Propellants case-bonded

Figure 4.17 Schematic of a case-bonded propellant. Reprinted from G.P.Sutton, Rocket Propulsion Elements An Introduction to the Engineering of Rockets, 1992 1992, John Wiley and Sons, Chichester, UK.

Since most rockets are regeneratively cooled, this loss is not felt. There is more loss in solid propellant rockets but even here with modern case bonded propellants the losses are almost entirely confined to the nozzle except at the very end of burning and are of no great significance. 

Propellants cast into rockets are commonly case-bonded to the motors to achieve maximum volumetric loading density. The interior of the motor is thoroughly cleaned, coated using an insulating material, and then lined with a composition to which the propellant binder adheres under the environmental stresses of the system. The insulation material is generally a mbber-type composition, filled with siUca, titanium dioxide, or potassium titanate. SiUca-filled nitrate mbber and vulcanizable ethylene—propylene mbber have been used. The liner generally consists of the same base polymer as is used in the propellant. It is usually appHed in a thin layer, and may be partially or fully cured before the propellant is poured into the rocket. 

The advanced applications for nitrocellulose plastisol propellants require that they be integrally bonded to the motor case. Successful case bonding for the multiyear storage life of a rocket calls for special adhesives and liners which are completely compatible with these highly plasticized propellants. Best results have been obtained with a combination of an impervious rubber liner and a crosslinked adhesive system with a limited affinity for the plasticizers used in the propellants. Examples of effective liners are silica-filled butyl rubber and chlorinated synthetic rubber. Epoxy polyamides, isocyanate-crosslinked cellulose esters, and combinations of crosslinked phenol-formaldehyde and polyvinyl formal varnishes have proved to be effective adhesives between propellant and impervious liners. Pressure curing of the propellants helps... 

The requirements, manufacturing procedures, designs of case-bonded solid propellant rockets, and the performance characteristics of a useful... 

Compatability of ingredients is always important. Particularly important to safe handling is the autoignition temperature and friction sensitivity of the propellant. Small changes in the formulation can often effect these important properties. An example is the sensitivity of some propellant formulations to extremely small amounts of chlorate. Safety precautions must always include consideration of the chemistry. By following this practice, the propellant industry has experienced an excellent safety record while making unusual progress in the application of viscoelastic materials in case-bonded solid propellant rockets. 

The other way of providing protection to a motor is to apply a polymeric material or rubber to its inner surface and cast propellant directly into the insulated motor. Before casting propellant mix, a liner material is also applied to the motor insulation in order to ensure a strong bond between them. Such propellant grain in known as case-bonded grain which is shown in Figure 4.17. 

It is characteristic of the technology of the manufacture of rocket charges containing composite propellants with thiokol that the semi-liquid mixture is poured directly into the rocket chamber lined from within with an insulating layer to which the charge adheres tightly ( case-bonded charge). This is a very cheap and rapid method of manufacture. 

Thermal variations. A solid propellant grain may be exposed to temperature extremes ranging from conditions of low temperature in the upper atmosphere, to tropical heat. The grain must not be too brittle at low temperature nor too soft at high temperature. The problem can be aggravated by case-bonding since different coefficients of expansion of the metal-case and the propellant can produce a very harmful effect. 

Case bonded -> Composite Propellants are unloaded from their casing by a remote controlled lathe or water gun also - Case Bonding Also -> Destruction of Explosive Materials... 

Star-shaped grooves in case-bonded charges tend to equalize the pressure and suppress the tendency to resonate. Other relevant keywords are -+ Burning Rate, - Solid Propellant Rockets. 

The difficulties encountered with inhibition of propellant powders have been mentioned, specifically of case-bonding i.e. the attachment of cylindrical powder grains to the envelope in order to achieve cigarette-type or internal-surface burning. The subject applies also to flare candles. 

In some applications the propellant can be cast directly on the primed chamber wall in most cases, however, the propellant is bonded to the wall or to the heat insulation through an intermediate layer—the liner (36). 

To ensure a strong bond between liner and insulation as well as propellant to liner, it is necessary that liner as well as propellant cure well at the interfaces. This means that in many cases the rubber insulation must undergo some treatment to remove substances which may interfere with the liner cure. Such substances are usually low molecular weight compounds and can often be removed by heating—e.g., water, which would otherwise react with isocyanate in a polyurethane liner. In addition the insulation and/or the cured liner surface may be washcoated with a cure catalyst which will increase the reaction rate of alcoholic hydroxyl groups over the rate of reaction of water with isocyanate to such an extent that the latter reaction can no longer compete with the cure reaction. 

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