Fuel and Oxidizer

Oxidizer

There is only one oxidizer which is easy to acquire, simple to use, indefinitely storable, and provides its own pressurization: Nitrous oxide (N2O).

Nitrous oxide is a molecule which can decompose into diatomic nitrogen and oxygen exothermically, providing a dense source of oxygen once it the decomposition reaction is started by an igniter. It has a few properties which make it the ideal choice for an amateur liquid rocket, and which make the Half Cat standard design as simple as it is.

The most important property of N2O is that it is used as a liquified gas - when stored in a closed container, it is in a saturated liquid state with a high vapor pressure around room temperature. This means that it is truly storable (unlike liquid oxygen, which constantly vents off to keep its temperature and pressure down) and self-pressurizes at normal temperatures. It is also not corrosive or toxic like hydrogen peroxide, nitric acid, or other more exotic oxidizers. See this page for safety information regarding nitrous oxide systems.

Automotive (racing) grade nitrous oxide is the most commonly available for amateurs. It typically contains up to 500 ppm sulfur dioxide, which does not have any effect on performance. Automotive N2O is available at many racing supply shops for prices which vary by supplier.

HCR-1100 standard for amateur liquid rocket oxidizer

The oxidizer shall be nitrous oxide (N2O) that is loaded, stored, and utilized in a saturated liquid condition. Oxidizer shall only be loaded into the rocket’s propellant tank through a remotely operated valve, when all personnel are at a safe distance.

Fuel

Many liquid fuels can be used, but alcohols are the best choice. They are generally cheap, easy to acquire, non-toxic, and evaporate cleanly so as to not leave residue. Ethanol specifically can easily be acquired in varying grades for low prices, especially those with a small gasoline content (E98 and E85). Almost any flammable liquid will work with varying levels of performance - see below for a full description of fuel options.

HCR-1100 standard for amateur liquid rocket fuel

The fuel shall be any non-toxic alcohol, ketone, or hydrocarbon which is a free-flowing flammable liquid at atmospheric pressure and temperature. The fuel may contain a maximum of 15% (nominally) by mass of gasoline, any amount of dissolved non-toxic solid additives, and up to 30% of water as dilutant.

Broadly speaking, there are two classes of fuels which can be used in Half Cat style rockets:  Organic solvents and hydrocarbons. As a rule of thumb, Half Cat type rockets can successfully launch with just about any free-flowing liquid that is flammable in open air. In one memorable incident, a collection of solvents was taken up late on a FAR Saturday to refill the 1Cat/3 fuel tank for a last-minute test fire attempt (the motor did not ignite, but it would have if the igniter had been better than solid propellant shavings pushed through the throat). In another instance – a launch attempt of 1Cat/4 – spare isopropyl alcohol was accidentally left behind, forcing the rocket to fly under the power of the only solvent which could be found in the moment: acetone. In yet another case, 1Cat/4 was launched with Klean Strip paintbrush cleaner, for no other reason than because it was available. Undoubtedly, even more obscure fuels will be tried in the future.

Half Cat Rocketry often experiments with weird, unconventional, or inadvisable fuels – this should not be taken as an endorsement of any particular fuel. All others building liquid rockets are strongly advised to stick with recommended fuels for safety. Recommended fuels are listed in the table above.

Gloves and eye protection should be worn at all times when handling any fuel. Always keep heat sources and open flame away from liquid fuels, and work in a well-ventilated area or outdoors to avoid fume inhalation. Always store fuel in secure, sealed containers away from heat or oxidizers.

Organic Solvents

Organic solvents are solvents which contain carbon and are typically represented by alcohols and ketones. A variety of organic solvents can be commonly found at home and hardware stores as disinfectants, paint removers, and surface cleaners. They are usually colorless, low-viscosity fluids with pungent odors. The biggest downside to organic solvents is that their fumes can cause harm if a great quantity is inhaled, so it is best to work with them outdoors or in a well-ventilated room.

