Hexaphenox

The term hexahenox is an artificial word. Hex stands for hexamine, phen for the chemical name of the catalyst phenolphthalein and ox for oxidizer KClO₃. Heaphenox is an experimental rocket propellant.

Potassium chlorate is a strong oxidizing agent and should be handled with care. It reacts vigorously, and in some cases spontaneously ignites or explodes, when mixed with many combustible materials. When mixed with a suitable fuel, it may form an explosive material, a so-called Sprengel explosive. Sulfur should be avoided in pyrotechnic compositions containing potassium chlorate, as these mixtures are prone to spontaneous deflagration. Also, mixtures of potassium chlorate with any compound with ignition promoting properties (for example Sb₂S₃) are very dangerous to prepare, as they are extremely shock sensitive.

Hexamine ((CH₂)₆N₄) is a white crystalline compound, which is highly soluble in water and polar organic solvents. It sublimes in a vacuum at 280 °C. Together with 1,3,5-trioxane, hexamine is a component of hexamine fuel tablets used by campers, hobbyists, the military and relief organizations for heating camping food or military rations. It burns smokeless, has a high energy density (30.0 MJ/kg), does not liquefy while burning and leaves no ashes. Hexamine has been used in colored fireworks compositions as a low-reactivity, accessory fuel. Indeed hexamine is not oxidizable by KNO₃, NaNO₃, LiNO₃, RbNO₃, Ba(NO₃)₂ and probably all other alkali or alkaline earth metal nitrates. Hexamine reacts though violently with AgNO₃. A hexamine/KNO₃ mixture form cyanides, highly basic compounds among others when enough energy in form of heat is applied, but does not combust. There are many color reactions used in cyanide determination. I have used the classical Prussian blue method:

A piece of the reaction product of hexamine and KNO₃ was dissolved in distilled water and filtered off. A fresh 10% iron(II) sulfate solution was added in shortfall. The solution turned green. After some drops of iron(III) chloride solution were added, the solution turned into deep blue:

Fe²⁺ + 6 CN → [Fe(CN)₆]^4-
Fe³⁺ + K₄[Fe(CN)₆] → K[Fe^IIFe^III(CN)₆] + 3 K⁺

Phenolphthalein, the catalyst, is a chemical compound with the formula C₂₀H₁₄O₄. Phenolphthalein is insoluble in water but solvable in some alcohols like methanol. It is primarily used as an indictor. It has been also used for over a century as a laxative but is now being removed because of concerns over carcinogenicity. It was to the best of my knowledge never used in rocketry as a catalyst.

Phenolphthalein is one of very less substances which enable the oxidation of hexamine by KNO₃ or other alkali or alkaline earth metal nitrates. Hexamine can be oxidized by KClO₃ without the presence of phenolphthalein, but a lot of heat needs to be applied to start the combustion. Phenolphthalein has a similar effect as sulfur in gun powder mixtures: It makes the powder easier to ignite, thereby increasing the rate of combustion. Besides phenolphthalein prevents the burning hexaphenox to form fuzed salt residues, which can block the rocket nozzle. It also prevents chemical reactions which make a combustion impossible (KNO₃/hexamine). The only residue left using phenolphthalein is some spongy carbon which can easily be blown out from the combustion chamber.

The current hexaphenox mixture contains 75 % w/w KClO₃, 15 % w/w hexamine and 10 % w/w phenolphthalein, very similar to gun powder. This ratio was found empirically. The calculated stoichiometric ratio might be different.

Hexaphenox it neither sensitive to shock, friction or electrostatic discharge.

The video below shows the first ever flight of a rocket propelled by hexaphenox. A C6 model rocket casing was re-used, filled with 10g  compressed hexaphenox. The grain had a core of 4mm dia. The nozzle throat was extended to a dia. of 4.8mm.

Hexaphenox propelled rocket clears launch pad: