Liquid Fuels
Any flammable liquid with a viscosity similar to or less than that of water is a potentially usable fuel in a liquid rocket engine. Here we will explore some of the fuel categories and learn why certain fuels are preferred for amateur use. Also found at the end of this page are custom fuel blends which have been used in Half Cat Rocketry engines.
Fuels
Alcohols
Most alcohols are stable and generally benign liquids which are easy to find off the shelf. Note that these are still solvents and can be harmful if ingested, inhaled, or splashed in the eyes.
Ethanol has a long history in rocketry going all the way back to the V2 - it is a very good coolant (relevant for regeneratively cooled engines) with a very respectable specific impulse, evaporates if spilled, poses almost no handling risk, and leaves no residue behind in fluid systems. Its properties are extremely well known, making it a predictable choice. One drawback to ethanol is that it can be fairly expensive in higher concentrations since it is taxed for its use as a beverage, unless denatured ethanol is used instead.
Isopropanol has all of the same benefits but no extra tax and can be found 99% pure in gallon jugs at hardware stores or online. Isopropanol performs only slightly worse than ethanol but is one of the safest possible fuels and therefore our fuel of choice.
Methanol theoretically provides somewhat better performance in terms of O/F ratio, density, and specific impulse, but it has more handling concerns that make it non-ideal compared to the two aforementioned alcohols. With proper consideration and PPE it is a perfectly valid choice for a liquid engine - notably, Calico 1 motors burn methanol for its superior performance (see Meowjave Green below).
There are many other exotic alcohols beyond these three, but they get more expensive and come with a host of safety issues that are simply not desirable for most amateurs. One particular mention will be made for furfuryl alcohol which is hypergolic with nitric acid - seen in use decades ago by the likes of Tom Mueller and FAR's Genesis rocket.
Hydrocarbons
Kerosene, although not as commonly seen with nitrous oxide, is perhaps the most widely used and versatile rocket fuel in existence. It has a high density and good cooling properties, although care must be taken to avoid building up residue in fluid passageways (since kerosene is a mixture of various length hydrocarbon chains). The main difference compared to alcohol, relevent for amateur rocketeers, is that kerosene doesn't readily evaporate. A fuel spill must be treated a potential fire hazard and cleaned up. This applies to clothing and skin contact as well: Whereas isopropanol will evaporate quickly, kerosene will stay present and potentially catch fire later.
Do not use gasoline. It has highly flammable vapors and no benefits compared to any of the other liquid fuels.
Propane
Solvents
Acetone
Limonene
Additives
Fuel Blends
Custom mixtures of fuels we have used in our motors.
Meowjave Green
Methanol + Boric Acid
Used in: Calico 1
Boric acid, sometimes labeled orthoboric acid, burns a bright green. Besides making the exhaust plume look exciting, it is helpful to give methanol a color when burning - since methanol flames are invisible in broad daylight, adding a colorant makes any residual fire outside of the combustion chamber more obvious.
Half Cat Jungle Juice
Isopropanol + Ethanol + Acetone + Paint Thinner
This came about due to not having enough isopropyl alcohol on hand for a test, and subsequently filling the fuel tank with whatever was on hand. The proportions are completely arbitrary, as the performance is so similar among all of the constituent fuels.
Meowjave Mix
Anything + Dirt, Dust, Sand
Used in: 1Cat/4
This is not a specific formulation or even an intentional one, but was included here to make the point that amateur motors can and should be resilient to some amount of FOD in the propellant. While thorough cleaning is always a good idea, we know from experience that the presence of unintended particulates has no meaningful effect on the performance or health of an engine.