Why Can Sugar be Used as Rocket Fuel?
What makes something suitable as rocket fuel? We look at the chemistry and science of rocket fuel.
Do you think of sugar as something highly flamable and explosive? Me neither, but one of the scenes in the movie, The Martian, got me wondering if I might be wrong.
In one of the movie scenes, an astronaut makes a bomb using only sugar and liquid oxygen. Not your typically bomb making material. Curious I investigated further and realized sugar is even used in amateur rockets. In fact all sorts of things may be used as fuel: ground up PVC tubes and gummy bears. The Myth Busters tried out the latter.
This all makes it seem as if it is totally random what might be used as rocket fuel. However there is a pattern. E.g. you can’t use gummy bears as they are. They have to be dried and made into powder.
Maximum Surface Area
Why is it often important that the fuel is in powder form? Here is a thought experiment to give you a clue: Imagine throwing a cloud of powder in the air and somehow being able to ignite it. Around every powder grain 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 of it would burn simultaneously. The key is to provide the fuel with as much surface area as possible getting in contact with 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 so that the fuel has as much surface area as possible exposed to air.
The same happens in the combustion chamber of a liquid fueled rocket engine. Some sort of injector like a pintle injector e.g. 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. Also 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.
Which begs the question, what exactly is an oxidizer?
An oxidizer is any chemical which is good at stealing electrons from another chemical. Oxygen is really good at it, 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 question is of course, why does an atoms 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 very strongly with almost anything, especially with certain metals.
Predicting Possible 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 we do experiments like trying to ionize different atoms. Then we discover that 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 atoms with 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 onto electrons tightly.
How do we predict which atoms behave in which way? Say we got the atomic number (number of protons or electrons in 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.
This is why we got the periodic table. It organizes atoms into these periods, so that each column in the periodic table represents atoms with similar chemical properties. To the left we got metals and on the right we got 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.
How Does This Help Pick Rocket Propellant?
Rocket propellant is the combination of rocket fuel and oxidizer which we are using to make the rocket propel 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.
Releasing Energy by Forming Strong Chemical Bonds
The closer the atoms can be pulled together the stronger the binding, or chemical bond.
When a strong chemical bond is formed, then more energy is released than when a weak bond is formed. One analogy I think is useful to think about is how e.g. the moon or a satellite is bound to the earth through the gravitational force. The atoms will be bound by the electrical force instead.
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. On the other hand if we give the satellite a hard push and increase its kinetic energy, then it will assume a higher orbit. If we give it a hard enough push then it will completely de-orbit the earth.
The analogy with chemistry would be that breaking a chemical bond, implies moving atoms too far away from each other for the electromagnetic force to snap them together again. As with the satellite example this requires adding energy. We typically do that by heating up the chemicals in some manner.
So 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.
So a good rocket propellant is thus a combination of fuel and oxidizer where there is as much difference as possible between the energy required to break the bonds of the reactants (fuel and oxidizer) and the energy released when forming the bonds of the products. Reactants are the inputs to a chemical reaction and products are the outputs. If we burn e.g. kerosene or sugar, the product would be water and carbon dioxide (CO₂).
The reaction for sugar and oxygen:
C₁₂H₂₂O₁₁ 12 O₂ ⟶ 12 CO₂ + 11 H₂O + energy
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 temperature. As you burn sugar and potassium nitrate, the reaction will produce excess heat which can further heat up remaining potassium nitrate and thus release more oxygen for further combustion.