Caramel fuel is a solid rocket fuel related to mixed fuels with an organic binder. It is named so because of the appearance and use of sugar or sorbitol in its composition. The English term "rocket candy" in the same way characterizes the attitude towards it. Bill Colburn, who used it for the first time in 1948, is considered to be the pioneer in the use of caramel fuel, and this fuel became widely known in the USA with the release of Bertrand Brinley's book in 1960. Widely used in makeshift rockets due to the availability of components.
Content
Composition and properties
The basic, most studied and frequently used composition is 65% KNO 3 and 35% sorbitol (by weight). Such a composition is close to the optimum in achievable specific impulse at small degrees of expansion characteristic of model solid propellant solid propellants . A moderate exponent in the law of combustion makes the fuel suitable for operation in a wide range of pressures, and, as a result, is suitable for artisanal solid propellant rocket motors with a noticeable variation in geometric characteristics.
The finished fuel consists of a solid solution of nitrate in sorbitol and fine particles of insoluble nitrate suspended in it. The melting point of the finished fuel is much lower than the starting components. The solubility of saltpeter in sorbitol in solid form is much less than in the melt, therefore, fuel cools gradually when it cools, as the volume of crystals is released from the solid solution, and a certain amount of heat is released. Large checkers remain soft for more than a day.
The energy characteristics of this composition are very moderate. The theoretical specific impulse of caramel fuel on potassium nitrate is 153 kgf × s / kg, and practically achievable does not exceed 125 kgf × s / kg. This is less than that of cheap ballistic fuel based on nitrocellulose, therefore, this composition is not used industrially. However, this is significantly more than that of smoky gunpowder, in addition, the manufacture of caramel fuel does not require the specific equipment necessary for the production of gunpowder, therefore it is popular with manufacturers of model rocket engines, both artisanal and serial commercial.
When sorbitol is replaced with sucrose in the fuel composition, the burning rate increases quite significantly, by 40% at atmospheric pressure, but other fuel properties (density, specific impulse, exponent in the law of combustion, etc.) almost do not change. The main disadvantage of the sucrose composition is the much more dangerous preparation process, since more intense heating of sucrose is required.
Weaknesses
Despite its relative safety, in comparison with other compositions, caramel fuels require the same precautions for use as any other rocket fuel, since it is a high-energy composition.
The main disadvantages of this fuel are hygroscopicity and a large amount of condensed phase in the combustion products. The fragility of this fuel should also be recognized as a disadvantage, which narrows the choice of solid propellant rocket constructions using it. Finally, the drawback is significant shrinkage (reduction in volume) during solidification, which can cause distortion of the shape of the checker or delamination of the reservation.
The initial fuel is low-toxic, but its combustion products can irritate the mucous membranes and respiratory organs, since potassium carbonate , which is released in a highly dispersed form and has an alkaline reaction, can cause a chemical burn even after cooling to room temperature. The combustion temperature of the base composition is approximately 1400 ℃, this is enough to soften the steel body of the solid propellant rocket when exposed to it without thermal protection.
See also
- Rocket fuel