Solid Rocket Fuel and Oxidant Agent

[fog37]

Hello Forum,

For a combustion to happen, there needs to be one material that plays the role of the fuel and another material that is the oxidant.
For example, fuel and oxygen react together to produce light and heat and expanding gases.

In making rocket fuel, sugar is used as fuel and potassium nitrate as oxidant. Why can the oxidant not be oxygen? Can the sugar not burn with oxygen? What is the role of the oxidant? Is the oxidant specific to the fuel being used?

Thanks,
fog37

 

[Borek]

And how are you going to mix the sugar with the oxygen?

 

[russ_watters]

Oxygen is the oxidant that potassium nitrate contains. Following-up on Borek's post: what's the difference in properties between molecular oxygen and potassium nitrate?

 

[fog37]

Well,

if we threw a match into a puddle of gasoline the oxygen is present in the atmosphere. It does not seem that the fuel and the oxygen mix...

 

[Borek]

It does not seem that the fuel and the oxygen mix

Hydrogen (another fuel) mixes with oxygen pretty well.

But you are on the right track. Things don't always mix the way we need them to be mixed.

 

[fog37]

I know that:
-- Combustion always produces carbon dioxide and water as byproduct.
-- Gasoline is made of hydrocarbons, i.e. hydrogen is present. Sugars contain carbon, oxygen and hydrogen too.
-- Combustion can be complete or incomplete depending on if the oxidant is present in sufficient amount or not
-- Turbulence helps the mixing process between the fuel and oxidizer

In a car engine, air and fuel are intentionally mixed to promote a complete combustion even if fuel can just burn without that mixing (puddle example). I think sugar would do the same but the reaction would not be very effective...

 

[Borek]

You are forgetting sugar is a solid, while gasoline is a liquid (and a highly volatile one).

Sugar/nitrates is a solid fuel, gasoline/air is not. While some general principles are identical, different setups require different approach.

Combustion always produces carbon dioxide and water as byproduct.

Combustion of sulfur produces neither. And water is hardly a byproduct - it is just a product.

 

[fog37]

Thanks Borek! I am learning a lot this morning.

-- By using a flame, I could burn sugar without mixing it with a nitrate. But I guess the combustion would not be complete.
-- The result from a combustion reaction is the generation of energy in the form of heat and light. The thermal energy heats up the air which expands abruptly and provides the propulsion of the rocket or car (pistons) by Newton's 3rd law.

-- You specify that water is just a product and not a byproduct. I have used the two terms interchangeably. What is the difference? A chemical reaction has reactants and products.

-- So the products are not always energy and carbon and hydrogen atoms that combine with other atoms to form water and CO2. Your sulphur, S, is not a hydrocarbon.

-- Does an explosion (rapid expansion or air) always presume that a combustion reaction has taken place?

Thanks.

 

[DrStupid]

Oxygen is the oxidant that potassium nitrate contains.

Isn't it nitrogen?

Does an explosion (rapid expansion or air) always presume that a combustion reaction has taken place?

No, explosions doesn't require any kind of reaction.

 

[russ_watters]

Isn't it nitrogen?

No, the nitrogen is sort of a carrier or binder:

The thought the OP never completed is that the thing that makes potassium nitrate more useful for solid rockets oxidizer is that it is...a solid.

 

[DrStupid]

No, the nitrogen is sort of a carrier:

Check the oxidation states.

 

[fog37]

We said that sugar is a solid and mixes well with another solid like potassium nitrate.

What about wood? that is a solid too which burns using the oxygen in air once a spark or flame is provided...

 

[Borek]

You need to select a solid that will burn without leaving an ash.

 

[fog37]

I see. Silly question: what is the problem with ash. Is it the waste of the reaction? Should there be no waste in combustion reactions?

 

[DrStupid]

what is the problem with ash.

It reduces the efficiency. To maximize thrust the resulting amount of gas should be maximized and the amount of liquids and solids minimized. It is possible to build rockets with propellants which produce ash (e.g. firework rockets powered with something like gun powder) but at a large scale it is not a good idea.

 

[SteamKing]

No, explosions doesn't require any kind of reaction.

You sure about that? Even a A-bomb requires that a nuclear reaction take place before an explosion occurs.

 

[Borek]

What about steam boiler explosion?

 

[SteamKing]

What about steam boiler explosion?

Point taken.