Propellant tailoring

To establish the relationship between current liquid propellant applications and the available propellant technology, this paper has been divided into three sections. A section on basic propellant considerations describes the normal parameters used to evaluate propellant candidates and their influence on the propulsion system. Although such considerations have been thoroughly discussed in many previous publications (e.g., Ref. 3), their importance in establishing the basic criteria for propellant system selection requires a limited review in this text as a background aid to the reader. Current liquid propellants and propellant candidates are discussed in a second section in terms of capabilities and limitations as well as potential application areas (the compositions of all propellants discussed are defined in the Nomenclature section at the end of this article). Finally, a section of propellant tailoring illustrates examples of propellant formulation and describes propellant problem-solving techniques. In conclusion, the results of these considerations are illustrated by the current liquid propellant systems. 

An example of propellant tailoring is the fuel used to launch the first U. S. satellite into orbit. The original fuel for the launch vehicle was ethyl alcohol. MAF-4 (also known as hydyne or U-DETA), a mixture of 60% UDMH and 40% diethylenetriamine (DETA), was formulated to simulate the physical properties of C2H5OH but provide the increased propellant performance (using liquid oxyen as the oxidizer) requirements of the mission. 

Propellant tailoring has also been studied as a means of upgrading the performance of present liquid oxygen systems by adding F2. This potential performance improvement (indicated in Table II) would apply to both hydrocarbon- and hydrogen-fueled systems. Experimental studies with both systems have verified the propellant system performance increases which can be realized. 

Hercopel a unique all-epoxide cure composite solid propellant with excellent mechanical and ballistic properties. Its outstanding performance in extended environments makes it well suited for tactical missiles Double-Base Solid Propellants a wide variety of physical and ballistic properties which can be tailored to meet specific performance requirements. Their high specific impulse and excellent reproducibility are two of the many reasons Hercules double-base propellants are found in many of our rocket motors and gas generators used for both military and space applications... 

A hydrocarbon prepolymer containing terminal carboxyl groups (28) is available to the propellant chemist. These polymers were synthesized to eliminate some of the variables found in the copolymers. The carboxyl groups can be made of the same types with like reactivity. These linear non-branched polymers impart greater extensibility to elastomeric formulations. The chemistry in propellants is similar to the random functionality polymer. As 20 years of the chemistry of crosslinked propellant binders is reviewed, one familiar with the art cannot fail to predict solid propellant formulations using these polymers tailored to the specific requirements of the solid rocket design with the confidence that any discipline of science can be practiced. 

The urethane reaction is particularly useful for solid propellant applications because of its quantitative nature, convenient rate which can be adjusted by proper choice of catalysts, and the availability of many suitable hydroxyl compounds which permit the tailoring of propellant mechanical properties. Despite the quantitative nature and apparent simplicity of the urethane reaction R NCO + ROH - R NHCOOR, its exact course has not been fully explored yet. It does not follow simple second-order kinetics as the above formula would suggest since its second-order rate constant depends on many factors, such as concentration of reactants and the nature of the solvent. Baker and co-workers (2) proposed that the reaction is initiated through the attack of an alkoxide ion on the carbon atom of the isocyanate group... 

Aziridine. Propellants cured with MAPO have excellent processing characteristics and satisfactory uniaxial tensile properties over a wide range of temperatures. However, the problems associated with the aging behavior of these propellants have led to the use of other types of curing systems which do not contain the P—N bond. These latter materials are di- and trifunctional aziridines, such as those shown in Table IV, and provide satisfactory propellants in which the uniaxial tensile properties can be tailored to a desired modulus. Such mixed aziridine-cured systems give satisfactory initial properties, reduce the postcure behavior, and improve the storage characteristics of CTPB propellants. 

In many situations, the propulsion and vehicle system requirements cannot be met by the available propellant combinations. Such problems are often solved by tailoring. This involves the formulation of a desirable set of characteristics by mixing selected ingredients. Several examples of new propellants that have been developed in this manner are noted below. 

A large number of formulations are available for use the formulation is decided and tailored depending on the requirements of users. However, no single formulation meets all requirements of the user. An ideal gun propellant should possess most of the following properties. 

Minimum Signature. Propellants whose exhaust characteristics are tailored to give not only minimum smoke properties, but also to have low visible, ultraviolet, or infrared emissions are termed minimum signature propellants. Minimum signature propellants are of interest from the standpoints of launch site and missile detectability and from considerations of through-plume guidance. 

The propellant chemist knows what is needed to make a truly advanced propellant—the energy of the cryogenics (fluorine/hydrogen), the density of solids, and the ability to tailor properties to the mission at hand. The energetics are a direct consequence of the simplified specific impulse relationship ... 

The products in Table 7-8 are made by the same procedure as those made for the aerosol hairspray, except no propellant is used and the above represents 100% of the formula. A spritz product can be made from the preceding formulas by proportioning all ingredients except solvent by about 150%. Adjustments will be necessary to meet the VOC regulations. Combinations of resins and tailored spray systems may be used to improve this system further. 

Both experimental and theoretical research aim to elucidate why for some molecule-substrate combinations the desired supramolecular structure formation develops and for others it does not. With the gathered knowledge, the tailoring of the molecular components to promote molecular recognition for noncovalent bond formation together with the choice of the right support is to be propelled. 

CUTEC developed a tailor-made hot gas ejector for anode offgas recycling that uses pressurised propane from standard gas bottles as propellant gas. Propane leaves the ejector nozzle at high velocity and hereby mixes with AOG. A Laval nozzle accelerates the propane stream to supersonic speed and enables a recycle ratio sufficient for soot-free reformer operation. As the ejector has no moving parts it is expected to work robust, even at the high operating temperatures of about 600 C. 

Combustion chemistry strongly depends on the gas molecules that leave the surface of the propellant. Section 3 of this article summarized most of the findings to date. Many of the small molecule products detected in our measurements are likely to be important in feeding the early flame zone. A practical use of the structure/decomposition relationships might then be to connect the synthetic organic chemist and propellant formulator to the combustion scientist. By knowing the flame characteristics of various gas combinations from the work of the combustion scientist, the desirable and undesirable combustion features could be tailored in and out by the synthetic organic chemist and formulator to produce the desired gas combinations. 

---