Isopropanol

Isopropyl alcohol is the “classic” amateur liquid fuel. It is commonly referred to by the abbreviation IPA, or the names Isopropanol or 2-Propanol. It is among the best performing liquid fuels, possesses favorable cooling properties for regenerative chambers, and can be obtained at 99% purity very cheaply and easily. While not quite as cost-effective as “dirty” ethanol mixtures, it is very safe and can be used to clean parts and sanitize surfaces.

Ethanol

Pure ethanol has virtually identical properties to isopropanol, but it comes with one major downside:  It is hard to find cheaply because it is taxed heavily for its use in alcoholic beverages. The next best option is a denatured ethanol, but these must be checked carefully because the additives can be hazardous and of wildly varying percentage (some denatured alcohols contain up to 50% methanol). For cost reasons, if pure ethanol is desired, then a mostly pure “dirty” mixture should be used. Dirty ethanols are mixtures that contain ethanol along with other flammable liquids, making them unsafe to drink and slightly less efficient than pure alcohol.

E98

E98 consists of (nominally) 98% ethanol, making it one of the purest dirty ethanols available. E98 can be found at racing shops for reasonable prices, making it only slightly less convenient to acquire than isopropanol or E85.

E85

First debuted in Mojave Sphinx’s initial launch, E85 is the undisputed king of cheap fuels. Nominally consisting of 85% ethanol and 15% gasoline (although this can vary substantially depending on location and season), E85 can be pumped directly from many gas stations at a price comparable to regular gasoline. An inexpensive test kit or simply a graduated cylinder can be used to measure the ethanol content, by mixing the E85 with a small amount of water and observing the stratification between the gasoline and water/ethanol mixture. The gasoline content lowers performance slightly but it still behaves just like ethanol for the most part. One minor safety concern for E85 compared to other solvent fuels is that the gasoline content gives it more flammable vapors than regular ethanol – it should go without saying that open flame and heat sources must be kept away from any fuel.

Methanol

Methanol should not be used. It has no real advantage in amateur liquid rockets, but it has the very real health hazard that it will cause sickness, blindness, and potentially death if inhaled or ingested. There are edge cases where the density and low ideal mixture ratio of methanol can be a benefit, but this is not the case most of the time. If methanol is used, personnel must wear protective equipment, including a respirator mask with an organic molecule filter cartridge.

Acetone

Acetone is the simplest ketone. It is a more powerful solvent than alcohol, and it is also slightly less efficient as a fuel. Acetone tends to swell O-rings and generally degrade the interior of plumbing systems, so if used it should be flushed out as soon as possible with isopropanol, water, or another more benign fuel. There is no real reason to use acetone, unless it is the only solvent on hand or “just because.”

Solvent Mixtures

There are a variety of products available from brands such as Klean Strip which are a mixture of other solvents. Paint thinner, lacquer thinner, brush cleaner, etc. will all work as fuel – however, it is important to look up and read the Safety Data Sheet (SDS) for a product to see if it contains methanol or other hazardous substances and take appropriate precautions.

Hydrocarbons

Hydrocarbons are organic compounds which contain only hydrogen and carbon. The most well-known hydrocarbons are the simple, short-chain molecules such as methane, propane, butane, etc. Oil refinement produces a plethora of long-chain molecules which are separated out by weight and classified under various names depending on the composition and average molecular weight.

The most famous hydrocarbon fuel for rockets is RP-1, a highly refined form of Kerosene which has a long history of use in many launch vehicles. Because hydrocarbons contain no oxygen of their own, they have higher ideal mixture ratios than organic solvents. For rockets burning nitrous oxide, this means that the mixture ratio needs to be very high due to the relatively low oxygen content of N2O. This is undesirable for a few reasons including increased cost and oxidizer feed system size. Realistically, organic solvents win out against hydrocarbons in amateur liquid rockets on every count.

The only reason hydrocarbons (namely diesel) are used in Mojave Sphinx is for the fiery, smokey exhaust plume that makes for a visual spectacle in launches.

