What exactly does "fuel" and "propellant" mean?

[Cesrate]

Are they same in definition?

[rodion_herrera]

While they may seem similar at the surface level, I think fuel pertains to something which has to be ignited in order to produce the desired force/thrust. A propellant can be something (i.e. gas or liquid) that can produce a desired force or thrust, but doesn't need to be in the act of combustion to produce the force.

[Cby]

I believe the fuel is what mixes with the oxidiser to create the thrust. Propellant is what they are together. I may be wrong.

Edited June 22, 2013 by Cby

[Canopus]

From nyrath's Atomic Rockets website:

In a rocket, there is a difference between "fuel" and "reaction mass." Rockets use Newton's third law of Action and Reaction in order to move. Mass is violently thrown away in the form of the rocket's exhaust and the reaction accelerates the rocket forward. This mass is of course the "reaction mass." It is sometimes also called "remass" or "propellant."

The "fuel" is what is burned or whatever to generated the energy to expel the reaction mass. For example, in a classic atomic rocket, the fuel is the uranium-235 rods in the nuclear reactor, the reaction mass is the hydrogen gas heated in the reactor and expelled from the exhaust nozzle.

There are only a few confusing cases where the fuel and the reaction mass are the same thing. This is the case with chemical rockets such as the Space Shuttle and the Saturn 5, which is how the misconception started in the first place.

http://www.projectrho.com/public_html/rocket/misconceptions.php

[philly_idle]

I believe the fuel is what mixes with the oxidiser to create the thrust. Propellant is what they are together. I may be wrong.

Exactly. Propellant is the combination of fuel and oxidizer, i.e:

• fuel = anything that burns (usually either some form of hydrocarbon like kerosene or hydrogen). SRBs use aluminium with some binder like HTBP or PBAN.
  
• oxidizer = oxygen in some form (pure or as a molecule like N2O or NTO) or - in case of SRBs - ammonium perchlorate
  
• propellant = used to describe the combination of both; general term to describe what drives your rocket
  

[K^2]

Due to conservation of momentum, if you want to change your velocity in empty space, you have to eject something. Whatever you eject is the propellant. It can be rocket exhaust, stream of ions from an ion drive, light from the photon drive, or even the wrench you decided to sacrifice to try and get yourself back to the ship you accidentally drifted away from. There is no such thing as propulsion without propellant, and the quantity of propellant you need for any particular velocity adjustment depends on the I_SP of your chosen propulsion method.

Fuel is any consumable that provides your propulsion system with energy. The fuel can be chemical, nuclear, or even anti-matter fuel. Or you can have no fuel at all. For example, if you are using a solar-powered ion drive.

Bipropellant fuels of conventional chemical rockets are the main source of confusion with these concepts. For starters, your fuel is your propellant. The same stuff you burn for energy, after undergoing a chemical reaction, becomes the stuff you eject to comply with conservation of momentum. Worse yet, bipropellant fuel consists of two components, one of which we usually call fuel and the other oxidizer. So the term "fuel" can mean either the individual compound or the combination of the two depending on context. In either case, fuel is either all or part of your propellant supply.

So in general, fuel and propellant aren't the same thing. But for most of the real world rockets it is, so the terms might end up being used interchangeably in many discussions.

[Cby]

Exactly. Propellant is the combination of fuel and oxidizer, i.e:

• fuel = anything that burns (usually either some form of hydrocarbon like kerosene or hydrogen). SRBs use aluminium with some binder like HTBP or PBAN.
  
• oxidizer = oxygen in some form (pure or as a molecule like N2O or NTO) or - in case of SRBs - ammonium perchlorate
  
• propellant = used to describe the combination of both; general term to describe what drives your rocket
  

Yay! I got it right!

Edited June 22, 2013 by Cby

[pizzaoverhead]

Propellant is for causing movement. Fuels are materials used as a consumable energy source in a reaction. This could be to produce hot gases in rocket exhausts for propulsion, but it could also be fuel burned to produce oxygen in a chemical oxygen generator, wood in a fire burned to produce heat, or even the food space explorers use to fuel themselves!

EDIT: Here's a good description.

Edited June 17, 2013 by pizzaoverhead

[architeuthis]

Exactly. Propellant is the combination of fuel and oxidizer, i.e:

• oxidizer = oxygen in some form (pure or as a molecule like N2O or NTO) or - in case of SRBs - ammonium perchlorate
  

Or even more generally any electron acceptor. In many cases fluorine is preferred to oxygen.

[philly_idle]

Come on guys, you make it way too complicated. Cesrate just wanted to know the difference between fuel and propellant.

Apart from ion thrusters, the only engine type that ever made it to service is the chemical engine. There has never been a production engine that uses nuclear 'fuel' (NERVA), antimatter, or any other wild idea. So yes, for 99.999% of all engines, propellant = fuel + oxidizer.

Or even more generally any electron acceptor. In many cases fluorine is preferred to oxygen.

Fluorine (FLOX) is very expensive and the exhaust is highly toxic. To my knowledge there were only small lab test engines to test it's performance and has never been deployed to service.

Edited June 18, 2013 by philly_idle

[Nibb31]

In the rocket world: propellant = fuel + oxydizer.

Rocket engineers get pissed off when you loosely use the term "fuel" instead of "propellant". Adding fuel will get you nowhere. Adding propellant increases Delta-v.