Compositions – Chemically interactive reactants
Reexamination Certificate
2001-03-09
2004-02-17
Kopec, Mark (Department: 1757)
Compositions
Chemically interactive reactants
C252S500000, C252S183110, C149S017000, C149S018000, C149S019200, C149S053000, C149S060000, C149S063000, C264S003400, C264S003500
Reexamination Certificate
active
06692655
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to multi-base propellants, and especially to cross-linked plastisol propellants suited for use in rocket motor assemblies. This invention further relates to rocket motor assemblies loaded with the multi-base propellants.
2. Description of the Related Art
A typical solid fuel rocket motor generally comprises a case of metal or reinforced polymeric composite material and a nozzle attached to the case. Within the case is a propellant grain, which upon firing undergoes combustion reactions to generate large quantities of combustion gases and particles (i.e., combustion products). The combustion products generated by the propellant grain are expelled through the nozzle attached to the case. Nozzles are designed to accelerate the combustion product gases from the propellant grain to the maximum velocity at exit. Most commonly, this design involves a provision in the nozzle pathway comprising a throat having a restricted cross-sectional area, and a frustoconical skirt aft of the throat. The throat and skirt collectively define a converging/diverging configuration to the nozzle pathway. A heat insulating layer and a liner are usually interposed between the grain and the outer case to protect the outer case from the high operating temperatures associated with rocket motor operation and the erosive high velocity particles generated during combustion of the propellant grain. The liner serves the additional function of enhancing grain-to-case or grain-to-insulation bonding.
Propellants containing nitrocellulose as the principle energetic polymeric binder plasticized with one or more plasticizers are commonly referred to in the art as double-base propellants. A typical formulation for a double-base propellant includes, as its main ingredients, 10-90 wt % nitrocellulose and 10-90 wt % plasticizer, more preferably 40-70 wt % nitrocellulose and 30-60 wt % plasticizer. Among the plasticizers most commonly used in the art for forming double-base propellants are nitroglycerine, butanetrioltrinitrate, and diglycol dinitrate.
Another common ingredient used with plasticized nitrocellulose-based propellants is nitroguanidine. Propellants containing nitrocellulose, one or more plasticizers, and nitroguanidine are commonly referred to in the art as a triple-base propellant. (The term triple-base propellant has also sometimes been used to denote propellants containing nitrocellulose, one or more plasticizers, and energetic fuels other than nitroguanidine.) It is common in the art to classify both double-base and triple-base propellants as multi-base propellants.
Another class of propellants is composite-modified multi-base propellants, in which the nitrocellulose serves the additional function of acting as a binder to immobilize oxidizer particles (e.g., ammonium perchlorate) and/or fuel (e.g., aluminum) particles.
It is known in the art to make multi-base propellants from plastisol-grade nitrocellulose. The term “pelletized nitrocellulose” (PNC) propellant refers to multi-base propellants made via a conventional slurry mixing technique in which the pelletized nitrocellulose is processed by slurry mixing and pouring the mixed slurry, in an uncured state, into casting molds or rocket motors in a casting step. The slurry is prepared by dispersing pelletized nitrocellulose having diameters generally on the order of 1 to 20 microns in a suitable non-solvent diluent, most commonly heptane. To the slurry is added a suitable nitrate ester plasticizer, such as nitroglycerin and/or butanetrioltrinitrate (BTTN). Other processing agents and chemical stabilizers, such as N-methyl-p-nitroaniline (MNA), are also added to the slurry at this stage. After removing a portion of the heptane from the top of the formulation, mixing is performed under vacuum conditions to remove remnants of the heptane from the slurry. Next, further ingredients are added and the formulation is mixed in an appropriate mixer, such as a vertical mixer. These ingredients include, among others, fibers, ballistic additives, energetic solid fuels, and, in the case of a composite multi-base propellant, oxidizer particles and/or fuel particles. After thoroughly mixing the formulation, a suitable cross-linker (e.g., a diisocyanate) may be added and the propellant is cast and cured to form a homogenous propellant.
Advantageous properties associated with multi-base propellants include their excellent ambient mechanical properties, low shock sensitivity, excellent ballistics, and operational characteristics, as well as their low signature plumes. These properties make multi-base propellants highly desirable for many rocket motor applications. However, the use of multi-base propellants is not without its problems.
Several hazards and time-consuming steps make the conventional plastisol production process undesirable for large-scale implementation. For example, although the pelletized nitrocellulose is relatively safe to handle when diluted in heptane, without the diluent the dry nitrocellulose is extremely sensitive to electrostatic discharge (ESD), especially prior to admixture of the nitrocellulose with plasticizer. The ESD sensitivity of the nitrocellulose is especially problematic with nitrocellulose in dry pellet form, since the pellets are characterized by a relatively high surface area. During normal handling of heptane-wet pelletized nitrocellulose, the heptane tends to evaporate due to its low boiling point. Evaporation of the heptane from the slurry tends to leave small quantities of hazardous (electrostatic-discharge sensitive) dry nitrocellulose on the surfaces of tooling and bulk container walls. Special precautions must be taken to avoid the deposition of hazardous dry nitrocellulose and, when such precautions are not fully effective and dry nitrocellulose is deposited on the tooling and bulk container walls, to safely remove the dry nitrocellulose without incident. Removal of the heptane diluent from the plasticized slurry during processing is also labor-intensive, time-consuming, and is usually performed at various stages of the conventional process, requiring repeated assaying of heptane concentration. Heptane is a low conductivity, flammable and hazardous solvent, and must be handled with caution.
Additionally, despite the excellent mechanical properties that multi-base propellants possess at ambient temperatures, multi-base propellants have consistently been found to exhibit inferior mechanical properties, such as tensile strength, at extreme low and elevated temperatures. Dramatic temperature changes that a multi-base propellant experiences in normal fabrication and use may generate mechanical strain in the propellant. If the multi-base propellant does not have satisfactory mechanical properties, these mechanical strains may increase the likelihood of fracture to the propellant grain, especially at low temperature ignition. Fractures in a propellant grain can, if widespread, significantly increase the propellant surface area available for combustion reaction. Attempting to anticipate the degree of fracture and the locations at which fractures will occur adds a large degree of uncertainty and unpredictability to motor performance. As a consequence, the chamber pressure created during combustion of a multi-base propellant grain can be increased to unanticipated levels.
SUMMARY OF THE INVENTION
The present invention is directed to a method of making a multi-base propellant by a suitable technique that substantially avoids the hazards and deleterious processing economies associated with the formation of dry nitrocellulose on processing equipment and tooling, yet produces a multi-base propellant that is mechanically robust, even over a wide range of operating temperatures such as −46° C. (−50° F.) to 66° C. (150° F.), which are normally experienced in rocket motor operation.
In accordance with the principles of this invention, a method for making multi-base propellants according to one embodiment of the invention in which pelletized nitrocellulose is
Cragun Richard B.
Lund Gary K.
Mancini Vincent E.
Martins Laura J.
Wells Michael V.
Alliant Techsystems Inc.
Kopec Mark
Vijayakumar Kallambella
LandOfFree
Method of making multi-base propellants from pelletized... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of making multi-base propellants from pelletized..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of making multi-base propellants from pelletized... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3322677