is a fast way of burning charcoal

Now, you are probably wondering: What exactly is an oxidizer?

Oxidizers are master electron thieves

An oxidizer is any chemical which is good at stealing electrons from another chemical. Oxygen is really good at that, but fluoride is even better.

The ability to grab electrons is referred to as electron negativity and is closely related to ionization energy, which is the energy required to knock an electron off an atom and turn it into a charged ion.

Atoms with high electron negativity such as oxygen and fluoride requires high ionization energy to turn into ions.

Now, the next question is: why does an atom's ability to grab electrons matter? Before people knew anything about electrons and knew much about how an atom behaved, one could simply observe that certain atoms like oxygen, chloride, and fluoride reacted strongly with almost anything, especially with certain metals.

Using the periodic table to predict which element is an oxidizer

In science, when we make observations, we want to be able to formulate rules which allow us to make predictions, about behaviors we haven't tried yet. So one does experiments like trying to ionize different atoms and find that all these gasses which happen to react very violently with everything also happens to require a lot of energy to be ionized. The metals which they affect most strongly happen to have very low ionization energy. In fact, there is a linear relationship: the gasses with the higher ionization energy react more strongly, and they react strongest with the elements which have the lowest ionization energy.

Thus, when pairing atoms into molecules, observations suggest that atoms which hold their electrons weakly easily react with atoms which hold on to electrons tightly.

How do we predict which atoms behave as electron thieves? Say we got the atomic number (number of protons or electrons in an atom), and arrange atoms by that. It turns out that the properties of the atoms, such as required ionization energy to free electrons, isn't entirely random. But it isn't linear by atom number, either. Instead, it occurs periodically. Electron negativity and ionization energy will keep increasing with atom number for a period, only to drop low again and start a new period of increasing electron negativity.

Shows how energy required to ionize an atom periodically falls and increases with the increasing atom size (atomic number).

This observation is what led us to organize elements into a periodic table. It organizes atoms into specific periods, so that each column in the periodic table represents atoms with similar chemical properties. The left side of the periodic table contains metals, and the right side contains gasses. The left-most metals have the lowest ionization energy, and the right-most gasses have the highest. Ionization energy drops as you move down a period and increases as you move up. The table has been purposefully arranged that way.

Periodic table has been organized based on the periodic occurance of chemical properties such as electron negativity and ionization energy. Elements have been colored according to their chemical properties.

How does the periodic table help us pick a rocket propellant?

Rocket propellant is the combination of rocket fuel and oxidizer which we are using to propel the rocket forward. How the fuel and oxidizer react with each other decides how much propulsion we get from the propellant. All other things being equal, anything which produces a lot of heat quickly when the fuel and oxidizer react will give us better propulsion.

Knowing the electron negativity or ionization energy of the fuel and oxidizer gives as a clue as to what atoms are likely to react with each other. When an atom steals electrons from another, it becomes slightly negative and the other atom lacking an electron becomes slightly positive. That makes the atoms snap together by the electrical forces, which causes opposite charges to attract each other and equal charges to repel each other.

Unless there is a big difference in electron negativity, the more greedy atom won't be able to fully pull away the electrons. Instead, the atoms will engage in a tug war over the electrons binding the atoms to each other.

The closer the atoms can be pulled together, the stronger the binding, or chemical bond between them, will be.

Understanding why creating chemical bonds release energy

When a strong chemical bond is formed, more energy is released than when a weak bond is formed. One analogy I think is useful to think about is how for instance the moon or a satellite is bound to the Earth through the gravitational force. If we want a satellite to get closer to the earth, it has to reduce its velocity or kinetic energy to fall to a lower orbit. If we, on the other hand, give the satellite a hard push and increase its kinetic energy, then it will assume a higher orbit. With a strong enough push, the satellite will leave Earth orbit entirely.

The analogy with chemistry would be to break the chemical bond, by moving the atoms too far away from each other. As with the satellite example, this requires adding energy. We typically do that by heating the chemicals in some manner. We can summarize the relation between energy and molecular bonds:

Breaking bonds, requires energy, while forming bonds releases energy. Strong bonds require a lot of energy to break, but they also release a lot of energy when formed.

Allow me to elaborate on what makes a good fuel and oxidizer combination as opposed to a bad one.

Both the fuel and the oxidizer will consist of molecules with chemical bonds. If you react methane (CH₄) with oxygen (O₂) you need first spend energy to break the bonds between carbon (C) and hydrogen (H) in the methane molecules. You also need to break the bonds between oxygen molecules to create free oxygen atoms. Later, oxygen atoms will bond with carbon and hydrogen to form carbon dioxide (CO₂) and water (H₂O). The reaction looks as follows:

CH₄ + 2O₂ ⟶ CO₂ + 2H₂O + energy

The input compounds on the left side of the arrow are called the reactants, while the outputs on the right side of the arrow are called the products.

Breaking chemical bonds in the reactants consumes energy, while forming bonds in the products release energy. Typically, energy is added to a mix of reactants by heating them up. The heating breaks the bonds in the reactants and allow new bonds to form, which further heats the mix and sustains the reaction.

In a good fuel and oxidizer combination, there is a big difference between energy spent to break the bonds in the reactants and the energy released in the products.

In other words, if the reactants are made up of elements with very weak bonds and the products have very strong chemical bonds, then much more energy will get released from the reaction than if the reactants had strong bonds.

Since we are discussing sugar as a rocket fuel, let us look at the reaction between sugar and oxygen:

C₁₂H₂₂O₁₁ 12 O₂ ⟶ 12 CO₂ + 11 H₂O + energy

Another very popular rocket fuel is RP-1 (highly refined Kerosene) and oxygen:

2 C₁₂H₂₆ 37 O₂ ⟶ 24 CO₂ + 26 H₂O + energy

CO₂ has very strong bonds, which means the forming of CO₂ releases a lot of energy.

With kerosene fuel, we usually use liquid oxygen as an oxidizer. For sugar, a solid oxidizer works better. Potassium nitrate (KNO₃) contains a lot of oxygen, which is released under high heat. As you burn sugar and potassium nitrate, the reaction will produce excess heat, which can further heat the remaining potassium nitrate and thus release more oxygen for further combustion.

Wrap up

What I have discussed in this story is what is generally known as Redox reactions, which is short for reduction-oxidization. A chemist would refer to the fuel as the reductant or reducing agent. To quote wikipedia:

Thus, in the reaction, the reductant or reducing agent loses electrons and is oxidized, and the oxidant or oxidizing agent gains electrons and is reduced. The pair of an oxidizing and reducing agent that is involved in a particular reaction is called a redox pair.

Here I have described redux reactions which work quite violently to help produce thrust to power rockets. However, a redox reaction can also be a much slower reaction, such as iron rusting on a surface. In this case, oxygen in the air oxidizes the iron and the product is iron oxide, better known as rust. The reaction still produces heat, but because the reaction happens, so slowly we normally don't notice that heat production.