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.

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

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.

A carburator mixes gasoline (fuel) with oxygen (ozidizer), to increase the amount of gasoline in close contact with oxygen.

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…