Diesel

Like E85, diesel can be pumped cheaply from many gas stations. While not exactly the same mixture of hydrocarbons as “actual” kerosene, it is basically kerosene. Its performance is lower than alcohols because of its very high ideal mixture ratio, so diesel’s only use is for show. Liquid hydrocarbons like diesel also have the downside that they don’t evaporate the way organic solvents do, so it will coat anything it touches in an oily film (including the inside of the plumbing, the outside of the rocket if any spills, hands, tools, etc.). Any other form of kerosene can be substituted in place of diesel.

Jet-A

Jet fuel is another, more pure form of kerosene. Its performance, like any other hydrocarbon, will be low compared to alcohols, and it is not as simple to acquire as diesel. Jet-A was only used in Mojave Sphinx because it was available for free.

Gasoline

Pure gasoline should not be used. It has all the downsides of diesel but the added danger of volatility, meaning that it is always evaporating and creating noxious, flammable vapors.

WD-40

WD-40 was used as a fuel to prove the point that any flammable liquid will work in Mojave Sphinx. It is a proprietary mixture of hydrocarbons, which give it a similar look and performance to kerosene.

Other Light Oils

Although other hydrocarbons have yet to be tried at the time of writing, any light, relatively inviscid oil which burns in air should work as a fuel.

Fuel Additives & Custom Blends

All fuels used in Mojave Sphinx will burn with a natural light orange flame color, which may be indistinct depending on the ambient brightness; in full sunlight, the flame produced by most organic solvent fuels will be largely invisible. Metal salt additives can be used to create brighter and more unique flame colors, which make launches more exciting and visually impressive. All custom blends are typically a mixture of 2% by mass of solute in E85, but they can be made with any alcohol which will dissolve the additive. 2% is mostly arbitrary, but was chosen because it is low enough to be fully soluble in ethanol (or in a small amount of water)  but enough to effectively change the flame color. The performance difference compared to unaltered fuel is negligible, and typically not possible to measure within the capabilities of amateur test setups. Higher or lower concentrations of additives may work just as well but have not been tried yet.

Meowjave Green  [ B(OH)3 ]

Meowjave Green contains boric acid (also called orthoboric acid, and sometimes sold as roach killer), which delivers a bright green flame from the combustion of boron. It is named in homage to the Aerotech solid propellant Mojave Green.

CatPunRed  [ LiCl ]

CatPunRed contains 2% by mass lithium chloride. Its name is the result of a social media suggestion, where the only requirement given was that it include a cat pun. CatPunRed burns a deep red due to the lithium content.

Garfield Gorange  [ CaCl2 ]

Garfield Gorange contains calcium chloride, which results in a vibrant orange flame. While most fuels without additives will burn some shade of orange due to luminescence of carbon in the exhaust, standard liquid rocket motors will typically only produce a pale orange plume due to mediocre efficiency. However, the calcium content causes a much more pronounced orange color and makes the Mach diamonds highly visible. It also contrasts well with a blue sky background in photographs.

Tony Torange  [ NaBr ]

Tony Torange was an attempt to make yellow flame with sodium bromide, but it burned a subdued orange instead. It is possible that a different concentration of the colorant, or in combination with other additives, may change the color to be more unique.

Meowrple  [ KI ]

Meowrple contains potassium iodide dissolved in a small amount of water, since it will not readily dissolve in ethanol on its own. Potassium burns purple, although it is a paler color than other blends; in broad daylight, it may appear more white than purple. The additive is also more expensive, although this is relative since a Mojave Sphinx fuel load costs next to nothing.

Other Additives

There are other metal salts and compounds which may offer additional flame colors, but have not been tried yet. Some compounds, like strontium chloride (and potassium iodide), are soluble in a small amount of water, which can then be mixed into ethanol to add the colorant without affecting performance much. In the future, further additives (and mixtures of additive) will undoubtedly be tried in attempts to make other colors, such as blue.